Resting Muscle Shear Modulus Measured With Ultrasound Shear-Wave Elastography as an Alternative Tool to Assess Muscle Fatigue in Humans

The shear modulus of the vastus lateralis muscle does not follow the passive residual torque enhancement in the knee extensors

In vitro experiments define passive force enhancement as the increase in steady-state passive force following the deactivation of an actively stretched muscle, in contrast to a purely passive stretch. This phenomenon, linked to residual force enhancement, is also observed in voluntarily contracted muscles as passive residual torque enhancement (RTEpass). While mechanisms remain unclear, titin stiffness likey plays a key role. Supersonic shear wave elastography (SSI) estimates tissue stiffness via the shear modulus (μ). The study aimed to assess whether RTEpass of the knee extensor muscles is accompanied by an increase in vastus lateralis stiffness (RμEpass) as measured by shear wave elastography. Passive torque was measured in 20 healthy young adults at a knee flexion angle of 100° before and after both isometric contractions (control protocol) and isometric contractions preceded by an eccentric contraction at 30°/s (from 70° to 100°). The comparison of protocols revealed a significant mean RTEpass of 1.03 N·m (16.5 %; p < 0.001), confirming the RTEpass in knee extensors. Although the experimental protocol showed a significant change in μ from the Before- to Post-contraction moment (5.89 %; p = 0.041), no differences in μ were observed between protocols at any post-contraction moments (p ≥ 0.191). Spearman correlation analysis indicated a weak, non-significant correlation between RTEpass and RμEpass (rs = 0.219; p = 0.352). These findings suggest that changes in vastus lateralis stiffness, as measured by SSI, are insufficient to explain RTEpass. While the literature identifies titin as a primary mechanism for passive residual torque enhancement, SSI elastography did not detect this phenomenon through solely vastus lateralis stiffness.

The interaction of post-activation potentiation and fatigue on skeletal muscle twitch torque and displacement

Introduction

Tensiomyography (TMG) assesses skeletal muscle contractile properties based on the electrically stimulated radial muscle displacement. As the relationship between twitch displacement and associated torque is poorly understood, it is unclear how it is affected by post-activation potentiation and muscle fatigue. This study investigated how the interaction of potentiation and fatigue affects the rectus femoris (RF) twitch displacement and associated torque.

Materials and methods

Sixteen resistance-trained men (n = 8) and women (n = 8) performed two sets of five and five sets of ten seated maximum voluntary isometric knee extensions to induce potentiation and fatigue. Twitch displacement and torque were measured at baseline before the first set, after each set, and every 2 min for 15 min after the last set.

Results

The exercise effectively induced potentiation and fatigue as peak twitch torque increased by 44.1% after the first set, decreased by 32.9% after the last set and remained decreased by 26.4% after 15 min. Twitch displacement was considerably less affected by the exercise. Consequently, TMG parameters could not accurately detect potentiated or fatigued participants as indicated by the peak twitch torque.

Discussion

The TMG parameters' insufficient diagnostic accuracy likely resulted from a reduced signal-to-noise ratio at 90° knee flexion and the associated longer muscle length of the RF, compared to more extended knee angles commonly employed in TMG studies. These results highlight an important methodological consideration as the joint angle, i.e. muscle length, appears to influence the TMG parameters' ability to detect exercise-induced changes in contractile properties.

Skeletal muscle elastic modulus in marathon distance runners

Skeletal muscle shear elastic modulus is a non-invasive surrogate for early detection of muscle damage and soreness consequent to unaccustomed eccentric muscle work. We investigated the influence of marathon distance running on skeletal muscle shear elastic modulus. Shear modulus of the rectus femoris was measured via ultrasound shear wave elastography in 80 participants (30 female, 50 male) before and after running a World Marathon Major. Experience level, muscle soreness and run readiness were surveyed. Pre-marathon shear elastic modulus was lower in competitive vs recreational runners (13.0 ± 4.6 vs 15.6 ± 5.6 kPa; P = 0.0014), lower for fastest vs slowest finish times (11.6 ± 3.0 vs 16.9 ± 6.5 kPa; P =  < 0.0001) and associated with marathon finish time (r = - 0.40; P < 0.0003). Marathon running increased shear modulus (~ 23%), irrespective of experience, sex or course, but was blunted in runners wearing highly cushioned footwear with plates who had matched finish times (Other =  ~ 31% vs Vaporfly =  ~ 17%). Muscle soreness was strongly associated with run readiness (R2 = 0.995; P = 0.0026), and marathon recovery time was longer in runners with greater increases in marathon-mediated shear modulus. Skeletal muscle elastic modulus may reflect both short- and long-term muscle adaptation as a function of marathon running, or factors occurring before or after a marathon itself, such as exercise capacity or recovery time. These data are consistent with marathon-mediated muscle damage and soreness, but equally highlight a possibility to monitor and modulate outcomes in favor of a runner.

Image-Derived Skeletal Muscle Activation Metric Map: A Forearm Study Using Ultra-Fast Ultrasound Imaging and High-Density Electromyography

OBJECTIVE

Quantification of the skeletal muscle response can help better understand the fundamentals of the musculoskeletal system and can serve as a diagnostic measure or recovery assessment tool during rehabilitation for neurological injuries. Surface electromyography (EMG) is commonly used to measure muscle activity, but it is limited to detecting myoelectric signals without anatomy associated information. In this study, we proposed to use ultra-fast ultrasound imaging and introduced a new image analysis methodology to quantify a muscle's spatial-temporal mechanical response.

METHODS

The methodology is based on analyzing the spatial-temporal change of the impulsive kinetic energy during the period of muscle contraction. The analysis can derive an anatomy-registered muscle activation metric map that localizes regions of muscle activation. To demonstrate this, we intentionally evoked regional muscle responses in five participants without disabilities by electrically stimulating the median nerve and individual forearm muscle groups, respectively. Both ultrasound images and high-density EMG (HD-EMG) data were recorded and processed.

RESULTS

We presented the ultrasound image-derived activation localization from five participants and compared the results with HD-EMG measurements.

CONCLUSION

The comparison indicates a good resemblance for describing muscle recruitment pattern.

SIGNIFICANCE

The proposed methodology can potentially become an alternative or complementary approach to surface EMG for the study of skeletal muscle activation and for diagnosis and prognosis in clinical settings.

The effects of spinal flexion exposure on lumbar muscle shear modulus and posture

PURPOSE

Spinal flexion exposure (SFE) leads to alterations in neuromuscular and mechanical properties of the trunk. While several studies reported changes in intrinsic trunk stiffness following SFE, there is a lack of studies evaluating the effects on lumbar muscle shear modulus (SM). Therefore, the aim of our study was to investigate the effects of SFE on lumbar muscle SM and posture.

METHODS

Sixteen young volunteers were included in this clinical study. Passive lumbar muscle SM, lumbar lordosis, lumbar flexion range of motion and sitting height were measured prior to and following a 60-min SFE protocol.

RESULTS

For SM, our results did not show a significant muscle × time interaction effect (p = 0.40). However, we found increased SM (from 6.75 to 15.43% - all p < 0.02) and maximal lumbar flexion (15.91 ± 10.88%; p < 0.01), whereas lumbar lordosis ( - 7.67 ± 13.97%; p = 0.03) and sitting height ( - 0.57 ± 0.32%; p < 0.01) decreased following SFE. Our results showed no significant correlations between the changes in the included outcome measures (p = 0.10-0.83).

