The functional significance of hamstrings composition: is it really a "fast" muscle group?

Exploring the biomechanics and fatigue patterns of eccentric quasi-isometric muscle actions in the knee extensors and flexors

PURPOSE

Eccentric quasi-isometric (EQI) resistance training is emerging as a promising option in sports medicine and rehabilitation. Despite prior research on EQI contractions in quadriceps and biceps brachii, their use in hamstring injury contexts is underexplored. Therefore, our study examines and contrasts the biomechanics and fatigue effects of EQI training on knee extensors and flexors.

METHODS

Following familiarization, 16 healthy, active participants (9 men, 7 women; 23.5 ± 2.6 years, 72.1 ± 12.8 kg, 173.4 ± 10.7 cm) performed, in random order, four EQI contractions for knee extensions and flexions, respectively. EQI contractions were isotonically loaded to 70% of concentric (60°·s-1) maximal voluntary contraction. Rest between repetitions was set at three minutes, while four minutes separated each muscle group. Peak torque, mean torque, and optimal angle were evaluated pre- and post-bouts. Inter-repetition contraction time and angular velocity were also assessed.

RESULTS

Average torque was 160.9 ± 44.2 and 71.5 ± 23.2 Nm for the extensors and flexors. Peak and mean torque significantly decreased for both extensors (p < 0.001, d = 0.70-0.71) and flexors (p ≤ 0.022, d = 0.36) after EQI contractions, respectively. However, the optimal angle increased for extensors (p < 0.001, d = 1.00) but not flexors (p = 0.811, d = 0.06). During EQI contractions, knee flexors exhibited greater intra-repetition velocity than extensors (p = 0.002; η2 = 0.50). Decreases in inter-repetition time and range of motion were more consistent for the extensors.

CONCLUSIONS

Distinct responses exist when comparing EQI contractions of the knee extensors and flexors, particularly their effect on peak torque angles. These findings suggest knee flexors may require lower relative intensities to align more closely with extensor EQI contractions.

Semitendinosus and biceps femoris long head activity during the single leg bridge test in healthy individuals

INTRODUCTION

The single leg bridge test (SLBT) has been suggested as a clinical test to examine function, screen injury risk, and monitor the effectiveness of rehabilitation programes targeting the hamstring. This study aimed to determine the inter-day reliability and repeatability of both SLBT performance, semitendinosus (ST), and biceps femoris long head (BFlh) surface electromyography (sEMG) responses and characterise the BFlh and ST electrical activity during the SLBT performed until exhaustion in healthy individuals.

METHODS

Twelve physically active young men without previous hamstring injury were tested for the number of repetitions attained, and sEMG signal median frequency and amplitude in both ST and BFlh of each lower limb, randomly in two sessions, with a seven-day interval between sessions.

RESULTS

High reliability [ICC = 0.85] was found for the number of SLBT repetitions attained. Reliability of sEMG outcomes showed better results for ST (ICC = 0.62-0.91) than for BFlh (ICC = 0.39-0.81), and a high to very-high repeatability was found for both ST (ICC = 0.91-0.84) and BFlh (ICC = 0.91-0.85). sEMG median frequency decreased and amplitude increased for both BFlh (p ≤ 0.001) and ST (p ≤ 0.039) at the end of SLBT, suggesting localised fatigue.

CONCLUSIONS

The SLBT performed by healthy individuals until exhaustion proved to be reliable and to induce fatigue in both BFlh and ST, where the sEMG median frequency and amplitude can be measured on different days with acceptable reliability and high repeatability, suggesting its potential future use in both practical and clinical settings.

Hamstrings on focus: Are 72 hours sufficient for recovery after a football (soccer) match? A multidisciplinary approach based on hamstring injury risk factors and histology

This study aimed to assess acute and residual changes in sprint-related hamstring injury (HSI) risk factors after a football (soccer) match, focusing on recovery within the commonly observed 72-h timeframe between elite football matches. We used a multifactorial approach within a football context, incorporating optical and ultrastructural microscopic analysis of BFlh (biceps femoris long head) muscle fibres, along with an examination of BFlh fibre composition. Changes in sprint performance-related factors and HSI modifiable risk factors were examined until 3 days after the match (MD +3) in 20 football players. BFlh biopsy specimens were obtained before and at MD +3 in 10 players. The findings indicated that at MD +3, sprint-related performance and HSI risk factors had not fully recovered, with notable increases in localized BFlh fibre disruptions. Interestingly, match load (both external and internal) did not correlate with changes in sprint performance or HSI risk factors nor with BFlh fibre disruption. Furthermore, our study revealed a balanced distribution of ATPase-based fibre types in BFlh, with type-II fibres associated with sprint performance. Overall, the results suggest that a 72-h recovery period may not be adequate for hamstring muscles in terms of both HSI risk factors and BFlh fibre structure following a football match.

The effects of a 6-month intervention aimed to reduce sedentary time on skeletal muscle insulin sensitivity: a randomized controlled trial

Sedentary behavior (SB) and physical inactivity associate with impaired insulin sensitivity. We investigated whether an intervention aimed at a 1-h reduction in daily SB during 6 mo would improve insulin sensitivity in the weight-bearing thigh muscles. Forty-four sedentary inactive adults [mean age 58 (SD 7) yr; 43% men] with metabolic syndrome were randomized into intervention and control groups. The individualized behavioral intervention was supported by an interactive accelerometer and a mobile application. SB, measured with hip-worn accelerometers in 6-s intervals throughout the 6-mo intervention, decreased by 51 (95% CI 22-80) min/day and physical activity (PA) increased by 37 (95% CI 18-55) min/day in the intervention group with nonsignificant changes in these outcomes in the control group. Insulin sensitivity in the whole body and in the quadriceps femoris and hamstring muscles, measured with hyperinsulinemic-euglycemic clamp combined with [18F]fluoro-deoxy-glucose PET, did not significantly change during the intervention in either group. However, the changes in hamstring and whole body insulin sensitivity correlated inversely with the change in SB and positively with the changes in moderate-to-vigorous PA and daily steps. In conclusion, these results suggest that the more the participants were able to reduce their SB, the more their individual insulin sensitivity increased in the whole body and in the hamstring muscles but not in quadriceps femoris. However, according to our primary randomized controlled trial results, this kind of behavioral interventions targeted to reduce sedentariness may not be effective in increasing skeletal muscle and whole body insulin sensitivity in people with metabolic syndrome at the population level.NEW & NOTEWORTHY Aiming to reduce daily SB by 1 h/day had no impact on skeletal muscle insulin sensitivity in the weight-bearing thigh muscles. However, successfully reducing SB may increase insulin sensitivity in the postural hamstring muscles. This emphasizes the importance of both reducing SB and increasing moderate-to-vigorous physical activity to improve insulin sensitivity in functionally different muscles of the body and thus induce a more comprehensive change in insulin sensitivity in the whole body.

