Longer Muscle Lengths Research Articles

Novel Supine Isometric Hamstring Test: A Reliability Study

Background: Hamstring strain injuries are highly prevalent, particularly when the hamstrings are in a lengthened position; however, monitoring strength deficits and fatigue throughout a season is currently difficult due to a lack of practical, reliable tests that do not cause undue fatigue. Objective: The aim of the current study was to investigate the intersession reliability of a novel 90°hip: 40°knee isometric hamstring test (IHT) in the supine position. Methodology: This was a prospective, observational cohort study conducted on nine male participants (24 ± 3 years, 81.9 ± 6.4kg, 181.8 ± 5.3cm). Each participant completed two testing sessions of the 90°hip: 40°knee IHT using force platforms. Relative and absolute reliability of isometric peak force (IPF) were assessed using intraclass correlation coefficient (ICC) and coefficient of variation (CV%), respectively. Results: Relative reliability of the 90°hip: 40°knee IHT was considered “good” in the dominant limb (ICC = 0.92, Lower bound 95% Confidence Interval (CI) = 0.72) and “poor” in the non-dominant limb (ICC = 0.76, Lower bound 95% CI = 0.28). Absolute reliability was “acceptable” in both legs with 5.81% in the dominant limb and 9.65% in the non-dominant limb. Conclusion: The 90°hip: 40°knee IHT demonstrated moderate reliability overall, further research is required to discover the optimum IHT configuration which can be reliably executed at longer hamstring muscle lengths. This study provides foundational information for practitioners on a novel method of reliably testing hamstring isometric strength at longer muscle lengths.

Submaximal eccentric resistance training increases serial sarcomere number and improves dynamic muscle performance in old rats.

The age-related loss of muscle mass is partly accounted for by the loss of sarcomeres in series, contributing to declines in muscle mechanical performance. Resistance training biased to eccentric contractions increases serial sarcomere number (SSN) in young muscle, however, maximal eccentric training in old rats previously did not alter SSN and worsened performance. A submaximal eccentric training stimulus may be more conducive to adaptation for aged muscle. The purpose of this study was to assess whether submaximal eccentric training can increase SSN and improve mechanical function in old rats. Twelve 32-month-old male F344/BN rats completed 4 weeks of submaximal (60% maximum) eccentric plantar-flexion training 3 days/week. Pre- and post-training, we assessed in-vivo maximum isometric torque at a stretched and neutral ankle angle, the passive torque-angle relationship, and the isotonic torque-velocity-power relationship. The soleus and medial gastrocnemius (MG) were harvested for SSN measurements via laser diffraction, with the untrained leg as a control. SSN increased 11% and 8% in the soleus and MG, respectively. Training also shifted optimal torque production towards longer muscle lengths, reduced passive torque 42%, and increased peak isotonic power 23%. Submaximal eccentric training was beneficial for aged muscle adaptations, increasing SSN, reducing muscle passive tension, and improving dynamic contractile performance.

Ankle Joint Angle Influences Relative Torque Fluctuation during Isometric Plantar Flexion.

The purpose of this study was to investigate the influence of changes in muscle length on the torque fluctuations and on related oscillations in muscle activity during voluntary isometric contractions of ankle plantar flexor muscles. Eleven healthy individuals were asked to perform voluntary isometric contractions of ankle muscles at five different contraction intensities from 10% to 70% of maximum voluntary isometric contraction (MVIC) and at three different muscle lengths, implemented by changing the ankle joint angle (plantar flexion of 26°-shorter muscle length; plantar flexion of 10°-neutral muscle length; dorsiflexion of 3°-longer muscle length). Surface electromyogram (EMG) signals were recorded from the skin surface over the triceps surae muscles, and rectified-and-smoothed EMG (rsEMG) were estimated to assess the oscillations in muscle activity. The absolute torque fluctuations (quantified by the standard deviation) were significantly higher during moderate-to-high contractions at the longer muscle length. Absolute torque fluctuations were found to be a linear function of torque output regardless of muscle length. In contrast, the relative torque fluctuations (quantified by the coefficient of variation) were higher at the shorter muscle length. However, both absolute and relative oscillations in muscle activities remained relatively consistent at different ankle joint angles for all plantar flexors. These findings suggest that the torque steadiness may be affected by not only muscle activities, but also by muscle length-dependent mechanical properties. This study provides more insights that muscle mechanics should be considered when explaining the steadiness in force output.