CONCLUSION

We hypothesized that increased lumbar muscle SM following SFE might be a compensation for decreased passive stability due to viscoelastic deformations of connective tissues, which are indicated by increased maximal lumbar flexion and decreased sitting height. However, there were no significant correlations between the changes of the included outcome measures, which implies that increased muscle SM and reduced lumbar lordosis are more likely an independent consequence of SFE.

Quantifying Active and Passive Stiffness in Plantar Flexor Muscles Following Intermittent Maximal Isometric Contractions Using Shear Wave Elastography

OBJECTIVE

This study aimed: (i) to investigate the impact of fatigue, triggered by maximal isometric contraction exercises, on the active and passive stiffness of plantar flexors (PF), and (ii) to examine the relationship between changes in mechanical parameters and neuromuscular alterations after fatigue.

METHODS

A healthy cohort (n = 12; age = 27.3 ± 5.5 y; BMI = 24.4 ± 2.35 kg/m²) was instructed to perform 60 isometric contractions, each lasting 4 s with a 1-s rest interval, using an ergometer. Several measures were taken before and after the fatigue protocol. First, the stiffness of the PF-tendon complex (PFC) was quantified during passive ankle mobilization both during and after the fatigue protocol using the ergometer. Additionally, from shear wave elastography, the active and passive stiffness of the gastrocnemius medialis (GM) were measured during passive ankle mobilization and isometric maximal voluntary contraction (MVC), respectively. Finally, the peak torque and the rate of torque development (RFD) of PF were assessed during the MVC using the ergometer. Ankle muscle activities (surface electromyograph [SEMG]) were recorded during all evaluations using electromyography.

RESULTS

After the fatigue protocol, the results revealed a decline in active stiffness, peak torque of PF, RFD and SEMG activity of the GM (p < 0.001). Furthermore, significant correlation was identified between the decrease of the peak torque of PF and the active stiffness of the GM (r = 0.6; p < 0.05). A decrease in the PFC stiffness (p < 0.001) and a decrease in the shear modulus of the GM at 20° (p < 0.001) were also observed.

CONCLUSION

Isometric fatiguing exercises modify the mechanical properties of both the contractile and elastic components. Notably, decreases in both passive and active stiffness may be critical for athletes, as these changes could potentially increase the risk of injury.

Effects of flywheel resistance training on countermovement jump performance and vastus lateralis muscle stiffness: A controlled study

This study aimed to investigate the effects of an 8-week resistance training using flywheel (FW) device on countermovement jump (CMJ) performance and resting stiffness of the vastus lateralis (VL) muscle. Physically active adults were randomly assigned to a training intervention group (T; n = 18) and a control group (C; n = 13), which received no intervention. Jump performance variables and ultrasound-assessed resting VL shear modulus were measured before and after the intervention. Analysis of covariance revealed statistically significant group differences in jump height (T =  +9%; C = -3%), rate of force development (T =  +32%; C =  +4%), peak power (T =  +9%; C = -1%), and peak force (T =  +7%; C = -1%). Jump performance improved only in the training group (all CMJ variables p < 0.05). Conversely, no significant changes within groups were observed in the resting shear wave modulus results (p > 0.05). VL stiffness decreased in the training group (-4%) and increased in the control group (+6%). Our results suggest that resistance training using FW device with individually allocated high-load FW inertia induces significant improvements in jump performance, which are not underpinned by changes in VL muscle stiffness.

Shear wave elastography characteristics of the gastrocnemius muscle in postural instability gait disorder vs tremor dominant Parkinson's disease patients

OBJECTIVES

This study explored the characteristics of muscle stiffness of lower gastrocnemius in resting and exercise states in patients with postural instability gait difficulty (PIGD) and tremor dominant (TD) Parkinson's disease patients using shear wave elastography (SWE).

DATA AND METHODS

75 PD patients from the Department of Parkinson's Disease Center in the Hospital from September 2021 to December 2022 were prospectively included, including 44 patients with PIGD and 31 with TD. In the same period, 40 healthy subjects matching gender and age were included as the control group. SWE was used to detect Young's modulus of both sides (right and left, R- and L-) of the lateral head of the gastrocnemius in resting (YM1) and exerciser states (YM2) in all participants and the absolute difference Young's modulus between resting and exercise state (ΔYM) was calculated.

RESULTS

R-YM2 and R-ΔYM were the highest in the normal controls, followed by the TD group, and lowest in the PIGD group. There were no differences in L-YM2 and L-ΔYM between the PIGD group and the TD group (all p > 0.05), but they were lower than those in the normal control group (all p < 0.05). In addition, R-YM2 and R-ΔYM were negatively correlated with disease duration and UPDRS III scores in the PIGD group (all p < 0.05). R-ΔYM has the highest value in the differential diagnosis of PIGD and TD patients. The area under the receiver operating characteristic curve (ROC) curve is 0.812 (95%CI, 0.730-0.893), and the diagnostic threshold is 120.5 Kpa with a sensitivity of 63.6%, a specificity of 90.1%, a positive predictive of 80%, and a negative predictive value of 80%.

CONCLUSION

Shear wave elastography is a sensitive ultrasound method for evaluating muscle strength in patients with PIGD and TD. It also provides a new biological indicator to distinguish between different phenotypes of patients with PD.

Fatiguing exercise reduces cellular passive Young's modulus in human vastus lateralis muscle

Previous studies demonstrated that acute fatiguing exercise transiently reduces whole-muscle stiffness, which might contribute to increased risk of injury and impaired contractile performance. We sought to elucidate potential intracellular mechanisms underlying these reductions. To that end, the cellular passive Young's modulus was measured in muscle fibres from healthy, young males and females. Eight volunteers (four male and four female) completed unilateral, repeated maximal voluntary knee extensions until task failure, immediately followed by bilateral percutaneous needle muscle biopsy of the post-fatigued followed by the non-fatigued control vastus lateralis. Muscle samples were processed for mechanical assessment and separately for imaging and phosphoproteomics. Fibres were passively (pCa 8.0) stretched incrementally to 156% of initial sarcomere length to assess Young's modulus, calculated as the slope of the resulting stress-strain curve at short (sarcomere length = 2.4-3.0 µm) and long (sarcomere length = 3.2-3.8 µm) lengths. Titin phosphorylation was assessed by liquid chromatography followed by high-resolution mass spectrometry. The passive modulus was significantly reduced in post-fatigued versus control fibres from male, but not female, participants. Post-fatigued samples showed altered phosphorylation of five serine residues (four located within the elastic region of titin) but did not exhibit altered active tension or sarcomere ultrastructure. Collectively, these results suggest that acute fatigue is sufficient to alter phosphorylation of skeletal titin in multiple locations. We also found reductions in the passive modulus, consistent with prior reports in the literature investigating striated muscle stiffness. These results provide mechanistic insight contributing to the understanding of dynamic regulation of whole-muscle tissue mechanics in vivo. HIGHLIGHTS: What is the central question of this study? Previous studies have shown that skeletal muscle stiffness is reduced following a single bout of fatiguing exercise in whole muscle, but it is not known whether these changes manifest at the cellular level, and their potential mechanisms remain unexplored. What is the main finding and its importance? Fatiguing exercise reduces cellular stiffness in skeletal muscle from males but not females, suggesting that fatigue alters tissue compliance in a sex-dependent manner. The phosphorylation status of titin, a potential mediator of skeletal muscle cellular stiffness, is modified by fatiguing exercise. Previous studies have shown that passive skeletal muscle stiffness is reduced following a single bout of fatiguing exercise. Lower muscle passive stiffness following fatiguing exercise might increase risk for soft-tissue injury; however, the underlying mechanisms of this change are unclear. Our findings show that fatiguing exercise reduces the passive Young's modulus in skeletal muscle cells from males but not females, suggesting that intracellular proteins contribute to reduced muscle stiffness following repeated loading to task failure in a sex-dependent manner. The phosphorylation status of the intracellular protein titin is modified by fatiguing exercise in a way that might contribute to altered muscle stiffness after fatiguing exercise. These results provide important mechanistic insight that might help to explain why biological sex impacts the risk for soft-tissue injury with repeated or high-intensity mechanical loading in athletes and the risk of falls in older adults.

