Responses of human elbow flexor muscles to electrically stimulated forced lengthening exercise

Neuromuscular electrical stimulation producing low evoked force elicits the repeated bout effect on muscle damage markers of the elbow flexors

This study examined the repeated bout effect (RBE) on muscle damage markers following two bouts of neuromuscular electrical stimulation (NMES) in untrained individuals. Following familiarization, participants received 45 consecutive NMES to the biceps brachii at an intensity that produced low evoked force for the elbow flexors. Muscle damage markers (maximal voluntary isometric contraction [MVIC], elbow range of motion [ROM], muscle soreness via visual analogue scale [VAS] scores, pressure pain threshold [PPT], and muscle thickness) were measured before (PRE), after (POST), 1 day after (24 POST), and 2 days after (48 POST) NMES. Following 1 week of rest, procedures were replicated. Separate repeated measures two-way ANOVAs examined each measure. There were no interactions or bout main effects for MVIC or ROM. Time main effects indicated PRE MVIC was greater than POST (p ​= ​0.002) and 24-POST (p ​= ​0.024), and PRE ROM was greater than POST (p ​= ​0.036). There was no interaction for muscle thickness. Respective time and bout main effects indicated muscle thickness at PRE was less than POST (p ​= ​0.017), and second-bout muscle thickness (p ​= ​0.050) was less compared to the initial-bout. For PPT, there was an interaction (p ​= ​0.019). Initial-bout PRE PPT was less than POST (p ​= ​0.033). Initial-bout 48-POST PPT was less than second-bout 48-POST (p ​= ​0.037). There was a significant interaction for VAS (p ​= ​0.009). Initial-bout PRE VAS was less than POST (p ​= ​0.033) and 24-POST (p ​= ​0.015). Initial-bout POST and 24-POST VAS were greater than second-bout POST (p ​= ​0.023) and 24-POST (p ​= ​0.006), respectively. The results support RBE on muscle damage markers related to inflammation, but not MVIC and ROM.

Precise instruction and consideration of the vertical and horizontal force component increases validity and reliability of the 90:20 Isometric Posterior Chain Test

Hamstring injuries are associated with decreased hamstring strength. Matinlauri et al.'s 90:20 Isometric Posterior Chain Test (90:20 IPCT) efficiently assesses hamstring strength, but has not been validated so far. Furthermore, their rather unprecise original instruction allows high variability in test execution. We added a new instruction and variables and examined, whether this measure leads to increased reliability and validity. We assessed hamstring strength of 23 sport students via the 90:20 IPCT under the original instruction, to exert vertical force, and our new instruction, to exert vertical and horizontal force. Instead of only using bare vertical force as variable under the original (Fz_V) and our new instruction (Fz_VH), we also calculated the resultant force (Fres_VH) and the applied torque onto the force place (M_F_ortho_VH). To test for validity, we correlated the outcome variables with peak torque of gold standard dynamometry. Furthermore, we measured muscle activities of the mm. rectus femoris, biceps femoris, semitendinosus, and gluteus maximus under our new instruction and compared them to those under the original variable (Fz_V) via one sample t-tests. To evaluate reliability, tests were repeated on two separate days, for which we calculated intra class correlation coefficients (ICCs) and coefficients of variation (CVs). Our new instruction and variables (Fz_VH, Fres_VH, M_F_ortho_VH) showed better validity (mean r = 0.77, r = 0.81, and r = 0.85) and equally good or better reliability (ICCs: 0.87, 0.89, and 0.94; CVs: 4.7%, 4.1%, and 4.7%) than the original instruction and variable (Fz_V) (mean r = 0.70; ICC: 0.91; CV: 5.6%). There were no differences in muscle activities between the variables and instructions of the 90:20 IPCT. We recommend our new instruction and the applied torque onto the force plate as it makes the 90:20 IPCT a more reliable and valid tool to assess hamstring strength.

Immediate Effects of Whole-Body versus Local Dynamic Electrostimulation of the Abdominal Muscles in Healthy People Assessed by Ultrasound: A Randomized Controlled Trial

Dynamic electrostimulation consists of the application of local or global electrostimulation together with physical exercise. This study aimed to investigate the immediate effects of a dynamic electrostimulation session on the thickness of the abdominal musculature, inter-rectus distance, heart rate, blood pressure, and body temperature, and to identify possible differences in its form of application. A total of 120 healthy participants were divided into three groups: the whole-body electrostimulation group, the local electrostimulation group, and the control group without electrical stimulation. All groups performed a single session with the same dynamic exercise protocol. Muscle thickness and inter-rectus distance were evaluated ultrasonographically using the Rehabilitative Ultrasound Imaging technique both at rest and in muscle contraction (the active straight leg raise test) to find the post-intervention differences. The results showed significant differences in immediate post-intervention heart rate, with a smaller increase in the local electrostimulation group compared to the control and whole-body electrostimulation groups. No significant differences were identified between the groups after the interventions in the rest of the variables analyzed. Therefore, a local application, with the same effects as a global application on the abdominal musculature, has fewer contraindications, which makes its use more advisable, especially in populations with cardiorespiratory disorders, for which more research is needed.

Repeated Bout Effect of Two Resistance Training Bouts on Bowling-Specific Performance in Male Cricketers

To examine the repeated bout effect (RBE) following two identical resistance bouts and its effect on bowling-specific performance in male cricketers. Male cricket pace bowlers (N = 10), who had not undertaken resistance exercises in the past six months, were invited to complete a familiarisation and resistance maximum testing, before participating in the study protocol. The study protocol involved the collection of muscle damage markers, a battery of anaerobic (jump and sprint), and a bowling-specific performance test at baseline, followed by a resistance training bout, and a retest of physical and bowling-specific performance at 24 h (T24) and 48 h (T48) post-training. The study protocol was repeated 7–10 days thereafter. Indirect markers of muscle damage were lower (creatine kinase: 318.7 ± 164.3 U·L⁻¹; muscle soreness: 3 ± 1), whilst drop jump was improved (~47.5 ± 8.1 cm) following the second resistance training bout when compared to the first resistance training bout (creatine kinase: 550.9 ± 242.3 U·L⁻¹; muscle soreness: 4 ± 2; drop jump: ~43.0 ± 9.7 cm). However, sport-specific performance via bowling speed declined (Bout 1: −2.55 ± 3.43%; Bout 2: 2.67 ± 2.41%) whilst run-up time increased (2.34 ± 3.61%; Bout 2: 3.84 ± 4.06%) after each bout of resistance training. Findings suggest that while an initial resistance training bout reduced muscle damage indicators and improved drop jump performance following a second resistance training bout, this RBE trend was not observed for bowling-specific performance. It was suggested that pace bowlers with limited exposure to resistance training should minimise bowling-specific practice for 1–2 days following the initial bouts of their resistance training program.