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).

Effects of Stretching on Injury Risk Reduction and Balance

ABSTRACT Evidence for the effectiveness of acute and chronic stretching for improving range of motion is extensive. Improved flexibility can positively impact performances in activities of daily living and both physical and mental health. However, less is known about the effects of stretching on other aspects of health such as injury incidence and balance. The objective of this review is to examine the existing literature in these areas. The review highlights that both pre-exercise and chronic stretching can reduce musculotendinous injury incidence, particularly in running-based sports, which may be related to the increased force available at longer muscle lengths (altered force-length relationship) or reduced active musculotendinous stiffness, among other factors. Evidence regarding the acute effects of stretching on balance is equivocal. Longer-term stretch training can improve balance, which may contribute to a decreased incidence of falls and associated injuries and may thus be recommended as an important exercise modality in those with balance deficits. Hence, both acute and chronic stretching seem to have positive effects on injury incidence and balance, but optimum training plans are yet to be defined.

Myosin cross-bridge kinetics slow at longer muscle lengths during isometric contractions in intact soleus from mice.

Muscle contraction results from force-generating cross-bridge interactions between myosin and actin. Cross-bridge cycling kinetics underlie fundamental contractile properties, such as active force production and energy utilization. Factors that influence cross-bridge kinetics at the molecular level propagate through the sarcomeres, cells and tissue to modulate whole-muscle function. Conversely, movement and changes in the muscle length can influence cross-bridge kinetics on the molecular level. Reduced, single-molecule and single-fibre experiments have shown that increasing the strain on cross-bridges may slow their cycling rate and prolong their attachment duration. However, whether these strain-dependent cycling mechanisms persist in the intact muscle tissue, which encompasses more complex organization and passive elements, remains unclear. To investigate this multi-scale relationship, we adapted traditional step-stretch protocols for use with mouse soleus muscle during isometric tetanic contractions, enabling novel estimates of length-dependent cross-bridge kinetics in the intact skeletal muscle. Compared to rates at the optimal muscle length (Lo), we found that cross-bridge detachment rates increased by approximately 20% at 90% of Lo (shorter) and decreased by approximately 20% at 110% of Lo (longer). These data indicate that cross-bridge kinetics vary with whole-muscle length during intact, isometric contraction, which could intrinsically modulate force generation and energetics, and suggests a multi-scale feedback pathway between whole-muscle function and cross-bridge activity.

Effects of whole-body vibration training on calf muscle function during maximal isometric voluntary contractions.

The purposes of this study were to determine the impact of 6weeks of whole-body vibration training (WBVT) on maximum voluntary plantar flexor strength, muscle activity via surface electromyography (EMG), and muscle architecture measured at rest and during maximal contraction at different ankle joint angles in young healthy adults. Using a single-blind study design, 28 healthy men and women were randomly assigned to control (CG; N=14, 7 women) or whole-body vibration training (WBVG; N=14, 7 women) groups. Vibration training (20-25minutes; standing with knees flexed) was performed 3week-1 for 6weeks (18 sessions). Maximum isometric plantar flexor torque, muscle activity (medial and lateral gastrocnemius EMG) and medial gastrocnemius fascicle angle and length at rest and maximum contraction were tested at four ankle joint angles (ranging 45° to -15°; 0°=anatomical) before and after training. Significant increases (24.7%-37.5%) were observed in peak torque (N∙m∙kg-1 ;%) at -15°, 0°, 15° and 30° joint angles from pre- to post-intervention in WBVG, which were different to CG (no change) and greater at longer muscle lengths. No between-group differences were observed in changes in EMG amplitudes measured during contraction or muscle architecture parameters at rest or during contraction. Six weeks of WBVT in young, healthy adults increased isometric plantarflexion strength at multiple joint angles, without detectible changes in EMG, muscle architecture, or body composition. Therefore, WBVT can significantly improve maximum plantar flexor strength at multiple joint angles (muscle lengths) in young healthy men, although the mechanisms underpinning the changes are currently unclear.

Development of a Novel Nordic Hamstring Exercise Device to Measure and Modify the Knee Flexors' Torque-Length Relationship.