Human skeletal muscle-specific atrophy with aging: a comprehensive review

Age-related skeletal muscle atrophy appears to be a muscle group-specific process, yet only a few specific muscles have been investigated and our understanding in this area is limited. This review provides a comprehensive summary of the available information on age-related skeletal muscle atrophy in a muscle-specific manner, nearly half of which comes from the quadriceps. Decline in muscle-specific size over ∼50 yr of aging was determined from 47 cross-sectional studies of 982 young (∼25 yr) and 1,003 old (∼75 yr) individuals and nine muscle groups: elbow extensors (-20%, -0.39%/yr), elbow flexors (-19%, -0.38%/yr), paraspinals (-24%, -0.47%/yr), psoas (-29%, -0.58%/yr), hip adductors (-13%, -0.27%/yr), hamstrings (-19%, -0.39%/yr), quadriceps (-27%, -0.53%/yr), dorsiflexors (-9%, -0.19%/yr), and triceps surae (-14%, -0.28%/yr). Muscle-specific atrophy rate was also determined for each of the subcomponent muscles in the hamstrings, quadriceps, and triceps surae. Of all the muscles included in this review, there was more than a fivefold difference between the least (-6%, -0.13%/yr, soleus) to the most (-33%, -0.66%/yr, rectus femoris) atrophying muscles. Muscle activity level, muscle fiber type, sex, and timeline of the aging process all appeared to have some influence on muscle-specific atrophy. Given the large range of muscle-specific atrophy and the large number of muscles that have not been investigated, more muscle-specific information could expand our understanding of functional deficits that develop with aging and help guide muscle-specific interventions to improve the quality of life of aging women and men.

Fatigue-induced changes in hamstrings' active muscle stiffness: effect of contraction type and implications for strain injuries

PURPOSE

Hamstring strain injuries may occur due to differential fatigue and compromised mechanical properties among the hamstring muscles. We examined (1) the effect of fatigue on hamstrings active muscle stiffness, and (2) whether contraction type affects active muscle stiffness changes during a submaximal fatiguing task.

METHODS

Nine healthy males completed 99 submaximal knee flexions in isometric (ISO), concentric (CON), and eccentric (ECC) conditions. We measured the knee flexor maximal voluntary torque (MVT) (pre/post), shear wave velocity (SWV) during contraction and transverse relaxation times (T2) (pre/post) in biceps femoris long head (BFlh), semitendinosus (ST), and semimembranosus (SM) muscles.

RESULTS

MVT decreased substantially after all conditions (- 18.4 to - 33.6%). The average relative torque sustained during the task was lower in CON than ISO and ECC, but absolute torque was similar. SWV interindividual responses were highly variable across muscles and contraction types. On average, BFlh SWV tended to increase in ISO (0.4 m/s, 4.5%, p = 0.064) but decreased in ECC condition (- 0.8 m/s, - 7.7%, p < 0.01). ST SWV decreased in CON (- 1.1 m/s, - 9.0%, p < 0.01), while it remained unchanged in ISO and ECC. SM SWV decreased in CON (- 0.8 m/s, - 8.1%, p < 0.01), but it was unaffected in ISO and variable in ECC.

CONCLUSION

Fatigue has a differential effect on the mechanical properties of the constituent hamstring muscles, as measured with shear wave elastography, depending upon contraction type. We found preliminary evidence that BFlh is more fatigued than ST or SM during eccentric contractions, which may explain its susceptibility to strain injuries.

Muscle group-specific skeletal muscle aging: a 5-yr longitudinal study in septuagenarians

There is some evidence that the age-associated change in skeletal muscle mass is muscle specific, yet the number of specific muscles that have been studied to form our understanding in this area is limited. In addition, few aging investigations have examined multiple muscles in the same individuals. This longitudinal investigation compared changes in skeletal muscle size via computed tomography of the quadriceps (rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius), hamstrings (biceps femoris short and long heads, semitendinosus, and semimembranosus), psoas, rectus abdominis, lateral abdominals (obliques and transversus abdominis), and paraspinal muscles (erector spinae and multifidi) of older individuals from the Health, Aging, and Body Composition (Health ABC) study at baseline and 5.0 ± 0.1 years later (n = 469, 73 ± 3 yr and 78 ± 3 yr, 49% women, 33% black). Skeletal muscle size decreased (P < 0.05) in quadriceps (-3.3%), hamstrings (-5.9%), psoas (-0.4%), and rectus abdominis (-7.0%). The hamstrings and rectus abdominis atrophied approximately twice as much as the quadriceps (P < 0.05), whereas the quadriceps atrophied substantially more than the psoas (P < 0.05). The lateral abdominals (+5.9%) and paraspinals (+4.3%) hypertrophied (P < 0.05) to a similar degree (P > 0.05) over the 5 years. These data suggest that older individuals experience skeletal muscle atrophy and hypertrophy in a muscle group-specific fashion in the eighth decade, a critical time period in the aging process. A broader understanding of muscle group-specific skeletal muscle aging is needed to better guide exercise programs and other interventions that mitigate decrements in physical function with aging.NEW & NOTEWORTHY These longitudinal analyses of six muscle groups in septuagenarians provide novel information on the muscle group-specific aging process. Although the quadriceps, hamstrings, psoas, and rectus abdominis atrophied with different magnitudes, the lateral abdominals and paraspinals hypertrophied over the 5 years. These findings contribute to a better understanding of the skeletal muscle aging process and highlight the need to complete studies in this area with a muscle-specific focus.