Effect of a sulforaphane supplement on muscle soreness and damage induced by eccentric exercise in young adults: A pilot study

OBJECTIVE

Excessive exercise increases the production of reactive oxygen species in skeletal muscles. Sulforaphane activates nuclear factor erythroid 2-related factor 2 (Nrf2) and induces a protective effect against oxidative stress. In a recent report, sulforaphane intake suppressed exercise-induced oxidative stress and muscle damage in mice. However, the effect of sulforaphane intake on delayed onset muscle soreness after eccentric exercise in humans is unknown. We evaluated the effect of sulforaphane supplement intake in humans regarding the delayed onset muscle soreness (DOMS) after eccentric exercise.

RESEARCH METHODS & PROCEDURES

To determine the duration of sulforaphane supplementation, continuous blood sampling was performed and NQO1 mRNA expression levels were analyzed. Sixteen young men were randomly divided into sulforaphane and control groups. The sulforaphane group received sulforaphane supplements. Each group performed six set of five eccentric exercise with the nondominant arm in elbow flexion with 70% maximum voluntary contraction. We assessed muscle soreness in the biceps using the visual analog scale, range of motion (ROM), muscle damage markers, and oxidative stress marker (malondialdehyde; MDA).

RESULTS

Sulforaphane supplement intake for 2 weeks increased NQO1 mRNA expression in peripheral blood mononuclear cells (PBMCs). Muscle soreness on palpation and ROM were significantly lower 2 days after exercise in the sulforaphane group compared with the control group. Serum MDA showed significantly lower levels 2 days after exercise in the sulforaphane group compared with the control group.

CONCLUSION

Our findings suggest that sulforaphane intake from 2 weeks before to 4 days after the exercise increased NQO1, a target gene of Nrf2, and suppressed DOMS after 2 days of eccentric exercise.

Skeletal Muscle Damage Produced by Electrically Evoked Muscle Contractions

Understanding the physiological/mechanical mechanisms leading to skeletal muscle damage remains one of the challenges in muscle physiology. This review presents the functional, structural, and cellular consequences of electrically evoked submaximal isometric contractions that can elicit severe and localized skeletal muscle damage. Hypotheses related to underlying physiological and mechanical processes involved in severe and localized muscle damage also are discussed.

Egg white hydrolysate improves fatigue due to short-term swimming load test in mice

We studied the effect of egg white hydrolysate (EWH) on swimming endurance in mice. 7-week-old male ddY mice (28-30 g) were divided into three groups and fed an AIN-93G diet supplemented with casein (n = 8), EWH (n = 7), or egg white protein (EWP, n = 8) for 14 days. From day 11, the mice underwent a swimming test daily with a weight load equivalent to 10% of their body weight, and the lengths of time they swam were recorded. Blood was sampled for testing on the last day of the study. We observed that increases in the swimming duration through day 14 were significantly greater in the EWH group than in the casein group (p = 0.049). As a factor underlying this, the hexanoyl-lysine level in blood was confirmed to be decreased in the former group (p = 0.013). These findings indicate that consumption of EWH extended the swimming duration and suggest the mechanistic involvement of an antifatigue effect mediated by its antioxidant activity.

Acute adaptations and subsequent preservation of strength and speed measures following a Nordic hamstring curl intervention: a randomised controlled trial

This randomised controlled trial investigated changes in eccentric hamstring strength, 10m sprint speed, and change-of-direction (COD) performance immediately post Nordic hamstring curl (NHC) intervention and following a 3-week detraining period. Fourteen male team sports athletes were randomised to a do-as-usual control group (CG; n = 7) or to a NHC intervention group (NHC; n = 7). Isokinetic dynamometry at 180°/s evaluated eccentric hamstring strength immediately post-intervention as the primary outcome measure. Secondary outcomes included 10 m sprint time and COD. Each outcome was measured, pre, immediately post-intervention and following a 3-week detraining period. Immediately post-intervention significant group differences were observed in the NHC group for eccentric hamstring strength (31.81 Nm^-1 vs. 6.44 Nm^-1, P = 0.001), COD (-0.12 s vs. 0.20 s; P = 0.003) and sprint (- 0.06 s vs. 0.05 s; P = 0.024) performance. Performance improvements were maintained following a detraining period for COD (-0.11 s vs. 0.20 s; P = 0.014) and sprint (-0.05 s vs. 0.03 s, P = 0.031) but not eccentric hamstring strength (15.67 Nm^-1 vs. 6.44 Nm^-1, P = 0.145) These findings have important implications for training programmes designed to reduce hamstring injury incidence, whilst enhancing physical qualities critical to sport.

Preconditioning contractions prevent the delayed onset of myofibrillar dysfunction after damaging eccentric contractions

KEY POINTS

We examined the mechanisms underlying the positive effect of preconditioning contractions (PCs) on the recovery of muscle force after damaging eccentric contractions (ECCs). The mechanisms underlying the immediate force decrease after damaging ECCs differ from those causing depressed force with a few days' delay, where reactive oxygen species (ROS) produced by invading immune cells play an important causative role. PCs counteracted the delayed onset force depression and this could be explained by prevention of immune cell invasion, which resulted in decreased myeloperoxidase-mediated ROS production, hence avoiding cell membrane disruption, calpain activation and degenerative changes in myosin and actin molecules.

ABSTRACT

Preconditioning contractions (PCs) have been shown to result in markedly improved contractile function during the recovery periods after muscle damage from eccentric contractions (ECCs). Here, we examined the mechanisms underlying the beneficial effect of PCs with a special focus on the myofibrillar function. Rat medial gastrocnemius muscles were exposed to 100 repeated damaging ECCs in situ and excised immediately (recovery 0, REC0) or after 4 days (REC4). PCs with 10 repeated non-damaging ECCs were applied 2 days before the damaging ECCs. PCs improved in situ maximal isometric torque at REC4. Skinned muscle fibres were used to directly assess changes in myofibrillar function. PCs prevented the damaging ECC-induced depression in maximum Ca²⁺ -activated force at REC4. PCs also prevented the following damaging ECC-induced effects at REC4: (i) the reduction in myosin heavy chain and actin content; (ii) calpain activation; (iii) changes in redox homeostasis manifested as increased expression levels of malondialdehyde-protein adducts, NADPH oxidase 2, superoxide dismutase 2 and catalase, and activation of myeloperoxidase (MPO); (iv) infiltration of immune cells and loss of cell membrane integrity. Additionally, at REC0, PCs enhanced the expression levels of heat shock protein (HSP) 70, HSP25, and αB-crystallin in the myofibrils and prevented the increased mRNA levels of granulocyte-macrophage colony-stimulating factor and interleukin-6. In conclusion, PCs prevent the delayed force depression after damaging ECCs by an HSP-dependent inhibition of degenerative changes in myosin and actin molecules caused by myeloperoxidase-induced membrane lysis and subsequent calpain activation, which were triggered by an inflammatory reaction with immune cells invading damaged muscles.