The Nordic hamstring exercise (NHE) has been shown to reduce hamstring injury risk when employed in training programs. This study investigates a novel device to modify the NHE torque-length relationship of the knee flexors, as targeting the hamstrings at a more extended length may have benefits for hamstring strain injury prevention and rehabilitation. Eighteen recreational male participants completed three bilateral NHE repetitions at a conventional 0° flat position, a 10° incline, and a 10° decline slope on a novel device (HALHAM°). Measures of peak torque and break-torque angle explored the effect of inclination on the knee flexors' length-tension relationship. Relative thigh-to-trunk angle and angular velocity of the knee joint were used to assess influence of inclination on technique and exercise quality. Break-torque angle increased when performed at an incline (134.1 ± 8.6°) compared to both the decline (112.1 ± 8.3°, p <0.0001, g = 2.599) and standard flat NHE positions (126.0 ± 9.8°, p = 0.0002, g = 0.885). Despite this, altering inclination did not affect eccentric knee flexor peak torque (decline = 132.0 ± 63.1 Nm, flat = 149.7 ± 70.1 Nm, incline = 148.9 ± 64.9 Nm, F = 0.952, p = 0.389), angular velocity of the knee joint at break-torque angle (decline = 23.8 ± 14.4°, flat = 29.2 ± 22.6°, incline = 24.5 ± 22.6°, F = 0.880, p = 0.418) or relative thigh-to-trunk angle at break-torque angle (decline = 20.4 ± 10.4°, flat = 16.7 ± 10.8°, incline = 20.2 ± 11.2°, F = 1.597, p = 0.207). The report recommends the use of arbitrary metrics such as break-torque angle that can be replicated practically in the field by practitioners to assess proxy muscle length changes i.e., the angular range over which the torque can be produced. Inclination of the Nordic hamstring exercise leads to hamstring muscle failure at longer muscle lengths without reductions in the maximal force exuded by the muscle. Therefore, the NHE performed on an incline may be a more effective training intervention, specific to the proposed mechanism of hamstring strain injury during sprinting that occurs whilst the muscle is rapidly lengthening. Using a graded training intervention through the inclinations could aid gradual return-to-play rehabilitation.

Regional Differences in Biceps Femoris Long Head Stiffness during Isometric Knee Flexion.

This study sought to investigate whether the stiffness of the biceps femoris long head differs between proximal and distal regions during isometric knee flexion at different contraction intensities and muscle lengths. Twelve healthy individuals performed knee flexion isometric contractions at 20% and 60% of maximum voluntary isometric contraction, with the knee flexed at 15 and 45 degrees. Muscle stiffness assessment was performed using ultrasound-based shear wave elastography. Proximal and distal regions of the biceps femoris long head were assessed. Biceps femoris long head muscle showed a greater stiffness (i) in the distal region, (ii) at higher contraction intensity, and (iii) at longer muscle length. The proximal-to-distal stiffness ratio was significantly lower than 1 (i.e., heterogenous) at lower contraction intensity regardless of the muscle length. However, this was not observed at higher contraction intensity. This study is the first to show heterogeneity in the active stiffness of the biceps femoris long head. Given the greater incidence of injury at the proximal region of biceps femoris long head, this study opens new directions for future research. Additionally, the present study results indicate that studies assessing muscle stiffness at one single muscle region should be interpreted with caution.

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

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

Force enhancement in the human vastus lateralis is muscle-length-dependent following stretch but not during stretch

PurposeForce enhancement is the phenomenon of increased forces during (transient force enhancement; tFE) and after (residual force enhancement; rFE) eccentric muscle actions compared with fixed-end contractions. Although tFE and rFE have been observed at short and long muscle lengths, whether both are length-dependent remains unclear in vivo.MethodsWe determined maximal-effort vastus lateralis (VL) force-angle relationships of eleven healthy males and selected one knee joint angle at a short and long muscle lengths where VL produced approximately the same force (85% of maximum). We then examined tFE and rFE at these two lengths during and following the same amount of knee joint rotation.ResultsWe found tFE at both short (11.7%, P = 0.017) and long (15.2%, P = 0.001) muscle lengths. rFE was only observed at the long (10.6%, P < 0.001; short: 1.3%, P = 0.439) muscle length. Ultrasound imaging revealed that VL muscle fascicle stretch magnitude was greater at long compared with short muscle lengths (mean difference: (tFE) 1.7 mm, (rFE) 1.9 mm, P ≤ 0.046), despite similar isometric VL forces across lengths (P ≥ 0.923). Greater fascicle stretch magnitude was likely to be due to greater preload forces at the long compared with short muscle length (P ≤ 0.001).ConclusionAt a similar isometric VL force capacity, tFE was not muscle-length-dependent at the lengths we tested, whereas rFE was greater at longer muscle length. We speculate that the in vivo mechanical factors affecting tFE and rFE are different and that greater stretch of a passive component is likely contributing more to rFE at longer muscle lengths.

