Can passive stretch inhibit motoneuron facilitation in the human plantar flexors?

Reduction and recovery of self-sustained muscle activity after fatiguing plantar flexor contractions

PURPOSE

Persistent inward calcium and sodium currents (PICs) are crucial for initiation and maintenance of motoneuron firing, and thus muscular force. However, there is a lack of data describing the effects of fatiguing exercise on PIC activity in humans. We simultaneously applied tendon vibration and neuromuscular electrical stimulation (VibStim) before and after fatiguing exercise. VibStim induces self-sustained muscle activity that is proposed to result from PIC activation.

METHODS

Twelve men performed 5-s maximal isometric plantar flexor contractions (MVC) with 5-s rests until joint torque was reduced to 70%MVC. VibStim trials consisted of five 2-s trains of neuromuscular electrical stimulation (20 Hz, evoking 10% MVC) of triceps surae with simultaneous Achilles tendon vibration (115 Hz) without voluntary muscle activation. VibStim was applied before (PRE), immediately (POST), 5-min (POST-5), and 10-min (POST-10) after exercise completion.

RESULTS

Sustained torque (Tsust) and soleus electromyogram amplitudes (EMG) measured 3 s after VibStim were reduced (Tsust: -59.0%, p < 0.001; soleus EMG: -38.4%, p < 0.001) but largely recovered by POST-5, and changes in MVC and Tsust were correlated across the four time points (r = 0.69; p < 0.001). After normalisation to values obtained at the end of the vibration phase to control for changes in fibre-specific force and EMG signal characteristics, decreases in Tsust (-42.9%) and soleus EMG (-22.6%) remained significant and were each correlated with loss and recovery of MVC (r = 0.41 and 0.46, respectively).

CONCLUSION

The parallel changes observed in evoked self-sustained muscle activity and force generation capacity provide motivation for future examinations on the potential influence of fatigue-induced PIC changes on motoneuron output.

Combined Effects of Static and Dynamic Stretching on the Muscle-Tendon Unit Stiffness and Strength of the Hamstrings

ABSTRACT

Takeuchi, K, Nakamura, M, Matsuo, S, Samukawa, M, Yamaguchi, T, and Mizuno, T. Combined effects of static and dynamic stretching on the muscle-tendon unit stiffness and strength of the hamstrings. J Strength Cond Res 38(4): 681-686, 2024-Combined static and dynamic stretching for 30 seconds is frequently used as a part of a warm-up program. However, a stretching method that can both decrease muscle-tendon unit (MTU) stiffness and increase muscle strength has not been developed. The purpose of this study was to examine the combined effects of 30 seconds of static stretching at different intensities (normal-intensity static stretching [NS] and high-intensity static [HS]) and dynamic stretching at different speeds (low-speed dynamic [LD] and high-speed dynamic stretching [HD]) on the MTU stiffness and muscle strength of the hamstrings. Thirteen healthy subjects (9 men and 4 women, 20.9 ± 0.8 years, 169.3 ± 7.2 cm, 61.1 ± 8.2 kg) performed 4 types of interventions (HS-HD, HS-LD, NS-HD, and NS-LD). Range of motion (ROM), passive torque, MTU stiffness, and muscle strength were measured before and immediately after interventions by using an isokinetic dynamometer machine. In all interventions, the ROM and passive torque significantly increased (p < 0.01). Muscle-tendon unit stiffness significantly decreased in HS-HD and HS-LD (both p < 0.01), but there was no significant change in NS-HD (p = 0.30) or NS-LD (p = 0.42). Muscle strength significantly increased after HS-HD (p = 0.02) and NS-LD (p = 0.03), but there was no significant change in HS-LD (p = 0.23) or NS-LD (p = 0.26). The results indicated that using a combination of 30 seconds of high-intensity static stretching and high-speed dynamic stretching can be beneficial for the MTU stiffness and muscle strength of the hamstrings.

Acute effects of static and dynamic stretching for ankle plantar flexors on postural control during the single-leg standing task

Static stretching (SS) and dynamic stretching (DS) are widely used as warm-ups before sports. However, whether stretching affects postural control remains unclear. We compared the effects of SS and DS on the plantar flexors and postural control during single-leg standing. Fifteen healthy young participants performed SS, DS, or no stretching (control). The stretch condition consisted of four sets lasting 30 s each. The control condition was a rest with standing for 210 s. Center of pressure (COP) displacement was measured using a force plate before and after each intervention to assess postural control during the single-leg standing task. The COP area, COP velocity, and anteroposterior (COPAP) and mediolateral (COPML) range were calculated. DS significantly decreased in the COPML range (21.5 ± 4.1 to 19.0 ± 2.5 mm; P = 0.02), COP velocity (33.8 ± 7.6 to 29.8 ± 6.5 mm/s; P < 0.01), and COP area (498.6 ± 148.3 to 393.3 ± 101.1 mm2; P < 0.01), whereas SS did not change in the COP parameters (COP area 457.2 ± 108.3 to 477.8 ± 106.1 mm2, P = .49; COP velocity 31.2 ± 4.2 to 30.7 ± 5.8 mm/s, P = 0.60; COPAP 25.4 ± 3.1 to 25.3 ± 3.2 mm, P = 0.02; COPML 20.7 ± 3.3 to 21.1 ± 2.5 mm, P = 0.94). Therefore, DS of the plantar flexors enhances postural control during single-leg standing and may be effective for both injury prevention and performance enhancement.

Effects of jaw clenching and mental stress on persistent inward currents estimated by two different methods

Spinal motoneuron firing depends greatly on persistent inward currents (PICs), which in turn are facilitated by the neuromodulators serotonin and noradrenaline. The aim of this study was to determine whether jaw clenching (JC) and mental stress (MS), which may increase neuromodulator release, facilitate PICs in human motoneurons. The paired motor unit (MU) technique was used to estimate PIC contribution to motoneuron firing. Surface electromyograms were collected using a 32-channel matrix on gastrocnemius medialis (GM) during voluntary, ramp, plantar flexor contractions. MU discharges were identified, and delta frequency (ΔF), a measure of recruitment-derecruitment hysteresis, was calculated. Additionally, another technique was used (VibStim) that evokes involuntary contractions that persist after cessation of combined Achilles tendon vibration and triceps surae neuromuscular electrical stimulation. VibStim measures of plantar flexor torque and soleus activity may reflect PIC activation. ΔF was not significantly altered by JC (p = .679, n = 18, 9 females) or MS (p = .147, n = 14, 5 females). However, all VibStim variables quantifying involuntary torque and muscle activity during and after vibration cessation were significantly increased in JC (p < .011, n = 20, 10 females) and some, but not all, increased in MS (p = .017-.05, n = 19, 10 females). JC and MS significantly increased the magnitude of involuntary contractions (VibStim) but had no effect on GM ΔF during voluntary contractions. Effects of increased neuromodulator release on PIC contribution to motoneuron firing might differ between synergists or be context dependent. Based on these data, the background level of voluntary contraction and, hence, both neuromodulation and ionotropic inputs could influence neuromodulatory PIC enhancement.

Prolonged static stretching increases the magnitude and decreases the complexity of knee extensor muscle force fluctuations

Static stretching decreases maximal muscle force generation in a dose-response manner, but its effects on the generation of task-relevant and precise levels of submaximal force, i.e. force control, is unclear. We investigated the effect of acute static stretching on knee extensor force control, quantified according to both the magnitude and complexity of force fluctuations. Twelve healthy participants performed a series of isometric knee extensor maximal voluntary contractions (MVCs) and targeted intermittent submaximal contractions at 25, 50 and 75% MVC (3 x 6 seconds contraction separated by 4 seconds rest, with 60 seconds rest between each intensity) prior to, and immediately after, one of four continuous static stretch conditions: 1) no stretch; 2) 30-second stretch; 3) 60-second stretch; 4) 120-second stretch. The magnitude of force fluctuations was quantified using the standard deviation (SD) and coefficient of variation (CV), while the complexity of fluctuations was quantified using approximate entropy (ApEn) and detrended fluctuation analysis (DFA) α. These measures were calculated using the steadiest 5 seconds of the targeted submaximal contractions at each intensity (i.e., that with the lowest SD). Significant decreases in MVC were evident following the 30, 60 and 120-second stretch conditions (all P < 0.001), with a significant correlation observed between stretch duration and the magnitude of decrease in MVC (r = -0.58, P < 0.001). The 120-second stretch resulted in significant increases in SD at 50% MVC (P = 0.007) and CV at 50% (P = 0.009) and 75% MVC (P = 0.005), and a significant decrease in ApEn at 75% MVC (P < 0.001). These results indicate that the negative effects of prolonged static stretching extend beyond maximal force generation tasks to those involving generation of precise levels of force during moderate- to high-intensity submaximal contractions.