Effect of Knee Angle, Contractile Activity, and Intensity of Force Production on Vastus Lateralis Stiffness: A Supersonic Shear Wave Elastography Pilot Study

Supersonic shear image (SSI) ultrasound elastography provides a quantitative assessment of tissue stiffness using the velocity of shear waves. SSI's great potential has allowed researchers in fields like biomechanics and muscle physiology to study the function of complex muscle groups in different conditions. The aim of this study is to use SSI to investigate changes in the stiffness of the vastus lateralis (VL) muscle as a consequence of passive elongation, isometric contraction, and repeated muscle activity. In a single session, 15 volunteers performed a series of isometric, concentric, and eccentric contractions. SSI images were collected from the VL to assess its stiffness before and after the contractions and at various knee angles. Two-way within-subjects ANOVA was used to test the effects of muscle contraction type and knee angle on VL stiffness. Linear regression analysis was employed to assess the relationship between muscle stiffness and the intensity of isometric contractions. After maximal contractions, VL stiffness increased by approximately 10% compared to baseline values, and following maximal isometric (p < 0.01) and eccentric contractions (p < 0.05). Yet, there was no change in VL shear modulus at the end of concentric contractions. The relaxed VL shear modulus increased with knee flexion both before and after the knee extensor contractions (p < 0.001). A linear relationship between the shear modulus and the degree of isometric contraction was observed, although with notable individual variation (R2 = 0.125). Maximal contractile activity produces modest increases in relaxed muscle stiffness. The SSI-measured shear modulus increases linearly with the degree of isometric contraction.

Wearable Ultrasound Imaging Device for Dynamic Dual-Direction Shear Wave Elastography of Shoulder Muscle

The shoulder is the most mobile joint in the human body, thus requiring intricate coordination of adjacent muscles. Patients suffered from rotator cuff muscle injuries have several typical symptoms including shoulder pain and difficulty raising the arm, thus reducing work efficiency and compromising the quality of life. Ultrasound has been used widely for shoulder soft tissue imaging as well as ultrasound elastography was introduced in shoulder examination for the dilemma of treating degenerative rotator cuff tears. However, most of the ultrasound examination was performed under a static condition. Providing dynamic information from shoulder muscle is important in clinical applications because the pains sometimes come from various positions of the shoulder during moving. In this study, a customized wearable T-shaped ultrasound transducer (128 + 128 elements) was proposed for shoulder dual-direction shear wave elastography (DDSWE), which provides the SWE for both longitudinal (SW along the muscle fiber) and transverse (SW cross the muscle fiber) directions dynamically. An optical tracking system was synchronized with an ultrasound imaging system to capture shoulder movements in 3-D space with their corresponding ultrasound images. The performance of DDSWE and the accuracy of optical tracking were verified by phantom experiments. Human studies were carried out by volunteers as they are moving their arms. The experimental results show that the bias and precision for the proposed DDSWE in elastic phantom were about 6% and 1.2% for both directions, respectively. A high accuracy of optical tracking was observed using a 3-D motor stage experimental setup. Human experiments show that the shear wave velocities (SWVs) were increased with the angles of shoulder abduction, and the average transverse and longitudinal SWVs were increased from 2.24 to 3.35 m/s and 2.95 to 5.95 m/s with abduction angle from 0° to 60°, respectively, which they are anisotropic-dependent. All the experimental results indicate that the proposed wearable ultrasound DDSWE can quantify the mechanical properties of shoulder muscles dynamically, thereby helping surgeons and physical therapists determine whether the intensity of rehabilitation shoulder be tuned down or escalated in the future.

Three-dimensional mapping of ultrasound-derived skeletal muscle shear wave velocity

Introduction: The mechanical properties of skeletal muscle are indicative of its capacity to perform physical work, state of disease, or risk of injury. Ultrasound shear wave elastography conducts a quantitative analysis of a tissue's shear stiffness, but current implementations only provide two-dimensional measurements with limited spatial extent. We propose and assess a framework to overcome this inherent limitation by acquiring numerous and contiguous measurements while tracking the probe position to create a volumetric scan of the muscle. This volume reconstruction is then mapped into a parameterized representation in reference to geometric and anatomical properties of the muscle. Such an approach allows to quantify regional differences in muscle stiffness to be identified across the entire muscle volume assessed, which could be linked to functional implications. Methods: We performed shear wave elastography measurements on the vastus lateralis (VL) and the biceps femoris long head (BFlh) muscle of 16 healthy volunteers. We assessed test-retest reliability, explored the potential of the proposed framework in aggregating measurements of multiple subjects, and studied the acute effects of muscular contraction on the regional shear wave velocity post-measured at rest. Results: The proposed approach yielded moderate to good reliability (ICC between 0.578 and 0.801). Aggregation of multiple subject measurements revealed considerable but consistent regional variations in shear wave velocity. As a result of muscle contraction, the shear wave velocity was elevated in various regions of the muscle; showing pre-to-post regional differences for the radial assessement of VL and longitudinally for BFlh. Post-contraction shear wave velocity was associated with maximum eccentric hamstring strength produced during six Nordic hamstring exercise repetitions. Discussion and Conclusion: The presented approach provides reliable, spatially resolved representations of skeletal muscle shear wave velocity and is capable of detecting changes in three-dimensional shear wave velocity patterns, such as those induced by muscle contraction. The observed systematic inter-subject variations in shear wave velocity throughout skeletal muscle additionally underline the necessity of accurate spatial referencing of measurements. Short high-effort exercise bouts increase muscle shear wave velocity. Further studies should investigate the potential of shear wave elastography in predicting the muscle's capacity to perform work.

Sprint Resisted and Assisted Priming for Peak Performance

ABSTRACT

Krzysztof, K, Aleksander, M, Adam, Z, and Krzysztofik, M. Sprint resisted and assisted priming for peak performance. J Strength Cond Res 37(12): 2354-2361, 2023-This study aimed to investigate resisted and assisted sprint, or a combination of both, as a conditioning activity (CA) on creatine kinase activity, biceps femoris stiffness, postactivation performance enhancement (PAPE), and priming in a sprint. Ten female sprinters (age: 20.1 ± 2.3 years, body mass: 56.4 ± 4.2 kg, body height: 171 ± 5 cm, training experience: 5.6 ± 2 years) participated. Before the warm-up, blood samples were drawn to determine creatine kinase at baseline. After the warm-up, biceps femoris stiffness was determined, and consequently, 50-m sprint time (with 0- to 20-m and 20- to50-m split time evaluation) was assessed 5 minutes before (pre-CA) and in the seventh minute after the completion of each CA (4 sets of 40 m either: 10% body mass resisted [RST]; 105% maximum velocity assisted [AST] sprint; or 2 sets of each sprint as the CA [COMB]), as well as after 48 hours of recovery. Furthermore, another blood sample was taken 48 hours later, before the warm-up, to assess changes in creatine kinase activity. A significantly higher 20-m sprint time at post-CA ( p = 0.006; ES = 0.4) and post-48 ( p = 0.011; ES = 0.59) compared with pre-CA in the RST condition was found, whereas a significantly lower sprint time was reported in post-48 compared with post-CA ( p = 0.019; ES = 0.44) in the COMB condition. However, a significantly lower 30-m flying sprint time at post-CA ( p < 0.001; ES = 0.28) and at post-48 ( p = 0.014; ES = 0.22) compared with pre-CA in the RST condition was revealed. Moreover, a significantly lower 30-m flying sprint time at post-48 compared with the pre-CA ( p = 0.007; ES = 0.7 and p = 0.003; ES = 0.77) and with the post-CA ( p = 0.019; ES = 0.61 and p = 0.009; ES = 0.68) in the AST condition and COMB was reported. Furthermore, a significantly lower 50-m sprint time at post-48 compared with pre-CA ( p = 0.015; ES = 0.51 and p = 0.011; ES = 0.64) and with post-CA ( p = 0.005; ES = 0.46 and p = 0.001; ES = 0.55) in the AST and COMB condition were found. The creatine kinase activity at post-48 during AST was significantly lower than during RST ( p = 0.028; ES = 0.73) and COMB ( p = 0.028; ES = 1.48). No significant changes were found for biceps femoris stiffness ( p = 0.085; η 2 = 0.199). The COMB and AST contributed to the improvement of the 50-m sprint time 48 hours after activation, which may be a good precompetition approach. However, during training sessions aiming to induce an acute PAPE effect and improve the 30-m flying sprint time, the best solution might be to use low-volume-resisted sprints.