Comparison of tensiomyographic contractile properties of the knee muscles between endurance and power athletes

BACKGROUND: Postactivation potentiation (PAP) enhances contractility of skeletal muscle whereas fatigue deteriorates it. Available evidence suggests that the two phenomena may express differently in endurance and power athletes. OBJECTIVE: To compare the patterns of change in knee muscle contractility induced by PAP and fatigue between endurance and power athletes. METHODS: Eleven endurance and ten power athletes (age: 18–33 years) performed isokinetic fatigue and isometric PAP protocols with knee extensors and flexors on computerised dynamometer. Tensiomyography (TMG) of the vastus medialis and semitendinosus muscle medialis was performed before the protocols and during a 10-min recovery. RESULTS: The changes in TMG profile were most pronounced in the vastus medialis of power athletes following the PAP protocol and least pronounced in the semitendinosus of the endurance athletes following the fatigue protocol. The differences between athlete types were most significant for the time-domain TMG parameters of vastus medialis. A significant correlation (r= 0.51–0.73) between the fatigue indices and changes in TMG parameters was observed for the vastus medialis muscle only. CONCLUSIONS: The results show that the TMG patterns of PAP and fatigue in the vastus muscle differ between endurance and power athletes. In this muscle, the changes in TMG parameters are also strongly associated with the degree of fatigue.

Exercise-Induced Fatigue in Hamstring versus Quadriceps Muscles and Consequences on the Torque-Duration Relationship in Men

PURPOSE

The present study investigated the mechanisms of neuromuscular fatigue in quadriceps and hamstring muscles and its consequences on the torque-duration relationship.

METHODS

Twelve healthy men performed a 5-min all-out exercise (3-s contraction, 2-s relaxation) with either quadriceps or hamstring muscles on separate days. Central fatigue and peripheral fatigue were quantified via changes in pre- to postexercise voluntary activation (VA) and potentiated twitch (P Tw ) torque evoked by supramaximal electrical stimulation, respectively. Critical torque was determined as the mean torque of the last six contractions, whereas W ' was calculated as the torque impulse done above critical torque.

RESULTS

After exercise, maximal voluntary contraction (MVC) decreased to a greater magnitude ( P < 0.001) in quadriceps (-67% ± 9%) compared with hamstring (-51% ± 10%). ∆P Tw was also greater in quadriceps compared with hamstring (-69% ± 15% vs 55% ± 10%, P < 0.01), whereas central fatigue only developed in quadriceps (∆VA, -25% ± 28%). Hamstring demonstrated reduced critical torque compared with quadriceps (60 ± 12 vs 97 ± 26 N·m, P < 0.001) as well as drastically lower W ' (1001 ± 696 vs 8111 ± 2073 N·m·s, P < 0.001). No correlation was found between quadriceps and hamstring for any index of neuromuscular fatigue (∆MVC, ∆P Tw , or ∆VA).

CONCLUSIONS

These findings revealed that hamstring presented different etiology and magnitude of neuromuscular fatigue compared with quadriceps. The absence of correlation observed between quadriceps and hamstring fatigue parameters (∆MVC, ∆P Tw , or ∆VA) suggests no interrelation in fatigue etiology between these two muscle groups within individuals and, therefore, highlights the need to investigate specifically hamstring muscle fatigue.

Effects of Neuromuscular Electrical Stimulation and Blood Flow Restriction in Rehabilitation after Anterior Cruciate Ligament Reconstruction

The present study aimed to examine and compare the effects of a rehabilitation exercise (RE) using neuromuscular electrical stimulation (NMES) and blood flow restriction (BFR) on muscle function and knee functional abilities in patients who underwent anterior cruciate ligament reconstruction (ACLR). A total of 45 patients who underwent ACLR (28.76 ± 0.8 years; 34 males and 11 females) were retrospectively divided into three groups: control (CON, n = 15), NMES (n = 15), and BFR (n = 15). All participants carried out the RE program for 60 min, thrice a week for 12 weeks. The Lysholm score, International Knee Documentation Committee (IKDC) subjective score, thigh circumference at 5 cm from the knee joint, Y-balance posterior medial, and lateral significantly increased in all groups via intervention (p < 0.05). However, NMES showed a higher thigh circumference at 15 cm from the knee joint than CON via intervention (p < 0.05), and the strength and endurance of quadriceps femoris and hamstrings and Y-balance anterior showed a significant increase via intervention in NMES and BFR compared with CON (p < 0.05). In conclusion, we confirmed that RE using NMES and BFR effectively enhances muscle function and balance in ACLR patients.

Factors of Muscle Quality and Determinants of Muscle Strength: A Systematic Literature Review

Muscle quality defined as the ratio of muscle strength to muscle mass disregards underlying factors which influence muscle strength. The aim of this review was to investigate the relationship of phase angle (PhA), echo intensity (EI), muscular adipose tissue (MAT), muscle fiber type, fascicle pennation angle (θf), fascicle length (lf), muscle oxidative capacity, insulin sensitivity (IS), neuromuscular activation, and motor unit to muscle strength. PubMed search was performed in 2021. The inclusion criteria were: (i) original research, (ii) human participants, (iii) adults (≥18 years). Exclusion criteria were: (i) no full-text, (ii) non-English or -German language, (iii) pathologies. Forty-one studies were identified. Nine studies found a weak−moderate negative (range r: [−0.26]−[−0.656], p < 0.05) correlation between muscle strength and EI. Four studies found a weak−moderate positive correlation (range r: 0.177−0.696, p < 0.05) between muscle strength and PhA. Two studies found a moderate-strong negative correlation (range r: [−0.446]−[−0.87], p < 0.05) between muscle strength and MAT. Two studies found a weak-strong positive correlation (range r: 0.28−0.907, p < 0.05) between θf and muscle strength. Muscle oxidative capacity was found to be a predictor of muscle strength. This review highlights that the current definition of muscle quality should be expanded upon as to encompass all possible factors of muscle quality.

Isotonic and Isometric Exercise Interventions Improve the Hamstring Muscles’ Strength and Flexibility: A Narrative Review

Background: Hamstring weakness has been associated with an increased risk of hamstring strain, a common sports injury that occurs when athletes perform actions such as quick sprints. The hamstring complex comprises three distinct muscles: the long and short heads of the bicep femoris, the semimembranosus, and the semitendinosus. Methods: The researchers collected the data from different electronic databases: PubMed, Google Scholar, and the Web of Science. Results: Many studies have been conducted on the numerous benefits of hamstring strength, in terms of athletic performance and injury prevention. Isotonic and isometric exercises are commonly used to improve hamstring strength, with each exercise type having a unique effect on the hamstring muscles. Isotonic exercise improves the muscles’ strength, increasing their ability to resist any force, while isometric training increases strength and the muscles’ ability to produce power by changing the muscle length. Conclusions: These exercises, when performed at low intensity, but with high repetition, can be used by the healthy general population to prepare for training and daily exercise. This can improve hamstring muscle strength and flexibility, leading to enhanced performance and reduced injury risk.