Relationship between disturbances of CO2 homeostasis and force output characteristics during isometric knee extension

To determine whether disturbances of CO₂ homeostasis alter force output characteristics of lower limb muscles, participants performed four isometric knee extension trials (MVC_30%, 10s each with 20-s rest intervals) in three CO₂ conditions (normocapnia [NORM], hypercapnia [HYPER], and hypocapnia [HYPO]). Respiratory frequency and tidal volume were matched between CO₂ conditions. In each MVC_30%, the participants exerted a constant force (30% of maximum voluntary contraction [MVC]). The force coefficient of variation (Fcv) during each MVC_30% and MVC before and after the four MVC_30% trials were measured. For the means of the four trials, Fcv was significantly lower in HYPER than in HYPO. However, within HYPER, a significant positive correlation was found between the increase in end-tidal CO₂ partial pressure and the increase in Fcv. MVCs in NORM and HYPO decreased significantly over the four trials, while no such reduction was observed in HYPER. These results suggest that perturbed CO₂ homeostasis influences the force output characteristics independently of breathing pattern variables.

The Effects of Static Stretching Intensity on Range of Motion and Strength: A Systematic Review

The aim of this study was to systematically review the evidence on the outcomes of using different intensities of static stretching on range of motion (ROM) and strength. PubMed, Web of Science and Cochrane controlled trials databases were searched between October 2021 and February 2022 for studies that examined the effects of different static stretching intensities on range of motion and strength. Out of 6285 identified records, 18 studies were included in the review. Sixteen studies examined outcomes on ROM and four on strength (two studies included outcomes on both ROM and strength). All studies demonstrated that static stretching increased ROM; however, eight studies demonstrated that higher static stretching intensities led to larger increases in ROM. Two of the four studies demonstrated that strength decreased more following higher intensity stretching versus lower intensity stretching. It appears that higher intensity static stretching above the point of discomfort and pain may lead to greater increases in ROM, but further research is needed to confirm this. It is unclear if high-intensity static stretching leads to a larger acute decrease in strength than lower intensity static stretching.

The Combined Effect of Static Stretching and Foam Rolling With or Without Vibration on the Range of Motion, Muscle Performance, and Tissue Hardness of the Knee Extensor

ABSTRACT

Nakamura, M, Konrad, A, Kasahara, K, Yoshida, R, Murakami, Y, Sato, S, Aizawa, K, Koizumi, R, and Wilke, J. The combined effect of static stretching and foam rolling with or without vibration on the range of motion, muscle performance, and tissue hardness of the knee extensor. J Strength Cond Res 37(2): 322-327, 2023-Although the combination of static stretching (SS) and foam rolling (FR) is frequently used for warm-up in sports, the effect of the intervention order is unclear. This study compared mechanical tissue properties, pain sensitivity, and motor function after SS and FR (with and without vibration) performed in different orders. Our randomized, controlled, crossover experiment included 15 healthy male subjects (22.5 ± 3.3 years) who visited the laboratory 5 times (inactive control condition, FR + SS, FR vibration + SS, SS + FR, and SS + FR vibration ) with an interval of ≥48 hours. In each session, subjects completed three 60-second bouts of FR and SS, targeting the anterior thigh. Pressure pain threshold, tissue hardness, knee flexion range of motion (ROM), maximal voluntary isometric (MVC-ISO), and concentric (MVC-CON) torque, as well as countermovement jump height, were determined before and after the intervention. All interventions significantly ( p < 0.01) increased knee flexion ROM ( d = 0.78, d = 0.87, d = 1.39, and d = 0.87, respectively) while decreasing tissue hardness ( d = -1.25, d = -1.09, d = -1.18, and d = -1.24, respectively). However, MVC-ISO torque was significantly reduced only after FR + SS ( p = 0.05, d = -0.59). Our results suggest that SS should be followed by FR when aiming to increase ROM and reduce tissue hardness without concomitant stretch-induced force deficits (MVC-ISO, MVC-CON, and countermovement jump height). Additionally, adding vibration to FR does not seem to affect the magnitude of changes observed in the examined outcomes.

Exogenous Caffeine Ingestion Does Not Increase Plantarflexor Torque in Older or Younger Men

Aging is associated with neurodegeneration and a loss of muscle function, especially in lower-limb muscles. While caffeine may augment muscle force generation through multiple effects on the central nervous system, no studies have yet compared the effects of caffeine on force-generating capacity between younger and older men, who might respond differently due to age-related changes in the structures on which caffeine acts. In a double-blind, controlled trial, 22 younger (25 ± 5 years) and 21 older (68 ± 6 years) men were tested for isometric plantarflexor torque on two separate days (2-7 days apart) before and 60 min after ingesting 3 mg/kg (∼2 cups of coffee) of caffeine or placebo. No effects of caffeine ingestion on peak torque or rate of torque development were detected in either older or younger men. Therefore, 3 mg/kg of caffeine may not acutely counteract age-related decreases in force capacity of the functionally important plantarflexor muscles.

Positional transversal release is effective as stretching on range of movement, performance and balance: a cross-over study

Background

The aim of this study was to compare the positional transversal release (PTR) technique to stretching and evaluate the acute effects on range of movement (ROM), performance and balance.

Methods

Thirty-two healthy individuals (25.3 ± 5.6 years; 68.8 ± 12.5 kg; 172.0 ± 8.8 cm) were tested on four occasions 1 week apart. ROM through a passive straight leg raise, jumping performance through a standing long jump (SLJ) and balance through the Y-balance test were measured. Each measure was assessed before (T0), immediately after (T1) and after 15 min (T2) of the provided intervention. On the first occasion, no intervention was administered (CG). The intervention order was randomized across participants and comprised static stretching (SS), proprioceptive neuromuscular facilitation (PNF) and the PTR technique. A repeated measure analysis of variance was used for comparisons.

Results

No differences across the T0 of the four testing sessions were observed. No differences between T0, T1 and T2 were present for the CG session. A significant time × group interaction for ROM in both legs from T0 to T1 (mean increase of 5.4° and 4.9° for right and left leg, respectively) was observed for SS, PNF and the PTR. No differences for all groups were present between T1 and T2. No differences in the SLJ and in measures of balance were observed across interventions.

Conclusions

The PTR is equally effective as SS and PNF in acutely increasing ROM of the lower limbs. However, the PTR results less time-consuming than SS and PNF. Performance and balance were unaffected by all the proposed interventions.

Effects of three neuromuscular electrical stimulation methods on muscle force production and neuromuscular fatigue

This study compared the acute responses of three neuromuscular electrical stimulation (NMES) methods on muscle torque-time integral (TTI) and neuromuscular fatigue. Narrow-pulse (0.2 ms; NP), wide-pulse (1 ms; WP), and tendon vibration superimposed onto wide-pulse (WP + VIB)-NMES conditions were applied to sixteen healthy individuals (n = 16) in three separate sessions in a randomized order. Stimulation intensity was set to elicit 20% of maximal voluntary contraction (MVC); the stimulus pattern comprised four sets of 20 repetitions (5 s On and 5 s Off) with a one-minute inter-set interval. TTI was measured for each NMES condition and MVC, voluntary activation (VA), peak twitch torque (Peak_twitch ), and peak soleus (EMG_SOL ), medial (EMG_MG ), and lateral gastrocnemius (EMG_LG ) electromyography were measured before and immediately after each NMES condition. TTI was higher during WP + VIB (19.63 ± 6.34 MVC.s, mean difference = 3.66, p < 0.001, Cohen's d = 0.501) than during WP (15.97 ± 4.79 MVC.s) condition. TTI was higher during WP + VIB (mean difference = 3.79, p < 0.001, Cohen's d = 0.626) than during NP (15.84 ± 3.73 MVC.s) condition. MVC and Peak_twitch forces decreased (p ≤ 0.001) immediately after all conditions. No changes were observed for VA (p = 0.365). EMG_SOL amplitude reduced (p = 0.040) only after NP, yet EMG_LG and EMG_MG amplitudes decreased immediately after all conditions (p = 0.003 and p = 0.013, respectively). WP + VIB produced a higher TTI than WP and NP-NMES, with similar amounts of neuromuscular fatigue across protocols. All NMES protocols induced similar amounts of peripheral fatigue and reduced EMG amplitudes.

The Time-Course Changes in Knee Flexion Range of Motion, Muscle Strength, and Rate of Force Development After Static Stretching

Previous studies have shown that longer-duration static stretching (SS) interventions can cause a decrease in muscle strength, especially explosive muscle strength. Furthermore, force steadiness is an important aspect of muscle force control, which should also be considered. However, the time course of the changes in these variables after an SS intervention remains unclear. Nevertheless, this information is essential for athletes and coaches to establish optimal warm-up routines. The aim of this study was to investigate the time course of changes in knee flexion range of motion (ROM), maximal voluntary isometric contraction (MVIC), rate of force development (RFD), and force steadiness (at 5 and 20% of MVIC) after three 60-s SS interventions. Study participants were sedentary healthy adult volunteers (n = 20) who performed three 60-s SS interventions of the knee extensors, where these variables were measured before and after SS intervention at three different periods, i.e., immediately after, 10 min, and 20 min the SS intervention (crossover design). The results showed an increase in ROM at all time points (d = 0.86-1.01). MVIC was decreased immediately after the SS intervention (d = -0.30), but MVIC showed a recovery trend for both 10 min (d = -0.17) and 20 min (d = -0.20) after the SS intervention. However, there were significant impairments in RFD at 100 m (p = 0.014, F = 6.37, η_p ² = 0.101) and 200 m (p < 0.01, F = 28.0, η_p ² = 0.33) up to 20 min after the SS intervention. Similarly, there were significant impairments in force steadiness of 5% (p < 0.01, F = 16.2, η_p ² = 0.221) and 20% MVIC (p < 0.01, F = 16.0, η_p ² = 0.219) at 20 min after the SS intervention. Therefore, it is concluded that three 60-s SS interventions could increase knee flexion ROM but impair explosive muscle strength and muscle control function until 20 min after the SS intervention.