Minimal Evidence for a Secondary Loss of Strength After an Acute Muscle Injury: A Systematic Review and Meta-Analysis

BACKGROUND

An immediate loss of strength follows virtually all types of muscle injury but there is debate whether the initial strength loss is maximal or if a secondary loss of strength occurs during the first 3 days post-injury.

OBJECTIVE

The objective of this analysis was to conduct a systematic review and meta-analysis of the research literature to determine if a secondary loss of strength occurs after an injurious initiating event.

METHODS

Literature searches were performed using eight electronic databases (e.g., PubMed, Cochrane Library). Search terms included skeletal muscle AND (injur* OR damage*) AND (strength OR force OR torque). The extracted strength data were converted to a standard format by calculating the standardized mean difference, which is reported as the effect size (ES) along with its 95 % confidence interval (CI). The calculation of ES was designed so that a negative ES that was statistically less than zero would be interpreted as indicating a secondary loss of strength.

RESULTS

A total of 223 studies with over 4000 human and animal subjects yielded data on 262 independent groups and a total of 936 separate ESs. Our overall meta-analysis yielded a small-to-medium, positive overall ES that was statistically greater than zero (overall ES = +0.34, 95 % CI 0.27-0.40; P < 0.00000001). Considerable variation in ES was observed among studies (I ² = 86 %), which could be partially explained by the research group conducting the study, sex of the subject, day of post-injury strength assessment, whether fatigue was present immediately post-injury, and the muscle group injured. From the subgroup meta-analyses probing these variables, 36 subgroup ESs were calculated and none were statistically less than zero.

CONCLUSION

Overall, our findings do not support the presence of a secondary loss of strength following an acute muscle injury, and strongly suggest that strength, on average, recovers steadily over the first 3 days post-injury.

Muscle plasticity related to changes in tubulin and αB-crystallin levels induced by eccentric contraction in rat skeletal muscles

We used the model of eccentric contraction of the hindlimb muscle by Ochi et al. to examine the role of eccentric contraction in muscle plasticity. This model aims to focus on stimulated skeletal muscle responses by measuring tissue weights and tracing the quantities of αB-crystallin and tubulin. The medial gastrocnemius muscle (GCM) responded to electrically induced eccentric contraction (EIEC) with significant increases in tissue weight (p < 0.01) and the ratio of tissue weight to body weight (p < 0.05); however, there was a decrease in soleus muscle weight after EIEC. EIEC in the GCM caused contractile-induced sustenance of the traced proteins, but the soleus muscle exhibited a remarkable decrease in α-tubulin and a 19% decrease in αB-crystallin. EIEC caused fast-to-slow myosin heavy chain (MHC) isoform type-oriented shift within both the GCM and soleus muscle. These results have shown that different MHC isoform type-expressing slow and fast muscles commonly undergo fast-to-slow type MHC isoform transformation. This suggests that different levels of EIEC affected each of the slow and fast muscles to induce different quantitative changes in the expression of αB-crystallin and α-tubulin.

Ultrasound imaging in women's arm flexor muscles: intra-rater reliability of muscle thickness and echo intensity

Background

Different ultrasound parameters have been frequently used to assess changes associated with training, aging, immobilization, and neuromuscular diseases. However, an exploratory reliability analysis of the echo intensity (EI) and muscle thickness (MT) of the forearm flexors is scarce, especially in women.

Objective

The purpose of the present study was to determine the intra-rater reliability of MT and EI assessed by ultrasound in young women.

Method

Ultrasonographic MT and EI were acquired in the forearm flexors of 41 young women (22±2 yrs). Reliability was calculated using intraclass correlation coefficient (ICC_2,1), standard error of measurement (SEM), coefficient of variation (CV), smallest detectable change (SDC), and Bland and Altman plot analysis.

Results

ICC values for MT and EI were 0.88 (95% CI: 0.78-0.93). The SEM and CV values were lower than 10%. Bland and Altman analysis revealed that ultrasound mean differences were 0.27 mm (Limits of Agreement - LOA 95%: - 2.6 to 3.2 mm) and -0.09 a.u. (LOA 95%: - 10.9 to 10.7 a.u.).

Conclusion

MT and EI assessed by ultrasonography in young women appear to be reliable and may be used to monitor changes in muscle mass induced by strength training when these changes exceed the precision of ultrasound.

Using skin temperature and muscle thickness to assess muscle response to strength training

ABSTRACT Introduction: Several studies already reported the response of many biomarkers after strength training, but studies using low cost diagnostic imaging tools are rare. Objective: To evaluate the usage of skin temperature and muscle thickness (MT) to monitor muscle response (until 96 hours after) to high-intensity strength training. Methods: This is a short-term longitudinal study with 13 trained, healthy male volunteers. Volunteers performed five sets of biceps bi-set exercise with their dominant arm with dumbbells, with load of 70% of one-repetition maximum (1RM). The ultrasound (US) and thermal images were acquired before and immediately after the last set, 24, 48, 72 and 96 hours after exercise. Results: The analysis was divided in two stages: acute muscle response (until 24 hours after training) and delayed muscle response (from 24 to 96 hours after training). The elbow flexors thickness showed the peak value immediately after the last set of training. Skin temperature (on elbow flexors) and the elbow flexors thickness grew continuously from 24 to 96 hours after strength training. There is a high correlation (r=0.941, p=0.017) between skin temperature and muscle thickness from the end of exercise until 96 hours after strength training. Conclusions: The US images showed high sensibility for muscle physiological changes on the first 24 hours after exercise. On the other hand, the thermal images had higher sensibility for muscle physiological changes than US images from 24 to 96 hours after training.

Reliability and sensitivity of a simple isometric posterior lower limb muscle test in professional football players

This study aimed (1) to determine the reliability of a simple and quick test to assess isometric posterior lower limb muscle force in professional football players and (2) verify its sensitivity to detect reductions in force following a competitive match. Twenty-nine professional football players performed a 3-s maximal isometric contraction of the posterior lower limb muscles for both legs with players lying supine. Both legs were tested using a knee angle of 90° and 30° measured on a force plate. Players were tested twice with one week between sessions to verify reliability. Sensitivity was tested following a full competitive football match. The test showed high reliability for dominant leg at 90° (CV = 4.3%, ICC = 0.95, ES = 0.15), non-dominant leg at 90° (CV = 5.4%, ICC = 0.95, ES = 0.14), and non-dominant leg at 30° (CV = 4.8%, ICC = 0.93, ES = 0.30) and good reliability for dominant leg at 30° (CV = 6.3%, ICC = 0.86, ES = 0.05). The measure was sensitive enough to detect reductions in force for dominant leg at 90° (P = 0.0006, ES > 1), non-dominant leg at 90° (P = 0.0142, ES = 1), and non-dominant leg at 30° (P = 0.0064, ES > 1) and for dominant leg at 30° (P = 0.0016, ES > 1). In conclusion, the present test represents a useful and practical field tool to determine the magnitude of match-induced fatigue of the posterior lower limb muscles and potentially to track their recovery.