Hamstring Strain Injury in Athletes

Hamstring strain injury (HSI) may result in considerable impairment, activity limitation, and participation restriction, including time lost from competitive sports. This CPG includes sports-related overloading and overstretching injuries to myofascial or musculotendinous structures in any combination of the 3 hamstring muscles (the semitendinosus, semimembranosus, and biceps femoris). J Orthop Sports Phys Ther 2022;52(3):CPG1-CPG44. doi:10.2519/jospt.2022.0301.

Muscle Adaptations to Heavy-Load and Blood Flow Restriction Resistance Training Methods

Resistance-based blood flow restriction training (BFRT) improves skeletal muscle strength and size. Unlike heavy-load resistance training (HLRT), there is debate as to whether strength adaptations following BFRT interventions can be primarily attributed to concurrent muscle hypertrophy, as the magnitude of hypertrophy is often minor. The present study aimed to investigate the effect of 7 weeks of BFRT and HLRT on muscle strength and hypertrophy. The expression of protein growth markers from muscle biopsy samples was also measured. Male participants were allocated to moderately heavy-load training (HL; n = 9), low-load BFRT (LL + BFR; n = 8), or a control (CON; n = 9) group to control for the effect of time. HL and LL + BFR completed 21 training sessions (3 d.week⁻¹) comprising bilateral knee extension and knee flexion exercises (HL = 70% one-repetition maximum (1-RM), LL + BFR = 20% 1-RM + blood flow restriction). Bilateral knee extension and flexion 1-RM strength were assessed, and leg muscle CSA was measured via peripheral quantitative computed tomography. Protein growth markers were measured in vastus lateralis biopsy samples taken pre- and post the first and last training sessions. Biopsy samples were also taken from CON at the same time intervals as HL and LL + BFR. Knee extension 1-RM strength increased in HL (19%) and LL + BFR (19%) but not CON (2%; p < 0.05). Knee flexion 1-RM strength increased similarly between all groups, as did muscle CSA (50% femur length; HL = 2.2%, LL + BFR = 3.0%, CON = 2.1%; TIME main effects). 4E-BP1 (Thr37/46) phosphorylation was lower in HL and LL + BFR immediately post-exercise compared with CON in both sessions (p < 0.05). Expression of other growth markers was similar between groups (p > 0.05). Overall, BFRT and HLRT improved muscle strength and size similarly, with comparable changes in intramuscular protein growth marker expression, both acutely and chronically, suggesting the activation of similar anabolic pathways. However, the low magnitude of muscle hypertrophy was not significantly different to the non-training control suggesting that strength adaptation following 7 weeks of BFRT is not driven by hypertrophy, but rather neurological adaptation.

Sex differences in semitendinosus muscle fiber-type composition

Sex differences in muscle fiber-type composition have been documented in several muscle groups while the hamstring muscle fiber-type composition has been poorly characterized. This study aimed to compare the semitendinosus muscle composition between men and women. Biopsy samples were obtained from the semitendinosus muscle of twelve men and twelve women during an anterior cruciate ligament reconstruction. SDH and ATPase activities as well as the size and the proportion of muscle fibers expressing myosin heavy chain (MyHC) isoforms were used to compare muscle composition between men and women. The proportion of SDH-positive muscle fibers was significantly lower (37.4 ± 11.2% vs. 49.3 ± 10.6%, p < 0.05), and the percentage of fast muscle fibers (i.e., based on ATPase activity) was significantly higher (65.8 ± 10.1% vs. 54.8 ± 8.3%, p < 0.05) in men versus women. Likewise, men muscles exhibited a lower percentage of the area that was occupied by MyHC-I labeling (35.6 ± 10.1% vs. 48.7 ± 8.9%; p < 0.05) and a higher percentage of the area that was occupied by MyHC-IIA (38.3 ± 6.7% vs. 32.5 ± 6.5%; p < 0.05) and MyHC-IIX labeling (26.1 ± 9.6% vs. 18.8 ± 8.5%; p = 0.06) as compared with women muscles. The cross-sectional area of MyHC-I, MyHC-IIA, and MyHC-IIX muscle fibers was 31%, 43%, and 50% larger in men as compared with women, respectively. We identified sex differences in semitendinosus muscle composition as illustrated by a faster phenotype and larger muscle size in men as compared with women. This sexual dimorphism might have functional consequences.

Effect of long-term maximum strength training on explosive strength, neural, and contractile properties

The purpose of this cross‐sectional study was to compare explosive strength and underpinning contractile, hypertrophic, and neuromuscular activation characteristics of long‐term maximum strength‐trained (LT‐MST; ie, ≥3 years of consistent, regular knee extensor training) and untrained individuals. Sixty‐three healthy young men (untrained [UNT] n = 49, and LT‐MST n = 14) performed isometric maximum and explosive voluntary, as well as evoked octet knee extension contractions. Torque, quadriceps, and hamstring surface EMG were recorded during all tasks. Quadriceps anatomical cross‐sectional area (QACSA_MAX; via MRI) was also assessed. Maximum voluntary torque (MVT; +66%) and QACSA_MAX (+54%) were greater for LT‐MST than UNT ([both] p < 0.001). Absolute explosive voluntary torque (25–150 ms after torque onset; +41 to +64%; [all] p < 0.001; 1.15≤ effect size [ES]≤2.36) and absolute evoked octet torque (50 ms after torque onset; +43, p < 0.001; ES = 3.07) were greater for LT‐MST than UNT. However, relative (to MVT) explosive voluntary torque was lower for LT‐MST than UNT from 100 to 150 ms after contraction onset (−11% to −16%; 0.001 ≤ p ≤ 0.002; 0.98 ≤ ES ≤ 1.11). Relative evoked octet torque 50 ms after onset was lower (−10%; p < 0.001; ES = 1.14) and octet time to peak torque longer (+8%; p = 0.001; ES = 1.18) for LT‐MST than UNT indicating slower contractile properties, independent from any differences in torque amplitude. The greater absolute explosive strength of the LT‐MST group was attributable to higher evoked explosive strength, that in turn appeared to be due to larger quadriceps muscle size, rather than any differences in neuromuscular activation. In contrast, the inferior relative explosive strength of LT‐MST appeared to be underpinned by slower intrinsic/evoked contractile properties.

Muscle- and Region-Specific Associations Between Muscle Size and Muscular Strength During Hip Extension and Knee Flexion in the Hamstrings

CONTEXT

Strength deficits of the hamstrings following sports injuries decrease athletic performance and increase the risk of injury recurrence. Previous studies have shown a high correlation between the muscular strength during hip-extension and knee-flexion and total muscle size of the hamstrings. However, it remains unclear which region of the individual hamstring muscles is closely associated with muscular strength.