Placing Greater Torque at Shorter or Longer Muscle Lengths? Effects of Cable vs. Barbell Preacher Curl Training on Muscular Strength and Hypertrophy in Young Adults

Muscular strength and hypertrophy following resistance training may be obtained in different degrees depending on the approach performed. This study was designed to compare the responses of the biceps brachii to two preacher curl exercises, one performed on a cable-pulley system (CAB; in which a greater torque was applied during the exercise when elbows were flexed and biceps shortened) and one performed with a barbell (BAR; in which greater torque was applied when the elbows were extended and biceps stretched). Thirty-five young adults (CAB: 13 men, 5 women; BAR: 12 men, 5 women; age = 24 ± 5 years) performed a resistance training program three times per week for 10 weeks, with preacher curl exercises performed in three sets of 8–12 repetitions. Outcomes measured included elbow flexion peak isokinetic torque at angles of 20°, 60°, and 100° (considering 0° as elbow extended), and biceps brachii thickness (B-mode ultrasound). Following the training period, there were significant increases for both groups in elbow flexion peak torque at the 20° (CAB: 30%; BAR = 39%; p = 0.046), 60° (CAB: 27%; BAR = 32%; p = 0.874), and 100° (CAB: 17%; BAR = 19%; p = 0.728), and biceps brachii thickness (CAB: 7%; BAR = 8%; p = 0.346). In conclusion, gains in muscular strength were greater for BAR only at longer muscle length, whereas hypertrophy was similar regardless of whether torque emphasis was carried out in the final (CAB) or initial (BAR) degrees of the range of motion of the preacher curl in young adults.

Muscle length: A new feature to investigate neural control of lengthening contractions.

Muscle length: A new feature to investigate neural control of lengthening contractions.

Cross-Bridge Cycling Kinetics Slow at Longer Muscle Length in Tetanic Contracting Mouse Soleus Muscle

Muscle contraction results from force-generating cross-bridge interactions between myosin and actin. Cross-bridge cycling kinetics underlie fundamental properties of muscle contraction, such as total force production and energetics, and factors that influence cross-bridge kinetics at the molecular level can propagate to critically alter whole-muscle function. Reciprocally, whole-muscle properties, including movement and the length of the muscle, can influence kinetics on the cross-bridge level. Previous research using single-molecule and single-fiber experiments has suggested that cross-bridge cycling is strain-dependent, and that increasing the strain on cross-bridges may slow their cycling rate and prolong their attachment duration. However, whether this property is maintained in whole-muscle to affect cross-bridge behavior under various strains and muscle lengths remains unclear. Here, we estimated cross-bridge cycling kinetics using small-amplitude step-length perturbations (1% total muscle-tendon length) in electrically-activated mouse soleus muscles. We separated the resulting force response into distinct phases associated with cross-bridge detachment and cross-bridge recruitment rates. The initial, fast single-exponential force decay is dominated by myosin detachment events and the slower force recovery phase is dominated by cross-bridge recruitment events. We found that cross-bridge detachment kinetics were ∼30% slower when muscles were stretched 10% longer than optimal length (Lo) (t1/2=8.75 ms vs 6.65 ms at Lo, 17°C). Detachment kinetics were ∼35% faster at 10% shorter length (t1/2=4.25ms at 90% of Lo). These findings suggest that cross-bridge kinetics vary with muscle length during intact, tetanic contraction in skeletal muscle, which may follow from strain-dependent mechanisms of cross-bridge cycling and could intrinsically modulate whole muscle force-generation and energetics during locomotion. This study advances our understanding of the classic Length-Tension relationship of skeletal muscle and implies that strain-dependent cross-bridge kinetics contribute another feedback pathway between whole-muscle function and cross-bridge activity.