Contraction induced muscle injury: towards personalized training and recovery programs

Skeletal muscles can be injured by their own contractions. Such contraction-induced injury, often accompanied by delayed onset of muscle soreness, is a leading cause of the loss of mobility in the rapidly increasing population of elderly people. Unlike other types of muscle injuries which hurt almost exclusively those who are subjected to intensive exercise such as professional athletes and soldiers in training, contraction induced injury is a phenomenon which may be experienced by people of all ages while performing a variety of daily-life activities. Subjects that experience contraction induced injury report on soreness that usually increases in intensity in the first 24 h after the activity, peaks from 24 to 72 h, and then subsides and disappears in a few days. Despite their clinical importance and wide influence, there are almost no studies, clinical, experimental or computational, that quantitatively relate between the extent of contraction induced injury and activity factors, such as number of repetitions, their frequency and magnitude. The lack of such quantitative information is even more emphasized by the fact that contraction induced injury can be used, if moderate and controlled, to improve muscle performance in the long term. Thus, if properly understood and carefully implemented, contraction induced injury can be used for the purpose of personalized training and recovery programs. In this paper, we review experimental, clinical, and theoretical works, attempting towards drawing a more quantitative description of contraction induced injury and related phenomena.

Attenuation of eccentric exercise-induced muscle damage by preconditioning exercises

PURPOSE

This study compared the effect of an initial exercise consisting of either low-intensity eccentric or maximal isometric contractions (ISOs) on protective effect against maximal eccentric contraction (MaxECC)-induced muscle damage.

METHODS

Untrained young men were placed into one of five groups (n = 13 per group): MaxECC, 10% ECC, 20% ECC, 90° ISO, and 20° ISO. The MaxECC, 10% ECC, and 20% ECC groups performed 30 ECCs of the elbow flexors using a dumbbell equivalent to 100%, 10%, and 20% of maximal voluntary isometric contraction strength, respectively. The 90° ISO and 20° ISO groups performed 30 ISOs at 90° and 20° of elbow flexion, respectively. Three weeks later, all subjects performed 30 MaxECCs with the arm used for the first bout. Changes in maximal voluntary isometric and concentric contraction strength, range of motion, upper arm circumference, plasma creatine kinase and myoglobin concentration, and muscle soreness before and for 5 d after the first and second exercise bouts were compared among groups by a two-way repeated-measure ANOVA.

RESULTS

Changes in all measures after the first bout were smaller (P < 0.05) for 10% ECC, 20% ECC, 90° ISO, and 20° ISO groups compared with MaxECC group, and the changes were smaller (P < 0.05) for 10% ECC and 90° ISO than 20° ISO and 20% ECC groups. When compared with the first bout of MaxECC group, changes in the measures after the second bout were smaller for 20% ECC and 20° ISO groups with greater protective effect evident for 20° ISO group, but the protective effect conferred by these was smaller (P < 0.05) compared with MaxECCs.

CONCLUSION

These results suggest that there is threshold intensity for ECCs to confer protective effect, and ISOs at a long muscle length provide preconditioning effect.

The spectral changes in EMG during a second bout eccentric contraction could be due to adaptation in muscle fibres themselves: a simulation study

The mechanism of marked reduction in damage symptoms after repeated bout of similar eccentric contractions is still unknown. The neuronal adaptation leading to reduction of muscle fibre propagation velocity (MFPV) due to increased activation of slow-twitch motor units (MUs), decrease in activation of fast-twitch MUs, and/or increase in MU synchronization was suggested as a cause for lower EMG frequency characteristics. However, the repeated bout effect could occur also after electrically stimulated exercise. Prolonged elevation of cytoplasmic Ca(2+) due to the increased membrane permeability after eccentric contractions was reported. Elevated Ca(2+) induced peripheral changes that included alteration of intracellular action potential and MFPV reduction. We simulated and compared changes in EMG frequency characteristics related to effects of central nervous system (CNS) or to peripheral changes. The simulations were performed for different electrode arrangements and positions. The results showed that the peripheral effects could be similar or even stronger than the effects related to CNS. We hypothesised that the repeated bout effect was a consequence of the adaptation in muscle fibres necessary for avoiding Ca(2+)-induced protein and lipid degradation due to Ca(2+) overload resulting from the increased membrane permeability after eccentric contraction. The possibilities for noninvasive testing of this hypothesis were discussed.

Comparison in muscle damage between maximal voluntary and electrically evoked isometric contractions of the elbow flexors

This study compared between maximal voluntary (VOL) and electrically stimulated (ES) isometric contractions of the elbow flexors for changes in indirect markers of muscle damage to investigate whether ES would induce greater muscle damage than VOL. Twelve non-resistance-trained men (23-39 years) performed VOL with one arm and ES with the contralateral arm separated by 2 weeks in a randomised, counterbalanced order. Both VOL and ES (frequency 75 Hz, pulse duration 250 μs, maximally tolerated intensity) exercises consisted of 50 maximal isometric contractions (4-s on, 15-s off) of the elbow flexors at a long muscle length (160°). Changes in maximal voluntary isometric contraction torque (MVC), range of motion, muscle soreness, pressure pain threshold and serum creatine kinase (CK) activity were measured before, immediately after and 1, 24, 48, 72 and 96 h following exercise. The average peak torque over the 50 isometric contractions was greater (P < 0.05) for VOL (32.9 ± 9.8 N m) than ES (16.9 ± 6.3 N m). MVC decreased greater and recovered slower (P < 0.05) after ES (15% lower than baseline at 96 h) than VOL (full recovery). Serum CK activity increased (P < 0.05) only after ES, and the muscles became more sore and tender after ES than VOL (P < 0.05). These results showed that ES induced greater muscle damage than VOL despite the lower torque output during ES. It seems likely that higher mechanical stress imposed on the activated muscle fibres, due to the specificity of motor unit recruitment in ES, resulted in greater muscle damage.