Effects of reciprocal inhibition and whole-body relaxation on persistent inward currents estimated by two different methods

Abstract

Persistent inward currents (PICs) are crucial for initiation, acceleration, and maintenance of motoneuron firing. As PICs are highly sensitive to synaptic inhibition and facilitated by serotonin and noradrenaline, we hypothesised that both reciprocal inhibition (RI) induced by antagonist nerve stimulation and whole‐body relaxation (WBR) would reduce PICs in humans. To test this, we estimated PICs using the well‐established paired motor unit (MU) technique. High‐density surface electromyograms were recorded from gastrocnemius medialis during voluntary, isometric 20‐s ramp, plantarflexor contractions and decomposed into MU discharges to calculate delta frequency (ΔF). Moreover, another technique (VibStim), which evokes involuntary contractions proposed to result from PIC activation, was used. Plantarflexion torque and soleus activity were recorded during 33‐s Achilles tendon vibration and simultaneous 20‐Hz bouts of neuromuscular electrical stimulation (NMES) of triceps surae. ΔF was decreased by RI (n = 15, 5 females) and WBR (n = 15, 7 females). In VibStim, torque during vibration at the end of NMES and sustained post‐vibration torque were reduced by WBR (n = 19, 10 females), while other variables remained unchanged. All VibStim variables remained unaltered in RI (n = 20, 10 females). Analysis of multiple human MUs in this study demonstrates the ability of local, focused inhibition to attenuate the effects of PICs on motoneuron output during voluntary motor control. Moreover, it shows the potential to reduce PICs through non‐pharmacological, neuromodulatory interventions such as WBR. The absence of a consistent effect in VibStim might be explained by a floor effect resulting from low‐magnitude involuntary torque combined with the negative effects of the interventions.

Key points

Spinal motoneurons transmit signals to skeletal muscles to regulate their contraction. Motoneuron firing partly depends on their intrinsic properties such as the strength of persistent (long‐lasting) inward currents (PICs) that make motoneurons more responsive to excitatory input.

In this study, we demonstrate that both reciprocal inhibition onto motoneurons and whole‐body relaxation reduce the contribution of PICs to human motoneuron firing. This was observed through analysis of the firing of single motor units during voluntary contractions.

However, an alternative technique that involves tendon vibration and neuromuscular electrical stimulation to evoke involuntary contractions showed less effect. Thus, it remains unclear whether this alternative technique can be used to estimate PICs under all physiological conditions.

These results improve our understanding of the mechanisms of PIC depression in human motoneurons. Potentially, non‐pharmacological interventions such as electrical stimulation or relaxation could attenuate unwanted PIC‐induced muscle contractions in conditions characterised by motoneuron hyperexcitability.

Concurrent Achilles tendon vibration and tibial nerve stimulation to estimate persistent inward current strength in motoneurons

Vibratory (Tvib) and sustained (Tsust) torque responses to concurrent Achilles tendon vibration and neuromuscular electrical stimulation applied over the muscle belly (vib+stim) are used as indicators of motoneuron facilitation and, theoretically, persistent inward current strength. However, neuromuscular electrical stimulation (NMES) applied to the nerve trunk may potentiate motoneuronal excitability more than muscle belly NMES, yet it remains unclear whether NMES applied over the nerve evokes robust Tvib and Tsust responses when used during the vib+stim protocol. This study tested whether a nerve-targeted vib+stim protocol elicits Tvib and Tsust responses in the ankle plantar flexors with acceptable intra- and inter-session reliability. Fifteen men performed the vib+stim protocol with NMES applied over the tibial nerve three times across two sessions; twice in a single session (5-min apart) to test intrasession reliability and then again after 48 h to test intersession reliability. Intraclass correlation coefficients (ICC3,1), within-participant coefficients of variation (CV) and pairwise comparisons were used to verify relative and absolute reliability as well as systematic bias. Thirteen men presented Tvib and Tsust responses (response rate of 87%). Intrasession Tvib and Tsust ICCs were >0.73 but inter-session ICCs were <0.5. Although no systematic bias was detected (p>0.05), both intra- and inter-session CVs were large (>10%) for Tvib and Tsust. The Vib+stim protocol with NMES applied over the nerve evoked Tvib and Tsust in almost all participants, but presented a large intra- and inter-session variability. The method does not appear to be effective for assessing motoneuron facilitation in the plantar flexors.

Enhancing Adaptations to Neuromuscular Electrical Stimulation Training Interventions

Neuromuscular electrical stimulation (NMES) applied to skeletal muscles is an effective rehabilitation and exercise training modality. However, the relatively low muscle force and rapid muscle fatigue induced by NMES limit the stimulus provided to the neuromuscular system and subsequent adaptations. We hypothesize that adaptations to NMES will be enhanced by the use of specific stimulation protocols and adjuvant interventions.

Neuromuscular Correlates of the Contralateral Stretch-induced Strength Loss

PURPOSE

The current study investigated the effects of unilateral passive stretching on the neuromuscular mechanisms involved in the force-generating capacity of the contralateral muscle.

METHODS

Twenty-six healthy men underwent unilateral passive stretching of the plantarflexors (5 × 45 s on + 15 s off; total stretching time, 225 s). Before and after the stretching protocol, contralateral ankle range of motion, maximum voluntary contraction (MVC) of the plantarflexors, and surface electromyographic root-mean-square (sEMG RMS) of the soleus and the gastrocnemii muscles were determined. Concurrently, V-wave, maximum and superimposed H-reflex, and M-wave were elicited via nerve stimulation to estimate the supraspinal, spinal, and peripheral mechanisms, respectively. sEMG RMS, V-wave, and H-reflex were normalized to the M-wave.

RESULTS

After passive stretching, contralateral ankle range of motion was increased (+8% [1%/15%], effect size [ES] = 0.43 [0.02/0.84], P < 0.001), MVC of the plantarflexors was decreased (-9% [-21%/-2%], ES = -0.96 [-1.53/-0.38], P < 0.001), and the sEMG RMS/M-wave of the soleus and the gastrocnemii muscles was decreased (≈-9%, ES ≈ -0.33, P < 0.05). Concurrently, the V-wave/M-wave superimposed was decreased in all muscles (≈-13%, ES = -0.81 to -0.52, P < 0.05). No change in H-reflex/M-wave and M-wave was observed under both maximum and superimposed condition. The decrease in the MVC and the sEMG RMS of the contralateral muscle was accompanied by a decrease in the V-wave/M-wave but not the H-reflex/M-wave ratios and the M-wave.

CONCLUSIONS

The present outcomes suggest that only supraspinal mechanisms might be involved in the contralateral decrease in the maximum force-generating capacity.

Enhancement of self-sustained muscle activity through external dead space ventilation appears to be associated with hypercapnia

We reported that external dead space ventilation (EDSV) enhanced self-sustained muscle activity (SSMA) of the human soleus muscle, which is an indirect observation of plateau potentials. However, the main factor for EDSV to enhance SSMA remains unclear. The purpose of the present study was to examine the effects of EDSV-induced hypercapnia, hypoxia, and hyperventilation on SSMA. In Experiment 1 (n = 11; normal breathing [NB], EDSV, hypoxia, and voluntary hyperventilation conditions) and Experiment 2 (n = 9; NB and normoxic hypercapnia [NH] conditions), SSMA was evoked by electrical train stimulations of the right tibial nerve and measured using surface electromyography under each respiratory condition. In Experiment 1, SSMA was significantly higher than that in the NB condition only in the EDSV condition (P < 0.05). In Experiment 2, SSMA was higher in the NH condition than in the NB condition (P < 0.05). These results suggest that the EDSV-enhanced SSMA is due to hypercapnia, not hypoxia or increased ventilation.

Responses to a combined dynamic stretching and antagonist static stretching warm-up protocol on isokinetic leg extension performance

Antagonist static-stretching and dynamic-stretching are both effective at improving muscular performance. The purpose of this study was to investigate responses to a dynamic stretching warm-up protocol, a static-stretching warm-up protocol and a combined dynamic-stretching and antagonist static stretching warm-up protocol on isokinetic leg extension performance. Twelve participants completed a baseline (PRE) isokinetic knee-extension test at 60°.s-1 and 300°.s-1, following a 5 min warm-up on a cycle ergometer. Subsequently, participants completed the following warm-up protocols randomly over a three-week period: dynamic-stretching (DS); antagonist muscle static-stretching (AMSS) and dynamic followed by antagonist muscle static-stretching (DS-AMSS). A repeated measures analysis of variance (ANOVA) was conducted to determine where significant differences existed for peak torque, total work, average power, time-to-peak-torque and relative peak torque between warm-up protocols. DS-AMSS facilitated a significantly higher peak torque and total work compared to PRE, DS and AMSS at 60°.s-1 and 300°.s-1 P < 0.05, respectively). DS-AMSS caused significantly greater relative peak torque than PRE for 60°.s-1 and 300°.s-1 (P < 0.05). DS-AMSS resulted in significantly reduced time-to-peak-torque and increased average power at 60°.s-1 compared to PRE, DS and AMSS (P < 0.05). DS-AMSS and AMSS resulted in a significant reduction in time-to-peak-torque and increased average power compared to the PRE and DS (P < 0.05) at 300°.s-1.