Extrinsic Laryngeal Muscle Tension in Primary Muscle Tension Dysphonia with Shear Wave Elastography

OBJECTIVES

It has been assumed that patients with primary muscle tension dysphonia (pMTD) have more extrinsic laryngeal muscle (ELM) tension, but tools to study this phenomenon lack. Shear wave elastography (SWE) is a potential method to address these shortcomings. The objectives of this study were to apply SWE to the ELMs, compare SWE measures to standard clinical metrics, and determine group differences in pMTD and typical voice users before and after vocal load.

METHODS

SWE measurements of the ELMs from ultrasound examinations of the anterior neck, supraglottic compression severities from laryngoscopic images, cepstral peak prominences (CPP) from voice recordings, and self-perceptual ratings of vocal effort and discomfort were obtained in voice users with (N = 30) and without (N = 35) pMTD, before and after a vocal load challenge.

RESULTS

ELM tension significantly increased from rest-to-voiced conditions in both groups. However, the groups were similar in their ELM stiffness levels at SWE at baseline, during vocalization, and post-vocal load. Levels of vocal effort and discomfort and supraglottic compression were significantly higher and CPP was significantly lower in the pMTD group. Vocal load had a significant effect on vocal effort and discomfort but not on laryngeal or acoustic patterns.

CONCLUSION

SWE can be used to quantify ELM tension with voicing. Although the pMTD group reported significantly higher levels of vocal effort and vocal tract discomfort and, on average, exhibited significantly more severe supraglottic compression and lower CPP values, there were no group differences in levels of ELM tension using SWE.

LEVEL OF EVIDENCE

2 Laryngoscope, 133:3482-3491, 2023.

Hamstring Stiffness and Strength Responses to Repeated Sprints in Healthy Nonathletes and Soccer Players With Versus Without Previous Injury

BACKGROUND

The effect of 10 × 30 m repeated sprints on passive and active stiffness of semitendinosus (ST) and biceps femoris long head (BFlh), and knee flexor maximal voluntary isometric contraction (MVIC) and rate of force development (RFD), and whether athletes with previous hamstring injury have a different response, is unknown.

HYPOTHESIS

Repeated sprints would (1) increase BFlh stiffness and decrease ST stiffness and knee flexors MVIC and RFD in healthy participants; and (2) greater magnitude of response would be seen in athletes with previous hamstring injury.

STUDY DESIGN

Case series (experiment I) and case control (experiment II) study designs.

LEVEL OF EVIDENCE

Level 3.

METHODS

Healthy nonathletes attended 2 replicated sessions (experiment I, n = 18), while soccer players with (n = 38) and without (n = 67) previous hamstring injury attended 1 testing session (experiment II).

RESULTS

In both experiments, the knee flexors MVIC and RFD decreased after the sprints (P < 0.05). In experiment I, the ST and BFlh passive stiffness reduced after the sprints (P < 0.02), while a small BFlh active stiffness increase was noted (P = 0.02); however, no correlation was observed between the 2 testing sessions for the postsprint muscle stiffness responses (r = -0.07-0.44; P > 0.07). In experiment II, only an ST passive stiffness reduction was observed after the sprints (P < 0.01). No differences were noted between injured and noninjured lower limbs for any variable (P > 0.10).

CONCLUSION

Repeated sprints are likely to decrease the knee flexor's maximal and rapid strength, and to alter the hamstring stiffness in the nonathlete population. Previous hamstring injury does not apparently affect the footballer's hamstring functional and mechanical responses to repeated sprints.

CLINICAL RELEVANCE

The responses of hamstring stiffness and knee flexor strength to repeated sprints are unlikely to be associated with hamstring injury.

Acute Effects of Isometric Conditioning Activity on the Viscoelastic Properties of Muscles and Sprint and Jumping Performance in Handball Players

ABSTRACT

Krzysztofik, M, Spieszny, M, Trybulski, R, Wilk, M, Pisz, A, Kolinger, D, Filip-Stachnik, A, and Stastny, P. Acute effects of isometric conditioning activity on the viscoelastic properties of muscles and sprint and jumping performance in handball players. J Strength Cond Res 37(7): 1486-1494, 2023-The effects of conditioning activity (CA) on muscle stiffness are currently unknown, suggesting that maximum CA effort can increase or decrease the stiffness of involved muscle groups. Therefore, this study aimed to investigate the effect of maximal isometric half-squats on the viscoelastic properties of muscles and postactivation performance enhancement (PAPE) in sprints and jumps. Twelve handball players underwent a standard warm-up and baseline assessment of muscle stiffness and tone of vastus lateralis and gastrocnemius medialis muscle, followed by 20-m sprint with intermediate measures at 5 and 10 m and countermovement jump. The PAPE was assessed by repeating the tests (at 4th, 8th, and 12th minute post-CA) after a CA protocol consisting of 3 sets of 3-second maximal isometric half-squats (EXP) or a control condition (CTRL) without any CA. The vastus lateralis stiffness in the 4th and 12th minute and muscle tone in the 4th minute post-CA significantly decreased compared with baseline ( p = 0.041, ES = 0.57; p = 0.013, ES = 0.52; p = 0.004, ES = 0.81, respectively) in the EXP condition. The 20-m sprint time significantly decreased at all post-CA time points compared with the baseline for the EXP condition ( p < 0.033) and the after values in the CTRL condition ( p < 0.036). In comparison to baseline, the 10-m sprint time decreased in the eighth minute post-CA ( p = 0.021; ES = 0.82) in the EXP condition. Moreover, it was significantly lower at the 8th and 12th minute post-CA ( p = 0.038; ES = 0.71 and p = 0.005; ES = 1.26) compared with that time points in the CTRL condition. The maximal isometric half-squats effectively improved sprint performance and significantly decreased vastus lateralis tone and stiffness. These findings offer new insights into the assessment of viscoelastic properties for evaluating the fatigue or potentiation state, which requires further investigation.