Comparison between alternating and pulsed current electrical muscle stimulation for muscle and systemic acute responses

This study compared alternating current and pulsed current electrical muscle stimulation (EMS) for torque output, skin temperature ( Tsk), blood lactate and hormonal responses, and skeletal muscle damage markers. Twelve healthy men (23–48 yr) received alternating current EMS (2.5 kHz delivered at 75 Hz, 400 μs) for the knee extensors of one leg and pulsed current (75 Hz, 400 μs) for the other leg to induce 40 isometric contractions (on-off ratio 5–15 s) at the knee joint angle of 100° (0°: full extension). The use of the legs for each condition was counterbalanced among subjects, and the two EMS bouts were separated by 2 wk. The current amplitude was consistently increased to maximally tolerable level, and the torque and perceived intensity were recorded over 40 isometric contractions. Tskof the stimulated and contralateral knee extensors were measured before, during, and for 30 min after EMS. Blood lactate, growth hormone, testosterone, insulin-like growth factor 1, testosterone, and cortisol were measured before, during, and for 45 min following EMS. Muscle damage markers included maximal voluntary isometric contraction torque, muscle soreness with a 100-mm visual analog scale, and plasma creatine kinase (CK) activity, which were measured before and 1, 24, 48, 72, and 96 h after EMS. No significant differences in the torque induced during stimulation (∼30% maximal voluntary isometric contraction) and perceived intensity were found, and changes in Tsk, blood lactate, and hormones were not significantly different between conditions. However, all of the measures showed significant ( P < 0.05) changes from baseline values. Skeletal muscle damage was evidenced by prolonged strength loss, development of muscle soreness, and increases in plasma CK activity; however, the changes in the variables were not significantly different between conditions. It is concluded that acute effects of alternating and pulsed current EMS on the stimulated muscles are similar.

Increasing muscle strength and mass of thigh in elderly people with the hybrid-training method of electrical stimulation and volitional contraction

The "Hybrid training" (HYBT) method utilizing combined electrical stimulation and voluntary muscle contraction has been developed as a muscle training method. It has already been shown that the method is technically sound and clinically effective in healthy young subjects. The purpose of this study was to investigate the effect of the HYBT method on the knee extensor strength considering safety for elderly people. Twenty subjects were randomly divided into two groups: the HYBT group and the weight machine training (WMT) group. All the subjects performed knee flexion and extension for 19 min per session, twice a week for 12 weeks. At the baseline and after the training, the subjects' maximal isometric torque of knee extension and cross-sectional area (CSA) of quadriceps femoris muscle were measured. The subjects completed the study without adverse effects. The knee extension torque significantly increased in both groups (39% in HYBT group and 42% in WMT group, P < 0.05). The CSA of quadriceps whole significantly increased in both groups (9% in HYBT group and 14% in WMT group, P < 0.05). These results indicate that the HYBT method increases muscle strength and mass, and that this method is as effective as the WMT. In addition, unlike the WMT, the HYBT device, which is portable and not large in size, is so easy to handle that it can be placed at the bedside. Therefore, the HYBT has potential to become a safe, effective method of muscle training for elderly people.

The decrease in electrically evoked force production is delayed by a previous bout of stretch-shortening cycle exercise

AIM

Unaccustomed physical exercise with a large eccentric component is accompanied by muscle damage and impaired contractile function, especially at low stimulation frequencies. A repeated bout of eccentric exercise results in less damage and improved recovery of contractile function. Here we test the hypotheses that (1) a prior stretch-shortening cycle (SSC) exercise protects against impaired muscle function during a subsequent bout of SSC exercise and (2) the protection during exercise is transient and becomes less effective as the exercise progresses.

METHODS

Healthy untrained men (n = 7) performed SSC exercise consisting of 100 maximal drop jumps at 30 s intervals. The same exercise was repeated 4 weeks later. Peak quadriceps muscle force evoked by electrical stimulation at 15 (P15) and 50 (P50) Hz was measured before exercise, after 10, 25, 50 and 100 jumps as well as 1 and 24 h after exercise.

RESULTS

P15 and P50 were higher during the initial phase of the repeated bout compared with the first exercise bout, but there was no difference between the bouts at the end of the exercise periods. P15 and P50 were again larger 24 h after the repeated bout. The P15/P50 ratio during exercise was not different between the two bouts, but it was higher after the repeated bout.

CONCLUSION

A prior bout of SSC exercise temporarily protects against impaired contractile function during a repeated exercise bout. The protection can again be seen after exercise, but the underlying mechanism then seems to be different.

Gross ultrastructural changes and necrotic fiber segments in elbow flexor muscles after maximal voluntary eccentric action in humans

Eccentric muscle actions are associated with ultrastructural changes. The severity and types of change depend on the nature of the stimulation protocol, and on the method for assessing such changes, and can be regarded as a continuum from mild changes to pathological-like changes. Most studies describing more severe changes have been performed on animals and only a few in humans, some using electrical stimuli. Hence, a debate has emerged on whether voluntary actions are associated with the pathological-like end of the continuum. The aim of this study was to determine whether severe muscle damage, i.e., extensive ultrastructural changes, is confined to animal studies and studies on humans using electrical stimuli. Second, because there is no generally approved method to quantify the degree of muscle damage, we compared two published methods, analyzing the Z disks or sarcomeres, as well as novel analyses of pathological-like changes. A group of untrained subjects performed 70 voluntary maximal eccentric muscle actions using the elbow flexors. On the basis of large reductions in maximal force-generating capacity (on average, −62 ± 3% immediately after exercise, and −35 ± 6% 9 days later), five subjects were selected for further analysis. Biopsies were taken from m. biceps brachii in both the exercised and nonexercised arm. In exercised muscle, more disrupted (13 ± 4 vs. 3 ± 3%) and destroyed (15 ± 6 vs. 0%) Z disks were found compared with nonexercised muscle. A significant proportion of exercised myofibers had focal (85 ± 5 vs. 11 ± 7%), moderate (65 ± 7 vs. 11 ± 6%), and extreme (38 ± 9 vs. 0%) myofibrillar disruptions. Hypercontracted myofibrils, autophagic vacuoles, granular areas, central nuclei, and necrotic fiber segments were found to various degrees. The present study demonstrates that the more severe end of the continuum of ultrastructural changes occurs in humans after voluntary exercise when maximal eccentric muscle actions are involved.

Force per active area and muscle injury during electrically stimulated contractions

Multiple mechanical factors have been implicated in the initiation of exercise-induced muscle injury. Although high absolute force levels are associated with greater injury, the importance of high force per active area independent of absolute force remains to be determined, especially in humans.

PURPOSE

This study sought to examine the role of specific force in muscle injury when peak eccentric force and total eccentric force were matched between two exercise bouts.