Influence of wide-pulse neuromuscular electrical stimulation frequency and superimposed tendon vibration on occurrence and magnitude of extra torque

Low-frequency and high-frequency wide-pulse neuromuscular electrical stimulation (NMES) can generate extra torque (ET) via afferent pathways. Superimposing tendon vibration (TV) to NMES can increase the activation of these afferent pathways and favor ET generation. Knowledge of the characteristics of ET is essential to implement these stimulation paradigms in clinical practice. Thus, we aimed to investigate the effects of frequency and TV superimposition on the occurrence and magnitude of ET in response to wide-pulse NMES. NMES-induced isometric plantar flexion torque was recorded in 30 healthy individuals who performed five NMES protocols: wide-pulse low-frequency (1 ms; 20 Hz; WPLF) and wide-pulse high-frequency (1 ms; 100 Hz; WPHF) without and with superimposed TV (1 mm; 100 Hz) and conventional NMES (50 µs; 20 Hz; reference protocol). Each NMES protocol consisted of three 20-s trains interspersed by 90 s of rest, with NMES intensity being adjusted to reach 10% of maximal voluntary contraction. The ET occurrence was similar for WPLF and WPHF (P = 0.822). In the responders, the ET magnitude was greater for WPHF than WPLF (P < 0.001). There was no effect of superimposed TV on ET characteristics. This study reported an effect of NMES frequency on ET magnitude, whereas TV superimposition did not affect this parameter. In the context of our experimental design decisions, the present findings question the clinical use of wide-pulse NMES and its combination with superimposed TV. Yet, further research is needed to maximize force production through the occurrence and magnitude of ET.NEW & NOTEWORTHY This study is the first to assess the effect of stimulation frequency and superimposed tendon vibration on extra torque characteristics generated by wide-pulse neuromuscular electrical stimulation. The percentage of subjects showing extra torque (i.e., considered as responders) was similar for low-frequency and high-frequency wide-pulse neuromuscular electrical stimulation. In the responders, the extra torque was greater for high-frequency than for low-frequency wide-pulse neuromuscular electrical stimulation. The superimposition of tendon vibration had no effect on extra torque occurrence or magnitude.

Static Stretching Reduces Motoneuron Excitability: The Potential Role of Neuromodulation

Prolonged static muscle stretching transiently reduces maximal muscle force, and this force loss has a strong neural component. In this review, we discuss the evidence suggesting that stretching reduces the motoneuron's ability to amplify excitatory drive. We propose a hypothetical model in which stretching causes physiological relaxation, reducing the brainstem-derived neuromodulatory drive necessary to maximize motoneuron discharge rates.

Plantar flexor muscle stretching depresses the soleus late response but not tendon tap reflexes

The purpose of this study was to investigate changes in muscle spindle sensitivity with early and late soleus reflex responses via tendon taps and transcranial magnetic stimulation, respectively, after an acute bout of prolonged static plantar flexor muscle stretching. Seventeen healthy males were tested before and after 5 min (5 × 60-s stretches) of passive static stretching of the plantar flexor muscles. Maximal voluntary isometric torque and M wave-normalized triceps surae muscle surface electromyographic activity were recorded. Both soleus tendon reflexes, evoked by percussion of the Achilles tendon during rest and transcranial magnetic stimulation-evoked soleus late responses during submaximal isometric dorsiflexion were also quantified. Significant decreases in maximal voluntary isometric plantar flexion torque (-19.2 ± 13.6%, p = .002) and soleus electromyographic activity (-20.1 ± 11.4%, p < .001) were observed immediately after stretching, and these changes were highly correlated (r = 0.76, p < .001). No changes were observed in tendon reflex amplitude or latency or peak muscle twitch torque (p > .05). Significant reductions in soleus late response amplitudes (-46.9 ± 36.0%, p = .002) were detected, although these changes were not correlated with changes in maximal electromyographic activity, torque or tendon reflex amplitudes. No changes in soleus late response latency were detected. In conclusion, impaired neural drive was implicated in the stretch-induced force loss; however, no evidence was found that this loss was related to changes in muscle spindle sensitivity. We hypothesize that the decrease in soleus late response indicates a stretch-induced reduction in a polysynaptic postural reflex rather than spindle reflex sensitivity.

Modulation of torque evoked by wide-pulse, high-frequency neuromuscular electrical stimulation and the potential implications for rehabilitation and training

The effectiveness of neuromuscular electrical stimulation (NMES) for rehabilitation is proportional to the evoked torque. The progressive increase in torque (extra torque) that may develop in response to low intensity wide-pulse high-frequency (WPHF) NMES holds great promise for rehabilitation as it overcomes the main limitation of NMES, namely discomfort. WPHF NMES extra torque is thought to result from reflexively recruited motor units at the spinal level. However, whether WPHF NMES evoked force can be modulated is unknown. Therefore, we examined the effect of two interventions known to change the state of spinal circuitry in opposite ways on evoked torque and motor unit recruitment by WPHF NMES. The interventions were high-frequency transcutaneous electrical nerve stimulation (TENS) and anodal transcutaneous spinal direct current stimulation (tsDCS). We show that TENS performed before a bout of WPHF NMES results in lower evoked torque (median change in torque time-integral: - 56%) indicating that WPHF NMES-evoked torque might be modulated. In contrast, the anodal tsDCS protocol used had no effect on any measured parameter. Our results demonstrate that WPHF NMES extra torque can be modulated and although the TENS intervention blunted extra torque production, the finding that central contribution to WPHF NMES-evoked torques can be modulated opens new avenues for designing interventions to enhance WPHF NMES.

Non-local acute stretching effects on strength performance in healthy young adults

BACKGROUND

Static stretching (SS) can impair performance and increase range of motion of a non-exercised or non-stretched muscle, respectively. An underdeveloped research area is the effect of unilateral stretching on non-local force output.

OBJECTIVE

The objective of this review was to describe the effects of unilateral SS on contralateral, non-stretched, muscle force and identify gaps in the literature.

METHODS

A systematic literature search following preferred reporting items for systematic review and meta-analyses Protocols guidelines was performed according to prescribed inclusion and exclusion criteria. Weighted means and ranges highlighted the non-local force output response to unilateral stretching. The physiotherapy evidence database scale was used to assess study risk of bias and methodological quality.

RESULTS

Unilateral stretching protocols from six studies involved 6.3 ± 2 repetitions of 36.3 ± 7.4 s with 19.3 ± 5.7 s recovery between stretches. The mean stretch-induced force deficits exhibited small magnitude effect sizes for both the stretched (-6.7 ± 7.1%, d = -0.35: 0.01 to -1.8) and contralateral, non-stretched, muscles (-4.0 ± 4.9%, d = , 0.22: 0.08 to 1.1). Control measures exhibited trivial deficits.

CONCLUSION

The limited literature examining non-local effects of prolonged SS revealed that both the stretched and contralateral, non-stretched, limbs of young adults demonstrate small magnitude force deficits. However, the frequency of studies with these effects were similar with three measures demonstrating deficits, and four measures showing trivial changes. These results highlight the possible global (non-local) effects of prolonged SS. Further research should investigate effects of lower intensity stretching, upper versus lower body stretching, different age groups, incorporate full warm-ups, and identify predominant mechanisms among others.

Non-local Acute Passive Stretching Effects on Range of Motion in Healthy Adults: A Systematic Review with Meta-analysis

BACKGROUND

Stretching a muscle not only increases the extensibility or range of motion (ROM) of the stretched muscle or joint but there is growing evidence of increased ROM of contralateral and other non-local muscles and joints.

OBJECTIVE

The objective of this meta-analysis was to quantify crossover or non-local changes in passive ROM following an acute bout of unilateral stretching and to examine potential dose-response relations.

METHODS

Eleven studies involving 14 independent measures met the inclusion criteria. The meta-analysis included moderating variables such as sex, trained state, stretching intensity and duration.

RESULTS

The analysis revealed that unilateral passive static stretching induced moderate magnitude (standard mean difference within studies: SMD: 0.86) increases in passive ROM with non-local, non-stretched joints. Moderating variables such as sex, trained state, stretching intensity, and duration did not moderate the results. Although stretching duration did not present statistically significant differences, greater than 240-s of stretching (SMD: 1.24) exhibited large magnitude increases in non-local ROM compared to moderate magnitude improvements with shorter (< 120-s: SMD: 0.72) durations of stretching.