OBJECTIVE

To investigate the relationship between the size of each region of the individual hamstring muscles and muscular strength during hip extension and knee flexion.

DESIGN

Within-subject repeated measures.

SETTING

University laboratory.

PARTICIPANTS

Twenty healthy young male volunteers who regularly engaged in sports activities.

OUTCOME MEASURES

Anatomical cross-sectional areas were acquired from the proximal, middle, and distal regions of the biceps femoris long head, biceps femoris short head, semitendinosus, and semimembranosus. Hip-extension and knee-flexion strength were measured during maximal voluntary isometric and concentric contractions (angular velocities of 60°/s and 180°/s).

RESULTS

The anatomical cross-sectional area of the distal regions in biceps femoris long head (r = .525-.642) and semitendinosus (r = .567) were significantly correlated with hip-extension strength under all conditions and only at an angular velocity of 180°/s, respectively. Meanwhile, anatomical cross-sectional areas of the distal regions in biceps femoris short head (r = .587-.684) and semimembranosus (r = .569-.576) were closely associated with knee-flexion strength under all conditions.

CONCLUSION

These results suggest that muscle size in the distal regions of biceps femoris long head and semitendinosus greatly contributes to the production of hip-extension strength, whereas that of biceps femoris short head and semimembranosus significantly contributes to the generation of knee-flexion strength. These findings could be useful for designing training and rehabilitation programs to efficiently improve strength deficits following sports injuries such as strain injury and anterior cruciate ligament tears.

Influence of submaximal isometric contractions of the hamstrings on electromyography activity and force while functioning as hip extensors

BACKGROUND: Although the medial and lateral hamstrings are clearly distinct anatomically and have different functions in the transverse plane, they are often considered as one muscle during rehabilitation. OBJECTIVE: The purpose of the study was to compare the electromyographic (EMG) activity between the prone position and the supine position during maximal isometric contraction and to additionally confirm the effect of submaximal isometric contractions on EMG activity of medial and lateral hamstrings, and force. METHODS: In the prone position, EMG activities of the long head of biceps femoris (BFLH) and semitendinosus (ST) were measured during the maximal isometric contraction. In the supine position, hip extension force with EMG activity were measured during the maximal and the submaximal isometric contractions. RESULTS: EMG activity in the prone position was significantly decreased in the supine position. In the supine position, there was a significant difference between the BFLH and ST during the maximal isometric contraction, but not during the submaximal isometric contractions. CONCLUSIONS: The dependence on the hamstrings could be relatively lower during hip extensions. When the medial and lateral hamstrings are considered separately, the lateral hamstrings may show a more active response, with increased muscle length, in clinical practice.

The Hamstrings: Anatomic and Physiologic Variations and Their Potential Relationships With Injury Risk

The incidence and recurrence of hamstrings injuries are very high in sports, posing elevated performance and financial-related costs. Attempts to identify the risk factors involved in predicting vulnerability to hamstrings injury is important for designing exercise-based programs that aim to mitigate the rate and severity of hamstrings injuries and improve rehabilitation strategies. However, research has shown that non-modifiable risk factors may play a greater role than modifiable risk factors. Recognizing non-modifiable risk factors and understanding their implications will afford the prescription of better suited exercise programs, i.e., that are more respectful of the individual characteristics. In a nutshell, non-modifiable risk factors can still be acted upon, even if indirectly. In this context, an underexplored topic is how intra and inter- individual anatomic and physiologic variations in hamstrings (e.g., muscle bellies, fiber types, tendon length, aponeurosis width, attachment sites, sex- and age-related differences) concur to alter hamstrings injuries risk. Some anatomic and physiologic variations may be modifiable through exercise interventions (e.g., cross-sectional area), while others may not (e.g., supernumerary muscle bellies). This apparent dichotomy may hide a greater complexity, i.e., there may be risk factors that are partially modifiable. Therefore, we explored the available information on the anatomic variations of the hamstrings, providing a deeper insight into the individual risk factors for hamstrings injuries and contributing with better knowledge and potential applications toward a more individualized exercise prescription.

Bilateral NIRS measurements of muscle mitochondrial capacity: Feasibility and repeatability

Background

Non‐invasive determination of mitochondrial capacity via near infrared spectroscopy (NIRS) typically involves voluntary exercise of a single muscle group followed by as many as 26 brief ischemic cuff occlusions to determine a single recovery rate constant (k).

Purpose

To determine the within‐ and between‐visit repeatability of a shortened bilateral NIRS protocol, and to establish the feasibility of hamstring k measurements.

Methods

Sixteen young (eight women, eight men; 22 ± 3 years) active adults underwent a bilateral electrical stimulation protocol in which multiple (n = 4) measurements of k for the vastus lateralis (VL) and medial hamstring (MH) muscles were determined on two visits. Repeatability (CV% and intraclass correlations, ICC) and equivalency across visits were assessed for both muscles.

Results

Mean k values in the VL were consistent with published values and within‐visit ICCs were moderately high for both muscles in both sexes. In men, average k values on visit 2 were within 1% (VL muscle) and 5% (MH muscle) of the values on visit 1 (all p > 0.78). In women, average k values were 10%–15% lower on visit 2 (p = 0.01 and p = 0.15 for MH and VL) with the largest between‐visit differences in a subset of participants with the most days between visits.

Conclusions

This bilateral NIRS protocol is time efficient and provides valid estimates of k in both sexes and muscle groups with acceptable within‐visit repeatability. Lower than expected between‐visit repeatability in some participants reinforces the need for further investigation of this newly developed protocol to identify and control for experimental and behavioral sources of variation.