METHODS

Ten subjects (six men, four women) performed 80 electrically stimulated (EMS) eccentric contractions of the right and left quadriceps femoris (QF) through a 90 degree arc at approximately 45 degrees x s(-1). Specific force was manipulated by applying 25-Hz EMS to one thigh and 100-Hz EMS to the contralateral thigh, whereas force production was matched by lowering the EMS amplitude during the 100-Hz bout. T2 magnetic resonance images of the QF were collected before and 3 d after the eccentric exercise bouts. Injury was assessed via changes in isometric force and ratings of soreness in the QF over the course of 28 d after exercise and by determining changes in T2 relaxation time and muscle volume 3 d after exercise.

RESULTS

The 100-Hz EMS induced a greater force loss (P < 0. 05), soreness (P < 0.05), change in muscle volume (P = 0.03), and volume of muscle demonstrating an increase in T2 (P = 0.005) compared with 25-Hz EMS.

CONCLUSIONS

Our findings suggest that in humans, high specific force potentially plays an important role in the initiation of exercise-induced muscle injury.

Muscle injury after repeated bouts of voluntary and electrically stimulated exercise

Repeated bouts of eccentric exercise reduce the amount of exercise-induced muscle injury.

PURPOSE

This study sought to evaluate the importance of neural adaptations by comparing the repeated bout effect on muscle injury caused by voluntary and electrically stimulated eccentric exercise.

METHODS

Sixteen subjects (nine men, seven women) were assigned into two groups; electrical stimulation (STIM) and voluntary (VOL). Each group performed 2 identical bouts of 80 eccentric contractions of the quadriceps femoris (QF) through a 90 degree arc at approximately 45 degrees x s(-1), separated by 7 wk. T2-weighted magnetic resonance images of the QF were obtained before and 3 d after each exercise bout. Injury was assessed by determining changes in T2 relaxation time and muscle volume 3 d after exercise, and changes in isometric force and ratings of soreness for 28 d after exercise.

RESULTS

The initial bout of exercise caused significant changes in T2 relaxation time, isometric force, and ratings of soreness in both STIM and VOL groups (P < 0.05). After the repeated bout, significantly smaller changes were noted in soreness ratings (P < 0.05), mean change in T2 (P<0.05), and percentage of the QF demonstrating an increase in T2 (P < 0.05) compared with the initial bout in both exercise groups.

CONCLUSIONS

A repeated-bout effect was observed after electrically stimulated exercise, and the magnitude of the effect was similar to that observed with voluntary exercise. This suggests that the primary mechanism for the reduction in muscle injury after repeated exercise bouts is not related to changes in muscle recruitment and is potentially related to structural changes within the muscles.

The effect of number of lengthening contractions on rat isometric force production at different frequencies of nerve stimulation

AIM

To test the effect of 3, 10, 60 and 240 lengthening contractions (LC) on maximal isometric force of rat plantar flexor muscles at different stimulation frequencies.

METHODS

Using a dynamometer and electrical nerve stimulation, maximally active skeletal muscles were stretched by ankle rotation to produce LC of the plantar flexor muscles in intact female rats. After the lengthening contraction protocols, maximal isometric force was measured at different frequencies of nerve activation to obtain frequency-dependent force deficits (weakness).

RESULTS

The magnitude of the force deficit, measured 1 h after the protocols at 80 Hz, increased as a function of repetition number (three LC, 33.3 +/- 1.7%; 10 LC, 37.2 +/- 2.3%; 60 LC, 67.6 +/- 1.5%; 240 LC, 77.7 +/- 1.2%). Force deficits were also measured at each stimulation frequency tested (5:120 Hz). Using a ratio of isometric force at 20:100 Hz stimulation, the relative depression of force at low frequency was determined. The relative depression of isometric force at low frequency was most prominent during the early repetitions.

CONCLUSION

As low-frequency force depression appears to result primarily from excitation-contraction (E-C) coupling failure, the early LC in a series of repeated contractions probably contribute most to damage of the cellular components involved in E-C coupling.

Impact of repeated bouts of eccentric exercise on sarcolemma disruption in human skeletal muscle

In animal models, unaccustomed eccentric exercise (EE) has been widely related to muscle fiber membrane (sarcolemma) damage. On the contrary, studies in humans reported that sarcolemma was not susceptible to damage following a single bout of EE. We hypothesized that the single bout of EE used by those studies was not sufficient to induce sarcolemma damage, in humans. In this study we examined muscle biopsies from untrained males who either performed six sets of 15 reps of maximum voluntary eccentric contractions (n=9), for six consecutive days, or served as control-group (n=6). Blood and biopsy samples were obtained one week prior to exercise, immediately after bout 3, and 24h after the last training session. In addition to standard haematoxylin-eosin staining, all biopsies were stained immunohistochemically using antibodies specific for fibronectin and desmin antigens. In the exercise-group, no biopsies taken at pre-exercise or post-exercise level showed evidence of sarcolemma damage as stained by anti-fibronectin antibody in eight of nine subjects. Serum creatine kinase (CK) and lactate dehydrogenase (LDH) activities increased significantly throughout the study despite the lack of sarcolemma damage.We suggest that in humans, repeated bouts of EE do not cause gross sarcolemma damage in the mid-belly of Vastus Lateralis.

Excitability at the motoneuron pool and motor cortex is specifically modulated in lengthening compared to isometric contractions

Neural control of muscle contraction seems to be unique during muscle lengthening. The present study aimed to determine the specific sites of modulatory control for lengthening compared with isometric contractions. We used stimulation of the motor cortex and corticospinal tract to observe changes at the spinal and cortical levels. Motor-evoked potentials (MEPs) and cervicomedullary MEPs (CMEPs) were evoked in biceps brachii and brachioradialis during maximal and submaximal lengthening and isometric contractions at the same elbow angle. Sizes of CMEPs and MEPs were lower in lengthening contractions for both muscles (by approximately 28 and approximately 16%, respectively; P < 0.01), but MEP-to-CMEP ratios increased (by approximately 21%; P < 0.05). These results indicate reduced excitability at the spinal level but enhanced motor cortical excitability for lengthening compared with isometric muscle contractions.

Comparison between voluntary and stimulated contractions of the quadriceps femoris for growth hormone response and muscle damage

This study aimed to compare voluntary and stimulated exercise for changes in muscle strength, growth hormone (GH), blood lactate, and markers of muscle damage. Nine healthy men had two leg press exercise bouts separated by 2 wk. In the first bout, the quadriceps muscles were stimulated by biphasic rectangular pulses (75 Hz, duration 400 μs, on-off ratio 6.25–20 s) with current amplitude being consistently increased throughout 40 contractions at maximal tolerable level. In the second bout, 40 voluntary isometric contractions were performed at the same leg press force output as the first bout. Maximal voluntary isometric strength was measured before and after the bouts, and serum GH and blood lactate concentrations were measured before, during, and after exercise. Serum creatine kinase (CK) activity and muscle soreness were assessed before, immediately after, and 24, 48, and 72 h after exercise. Maximal voluntary strength decreased significantly ( P < 0.05) after both bouts, but the magnitude of the decrease was significantly ( P < 0.05) greater for the stimulated contractions (−22%) compared with the voluntary contractions (−9%). Increases in serum GH and lactate concentrations were significantly ( P < 0.05) larger after the stimulation compared with the voluntary exercise. Increases in serum CK activity and muscle soreness were also significantly ( P < 0.05) greater for the stimulation than voluntary exercise. It was concluded that a single bout of electrical stimulation exercise resulted in greater GH response and muscle damage than voluntary exercise.