CONCLUSION

Passive static stretching of one muscle group can induce moderate magnitude, global increases in ROM. Stretching durations greater than 240 s may have larger effects compared with shorter stretching durations.

Involuntary sustained firing of plantar flexor motor neurones: effect of electrical stimulation parameters during tendon vibration

PURPOSE

Simultaneous application of tendon vibration and neuromuscular electrical stimulation (NMES) induces an involuntary sustained torque. We examined the effect of different NMES parameters (intensity, pattern of stimulation and pulse width) on the magnitude of the evoked involuntary torque.

METHODS

Plantar flexor torque was recorded during 33-s Achilles tendon vibration with simultaneous 20-Hz NMES bouts on triceps surae (n = 20; 13 women). Intensity was set to elicit 10, 20 or 30% of maximal voluntary contraction torque (MVC), pulse width was narrow (0.2 ms) or wide (1 ms), and the stimulus pattern varied (5 × 2-s or 10 × 1-s). Up to 12 different trials were performed in a randomized order, and then repeated in those who produced a sustained involuntary torque after the cessation of vibration.

RESULTS

Six of 7 men and 5 of 13 women produced a post-vibration sustained torque. Eight of 20 participants did not complete the 30% trials, as they were perceived as painful. Torque during vibration at the end of NMES and the increase in torque throughout the trial were significantly higher in 20 than 10% trials (n = 11; 9.7 ± 9.0 vs 7.1 ± 6.1% MVC and 4.3 ± 4.5 vs 3.6 ± 3.5% MVC, respectively). Post-vibration sustained torque was higher in wide pulse-width trials (5.4 ± 5.9 vs 4.1 ± 4.3% MVC). Measures of involuntary torque were not different between 20 and 30% trials (n = 8).

CONCLUSION

Bouts of 5 × 2-s NMES with wide pulse width eliciting 20% MVC provides the most robust responses and could be used to maximise the production of involuntary torque in triceps surae.

Effect of ankle joint position on triceps surae contractile properties and motor unit discharge rates

The triceps surae (TS) length-tension relationship can be altered by changing the knee joint position, ankle joint position or both. However, studies exploring the effect of muscle length on neuromuscular properties have focused only on knee joint position changes affecting two of the three muscle components of the TS. Thus, the purpose of this study is to compare the neuromuscular properties of the three TS muscles during plantar flexion contractions at two ankle joint positions, 20° dorsiflexed (DF) and 20° plantar flexed (PF). Maximal isometric voluntary strength (MVC), voluntary activation, and evoked contractile properties of the ankle plantar flexors were compared between both ankle joint positions. Additionally, soleus, medial (MG), and lateral (LG) gastrocnemii motor unit discharge rates (MUDRs) were sampled during plantar flexion contractions at 25%, 50%, 75%, and 100% MVC using indwelling tungsten electrodes. MVC and peak twitch torque were lower by ~61% and 70%, respectively, whereas the maximal rate of torque relaxation was 39% faster in the PF compared with the DF position. Voluntary activation (~95%) was unaffected by changes in ankle joint position. LG MUDRs showed no differences between ankle joint positions, regardless of contraction intensity. Submaximal MG and soleus MUDRs showed no differences between the two ankle joint positions, however both muscles had 9% and 20% higher MUDRs in the DF position, respectively. These results provide further evidence for the differential activation among the three components of the TS with the greatest increases in soleus MUDRs compared with the gastrocnemii when the muscles are lengthened.

Test-Retest Reliability of Plantar Flexion Torque Generation During a Functional Knee Extended Position in Older and Younger Men

Measuring ankle torque is of paramount importance. This study compared the test-retest reliability of the plantar flexion torque-generating capacity between older and younger men. Twenty-one older (68 ± 6 years) and 22 younger (25 ± 5 years) men were tested twice for maximal isometric plantar flexion. Peak torque (PT), rate of torque development, and contractile impulses (CI) were obtained from 0 to 50 ms (rate of torque development0-50; CI0-50) and from 100 to 200 ms (rate of torque development100-200; CI100-200). Typical error as the coefficient of variation (CVTE) and intraclass correlation coefficient were used to assess test-retest reliability. Student's t test was applied to investigate systematic errors. The CVTE ratio was used for between-group comparisons. Only PT demonstrated acceptable reliability (intraclass correlation coefficient ≥ .75 and CV ≤ 10%). Older men demonstrated greater CVTE than younger men for PT (ratio = 2.24), but lesser for rapid torque (ratio ≤ 0.84). Younger men demonstrated systematic error for PT (6.5%) and CI100-200 (-8.9%). In conclusion, older men demonstrated greater variability for maximal torque output, but lesser for rapid torque.

Mechanisms underlying performance impairments following prolonged static stretching without a comprehensive warm-up

Whereas a variety of pre-exercise activities have been incorporated as part of a "warm-up" prior to work, combat, and athletic activities for millennia, the inclusion of static stretching (SS) within a warm-up has lost favor in the last 25 years. Research emphasized the possibility of SS-induced impairments in subsequent performance following prolonged stretching without proper dynamic warm-up activities. Proposed mechanisms underlying stretch-induced deficits include both neural (i.e., decreased voluntary activation, persistent inward current effects on motoneuron excitability) and morphological (i.e., changes in the force-length relationship, decreased Ca²⁺ sensitivity, alterations in parallel elastic component) factors. Psychological influences such as a mental energy deficit and nocebo effects could also adversely affect performance. However, significant practical limitations exist within published studies, e.g., long-stretching durations, stretching exercises with little task specificity, lack of warm-up before/after stretching, testing performed immediately after stretch completion, and risk of investigator and participant bias. Recent research indicates that appropriate durations of static stretching performed within a full warm-up (i.e., aerobic activities before and task-specific dynamic stretching and intense physical activities after SS) have trivial effects on subsequent performance with some evidence of improved force output at longer muscle lengths. For conditions in which muscular force production is compromised by stretching, knowledge of the underlying mechanisms would aid development of mitigation strategies. However, these mechanisms are yet to be perfectly defined. More information is needed to better understand both the warm-up components and mechanisms that contribute to performance enhancements or impairments when SS is incorporated within a pre-activity warm-up.

Neuromuscular versus Mechanical Stretch-induced Changes in Contralateral versus Ipsilateral Muscle

PURPOSE

Whether or not the homologous contralateral muscle (CM) undergoes stretch-induced force reduction as the stretched muscle (SM) is still unclear. The neuromuscular and mechanical factors underlying the force reduction in CM and SM were investigated.

METHODS

Twenty-one participants underwent unilateral knee extensors passive stretching. In both CM and SM, before, immediately after (POST), 5 (POST5), and 10 min (POST10) after passive stretching, maximum voluntary contraction (MVC), peak force (pF), and voluntary activation (VA) were measured. During MVC, the electromyographic and mechanomyographic root mean square (EMG RMS and MMG RMS, respectively) was calculated in rectus femoris, vastus lateralis, and vastus medialis, together with M-wave. The total electromechanical delay (EMD), divided in time delay (Δt) EMG-MMG and Δt MMG-F was calculated.

RESULTS

In CM at POST, the decrease in MVC (-11%; 95% confidence interval [CI], -13 to -9; effect size [ES], -2.27) was accompanied by a fall in VA (-7%; 95% CI, -9 to -4; ES, -2.29), EMG RMS (range, -22% to -11%; ES, -3.92 to -2.25), MMG RMS (range, -10% to -8%; ES, -0.52 to -0.39) and an increase in Δt EMG-MMG (≈+10%; ES, 0.73 to 0.93). All changes returned to baseline at POST5. In SM, decrease in MVC (-19%; 95% CI, -24 to -18; ES, -3.08), pF (-25%; 95% CI, -28 to -22; ES, -4.90), VA (-10%; 95% CI, -11 to -9; ES, -5.71), EMG RMS (≈-33%; ES, -5.23 to -3.22) and rise in MMG RMS (range, +25% to +32%; ES, 4.21 to 4.98) and EMD (≈+28%; ES, 1.59 to 1.77) were observed at POST and persisted at POST10. No change in M-wave occurred.

CONCLUSIONS

The contralateral central motor drive stretch-induced inhibition seems to account for the force reduction in CM. In SM, both central inhibition and mechanical factors concurred.

Passive muscle stretching reduces estimates of persistent inward current strength in soleus motor units

Prolonged (≥60 s) passive muscle stretching acutely reduces maximal force production at least partly through a suppression of efferent neural drive. The origin of this neural suppression has not been determined; however, some evidence suggests that reductions in the amplitude of persistent inward currents (PICs) in the motoneurons may be important. The aim of the present study was to determine whether acute passive (static) muscle stretching affects PIC strength in gastrocnemius medialis (GM) and soleus (SOL) motor units. We calculated the difference in instantaneous discharge rates at recruitment and de-recruitment (ΔF) for pairs of motor units in GM and SOL during triangular isometric plantar flexor contractions (20% maximum) both before and immediately after a 5 min control period and immediately after five 1 min passive plantar flexor stretches. After stretching, there was a significant reduction in SOL ΔF (-25.6%; 95% confidence interval, CI=-45.1% to -9.1%, P=0.002) but not GM ΔF These data suggest passive muscle stretching can reduce the intrinsic excitability, via PICs, of SOL motor units. These findings (1) suggest that PIC strength might be reduced after passive stretching, (2) are consistent with previously established post-stretch decreases in SOL but not GM EMG amplitude during contraction, and (3) indicate that reductions in PIC strength could underpin the stretch-induced force loss.