Acute Effects of Varied Back Squat Activation Protocols on Muscle-Tendon Stiffness and Jumping Performance

ABSTRACT

Krzysztofik, M, Wilk, M, Pisz, A, Kolinger, D, Tsoukos, A, Zając, A, Stastny, P, and Bogdanis, GC. Acute effects of varied back squat activation protocols on muscle-tendon stiffness and jumping performance. J Strength Cond Res 37(7): 1419-1427, 2023-Intensity, movement velocity, and volume are the principal factors to successfully use postactivation performance enhancement. Therefore, 15 resistance-trained volleyball players completed 3 different back squat configurations as a conditioning activity (CA) in randomized order: (a) 3 sets of 3 repetitions at 85% 1RM (HL); (b) a single set of back squats at 60% 1RM until 10% mean velocity loss (VB); (c) and 2 sets of back squats at 60% 1RM until 10% mean velocity loss (2VB) on subsequent countermovement jump performance, Achilles tendon, and vastus lateralis stiffness with concomitant front thigh skin surface temperature assessment. The measurements were performed 5 minutes before the CA and at 2, 4, 6, 8, and 10 minutes. The jump height was significantly increased in the second minute and at peak, post-CA compared with baseline for all conditions ( p = 0.049; ES = 0.23 and p < 0.001; ES = 0.37). Skin surface temperature was significantly increased for all post-CA time points compared with baseline in the 2VB condition ( p from <0.001-0.023; ES = 0.39-1.04) and in the fourth minute and at peak post-CA in HL condition ( p = 0.023; ES = 0.69 and p = 0.04; ES = 0.46), whereas for the VB condition, a significant decrease in peak post-CA was found ( p = 0.004; ES = -0.54). Achilles tendon stiffness was significantly decreased for second, fourth, eighth, 10th, and peak post-CA in comparison to baseline for all conditions ( p from p = 0.004-0.038; ES = -0.47 to -0.69). Vastus lateralis stiffness was significantly decreased for peak post-CA compared with baseline for all conditions ( p = 0.017; ES = -0.42). We recommend using a single set of barbell squats with a 10% velocity loss as a mechanism of fatigue control to acutely improve jump height performance and avoid unnecessary increases in training volume.

Contractile properties are less affected at long than short muscle length after eccentric exercise

PURPOSE

The aim of the present study was to investigate whether the electrically evoked muscle responses are differently affected over time by the knee joint angle after an exercise-induced muscle damage (EIMD). We hypothesized that low-frequency-evoked responses would be less affected at long than short muscle length, and that mechanisms located within the muscle and tendinous tissues would be involved.

METHODS

Fifteen males performed 45 min loaded downhill walking (DW) exercise. Maximal voluntary contraction torque (MVC), optimal angle for torque production, voluntary activation level (VAL), twitch, doublet at 10 and 100 Hz (Db10 and Db100, respectively), rate of torque development (RTD), post-activation potentiation (PAP), muscle shear elastic modulus (µ) and aponeurosis stiffness were assessed before, after, and 4, 24, 48, 72 and 168 h after the exercise at a knee angle of 40°, 90° and 120° (0°: full extension).

RESULTS

MVC, VAL and Db100 were similarly decreased across joint angles after the DW and optimal angle was not affected. Twitch, Db10, Db10/Db100, PAP and RTD were less affected and muscle µ more increased at long than short muscle lengths (p < 0.05), especially during the first 24 h after the DW exercise.

CONCLUSION

Low-frequency-evoked responses were more preserved at long than short muscle length the first 24 h after the DW exercise, suggesting that joint angle should be taken into account to assess muscular alterations after EIMD. This length-dependence could be associated to the higher sensitivity to Ca²⁺ and the higher increase in muscle stiffness at long than short muscle length.

Constitutive parameter identification of transtibial residual limb soft tissue using ultrasound indentation and shear wave elastography

Finite element analysis (FEA) can be used to evaluate applied interface pressures and internal tissue strains for computational prosthetic socket design. This type of framework requires realistic patient-specific limb geometry and constitutive properties. In recent studies, indentations and inverse FEA with MRI-derived 3D patient geometries were used for constitutive parameter identification. However, long computational times and use of specialized equipment presents challenges for clinical, deployment. In this study, we present a novel approach for constitutive parameter identification using a combination of FEA, ultrasound indentation, and shear wave elastography. Local shear modulus measurement using elastography during an ultrasound indentation experiment has particular significance for biomechanical modeling of the residual limb since there are known regional dependencies of soft tissue properties such as varying levels of scarring and atrophy. Beyond prosthesis design, this work has broader implications to the fields of muscle health and monitoring of disease progression.

The Effects of Isometric Fatigue on Trunk Muscle Stiffness: Implications for Shear-Wave Elastography Measurements

Muscle stiffness has been implicated as a possible factor in low back pain risk. There are few studies on the effects of isometric fatigue on the shear modulus of trunk muscles. This study aimed to investigate the effects of trunk isometric fatigue on the passive and active (during low and high-level contractions) shear moduli of the erector spinae (ES) and superficial and deep multifidus (MF) muscles. We assessed passive and active shear modulus using shear-wave elastography in healthy young participants (n = 22; 11 males, 11 females), before and after an isometric trunk extension fatigue protocol. Maximal voluntary force decreased from 771.2 ± 249.8 N before fatigue to 707.3 ± 204.1 N after fatigue (-8.64%; p = 0.003). Passive shear modulus was significantly decreased after fatigue in the MF muscle (p = 0.006-0.022; Cohen's d = 0.40-46), but not the ES muscle (p = 0.867). Active shear modulus during low-level contraction was not affected by fatigue (p = 0.697-0.701), while it was decreased during high-level contraction for both muscles (p = 0.011; d = 0.29-0.34). Sex-specific analysis indicated the decrease in ES shear modulus was significant in males (p = 0.015; d = 0.31), but not in females (p = 0.140). Conversely, the shear modulus in superficial MF had a statistically significant decrease in females (p = 0.002; d = 0.74) but not in males (p = 0.368). These results have important implications for further investigations of the mechanistic interaction between physical workloads, sex, muscle stiffness (and other variables affecting trunk stability and neuromuscular control), and the development/persistence of low back pain.

Five-Compressions Protocol as a Valid Myotonometric Method to Assess the Stiffness of the Lower Limbs: A Brief Report

The objective of this study was to evaluate the validity of a short assessment MyotonPRO protocol to measure the stiffness of the superficial muscles and tendons of the lower limbs. The stiffness of the dominant lower limb vastus lateralis (VL), rectus femoris (RF) and patellar tendon (PT) was evaluated in 52 healthy participants (26.9 ± 3.4 years) with two MyotonPRO protocols: the standard protocol (10 mechanical taps) and the short protocol (five mechanical taps). The myotonometry was performed at the midpoint of the length from the upper pole of the patella to the greater trochanter for the VL, and to the anterior superior iliac spine for the RF. The PT was evaluated 1 cm caudal from the inferior pole of the patella. Pearson's correlation coefficients were calculated to determine the relationships between protocols. The validity of the short protocol was evaluated with Student's t-test. High positive correlations were observed between the short and standard protocols in the stiffness of the VL (r = 0.959; p < 0.001), the RF (r = 0.967; p < 0.001) and the PT (r = 0.953; p < 0.001) and no differences were found between both protocols in the stiffness assessment of the VL, RF and PT (p > 0.05). Therefore, the five-compressions protocol is a valid protocol for the assessment of lower limb mechanical properties.