Changes in Torque-Angle Profiles of the Hamstrings and Hamstrings-to-Quadriceps Ratio After Two Hamstring Strengthening Exercise Interventions in Female Hockey Players.

Delextrat, A, Bateman, J, Ross, C, Harman, J, Davis, L, Vanrenterghem, J, and Cohen, DD. Changes in torque-angle profiles of the hamstrings and hamstrings-to-quadriceps ratio after two hamstring strengthening exercise interventions in female hockey players. J Strength Cond Res 34(2): 396-405, 2020-The aim of this study was to compare the effects of 2 hamstring strengthening interventions (Nordic hamstrings [NHE] vs. eccentric leg curl [ELC]) on the hamstring torque-angle profiles and functional hamstrings-to-quadriceps ratio (Hecc:Qcon) in female hockey players. Female university-level players were randomly allocated to an NHE group (n = 9, 19.7 ± 1.4 years; 168.4 ± 4.4 cm; 66.2 ± 7.2 kg, 26.0 ± 4.4%), an ELC group (n = 8, 19.5 ± 1.0 years; 168.1 ± 3.4 cm; 66.7 ± 4.5 kg, 24.8 ± 3.5%), or a control (C) group (n = 8, 19.6 ± 1.4 years; 169.9 ± 7.5 cm; 70.7 ± 13.0 kg, 25.9 ± 5.2%). They performed baseline isokinetic concentric strength tests of the quadriceps (Qcon) and eccentric strength of the hamstrings (Hecc) at 120°·s, followed by a 6-week intervention with exercises (NHE or ELC) performed 3 times weekly, before post-tests. Analyses of variance with repeated measures were used to assess the effects of knee position angle (from 90° of knee flexion to 10° close to extension), group, and time on Qcon, Hecc, and Hecc:Qcon. There were no interactions between independent variables. Significant increases in Hecc and Hecc:Qcon were shown after NHE (+29.9 and +27.8%) and ELC (+30.5 and +38.3%) in the nondominant leg only. Furthermore, significant shifts in the hamstring eccentric angle of peak torque toward a longer muscle length were shown in both legs (14.3-28.6%). These findings suggest that NHE and ELC both resulted in significant improvements in peak and muscle-length-specific neuromuscular risk factors in the nondominant (ND) limb, thereby reducing interlimb peak strength asymmetries. Strength and conditioning specialists could therefore use both the NHE and ELC exercises in female hockey players.

Non-uniformity of displacement and strain within the Achilles tendon is affected by joint angle configuration and differential muscle loading

Although the Achilles tendon (AT) has been studied for more than a century, a complete understanding of the mechanical and functional consequences of AT structural organization is currently lacking. The aim of this study was to assess how joint angle configuration affects subtendon displacement and strain of soleus (SOL) and lateral gastrocnemius (LG) muscles. Knots sutured onto SOL and LG subtendons of 12 Wistar rats, were videotaped to quantify displacements and the ankle torque was assessed for different isometric activation conditions (i.e., individual and simultaneous) of the triceps surae muscles. Changing ankle and knee joint angle affected the magnitude of displacement, relative displacement and strain of both SOL and LG subtendons. SOL subtendon behavior was not only affected by changes in ankle angle, but also by changes in knee angle. Displacement of SOL subtendon decreased (28–49%), but strain increased in response to knee extension. Independent of joint angle configuration, stimulation of any combination of the muscles typically resulted in displacements and strains of LG and SOL subtendons. Typically SOL displaced more but LG displaced more when stimulated at longer muscle lengths. Our results demonstrate that the distinct subtendons of the Achilles tendon can move and deform differently, but are not fully independent. Within the AT, there appears to be a precarious balance between sliding allowance and mechanical connectivity between subtendons.