Minute oscillation stretching: A novel modality for reducing musculo-tendinous stiffness and maintaining muscle strength

A novel stretching modality was developed to provide repetitive small length changes to the plantar flexors undergoing passive stretch defined as "minute oscillation stretching" (MOS). This study investigated the effects of MOS on neuromuscular activity during force production, the rate of torque development (RTD), and the elastic properties of the plantar flexors and Achilles tendon. Ten healthy males participated in this study. The neuromuscular activity of the triceps surae and tibialis anterior muscles during maximal voluntary plantar flexion torque [MVT], RTD of plantar flexion, Achilles tendon stiffness, and muscle stiffness were measured before and after two types of interventions for a total of 5 minutes: static stretching (SS) and MOS at 15 Hz and without intervention (control). Achilles tendon stiffness was calculated from the tendon elongation measuring by ultrasonography. Muscle stiffness was determined for the medial gastrocnemius [MG] using shear wave elastography. The MVT, mean electromyographic amplitudes [mEMG] of MG and lateral gastrocnemius [LG], and RTD were significantly decreased following SS (MVT: -7.2 ± 7.9%; mEMG of MG: -8.7 ± 10.2%; mEMG of LG: -12.4 ± 10.5%; RTD: -6.6 ± 6.8%), but not after MOS. Achilles tendon stiffness significantly decreased after SS (-13.4 ± 12.3%) and MOS (-9.7 ± 11.5%), with no significant differences between them. Muscle stiffness significantly decreased in SS and MOS, with relative changes being significantly greater for MOS (-7.9 ± 8.3%) than SS (-2.3 ± 2.9%) interventions. All variables remained unchanged in the controls. In conclusion, MOS changed muscle-tendon compliance without loss of muscle function.

The optimal duration of high-intensity static stretching in hamstrings

Objectives

The purpose of this study was to compare the duration of high-intensity static stretching on flexibility and strength in the hamstrings.

Methods

Fourteen healthy males (20.8 ± 0.6 years, 170.7 ± 6.5 cm, 66.4 ± 9.9 kg) underwent high-intensity static stretching for three different durations (10, 15, and 20 seconds). The intensity of static stretching was set at the maximum point of discomfort. To examine the change in flexibility and strength, range of motion, peak passive torque, relative passive torque, muscle-tendon unit stiffness, peak torque of isokinetic knee flexion, and knee angle at peak torque of isokinetic knee flexion were measured. To evaluate a time course of pain, a numerical rating scale was described.

Results

Range of motion (P < 0.01), peak passive torque (P < 0.01), and knee angle at peak torque were increased at all interventions. Relative passive torque (P < 0.01) and muscle-tendon unit stiffness (P < 0.01) were decreased at all interventions. Peak torque decreased after 10 seconds of stretching (P < 0.05). Numerical rating scale during stretching was 8–9 levels in all interventions, the pain disappeared immediately after the post-measurements (median = 0).

Conclusion

The results suggested that muscle-tendon unit stiffness decreased regardless of duration of high-intensity static stretching. However, peak torque of isokinetic knee flexion decreased after 10 seconds of high-intensity static stretching, though it was no change after for more than 15 seconds of stretching.

Effects of Instrument-assisted Soft Tissue Mobilization on Musculoskeletal Properties

Purpose

Instrument-assisted soft tissue mobilization (IASTM) has been reported to improve joint range of motion (flexibility). However, it is not clear whether this change in the joint range of motion is accompanied by any alterations in the mechanical and/or neural properties. This study aimed to investigate the effects of IASTM in plantarflexors and Achilles tendon on the mechanical and neural properties of them.

Methods

This randomized, controlled, crossover study included 14 healthy volunteers (11 men and 3 women, 21–32 yr). IASTM was performed on the skin over the posterior part of the lower leg for 5 min and targeted the soft tissues (gastrocnemii, soleus, and tibialis posterior muscles; overlying deep fascia; and Achilles tendon). As a control condition, the same participants rested for 5 min between pre- and postmeasurements without IASTM on a separate day. The maximal ankle joint dorsiflexion angle (dorsiflexion range of motion), the peak passive torque (stretch tolerance), and the ankle joint stiffness (slope of the relationship between passive torque and ankle joint angle) during the measurement of the dorsiflexion range of motion and muscle stiffness of the triceps surae (using shear wave elastography) were measured before and immediately after the interventions.

Results

After IASTM, the dorsiflexion range of motion significantly increased by 10.7% ± 10.8% and ankle joint stiffness significantly decreased by −6.2% ± 10.1%. However, peak passive torque and muscle stiffness did not change. All variables remained unchanged in the repeated measurements of controls.

Conclusion

IASTM can improve joint range of motion, without affecting the mechanical and neural properties of the treated muscles.

Lack of cortical or Ia-afferent spinal pathway involvement in muscle force loss after passive static stretching

This study investigated whether modulation of corticospinal-motoneuronal excitability and/or synaptic transmission of the Ia afferent spinal reflex contributes to decreases in voluntary activation and muscular force after an acute bout of prolonged static muscle stretching. Fifteen men performed five 60-s constant-torque stretches (15-s rest intervals; total duration 5 min) of the plantar flexors on an isokinetic dynamometer and a nonstretching control condition in random order on 2 separate days. Maximum isometric plantar flexor torque and triceps surae muscle electromyographic activity (normalized to M wave; EMG/M) were simultaneously recorded immediately before and after each condition. Motor-evoked potentials (using transcranial magnetic stimulation) and H-reflexes were recorded from soleus during EMG-controlled submaximal contractions (23.4 ± 6.9% EMG maximum). No changes were detected in the control condition. After stretching, however, peak torque (mean ± SD; -14.3 ± 7.0%) and soleus EMG/M (-17.8 ± 6.2%) decreased, and these changes were highly correlated (r = 0.83). No changes were observed after stretching in soleus MEP or H-reflex amplitudes measured during submaximal contractions, and interindividual variability of changes was not correlated with changes in EMG activity or maximum torque. During EMG-controlled submaximal contractions, torque production was significantly decreased after stretching (-22.7 ± 15.0%), indicating a compromised muscular output. These data provide support that changes in the excitability of the corticospinal-motoneuronal and Ia afferent spinal reflex pathways do not contribute to poststretch neural impairment.NEW & NOTEWORTHY This study is the first to specifically examine potential sites underlying the decreases in neural activation of muscle and force production after a bout of muscle stretching. However, no changes were found in either the H-reflex or motor-evoked potential amplitude during submaximal contractions.

Static stretch and dynamic muscle activity induce acute similar increase in corticospinal excitability

Even though the acute effects of pre-exercise static stretching and dynamic muscle activity on muscular and functional performance have been largely investigated, their effects on the corticospinal pathway are still unclear. For that reason, this study examined the acute effects of 5×20 s of static stretching, dynamic muscle activity and a control condition on spinal excitability, corticospinal excitability and plantar flexor neuromuscular properties. Fifteen volunteers were randomly tested on separate days. Transcranial magnetic stimulation was applied to investigate corticospinal excitability by recording the amplitude of the motor-evoked potential (MEP) and the duration of the cortical silent period (cSP). Peripheral nerve stimulation was applied to investigate (i) spinal excitability using the Hoffmann reflex (H_max), and (ii) neuromuscular properties using the amplitude of the maximal M-wave (M_max) and corresponding peak twitch torque. These measurements were performed with a background 30% of maximal voluntary isometric contraction. Finally, the maximal voluntary isometric contraction torque and the corresponding electromyography (EMG) from soleus, gastrocnemius medialis and gastrocnemius lateralis were recorded. These parameters were measured immediately before and 10 s after each conditioning activity of plantar flexors. Corticospinal excitability (MEP/M_max) was significantly enhanced after static stretching in soleus (P = 0.001; ES = 0.54) and gastrocnemius lateralis (P<0.001; ES = 0.64), and after dynamic muscle activity in gastrocnemius lateralis (P = 0.003; ES = 0.53) only. On the other hand, spinal excitability (H_max/M_max), cSP duration, muscle activation (EMG/M_max) as well as maximal voluntary and evoked torque remained unaltered after all pre-exercise interventions. These findings indicate the presence of facilitation of the corticospinal pathway without change in muscle function after both static stretching (particularly) and dynamic muscle activity.

Ankle dysfunction in multiple sclerosis and the effects on walking

PURPOSE

Even in the early stage of the disease, for patients suffering from multiple sclerosis (MS), the most common and reported biomechanical alterations in the lower limb are located at the ankle joint. However, the effects of these impairments on gait deterioration should be discussed.

MATERIALS AND METHODS

This review was written according to the PRISMA guidelines. The search focussed on biomechanical changes (kinetic, kinematic, and electromyographic data) at the ankle during gait in MS patients. The search was performed in the databases: Pubmed, Web of Science, and Cochrane Library.