Changes in the soleus muscle architecture after exhausting stretch-shortening cycle exercise in humans

This study focused on the architectural changes in the muscle-tendon complex during the immediate and secondary (delayed) reductions of performance (bimodal recovery) caused by an exhaustive rebound type stretch-shortening cycle (SSC) exercise. The isometric plantar flexor torque during maximum voluntary contraction (MVC) was measured together with recording of electromyography (EMG) and ultrasonography from the soleus muscle before (BEF), after (AFT), 2 h (2H), 2 and 8 days (2D, 8D) after the SSC exercise (n=8). The performance variables (MVC torque and EMG activation) followed the bimodal recovery patterns. This was not the case in the changes of the fascicle length and muscle thickness. The relative torque changes in MVC correlated positively (R=0.78, P=0.02) to the corresponding averaged EMG changes between BEF and 2H (BEF-->2H); the significance disappeared in the comparison between 2H and 2D (2H-->2D), during which period MVC showed a secondary reduction. The relative torque changes in MVC showed no correlation with the changes in muscle thickness between BEF-2H. However, this correlation between 2H-2D was negative (R=-0.85, P<0.01). The fascicle shortening/average EMG ratio in MVC increased at 2H, and then decreased more at 2D than 2H (P<0.05). Thus, the secondary performance decline was not related to the corresponding EMG reduction but to the increased muscle thickness, which peaked at 2D. The results suggest clearly that the secondary decline in MVC could be related to the increase in muscle volume.

Effects of a hybrid exercise on the activities of myogenic enzymes in plasma

"Hybrid" exercise uses forces generated by an electrically stimulated muscle to resist the motion of a volitionally contracting agonist. This approach is effective in increasing muscle bulk and strength. Its safety, however, has not been studied and the purpose of this study was to address this lack by assessing the impact of a hybrid strengthening regimen on the serological markers of exercise associated muscle damage. Twelve healthy, sedentary men between the ages of 20 and 27 years were divided into two groups of 6. The members of each group completed an exercise session that consisted of 10 sets of 10 reciprocal elbow flexor and extensor contractions separated by 1-minute rest intervals. In one group, the subjects underwent a hybrid exercise protocol. Subjects in the second group underwent a conventional isometric electrical stimulation program with the same stimulation intensities. Plasma creatine kinase (CK), aspartate aminotransferase (AST), and lactate dehydrogenase (LDH) activities were determined immediately before and after the exercise session as well as at follow-up 1, 2, and 7 days later. Secondary variables included active and passive range of motion and the subjects' perceived levels of "soreness" . Evaluation revealed that neither enzymatic activities nor range of motion changed in a statistically significant manner with time within the groups or between the groups. In conclusion, the stimulation intensities involved in hybrid exercise programs of the upper extremity do not appear to be associated with either overt or covert signs of muscle injury.

Effects of Muscle Damage Induced by Eccentric Exercise on Muscle Fatigue

PURPOSE

The present study was designed to determine to what extent muscle damage induced by repetitive eccentric exercise with maximal voluntary effort (ECC) affects the time course of central and peripheral fatigue during sustained maximal voluntary contraction (MVC).

METHODS

Ten healthy male volunteers were asked to perform brief (control MVC) and sustained MVC (fatigue test of 60 s in duration) with elbow flexion before and 2 and 4 d after ECC. Transcranial magnetic stimulation (TMS) was applied to the motor cortex to determine changes in voluntary activation (VA), the size of the motor evoked potential (MEP), and length of electromyographic (EMG) silencing. The ratio of the root mean square value for the surface EMG of the biceps brachii and exerted force within 50 ms before TMS was also calculated (RMS/F).

RESULTS

In two subjects, no significant changes in MVC and muscle soreness were seen after ECC so that their data was excluded from further analysis. Control MVC and muscle soreness was significantly decreased and increased, respectively, 2 and 4 d after ECC compared with that before ECC (P < 0.001). During the fatigue test, VA, which was determined by a phasic increase in the twitch force after TMS, significantly decreased 2 and 4 d after ECC compared with that beforehand (P < 0.01). In addition, the RMS/F was significantly increased 2 and 4 d after ECC (P < 0.001). Although the degree of facilitation of the MEP was significantly increased (P < 0.05), the length of EMG silencing was less affected by ECC.

CONCLUSIONS

Muscle damage and/or muscle soreness induced by repetitive eccentric exercise with maximal effort may be a strong modifier of central and peripheral fatigue during sustained MVC.

Changes in the angle-force curve of human elbow flexors following eccentric and isometric exercise

The aim of this study was to explore and compare the magnitude and time-course of the shift in the angle-force curves obtained from maximal voluntary contractions of the elbow flexors, both before and 4 consecutive days after eccentric and isometric exercise. The maximal isometric force of the elbow flexors of fourteen young male volunteers was measured at five different elbow angles between 50 degrees and 160 degrees . Subjects were then divided into two groups: the eccentric group (ECC, n=7) and the isometric group (ISO, n=7). Subjects in the ECC group performed 50 maximal voluntary eccentric contractions of the elbow flexors on an isokinetic dynamometer (30 degrees x s(-1)), while subjects in the ISO group performed 50 maximal voluntary isometric muscle contractions with the elbow flexors at a lengthened position. Following the ECC and ISO exercise protocols, maximal isometric force at the five angles, muscle soreness, and the relaxed (RANG) and flexed (FANG) elbow angles were measured at 24 h intervals for 4 days. All results were presented as the mean and standard error, and a quadratic curve was used to model the maximal isometric force data obtained at the five elbow angles. This approach not only allowed us to mathematically describe the angle-force curves and estimate the peak force and optimum angle for peak force generation, but also enabled us to statistically compare the shift of the angle-force curves between and within groups. A large and persistent shift of the angle-force curve towards longer muscle lengths was observed 1 day after eccentric exercise ( P<0.01). This resulted in a approximately 16 degrees shift of the optimum angle for force generation, which remained unchanged for the whole observation period. A smaller but also persistent shift of the angle-force curve was seen after isometric exercise at long muscle length ( P<0.05; shift in optimum angle approximately 5 degrees ). ECC exercise caused more muscle damage than ISO exercise, as indicated by the greater changes in RANG and ratings of muscle soreness ( P<0.05). It was suggested that the shift in the angle-force curve was proportional to the degree of muscle damage and may be explained by the presence of overstretched sarcomeres that increased in series compliance of the muscle.