Differences in Medial and Lateral Gastrocnemius Stiffness after Exercise-Induced Muscle Fatigue

Muscles are affected at the cellular level by exercised-induced fatigue, inducing changes in their stiffness. Examining muscle stiffness can improve the knowledge of various pathologic conditions, such as pain and injury. The objective of this study was to examine the stiffness of the medial gastrocnemius (MG) muscle and the lateral gastrocnemius (LG) muscle to determine the changes in stiffness, and to assess the differences in the stiffness between the MG and the LG, as affected by muscle fatigue measured using shear wave elastography (SWE) and a MyotonPRO after inducing muscle fatigue. A total of 35 healthy young adults participated in the study. The stiffness of the MG and the LG were assessed before and after a muscle fatigue protocol (MFP), which included three sets of 50 eccentric contractions of the calf muscles of the dominant leg, at rest, and at maximum voluntary contraction (MVC). The measurements were taken with SWE and the MyotonPRO simultaneously. Compared to baseline, the resting stiffness of the MG and the LG significantly increased immediately, 24 h, and 48 h after muscle fatigue (p < 0.05); however, during MVC, the stiffness of the MG decreased (p < 0.05) and that of the LG showed no change (p > 0.05). When the stiffness of the MG and the LG were compared before and after the MFP, changes in the stiffness of the MG were significantly greater than those in the LG (p < 0.05). This signifies that the MG was more affected by the exercise-induced muscle fatigue than was the LG. The assessment of musculoskeletal tissue and its characteristics, before and after eccentric exercise, is crucial in the prevention of overuse injuries associated with repeated exposure to both low and high levels of force.

The Applicability of Shear Wave Elastography to Assess Myotendinous Stiffness of Lower Limbs during an Incremental Isometric Strength Test

The aim of the study was to describe the applicability of shear wave elastography to assess muscular and tendinous stiffness of the lower limbs during an incremental isometric strength test and to differentiate the stiffness evolution between superficial and deep muscle regions. Dominant rectus femoris and patellar tendons of 30 physically active people (28.3 ± 9.2 years, 173.2 ± 7.7 cm, 76.2 ± 12.6 kg) were measured in different isometric strength conditions (relaxed muscle, and at 10%, 20%, 30%, 40%, 50% and 60% of maximal voluntary contraction (MVC)). The percentage of success was >85% at all muscle contraction intensities for rectus femoris muscles but only in a relaxed condition for patellar tendons. Rectus femoris stiffness significantly increased compared to the relaxed condition from 30% to 60% MVC (p ≤ 0.011) in superficial muscle regions, and from 10% to 60% MVC (p ≤ 0.002) in deep muscle regions. Deep muscle regions showed higher stiffness values than superficial muscle regions at 30% MVC (51.46 ± 38.17 vs. 31.83 ± 17.05 kPa; p = 0.019), 40% MVC (75.21 ± 42.27 vs. 51.25 ± 28.90 kPa; p = 0.018), 50% MCV (85.34 ± 45.05 vs. 61.16 ± 37.03 kPa; p = 0.034) and 60% MVC (109.29 ± 40.04 vs. 76.67 ± 36.07 kPa; p = 0.002). Rectus femoris stiffness increased during the incremental isometric contraction test, and inter-region differences were found at 30% MVC.

Using Shear-Wave Elastography to Assess Exercise-Induced Muscle Damage: A Review

Shear-wave elastography is a method that is increasingly used to assess muscle stiffness in clinical practice and human health research. Recently, shear-wave elastography has been suggested and used to assess exercise-induced muscle damage. This review aimed to summarize the current knowledge of the utility of shear-wave elastography for assessment of muscle damage. In general, the literature supports the shear-wave elastography as a promising method for assessment of muscle damage. Increases in shear modulus are reported immediately and up to several days after eccentric exercise, while studies using shear-wave elastography during and after endurance events are showing mixed results. Moreover, it seems that shear modulus increases are related to the decline in voluntary strength loss. We recommend that shear modulus is measured at multiple muscles within a muscle group and preferably at longer muscle lengths. While further studies are needed to confirm this, the disruption of calcium homeostasis seems to be the primary candidate for the underlying mechanism explaining the increases in shear modulus observed after eccentric exercise. It remains to be investigated how well the changes in shear modulus correlate with directly assessed amount of muscle damage (biopsy).

Sex-related differences and effects of short and long trail running races on resting muscle-tendon mechanical properties

The purpose of the study was to assess sex-related differences in resting mechanical properties and adaptations of skeletal muscles and tendons in response to trail running races of different distances using multi-site shear wave elastography assessments of the lower limb, force capacity and blood analyses. Sex differences in resting mechanical properties of knee extensor and plantar flexor muscles and tendons were characterized by shear wave velocity measurements in healthy males (N=42) and females (N=25) trained in long distance running. Effects of running distance on muscle and tendon properties were assessed in short (<60km, N=23) vs. long (>100km, N=26) distance races. Changes in isometric maximal voluntary contraction torque, serum C-reactive protein and creatine kinase activity were also quantified after running races. Higher shear wave velocity of relaxed triceps surae muscle was detected in females as compared to males before running races (+4.8%, p=0.006), but the significant increases in triceps surae muscle group (+7.0%, p=0.001) and patellar tendon shear wave velocity (+15.4%, p=0.001) after short-distance races were independent of sex. A significant decrease in triceps surae muscle shear wave velocity was found after long-distance races in the whole experimental population (-3.1%, p=0.049). Post-races increase in C-reactive protein and creatine kinase activity were significantly correlated to the relative decreases in triceps surae and quadriceps femoris skeletal muscle shear wave velocity (ρ=-0.56, p=0.001 and ρ=-0.51, p=0.001, respectively). Resting mechanical properties of muscles and tendons are affected by sex, and that adaptations to trail races are related to running distance. Exercise-induced changes in resting skeletal muscle mechanical properties are associated with enhanced indirect markers of inflammation and muscle damage.

Assessing the differences in muscle stiffness measured with shear wave elastography and myotonometer during the menstrual cycle in young women

This study assessed the differences in muscle stiffness of the medial gastrocnemius (MG) and tibialis anterior (TA) muscles at rest and contraction during ovulation and follicular phase (menstruation) in women with regular menstrual cycle. Thirty-four young healthy women (mean age 21.3 ± 1.3 years) with regular menstrual cycles participated in this study. Stiffness of the TA and MG muscles at rest and voluntary contraction during ovulation and follicular phase in young women were measured using shear-wave elastography (SWE) and the handheld myotonometer MyotonPRO. The absolute stiffness difference between resting and contraction was expressed as the stiffness increase rate (SIR). The stiffness of the MG and TA at the resting position was not significantly different between the two phases of the menstrual cycle (p > .05). A significantly greater stiffness of both muscles measured using MyotonPRO in the follicular phase than during ovulation was found (p < .05), while stiffness measured by SWE showed a difference only in the TA muscle during contraction (p < .05). In addition, there were no significant differences in the SIR of both muscles between the two phases (p > .05). The results of our study showed a significantly greater stiffness of the MG and TA muscles at the follicular phase than at ovulation during contraction only. As muscle stiffness affects the risk of injury owing to reduced stability during sports activities, these changes in mechanical properties during the menstrual cycle should be noted, and training strategies should be used in female athletes. This article is protected by copyright. All rights reserved.

Verification of surface electromyographic activity of the oblique externus abdominis using ultrasound shear wave elastography

This study used ultrasound shear wave elastography (SWE) to revalidate whether surface electromyographic (EMG) electrodes placed on the oblique externus abdominis (OE) can detect only the OE activity without the confounding activity of the underlying oblique internus abdominis (OI). During left and right trunk rotations, the EMG activity was acquired using surface EMG electrodes placed on the right OE. Shear wave velocity (V_s ) values of the right OE and OI were acquired using SWE. The EMG activity during the left and right trunk rotations significantly increased as the level of exertion increased (25%, 50%, 75%, and 100% of the one-repetition maximum [1RM]). The V_s of the right OE was significantly different only between 25% and 75% 1RM in the right trunk rotation, but significantly increased from 25% to 75% 1RM during the left trunk rotation. The V_s of the right OI during the right trunk rotation significantly increased with increased levels of exertion, except between 50% and 75% 1RM. The results for the V_s of the OE and OI in the right trunk rotation suggest that surface EMG electrodes placed on the OE would detect not only the antagonistic OE activity but also the agonistic OI activity.