RESULTS

Eleven studies were included. The reduction in the ankle range of motion (RoM) induced by increased cocontractions of the tibialis anterior and triceps surae muscles could be a compensatory strategy to improve body-weight support and balance during the stance phase.

CONCLUSIONS

Future rehabilitation programmes should consider the control of weight support at the ankle during gait training.Implications for rehabilitationThe ankle supports and stabilises the body during the stance phase of gait.The reduced ankle range of motion in multiple sclerosis (MS), even at an early stage of the disease, is due to cocontractions of tibialis anterior and triceps surae and could be a compensatory strategy to be more stable.Rehabilitation programmes for MS patients should focus on the control of body segments motion during the weight transfer above the ankle.

Acute Effects of Static Stretching on Muscle Strength and Power: An Attempt to Clarify Previous Caveats

The effects of static stretching (StS) on subsequent strength and power activities has been one of the most debated topics in sport science literature over the past decades. The aim of this review is (1) to summarize previous and current findings on the acute effects of StS on muscle strength and power performances; (2) to update readers’ knowledge related to previous caveats; and (3) to discuss the underlying physiological mechanisms of short-duration StS when performed as single-mode treatment or when integrated into a full warm-up routine. Over the last two decades, StS has been considered harmful to subsequent strength and power performances. Accordingly, it has been recommended not to apply StS before strength- and power-related activities. More recent evidence suggests that when performed as a single-mode treatment or when integrated within a full warm-up routine including aerobic activity, dynamic-stretching, and sport-specific activities, short-duration StS (≤60 s per muscle group) trivially impairs subsequent strength and power activities (∆1–2%). Yet, longer StS durations (>60 s per muscle group) appear to induce substantial and practically relevant declines in strength and power performances (∆4.0–7.5%). Moreover, recent evidence suggests that when included in a full warm-up routine, short-duration StS may even contribute to lower the risk of sustaining musculotendinous injuries especially with high-intensity activities (e.g., sprint running and change of direction speed). It seems that during short-duration StS, neuromuscular activation and musculotendinous stiffness appear not to be affected compared with long-duration StS. Among other factors, this could be due to an elevated muscle temperature induced by a dynamic warm-up program. More specifically, elevated muscle temperature leads to increased muscle fiber conduction-velocity and improved binding of contractile proteins (actin, myosin). Therefore, our previous understanding of harmful StS effects on subsequent strength and power activities has to be updated. In fact, short-duration StS should be included as an important warm-up component before the uptake of recreational sports activities due to its potential positive effect on flexibility and musculotendinous injury prevention. However, in high-performance athletes, short-duration StS has to be applied with caution due to its negligible but still prevalent negative effects on subsequent strength and power performances, which could have an impact on performance during competition.

Could inter-set stretching increase acute neuromuscular and metabolic responses during resistance exercise?

This study investigated the acute effects of inter-set static stretching (ISS) during resistance exercise (RE) on the subsequent neuromuscular and metabolic responses. Twelve resistance-trained men performed three different knee extension RE protocols comprised of seven sets of 10 repetitions in a counterbalanced fashion. The three protocols were: 1) ISS (subjects performed 25 sec of quadriceps stretching between sets during 40 sec rest interval); 2) control (CON, subject passively rested between sets for 40 sec); 3) traditional (TRA, subject passively rested between sets for 120 sec). Total work was lower (p < 0.05) in ISS than CON and TRA (p <0.05). The fatigue index was greater (p < 0.05) in ISS compared with CON and TRA. ISS also resulted in lower (p < 0.05) electromyography (EMG) amplitude during the 6th and 7th sets compared with TRA. Additionally, EMG frequency was lower (p < 0.05) from the 3rd to 5th sets during ISS compared to CON, and from the 3rd to 7th sets compared to TRA. Muscle swelling and blood lactate similarly increased (p > 0.05) in response to all protocols. These results indicate that ISS negatively impacts neuromuscular performance, and does not increase the metabolic stress compared to passive rest intervals.

Passive muscle stretching impairs rapid force production and neuromuscular function in human plantar flexors

PURPOSE

We examined the effect of muscle stretching on the ability to produce rapid torque and the mechanisms underpinning the changes.

METHODS

Eighteen men performed three conditions: (1) continuous stretch (1 set of 5 min), (2) intermittent stretch (5 sets of 1 min with 15-s inter-stretch interval), and (3) control. Isometric plantar flexor rate of torque development was measured during explosive maximal voluntary contractions (MVC) in the intervals 0-100 ms (RTD_V100) and 0-200 ms (RTD_V200), and in electrically evoked 0.5-s tetanic contractions (20 Hz, 20 Hz preceded by a doublet and 80 Hz). The rate of EMG rise, electromechanical delay during MVC (EMD_V) and during a single twitch contraction (EMD_twitch) were assessed.

RESULTS

RTD_V200 was decreased (P < 0.05) immediately after continuous (- 15%) and intermittent stretch (- 30%) with no differences between protocols. The rate of torque development during tetanic stimulations was reduced (P < 0.05) immediately after continuous (- 8%) and intermittent stretch (- 10%), when averaged across stimulation frequencies. Lateral gastrocnemius rate of EMG rise was reduced after intermittent stretch (- 27%), and changes in triceps surae rate of EMG rise were correlated with changes in RTD_V200 after both continuous (r = 0.64) and intermittent stretch (r = 0.65). EMD_V increased immediately (31%) and 15 min (17%) after intermittent stretch and was correlated with changes in RTD_V200 (r = - 0.56). EMD_twitch increased immediately after continuous (4%), and immediately (5.4%), 15 min (6.3%), and 30 min after (6.4%) intermittent stretch (P < 0.05).

CONCLUSIONS

Reductions in the rate of torque development immediately after stretching were associated with both neural and mechanical mechanisms.

Can transcranial direct current stimulation improve range of motion and modulate pain perception in healthy individuals?

The objective of the present study was to investigate the effects of different electrode assemblies and electric current polarity on the ROM of the hip and pain perception. Ten healthy male, sedentary, right-leg-dominant, and aged between 19 and 30 years (24.0 ± 4.0 years) subjects were recruited. For the experimental conditions, the application of transcranial direct current stimulation (tDCS) was performed with the following montages. In the montage 1, the cathodal electrode was placed over the motor cortex (MC) horizontally, and the anodal electrode was positioned over the left dorsolateral prefrontal cortex (DLPFC). In the montage 2, the anodal electrode was placed over the MC bilaterally, and the cathode electrode was positioned over the left DLPFC. The sham montage was the same as the montage 1. In the montage 1 and 2 stimulation was applied with 2 mA current intensity for 20 min. In the Sham condition, the stimulator was turned off after 30 s of active stimulation and the electrodes remained on the participants for 20 min. Before and after experimental conditions (Pre-stimulation, Post-stimulation), the maximum Hip ROM and pain perception was measured. For the Montage 1, the maximum Hip ROM increased in post-stimulation compared to pre-stimulation, and in the Montage 2, the maximum Hip ROM decreased in post-stimulation compared to pre-stimulation. The pain perception in the Montage 1 decreased in the post-stimulation compared to pre-stimulation. In the post-stimulation, pain perception for the Montage 1 was lower compared to Montage 2 (p = 0.005), and sham (p = 0.004). When the anodic stimulus was applied on the left DLPFC and the cathodic stimulus on the motor cortex, an increase in ROM and a reduction in the pain perception was observed. This montage may to modulate pain perception and joint flexibility.

Neuromuscular Factors Contributing to Reductions in Muscle Force After Repeated, High-Intensity Muscular Efforts

Multiple neuromuscular processes contribute to the loss of force production following repeated, high-intensity muscular efforts; however, the relative contribution of each process is unclear. In Experiment 1, 16 resistance trained men performed six sets of unilateral isometric plantar flexor contractions of the right leg (3 s contraction/2 s rest; 85% maximal voluntary contraction torque; 90-s inter-set rest) until failure with and without caffeine ingestion (3 mg kg^-1) on two separate days. Corticospinal excitability and cortical silent period (cSP) were assessed before and immediately, 10 and 20 min after the exercise. In Experiment 2, electrically evoked tetanic force and persistent inward current (PIC)-mediated facilitation of the motor neuron pool (estimated using neuromuscular electrical stimulation with tendon vibration) were assessed before and after the same exercise intervention in 17 resistance trained men. Results showed decreases in peak plantar flexion torque (Experiment 1: -12.2%, Experiment 2: -16.9%), electrically evoked torque (20 Hz -15.3%, 80 Hz -15.3%, variable-frequency train -17.9%), and cSP (-3.8%; i.e., reduced inhibition) post-exercise which did not recover by 20 min. Electromyographic activity (EMG; -6%), corticospinal excitability (-9%), and PIC facilitation (-24.8%) were also reduced post-exercise but recovered by 10 min. Caffeine ingestion increased torque and EMG but did not notably affect corticospinal excitability, PIC amplification, or electrically evoked torque. The data indicate that a decrease in muscle function largely underpins the loss of force after repeated, high-intensity muscular efforts, but that the loss is exacerbated immediately after the exercise by simultaneous decreases in corticospinal excitability and PIC amplitudes at the motor neurons.