Functional and molecular adaptations of quadriceps and hamstring muscles to blood flow restricted training in patients with ACL rupture

Effects of low-load blood flow restricted (LL-BFR) training remain unexplored in patients with ACL rupture. Our hypothesis was that LL-BFR training triggers augmented gains in knee muscle strength and size, which are paralleled with transcriptional responses of hypoxia-regulated genes and myokines. Eighteen volunteers (age 37.5 ± 9 years) planned for ACL reconstruction, participated in the study. Twelve were divided between BFR group, performing 9 sessions of LL-BFR exercise, and SHAM-BFR group performing equal training with sham vascular occlusion. Six subjects served as a control for muscle biopsy analysis. Cross-sectional area (CSA) and isokinetic strength of knee muscles were assessed before and after the training. Change in CSA_quad was significantly (p < 0.01) larger in BFR (4.9%) compared with SHAM-BFR (1.3%). Similarly, change in peak torque of knee extensors was significantly (p < 0.05) larger in BFR (14%) compared with SHAM-BFR (-1%). The decrease in fatigue index of knee extensors (6%) was larger (p < 0.01) in BFR than in SHAM-BFR (2%). mRNA expression of HIF-1α in the vastus lateralis was reduced (p < 0.05) in SHAM-BFR, while VEGF-A mRNA tended to be higher in BFR. The mRNA expression of myostatin and its receptor were reduced (p < 0.05) in the semitendinosus after both types of training. Expression of IL-6, its receptors IL-6Rα and gp130, as well as musclin were similar in control and training groups. In conclusion, our results show augmented strength and endurance of knee extensors but less of the flexors. LL-BFR training is especially effective for conditioning of knee extensors in this population.

Neuromuscular fatigue and recovery after strenuous exercise depends on skeletal muscle size and stem cell characteristics

Hamstring muscle injury is highly prevalent in sports involving repeated maximal sprinting. Although neuromuscular fatigue is thought to be a risk factor, the mechanisms underlying the fatigue response to repeated maximal sprints are unclear. Here, we show that repeated maximal sprints induce neuromuscular fatigue accompanied with a prolonged strength loss in hamstring muscles. The immediate hamstring strength loss was linked to both central and peripheral fatigue, while prolonged strength loss was associated with indicators of muscle damage. The kinematic changes immediately after sprinting likely protected fatigued hamstrings from excess elongation stress, while larger hamstring muscle physiological cross-sectional area and lower myoblast:fibroblast ratio appeared to protect against fatigue/damage and improve muscle recovery within the first 48 h after sprinting. We have therefore identified novel mechanisms that likely regulate the fatigue/damage response and initial recovery following repeated maximal sprinting in humans.

The Muscle Morphology of Elite Sprint Running

PURPOSE

This study aimed to investigate the differences in muscle volumes and strength between male elite sprinters, sub-elite sprinters, and untrained controls and to assess the relationships of muscle volumes and strength with sprint performance.

METHODS

Five elite sprinters (100-m season's best equivalent [SBE100], 10.10 ± 0.07 s), 26 sub-elite sprinters (SBE100, 10.80 ± 0.30 s), and 11 untrained control participants underwent 1) 3-T magnetic resonance imaging scans to determine the volume of 23 individual lower limb muscles/compartments and 5 functional muscle groups and 2) isometric strength assessment of lower body muscle groups.

RESULTS

Total lower body muscularity was distinct between the groups (controls < sub-elite +20% < elite +48%). The hip extensors exhibited the largest muscle group differences/relationships (elite, +32% absolute and +15% relative [per kg] volume, vs sub-elite explaining 31%-48% of the variability in SBE100), whereas the plantarflexors showed no differences between sprint groups. Individual muscle differences showed pronounced anatomical specificity (elite vs sub-elite absolute volume range, +57% to -9%). Three hip muscles were consistently larger in elite vs sub-elite (tensor fasciae latae, sartorius, and gluteus maximus; absolute, +45%-57%; relative volume, +25%-37%), and gluteus maximus volume alone explained 34%-44% of the variance in SBE100. The isometric strength of several muscle groups was greater in both sprint groups than controls but similar for the sprint groups and not related to SBE100.

CONCLUSIONS

These findings demonstrate the pronounced inhomogeneity and anatomically specific muscularity required for fast sprinting and provides novel, robust evidence that greater hip extensor and gluteus maximus volumes discriminate between elite and sub-elite sprinters and are strongly associated with sprinting performance.

Associations Between Measures of Physical Activity and Muscle Size and Strength: A Systematic Review

Objective

To determine whether physical activity is associated with lower limb muscle size and strength within the general population.

Data Sources

Six databases were systematically searched from inception using 3 main constructs: lower extremity, muscle volume, and muscle strength.

Study Selection

Studies that measured physical activity (using either objective or subjective measurements), lower limb muscle size, and strength were included. Available discrete group data were standardized using previously published age- and sex-specific normative values prior to analysis.

Data Extraction

The final analysis included 47 studies from an initial yield of 5402 studies. Standardized scores for outcome measures were calculated for 97 discrete groups.

Data Synthesis

As anticipated, lower limb muscle size was positively correlated with lower limb muscle strength (r=0.26, P<.01; n=4812). Objectively measured physical activity (ie, accelerometry, pedometry) (n=1944) was positively correlated with both lower limb muscle size (r=0.30, P<.01; n=1626) and lower limb strength (r=0.24, P<.01; n=1869). However, subjectively measured physical activity (ie, questionnaires) (n=3949) was negatively associated with lower limb muscle size (r=–0.59, P<.01; n=3243) and lower limb muscle strength (r=–0.48, P<. 01; n=3882).

Conclusions

This review identified that objective measures of physical activity are moderately associated with lower limb muscle size and muscle strength and can, therefore, be used to predict muscle changes within the lower limbs associated with exercise-based rehabilitation programs.

Influence of Fatigue on the Rapid Hamstring/Quadriceps Force Capacity in Soccer Players

The objective of this study was to examine the effect of fatigue on maximal and rapid force capacities and muscular activation of the knee extensors and flexors. Seventeen professional soccer players volunteered to participate in this study. Peak torque (T_peak) and rate of torque development (RTD) of knee flexor (90°. s^-1, -30°. s^-1) and extensor (90°. s^-1) muscles were measured before and after fatigue (i.e., 30 maximal knee extension and flexion repetitions at 180°s^-1) performed on an isokinetic dynamometer. Hamstring to quadriceps peak strength and RTD ratios were calculated. Besides, using surface EMG, the mean level of activation (RMS_mean), Rate of EMG Rise (RER), and EMG Frequency-Time maps were measured on quadriceps and hamstring muscles. Following fatigue, T_peak, RTD, RER declined significantly in the two muscle groups (all p < 0.05) without modification of RMS_mean. No decrease in conventional and functional H/Q ratios was observed after fatigue except for a significant increase in the H _ecc30/Q _con180 ratios (1.03 ± 0.19 vs. 1.36 ± 0.33, p < 0.001). Besides, the RTD H/Q ratios decreased significantly after fatigue, and the statistical parametric mapping analysis (SPM) performed on the EMG/angle curves, and EMG Frequency-Time maps showed that fatigue strongly influenced the muscle activation during the first 100 ms of the movement, following the higher EMG frequency component shift toward the lower frequency component. Our results show that the reduction of RTD and RER during the first 100 ms of the contraction after fatigue exercise makes more sense than any H/Q ratio modification in understanding injury risk in soccer players.