Muscle Shear Elastic Modulus Provides an Indication of the Protection Conferred by the Repeated Bout Effect

Background: The neuromuscular system is able to quickly adapt to exercise-induced muscle damage (EIMD), such that it is less affected by subsequent damaging exercise, a phenomenon known as the repeated bout effect (RBE). The objective was to determine whether the mechanical properties of the quadriceps, as evaluated by shear wave elastography (SWE), were less affected when a second bout of eccentric-biased exercise was performed 2 weeks later. It was hypothesized that the first bout would confer protection against extensive muscle damage through an adaptation of the muscle stiffness before the second bout (i.e., higher muscle stiffness).

Methods: Sixteen males performed two identical bouts of downhill walking separated by 2 weeks (45 min at 4.5 km.h−1; gradient: 25%; load: 30% of the body mass). Rectus femoris (RF) and vastus lateralis (VL) resting shear elastic modulus (µ) and EIMD symptoms were measured before and up to 7 days following the exercise bouts. Changes in neuromuscular function was evaluated by maximal voluntary contraction torque, voluntary activation level, evoked mechanical response to single and double (10 and 100 Hz doublets) electrical stimulation. An index of protection (IP) was calculated for EIMD symptoms to assess magnitude the RBE.

Results: EIMD symptoms were less affected after the second than the first exercise bout. RF and VL-µ increased (p &lt; 0.001) only after the first exercise. RF µ was elevated up to 2 weeks after the end of the first exercise (p &lt; 0.001) whereas VL µ was only increased up to 24 h. The increase in µ observed 2 weeks after the end of the first exercise was correlated with the IP; i.e., attenuation of alterations in muscle µ, 10 Hz-doublet amplitude and rate of torque development after the second exercise bout (p &lt; 0.05).

Conclusion: We showed that muscle µ assessed by SWE was sensitive to the RBE, with a differential effect between VL and RF. The persistent increase in µ was associated with the attenuation of neuromuscular impairments observed after the second bout, suggesting that the increased muscle stiffness could be a “protective” adaptation making muscles more resistant to the mechanical strain associated to eccentric contractions.

Heavy-intensity cycling and running work-rate associated to VO2max affects isokinetic strength, the dynamic control ratio but not the conventional H:Q ratio

BACKGROUND: Bilateral strength asymmetry and fatigue predispose athletes to various injuries and conventional methods appear to be poor predictors of lower extremity muscular performance under NF conditions. OBJECTIVE: The purpose of the study was to compare the conventional Hcon/Qcon (HQR) ratio and the dynamic control ratio (DCR: Hecc/Qcon) under non-fatiguing (NF) and fatiguing (F) conditions and verify the effects of heavy-intensity constant running and cycling exercise on the isokinetic performance. METHODS: Twenty healthy male participants performed running and cycling VO2max at work-rate associated with the achievement of VO2max (TTE). Isokinetic muscle strength performance was tested at 60 and 180∘/s before and after these sessions with 48-hour intervals. Quadriceps (QFR) and hamstring (HFR) muscle fatigue rates were also calculated during these sessions. Blood lactate concentration was measured before and two-minutes after running and cycling TTE. RESULTS: No between-condition differences were found for the HQR while the DCR decreased significantly at 180∘/s following cycling and running sessions (p< 0.05). Cycling TTE was positively correlated with in dominant (r= 0.535, p= 0.015) and non-dominant (r= 0.446, p= 0.048) QFR. Positive correlations were also found between running TTE and dominant (r= 0.500, p= 0.25) and non-dominant (r= 0.465, p= 0.039) HFR. CONCLUSIONS: The DCR obtained at fast angular velocities following a strenuous exercise seems to be the best indicator of muscle performance while its assessment under F conditions reveals higher ratios compared to NF conditions and conventional methods.

Ultrasound Shear Modulus and Thickness of Lateral Abdominal Muscles in Different Contractile States in Relation to Self‐ Reported Hip/Groin Problems in Youth Soccer Players

To date, no studies have assessed lateral abdominal muscles’ (LAM) elasticity and thickness in relation to hip and groin symptoms in any population. The objectives were to a) assess the relationship between LAM ultrasound measurements (elasticity and thickness) and self-reported subscales of the Copenhagen Hip and Groin Outcome Score (HAGOS) and b) compare LAM elasticity and thickness between asymptomatic and symptomatic sides. Shear modulus and thickness of the oblique external (OE), internal (OI) and transversus abdominis (TrA) muscles in 25 young soccer players were assessed at rest and during isometric contraction using ultrasound shear wave elastography. HAGOS subscales were used to assess self-reported hip/groin problems. There was a significant (p < 0.05) moderate correlation between allometric-scaled OI resting thickness (mean of right and left) and the Activities of Daily Living (r = 0.40), Sport (r = 0.57) and Quality of Life (QOL) (r = 0.41) HAGOS subscales. Also, a moderate significant correlation was found between allometric-scaled TrA resting thickness and the QOL subscale (r = 0.47). Moderate correlations were found between resting OI shear modulus and the QOL (r = 0.44), between right TrA shear modulus during contraction and Symptoms (r = 0.57), and between the left TrA shear modulus during contraction and Physical Activity (r = 0.41) subscales. No differences were found between the symptomatic and asymptomatic side in thickness and elasticity measurements among soccer players with unilateral symptoms (p > 0.05). The relationships found between LAM and hip/groin problems in youth male soccer players indicate that muscles are thinner and more elastic (less stiff) in more symptomatic athletes.

Effect of sex and fatigue on quiet standing and dynamic balance and lower extremity muscle stiffness

PURPOSE

The purpose of the present study was to determine whether there are sex differences in fatigue-induced changes in quiet standing and dynamic balance and establish whether changes in muscle torque and resting stiffness may explain the potential sex differences in balance responses.

METHODS

Sixteen recreationally active men (age; 24.8 ± 5.0 years, height; 178.2 ± 5.6 cm, mass; 77.8 ± 13.2 kg) and 10 women (age; 21.0 ± 1.6 years, height; 167 ± 5.3 cm, mass; 61.3 ± 8.9 kg) were assessed for postural sway, Y balance test performance, isokinetic and isometric knee extensor torque and resting stiffness of the vastus lateralis (VL), gastrocnemius lateralis (GL) and Achilles tendon (AT) before and immediately after fatiguing exercise. The fatigue protocol consisted of five sets of 20-drop jumps.

RESULTS

The fatiguing exercise elicited similar magnitude (effects size; ES) reductions in muscle torque (men; ES = 0.45-0.80, women; ES = 0.46-0.52), dynamic balance (men; ES = 0.45-0.74, women; ES = 0.47-0.79) and resting VL stiffness (men; ES = 0.46, women; ES = 0.36) in men and women (all p < 0.05). For quiet standing balance, fatigue induced an increase in postural sway metrics (ES = 0.64-1.28) and reduction in resting GL stiffness (ES = 0.40) in men (both p < 0.001) but not women (p > 0.05).

CONCLUSION

Fatiguing exercise, when producing a similar level of force reduction, induces similar magnitude reductions in dynamic postural control and resting VL stiffness in men and women. Distinct deteriorations in quiet standing balance in men but not women were accompanied by modifications in calf muscle stiffness following exercise-induced muscle fatigue.