Neurophysiological Mechanisms Underpinning Stretch-Induced Force Loss

It is well known that prolonged passive muscle stretch reduces maximal muscle force production. There is a growing body of evidence suggesting that adaptations occurring within the nervous system play a major role in this stretch-induced force reduction. This article reviews the existing literature, and some new evidence, regarding acute neurophysiological changes in response to passive muscle stretching. We discuss the possible contribution of supra-spinal and spinal structures to the force reduction after passive muscle stretch. In summary, based on the recent evidence reviewed we propose a new hypothesis that a disfacilitation occurring at the motoneuronal level after passive muscle stretch is a major factor affecting the neural efferent drive to the muscle and, subsequently, its ability to produce maximal force.

Acute Effects of the Different Intensity of Static Stretching on Flexibility and Isometric Muscle Force

Kataura, S, Suzuki, S, Matsuo, S, Hatano, G, Iwata, M, Yokoi, K, Tsuchida, W, Banno, Y, and Asai, Y. Acute effects of the different intensity of static stretching on flexibility and isometric muscle force. J Strength Cond Res 31(12): 3403-3410, 2017-In various fields, static stretching is commonly performed to improve flexibility, whereas the acute effects of different stretch intensities are unclear. Therefore, we investigated the acute effects of different stretch intensities on flexibility and muscle force. Eighteen healthy participants (9 men and 9 women) performed 180-second static stretches of the right hamstrings at 80, 100, and 120% of maximum tolerable intensity without stretching pain, in random order. The following outcomes were assessed as markers of lower limb function and flexibility: static passive torque (SPT), range of motion (ROM), passive joint (muscle-tendon) stiffness, passive torque (PT) at onset of pain, and isometric muscle force. Static passive torque was significantly decreased after all stretching intensities (p ≤ 0.05). Compared with before stretching at 100 and 120% intensities, ROM and PT were significantly increased after stretching (p ≤ 0.05), and passive stiffness (p = 0.05) and isometric muscle force (p ≤ 0.05) were significantly decreased. In addition, ROM was significantly greater after stretching at 100 and 120% than at 80%, and passive stiffness was significantly lower after 120% than after 80% (p ≤ 0.05). However, all measurements except SPT were unchanged after 80% intensity. There was a weak positive correlation between the intensities of stretching and the relative change for SPT (p ≤ 0.05), a moderate positive correlation with ROM (p ≤ 0.05), and a moderate positive correlation with passive stiffness (p ≤ 0.05). These results indicate that static stretching at greater intensity is more effective for increasing ROM and decreasing passive muscle-tendon stiffness.

Flexibility responses to different stretching methods in young elite basketball players

Introduction

The aims of study were: 1) to verify the effectiveness of different stretching methods and training; 2) to compare the effects with only training on the flexibility of joints in basketball players.

Methods

30 males basketball players (age: 17±1yrs; BMI: 23.4±3.1), divided into 2 groups (15 experimental group - EG - and 15 control group, CG), participated to study. EG performed 5 different stretching method: passive stretching, active stretching, postural protocol, PNF and dynamic stretching. To assess differences (p<0.05) between groups, an ANOVA was applied to anthropometrics characteristic (age; height; weight and BMI) and flexibility performances (leg raise in a supine position; forward trunk bending). ANOVA for repeated measurements was conducted to asses differences in each group with time (i.e., pre-post).

Results

Results showed a variation linked to time (F=21.9; p<0.0001) and an effect of the treatment of the leg raise in a supine position test (F=25.1; p<0.0001). Also in flexion test of trunk, the average values could be linked to time of measurement (F=9.96; p<0.0001) and group (F=8.65; p<0.0001).

Conclusion

The results suggest that a specific different stretching protocol should be used in different part of body to offer performance benefit and decreasing of the incidents of injuries.

Level of evidence

IV.

The effects of different durations of static stretching within a comprehensive warm-up on voluntary and evoked contractile properties

Evidence for performance decrements following prolonged static stretching (SS) has led to a paradigm shift in stretching routines within a warm-up. Rather than SS, dynamic stretching (DS) and dynamic activity (DA) have replaced SS within warm-up routines. The objective of the present study was to compare the effect of differing lower limb SS durations (30 [SS30s], 60 [SS60s] or 120 s [SS120s] of SS per muscle group or no-stretch control) within a comprehensive warm-up protocol consisting of aerobic activity, DS and DA. Sixteen male participants completed the four stretching conditions in a randomized order, after a 5-min low-intensity (cycle) warm-up and before a DS/DA component on separate days. Tests included passive hip and knee ranges of motion (ROM), maximum voluntary knee extensor/flexor force, force produced at 100 ms (F100), vertical jump height and evoked knee extensor contractile properties. For hip flexion (hamstrings) ROM, SS120s provided the largest increase (5.6-11.7%) followed by SS60s (4.3-11.4%), control (4.4-10.6%) and SS30s (3.6-11.1%). For knee flexion (quadriceps) ROM, SS30s provided the largest increase (9.3-18.2%) followed by SS120s (6.5-16.3%), SS60s (7.2-15.2%) and control (6.3-15.2%). There were decreases in quadriceps F100 following SS in SS120s (29.6%) only. There were increases in vertical jump performance in the control (6.2%), SS60s (4.6%) and SS30s (3.3%). While 120 s SS per muscle increased ROM, even within a comprehensive warm-up routine, it also elicited notable performance decrements. However, moderate durations of SS were observed to improve ROM whilst either having negligible or beneficial (but not detrimental) effects on specific aspects of athletic performance.

Acute effects of unilateral static stretching on handgrip strength of the stretched and non-stretched limb

PURPOSE

To determine the effects of an acute bout of unilateral static stretching on handgrip strength of both the stretched and non-stretched limb. It was reasoned that if the non-stretched limb experienced a decrease in force output, further evidence for a neural mechanism to explain a post-stretch force reduction would be obtained as no mechanical adaptation would have occurred.

METHODS

Thirty participants performed maximum voluntary unilateral handgrip contractions of both limbs before and after stretching the finger flexors of the strength-dominant side only. Each trial was assessed for peak force, muscle activity (iEMG), and rate of force generation.

RESULTS

Following the stretching bout, peak force and iEMG decreased by 4.4% (p = 0.001) and 6.4% (p = 0.000) respectively in the stretched limb only. However, rate of force generation was significantly impaired in both the stretched (- 17.3%; p = 0.000) and non-stretched limbs (- 10.8%; p = 0.003) 1 min post-stretch, and remained similarly depressed for both limbs 15 min later.

CONCLUSION

Acute stretching negatively impacts rate of force generation more than peak force. Moreover, a reduced rate of force generation from the non-stretched limb indicates the presence of a cross-over inhibitory effect through the nervous system, which provides additional evidence for a neural mechanism.

Effect of tendon vibration during wide-pulse neuromuscular electrical stimulation (NMES) on muscle force production in people with spinal cord injury (SCI)

Background

Neuromuscular electrical stimulation (NMES) is commonly used in skeletal muscles in people with spinal cord injury (SCI) with the aim of increasing muscle recruitment and thus muscle force production. NMES has been conventionally used in clinical practice as functional electrical stimulation (FES), using low levels of evoked force that cannot optimally stimulate muscular strength and mass improvements, and thus trigger musculoskeletal changes in paralysed muscles. The use of high intensity intermittent NMES training using wide-pulse width and moderate-intensity as a strength training tool could be a promising method to increase muscle force production in people with SCI. However, this type of protocol has not been clinically adopted because it may generate rapid muscle fatigue and thus prevent the performance of repeated high-intensity muscular contractions in paralysed muscles. Moreover, superimposing patellar tendon vibration onto the wide-pulse width NMES has been shown to elicit further increases in impulse or, at least, reduce the rate of fatigue in repeated contractions in able-bodied populations, but there is a lack of evidence to support this argument in people with SCI.

Methods

Nine people with SCI received two NMES protocols with and without superimposing patellar tendon vibration on different days (i.e. STIM and STIM+vib), which consisted of repeated 30 Hz trains of 58 wide-pulse width (1000 μs) symmetric biphasic pulses (0.033-s inter-pulse interval; 2 s stimulation train; 2-s inter-train interval) being delivered to the dominant quadriceps femoris. Starting torque was 20% of maximal doublet-twitch torque and stimulations continued until torque declined to 50% of the starting torque. Total knee extensor impulse was calculated as the primary outcome variable.

Results

Total knee extensor impulse increased in four subjects when patellar tendon vibration was imposed (59.2 ± 15.8%) but decreased in five subjects (− 31.3 ± 25.7%). However, there were no statistically significant differences between these sub-groups or between conditions when the data were pooled.

Conclusions

Based on the present results there is insufficient evidence to conclude that patellar tendon vibration provides a clear benefit to muscle force production or delays muscle fatigue during wide-pulse width, moderate-intensity NMES in people with SCI.

Trial registration

ACTRN12618000022268. Date: 11/01/2018. Retrospectively registered.