Muscle architecture and morphology as determinants of explosive strength

PURPOSE

Neural drive and contractile properties are well-defined physiological determinants of explosive strength, the influence of muscle architecture and related morphology on explosive strength is poorly understood. The aim of this study was to examine the relationships between Quadriceps muscle architecture (pennation angle [Θ_P] and fascicle length [F_L]) and size (e.g., volume; Q_VOL), as well as patellar tendon moment arm (PT_MA) with voluntary and evoked explosive knee extension torque in 53 recreationally active young men.

METHOD

Following familiarisation, explosive voluntary torque at 50 ms intervals from torque onset (T₅₀, T₁₀₀, T₁₅₀), evoked octet at 50 ms (8 pulses at 300-Hz; evoked T₅₀), as well as maximum voluntary torque, were assessed on two occasions with isometric dynamometry. B-mode ultrasound was used to assess Θ_P and F_L at ten sites throughout the quadriceps (2-3 sites) per constituent muscle. Muscle size (Q_VOL) and PT_MA were quantified using 1.5 T MRI.

RESULT

There were no relationships with absolute early phase explosive voluntary torque (≤ 50 ms), but θ_P (weak), Q_VOL (moderate to strong) and PT_MA (weak) were related to late phase explosive voluntary torque (≥ 100 ms). Regression analysis revealed only Q_VOL was an independent variable contributing to the variance in T₁₀₀ (34%) and T₁₅₀ (54%). Evoked T₅₀ was also related to Q_VOL and θ_P. When explosive strength was expressed relative to MVT there were no relationships observed.

CONCLUSION

It is likely that the weak associations of θ_P and PT_MA with late phase explosive voluntary torque was via their association with MVT/Q_VOL rather than as a direct determinant.

Does Muscle-Tendon Unit Structure Predispose to Hamstring Strain Injury During Running? A Critical Review

Hamstring strain injury (HSI) remains the most common muscle injury in high-intensity running in humans. The majority of acute HSI occur specifically within the proximal region of the long head of biceps femoris and there is a sustained interest among researchers in understanding the factors that predispose to HSI. The present critical review describes the current understanding of biceps femoris long head (BFlh) structural features that might influence strain injury risk. Inter-individual differences in muscle-tendon architecture and interactions, muscle fiber type and region-specific innervation are likely to influence biceps femoris long head injury risk and might inform why some individuals are at an increased risk of sustaining a HSI during running. However, more research is needed, with future studies focusing on prospective data acquisition, improved computer simulations and direct imaging techniques to better understand the relationship between structural features, hamstring muscle function, and injury risk.

Effects of knee flexor submaximal isometric contraction until exhaustion on semitendinosus and biceps femoris long head shear modulus in healthy individuals

This study examined whether a knee flexor isometric contraction at 20% of maximal voluntary isometric contraction until exhaustion would alter the biceps femoris long head (BFlh) and semitendinosus (ST) active stiffness, assessed using ultrasound-based shear wave elastography. Twelve healthy individuals participated in 2 sessions separated by 7 days. Time to exhaustion was similar in both sessions (day 1: 443.8 ± 192.5 s; day 2: 474.6 ± 131.7 s; p = 0.323). At the start of the fatigue task, the ST showed greater active stiffness than the BFlh (p < 0.001), with no differences between days (p = 0.08). The ST active stiffness then decreased from 40% of the task time to exhaustion (− 2.2 to − 13.3%, p = 0.027) until the end of the task (− 16.1 to − 22.9%, p = 0.012), while no significant changes were noted in the BFlh (p = 0.771). Immediately after the fatigue task, a decrease in active stiffness was observed in the ST (− 11.8 to − 17.8%, p < 0.001), but not in the BFlh (p = 0.551). Results were consistent between the 2 testing sessions (p = 0.07–0.959). The present results indicate that fatigue alters the hamstring active stiffness pattern.

Assessment of muscle fiber adaptation in footballers using a new ELISA assay of myosin isoforms

BACKGROUND

To measure the impact of training models on injury incidence, data of health and performance were integrated to study fiber adaptation during a competitive season. We studied football players over a season, analyzing hours of exposure to sport by serum changes in fast and slow myosin, creatine kinase and lactate dehydrogenase.

METHODS

A new assay was developed to measure the myosin isoforms in 49 non-sporting volunteers and in 27 professional football players.

RESULTS

Myosin isoforms in volunteers with mean ages of 30±8 were 1553 µg/L fast and 1284 µg/L slow; in the group with of 56±7 were 1426 µg/L fast and 1046 µg/L slow. Slow myosin was significantly lower in older subjects (-18%). Samples from the players in preseason had lower mean scores for fast myosin (1123 µg/L) and higher for slow myosin (2072 µg/L) than reference volunteers. During the season, myosins reached the maximum with the maximum load (1537 µg/L fast, 2195 µg/L slow but decreased and adapted to the high level of demand (425 µg/L fast, 1342 µg/L slow). CK and LDH were maximal at the pre-season (227 U/L, 333 U/L) while myosin levels were maximal at the beginning of season (1537 µg/L, 2195 µg/L).

CONCLUSIONS

Measuring serum myosin isoforms we identify the type and amount of damage caused by training and matches, making it a new control tool capable of advising training towards a minimum of blood slow myosin but controlling the fast fiber participating and be able to improve the performance of the players.

Biceps Femoris Long-Head Architecture Assessed Using Different Sonographic Techniques

PURPOSE

To assess the repeatability of, and measurement agreement between, four sonographic techniques used to quantify biceps femoris long head (BFlh) architecture: (i) static-image with linear extrapolation; extended field-of-view (EFOV) with linear ultrasound probe path (linear-EFOV), using either (ii) straight or (iii) segmented analyses; and (iv) EFOV with nonlinear probe path and segmented analysis (nonlinear-EFOV) to follow the complex fascicle trajectories.

METHODS

Twenty individuals (24.4 ± 5.7 yr; 175 ± 0.8 cm; 73 ± 9.0 kg) without history of hamstrings strain injury were tested in two sessions separated by 1 h. An ultrasound scanner coupled with 6-cm linear probe was used to assess BFlh architecture in B-mode.