Greater Muscle Stiffness during Contraction at Menstruation as Measured by Shear-Wave Elastography

It is important to measure mechanical properties of muscle, since muscle stiffness is an important component of stabilizing or controlling joint stability. The levels of sex hormones especially estrogen vary over the phase of the menstrual cycle and impact the mechanical properties of soft tissue such as muscle, tendon, and ligaments due to the presence of 17-β estradiol receptor in human connective tissues. Recently, shear-wave elastography (SWE), based on ultrasound imaging, has been used as an accurate technique for visualizing and assessing tissue stiffness. The purpose of this study was to compare the muscle stiffness at rest and during contraction condition between the early follicular phase (menstruation) and ovulation in young women, measured using SWE. Thirty-seven young women with regular menstrual cycles completed this study throughout one full menstrual cycle. Stiffness of lower limb muscles such as the rectus femoris, biceps femoris, tibialis anterior, and medial gastrocnemius was measured at resting and during contraction conditions using SWE during menstruation and ovulation. All muscles showed significantly greater stiffness during the menstruation than ovulation when muscles were actively contracted (P < 0.05), whereas no significant differences in muscle stiffness at rest were noted across phase of the menstrual cycle. These significant findings suggest that muscular factors are changed with estradiol fluctuations; muscles are less stiff during ovulation where the levels of estradiol peak when muscles in a contraction condition. As muscle stiffness is an important part of joint stability, these differences should be recognized to prevent the risk of musculoskeletal injuries.

Skeletal muscle stiffness as measured by magnetic resonance elastography after chronic spinal cord injury: a cross-sectional pilot study

Skeletal muscle stiffness is altered after spinal cord injury (SCI). Assessing muscle stiffness is essential for rehabilitation and pharmaceutical interventions design after SCI. The study used magnetic resonance elastography to assess the changes in stiffness after chronic SCI compared to matched able-bodied controls and determine its association with muscle size, spasticity, and peak torque in persons with SCI. Previous studies examined the association between muscle stiffness and spasticity, however, we are unaware of other studies that examined the effects of muscle composition on stiffness after SCI. Ten participants (one female) with chronic SCI and eight (one female) matched able-bodied controls participated in this cross-sectional study. Magnetic resonance elastography was utilized to monitor stiffness derived from shear waves propagation. Modified Ashworth scale was used to evaluate spasticity scores in a blinded fashion. Peak isometric and isokinetic torques were measured using a biodex dynamometer. Stiffness values were non-significantly lower (12.5%; P = 0.3) in the SCI group compared to able-bodied controls. Moreover, stiffness was positively related to vastus lateralis whole muscle cross-sectional area (CSA) (r² = 0.64, P < 0.005) and vastus lateralis absolute muscle CSA after accounting for intramuscular fat (r² = 0.78, P < 0.0007). Stiffness was also positively correlated to both isometric (r²= 0.55-0.57, P < 0.05) and isokinetic peak (r²= 0.46-0.48, P < 0.05) torques. Our results suggest that larger clinical trial is warranted to confirm the preliminary findings that muscle stiffness is altered after SCI compared to healthy controls. Stiffness appeared to be influenced by infiltration of intramuscular fat and modestly by the spasticity of the paralyzed muscles. The preliminary data indicated that the relationship between muscle stiffness and peak torque is not altered with changing the frequency of pulses or angular velocities. All study procedures were approved by the Institutional Review Board at the Hunter Holmes McGuire VA Medical Center, USA (IRB #: 02314) on May 3, 2017.

Hip position influences shear wave elastography measurements of the hamstring muscles in healthy subjects

Ultrasound shear wave elastography (SWE) has recently emerged as a non-invasive tool for assessing muscle stiffness. The majority of studies utilizing SWE have focused primarily on upper-extremity muscles, with little attention attributed to lower-extremity muscles. In addition, of the studies that have been published, various joint and muscle positions have been examined, rendering it difficult to compare results across studies. Thus, the purpose of this investigation was to examine lower extremity medial hamstring muscles (semitendinosus and semimembranosus) and to determine how hip position (0° versus 90°) and muscle position (knee flexed versus extended) impacted resulting shear modulus values. Ten subjects varying widely in age participated in this study, and their hamstring stiffness was assessed in four separate positions: seated with the knee flexed and extended, and lying prone with the knee flexed and extended. Higher shear modulus values were found at the group-level when participants were seated compared to prone (hip placed at 90° compared to 0°). In addition, higher values were also found when the knee was extended compared to flexed, but only when the hip was placed at 90° (not 0°). These results demonstrate that joint and muscle position, particularly when assessing the hamstrings, largely impact resulting shear modulus values. Therefore, joint and muscle position need to be systematically controlled for and reported when establishing normative ranges for shear modulus values across specific age groups. This will enable physicians to more precisely determine whether patients' shear modulus values indicate clinically meaningful differences in comparison to normative data.

Ultra-High-Frame-Rate Ultrasound Monitoring of Muscle Contractility Changes Due to Neuromuscular Electrical Stimulation

The quick onset of muscle fatigue is a critical issue when applying neuromuscular electrical stimulation (NMES) to generate muscle contractions for functional limb movements, which were lost/impaired due to a neurological disorder or an injury. For in situ assessment of the effect of NMES-induced muscle fatigue, a novel noninvasive sensor modality that can quantify the degraded contractility of a targeted muscle is required. In this study, instantaneous strain maps of a contracting muscle were derived from ultra-high-frame-rate (2 kHz) ultrasound images to quantify the contractility. A correlation between strain maps and isometric contraction force values was investigated. When the muscle reached its maximum contraction, the maximum and the mean values of the strain map were correlated with the force values and were further used to stage the contractility change. During the muscle activation period, a novel methodology based on the principal component regression (PCR) was proposed to explore the strain-force correlation. The quadriceps muscle of 3 able-bodied human participants was investigated during NMES-elicited isometric knee extension experiments. Strong to very strong correlation results were obtained and indicate that the proposed measurements from ultrasound images are promising to quantify the muscle contractility changes during NMES.

Changes in the Viscoelastic Properties of the Vastus Lateralis Muscle With Fatigue

We investigated the in vivo effects of voluntary fatiguing isometric contractions of the knee extensor muscles on the viscoelastic properties of the vastus lateralis (VL). Twelve young males (29.0 ± 4.5 years) performed an intermittent voluntary fatigue protocol consisting of 6 sets × 10 repetitions of 5-s voluntary maximal isometric contractions with 5-s passive recovery periods between repetitions. Voluntary and evoked torque were assessed before, immediately after, and 20 min after exercise. The shear modulus (μ) of the VL muscle was estimated at rest and during a ramped isometric contraction using a conventional elastography technique. An index of active muscle stiffness was then calculated (slope from the relationship between shear modulus and absolute torque). Resting muscle viscosity (η) was quantified using a shear-wave spectroscopy sequence to measure the shear-wave dispersion. Voluntary and evoked torque decreased by ∼37% (P < 0.01) immediately after exercise. The resting VL μ was lower at the end of the fatigue protocol (-57.9 ± 5.4%, P < 0.001), whereas the resting VL η increased (179.0 ± 123%, P < 0.01). The active muscle stiffness index also decreased with fatigue (P < 0.05). By 20 min post-fatigue, there were no significant differences from the pre-exercise values for VL η and the active muscle stiffness index, contrary to the resting VL μ. We show that the VL μ is greatly reduced and η greatly enhanced by fatigue, reflecting a more compliant and viscous muscle. The quantification of both shear μ and η moduli in vivo may contribute to a better understanding of the mechanical behavior of muscles during fatigue in sports medicine, as well as in clinical situations.