Neuromuscular function after exercise-induced muscle damage: theoretical and applied implications

Exercise-induced muscle damage is a well documented phenomenon particularly resulting from eccentric exercise. When eccentric exercise is unaccustomed or is performed with an increased intensity or duration, the symptoms associated with muscle damage are a common outcome and are particularly associated with participation in athletic activity. Muscle damage results in an immediate and prolonged reduction in muscle function, most notably a reduction in force-generating capacity, which has been quantified in human studies through isometric and dynamic isokinetic testing modalities. Investigations of the torque-angular velocity relationship have failed to reveal a consistent pattern of change, with inconsistent reports of functional change being dependent on the muscle action and/or angular velocity of movement. The consequences of damage on dynamic, multi-joint, sport-specific movements would appear more pertinent with regard to athletic performance, but this aspect of muscle function has been studied less often. Reductions in the ability to generate power output during single-joint movements as well as during cycling and vertical jump movements have been documented. In addition, muscle damage has been observed to increase the physiological demand of endurance exercise and to increase thermal strain during exercise in the heat. The aims of this review are to summarise the functional decrements associated with exercise-induced muscle damage, relate these decrements to theoretical views regarding underlying mechanisms (i.e. sarcomere disruption, impaired excitation-contraction coupling, preferential fibre type damage, and impaired muscle metabolism), and finally to discuss the potential impact of muscle damage on athletic performance.

The effect of a repeat bout of exercise on muscle injury in persons with spinal cord injury

Following an initial bout of damaging exercise, a successive bout of similar exercise typically results in less injury, known as the "protective effect". Unloading due to spinal cord injury (SCI) increases the susceptibility to contraction-induced muscle injury. We tested the hypothesis that two bouts of isometric actions would evoke the same damage in the quadriceps femoris (QF) of patients with SCI. Six male subjects [32 (5) years old, 182 (9) cm, 81 (21) kg, injury level C6-T7, 6 (2) years post-injury, mean (SD)] were tested at two time points (Time1, Time2), separated by 8 weeks. Magnetic resonance images were taken of the QF prior to, immediately after, and 3 days after electromyostimulation (EMS) that evoked isometric knee extension. EMS (50 Hz) consisted of five sets of ten contractions (2 s on/6 s off, 1 min b/t sets) followed by three sets of ten contractions (1 s on/1 s off, 30 s b/t sets). Relative cross-sectional area of stimulated and injured skeletal muscle was obtained by quantifying pixels with an elevated T2. Relative area of stimulated QF was the same for both time points [92 (6)% and 89 (7)%] as was torque loss (approximately 55%). Three days post-EMS, the relative area of stimulated QF injured was not different between time points [30 (14)% vs 29 (17)%, P>0.05]. These results indicate an absence of a protective effective for repeat exercise bouts separated by 8 weeks in SCI patients using EMS.

Angle-specific impairment of elbow flexors strength after isometric exercise at long muscle length

In this study, we examined the long-term reductions in maximal isometric force (MIF) caused by a protocol of repeated maximal isometric contractions at long muscle length. Furthermore, we wished to ascertain whether the reductions in MIF are dependent on muscle length--that is, are the reductions in MIF more pronounced when the muscle contracts at a short length. The MIF of the elbow flexors of seven young male volunteers was measured at five different elbow angles between 50 degrees and 160 degrees. On a separate day, the participants performed 50 maximal voluntary isometric muscle contractions with the elbow flexors at a lengthened position; that is, with the shoulder hyperextended at 45 degrees and the elbow joint fixed at 140 degrees. Following this exercise, the MIF at the five elbow angles, range of motion, muscle soreness and plasma creatine kinase activity were measured at 24 h intervals for 4 days. On day 1, the decline in MIF was higher at the more acute elbow angles of 50 degrees (42 +/- 8%) and 70 degrees (39 +/- 8%; both P<0.01) than at 90 degrees (26 +/- 4%) and 140 degrees (16 +/- 3%; both P<0.01). No significant reduction in MIF was evident at an elbow angle of 160 degrees. Maximal isometric force at an elbow angle of 140 degrees was fully restored on day 3, whereas at an angle of 50 degrees it remained depressed for the 4 day observation period. Restoration of MIF was a function of the elbow angle, with force recovery being less at the smaller angles. The range of motion was decreased by 14 +/- 2 degrees on day 1 (P<0.01) and did not return to baseline values by day 4. Muscle soreness ratings remained significantly elevated for the 4 day period. Serum creatine kinase peaked on day 1 (522 +/- 129 IU, P<0.01) and decreased thereafter. We conclude that the disproportionate decrease in MIF at the small elbow angles and the length-specific recovery in MIF after repeated maximal isometric contractions at long muscle length may be explained by the presence of overstretched sarcomeres that increased in series compliance of the muscle, therefore causing a rightward shift of the force-length relationship.

Recent advances in the understanding of the repeated bout effect: the protective effect against muscle damage from a single bout of eccentric exercise

The repeated bout effect refers to the adaptation whereby a single bout of eccentric exercise protects against muscle damage from subsequent eccentric bouts. While the mechanism for this adaptation is poorly understood there have been significant recent advances in the understanding of this phenomenon. The purpose of this review is to provide an update on previously proposed theories and address new theories that have been advanced. The potential adaptations have been categorized as neural, mechanical and cellular. There is some evidence to suggest that the repeated bout effect is associated with a shift toward greater recruitment of slow twitch motor units. However, the repeated bout effect has been demonstrated with electrically stimulated contractions, indicating that a peripheral, non-neural adaptation predominates. With respect to mechanical adaptations there is evidence that both dynamic and passive muscle stiffness increase with eccentric training but there are no studies on passive or dynamic stiffness adaptations to a single eccentric bout. The role of the cytoskeleton in regulating dynamic stiffness is a possible area for future research. With respect to cellular adaptations there is evidence of longitudinal addition of sarcomeres and adaptations in the inflammatory response following an initial bout of eccentric exercise. Addition of sarcomeres is thought to reduce sarcomere strain during eccentric contractions thereby avoiding sarcomere disruption. Inflammatory adaptations are thought to limit the proliferation of damage that typically occurs in the days following eccentric exercise. In conclusion, there have been significant advances in the understanding of the repeated bout effect, however, a unified theory explaining the mechanism or mechanisms for this protective adaptation remains elusive.