RESULTS

The ultrasound probe was positioned at 52.0% ± 5.0% of femur length and 57.0% ± 6.0% of BFlh length. We found an acceptable repeatability when assessing BFlh fascicle length (ICC3,k = 0.86-0.95; SEM = 1.9-3.2 mm) and angle (ICC3,k = 0.86-0.97; SEM = 0.8°-1.1) using all sonographic techniques. However, the nonlinear-EFOV technique showed the highest repeatability (fascicle length ICC3,k = 0.95; fascicle angle, ICC3,k = 0.97). The static-image technique, which estimated 35.4% ± 7.0% of the fascicle length, overestimated fascicle length (8%-11%) and underestimated fascicle angle (8%-9%) compared with EFOV techniques. Also, the rank order of individuals varied by approximately 15% between static-image and nonlinear-EFOV (segmented) when assessing the fascicle length.

CONCLUSIONS

Although all techniques showed good repeatability, absolute errors were observed using static-image (7.9 ± 6.1 mm for fascicle length) and linear-EFOV (between 3.7 ± 3.0 and 4.2 ± 3.7 mm), probably because the complex fascicle trajectories were not followed. The rank order of individuals for fascicle length and angle were also different between static-image and nonlinear-EFOV, so different muscle function and injury risk estimates could likely be made when using this technique.

Intra- and Inter-Muscular Variations in Hamstring Architecture and Mechanics and Their Implications for Injury: A Narrative Review

Understanding the architecture, anatomy, and biomechanics of the hamstrings may assist in explaining the mechanisms that affect and improve their function. The aim of this review is to specifically examine intra- and inter-muscular variations in architecture and mechanical properties of the hamstrings. Of the hamstrings, the long head of the biceps femoris shows the shortest and more pennated fibers. The semimembranosus has a similar muscle architecture with a long head of the biceps femoris but it has a different proximal attachment as well as a different moment arm compared with the long head of the biceps femoris. For the same joint motion, the semitendinosus displays less relative strain than the other hamstrings probably owing to a greater length, longer fascicles and, possibly, a longer tendon. Intra-muscular variations in architecture are documented but their implications are currently unclear. Proximally, the long head of the biceps femoris has shorter and more pennated fibers coupled with a narrower aponeurosis than distally, while the semitendinosus is the only muscle that entails a tendinous inscription. In conclusion, some of the identified intra- and inter-muscular variations in architecture may help explain why some muscles sustain injuries more than others. In the same line, exercises designed for the hamstrings may not provide the same stimulus for all components of this muscle group. Future research could examine whether intervention strategies that target specific muscles or specific areas of the hamstrings may offer additional benefits for injury prevention or rehabilitation of their function.

Analysis of the Relationship between Elite Wrestlers' Leg Strength and Balance Performance, and Injury History

The purpose of this study is to examine the correlation between leg power and balance performance in elite wrestlers and injury history. In the research group, there are 18 elite freestyle male wrestlers at the ages of 24.27 ± 3.18 years, with a height of 171.86 ± 5.44 cm and a body weight of 79.27 ± 11.16 kg. Information on the injury history of the athletes’ upper legs for the past year was collected via interviews with the club’s physiotherapist. Laboratory tests to measure performance assessed height, body weight, Y balance and isokinetic leg strength. Data obtained from the study are presented as mean and standard deviation. The test of normality was carried out by the Shapiro-Wilk test. The Pearson Correlation Test was performed for all parameters with normal distribution, and significance level was accepted as p < 0.05. It was found that there is a relationship between the wrestlers’ right leg ratio and hamstring strength and injury history. However, there is no statistically significant relationship between left leg hamstring, quadriceps, ratio, right leg quadriceps, or right and left leg balance performance, and injury history. The resulting data shows that the proportioning between hamstring and quadriceps muscles in freestyle wrestlers’ upper leg strength values is not ideal. This finding provides evidence that injury risk increases with the additional impact of loss of strength.

The effect of long-term confinement and the efficacy of exercise countermeasures on muscle strength during a simulated mission to Mars: data from the Mars500 study

Background

Isolation and long duration spaceflight are associated with musculoskeletal deconditioning. Mars500 was a unique, high-fidelity analogue of the psychological challenges of a 520-day manned mission to Mars. We aimed to explore the effect of musculoskeletal deconditioning on three outcome measures: (1) if lower limb muscle strength was reduced during the 520-day isolation; (2) if type I or II muscle fibres were differentially affected; and (3) whether any 70-day exercise interventions prevented any isolation-induced loss of strength.

Methods

Six healthy male subjects (mean ± SEM) (34 ± 3 years; 1.76 ± 0.02 metres; 83.7 ± 4.8 kg) provided written, informed consent to participate. The subjects’ maximal voluntary contraction (MVC) was assessed isometrically in the calf (predominantly type I fibres), and maximal voluntary isokinetic force (MVIF) was assessed in the quadriceps/hamstrings (predominantly type II fibres) at 0.2 and 0.4 ms⁻¹ using the Multifunctional Dynamometer for Space (MDS) at 35-day intervals throughout Mars500. Exercise interventions were completed 3–7 days/week throughout the 520-day isolation in a counterbalanced design excluding 142–177 days (rest period) and 251–284 days (simulated Mars landing). Exercise interventions included motorized treadmill running, non-motorized treadmill running, cycle ergometry, elastomer-based resistance exercise, whole-body vibration (WBV), and resistance exercise using MDS.

Results

Calf MVC did not reduce across the 520-day isolation and MDS increased strength by 18% compared to before that of 70-day exercise intervention. In contrast, there was a significant bilateral loss of MVIF across the 520 days at both 0.2 ms⁻¹ (R² = 0.53; P = 0.001) and 0.4 ms⁻¹ (0.4 ms⁻¹; R² = 0.42; P = 0.007). WBV (+ 3.7 and 8.8%) and MDS (+ 4.9 and 5.2%) afforded the best protection against isolation-induced loss of MVIF, although MDS was the only intervention to prevent bilateral loss of calf MVC and leg MVIF at 0.2 and 0.4 ms⁻¹.

Conclusions

Mars500 induced significant loss of quadriceps/hamstrings MVIF but not calf MVC. Collectively, these data suggest that muscles with predominantly type I fibres were affected less by isolation compared to type II dominant muscles. MDS and WBV afforded the best protection against isolation-induced loss of strength and thus may have virtue in exploration class missions.

Electronic supplementary material

The online version of this article (10.1186/s40798-017-0107-y) contains supplementary material, which is available to authorized users.