Influence of muscle length on the stretch-shortening cycle in skinned rabbit soleus

Muscle force generated during shortening is instantaneously increased after active stretch. This phenomenon is called as stretch-shortening cycle (SSC) effect. It has been suggested that residual force enhancement contributes to the SSC effect. If so, the magnitude of SSC effect should be larger in the longer muscle length condition, because the residual force enhancement is prominent in the long muscle length condition. This hypothesis was examined by performing the SSC in the short and long muscle length conditions. Skinned fibers obtained from rabbit soleus (N = 20) were used in this study. To calculate the magnitude of SSC effect, the SSC trial (isometric-eccentric-concentric-isometric) and the control trial (isometric-concentric-isometric) were conducted in the short (within the range of 2.4 to 2.7 μm) and long muscle (within the range of 3.0 to 3.3 μm). The magnitude of SSC effect was calculated as the relative increase in the mechanical work attained during the shortening phase between control and SSC trials. As a result, the magnitude of SSC effect was significantly larger in the long (176.8 ± 18.1%) than in the short muscle length condition (157.4 ± 8.5%) (p < 0.001). This result supports our hypothesis that the magnitude of SSC effect is larger in the longer muscle length condition, possibly due to the larger magnitude of residual force enhancement.

Characterization of the passive mechanical properties of spine muscles across species

Passive mechanical properties differ between muscle groups within a species. Altered functional demands can also shift the passive force-length relationship. The extent that passive mechanical properties differ within a muscle group (e.g. spine extensors) or between homologous muscles of different species is unknown. It was hypothesized that multifidus, believed to specialize in spine stabilization, would generate greater passive tensile stresses under isometric conditions than erector spinae, which have more generalized functions of moving and stabilizing the spine; observing greater multifidus moduli in different species would strengthen this hypothesis. Permeabilized fibre bundles (n = 337) from the multifidus and erector spinae of mice, rats, and rabbits were mechanically tested. A novel logistic function was fit to the experimental data to fully characterize passive stress and modulus. Species had the greatest effect on passive muscle parameters with mice having the largest moduli at all lengths. Rats generated less passive stress than rabbits due to a shift of the passive force-length relationship towards longer muscle lengths. Rat multifidus generated slightly greater stresses than erector spinae, but no differences were observed between mouse muscles. The secondary objective was to determine the parameters required to simulate the passive force-length relationship. Experimental data were compared to the passive muscle model in OpenSim. The default OpenSim model, optimized for hindlimb muscles, did not fit any of the spine muscles tested; however, the model could accurately simulate experimental data after adjusting the input parameters. The optimal parameters for modelling the passive force-length relationships of spine muscles in OpenSim are presented.

Impact of Hip Flexion Angle on Unilateral and Bilateral Nordic Hamstring Exercise Torque and High-Density Electromyography Activity

In the bilateral Nordic hamstring exercise (NHE), hamstrings operate at relatively short lengths, which may limit the efficacy of the NHE in hamstring injury prevention. To examine knee flexion torque and biceps femoris long head (BFLH) and semitendinosus (ST) high-density electromyography (EMG) activity during the unilateral and bilateral NHE, performed with either neutral (NHE0) or 90°-flexed (NHE90) hips. In this laboratory study, exercises were performed on a novel device at the eccentric 1-repetition maximum load defined for 90° to 15° of knee range of motion. Torque and EMG signals normalized to maximal voluntary isometric activity were compared in different phases of the exercises with statistical parametric mapping. The EMG levels were lower in NHE90 than in NHE0, mainly in the second half of the movement. Knee flexor eccentric torque was higher in NHE90 than in NHE0 from the beginning to 87% of the bilateral movement, and over the entire unilateral movement. In NHE0, ST activity compared to BFLH activity was higher during the initial movement phase and lower when the movement was close to knee extension. Torque and EMG activity were generally similar in the bilateral and unilateral modes. If performed with neutral hips, the NHE selectively activates the BFLH near full knee extension. Performing the NHE with hips flexed to 90° is preferable when higher passive torque and ST selectivity are targeted at a longer muscle length. Performing these exercises unilaterally could help train each limb separately, with similar torque and EMG output to those of the bilateral conditions. J Orthop Sports Phys Ther 2019;49(8):584-592. Epub 26 Mar 2019. doi:10.2519/jospt.2019.8801.

Isometric training and long-term adaptations: Effects of muscle length, intensity, and intent: A systematic review.

Isometric training is used in the rehabilitation and physical preparation of athletes, special populations, and the general public. However, little consensus exists regarding training guidelines for a variety of desired outcomes. Understanding the adaptive response to specific loading parameters would be of benefit to practitioners. The objective of this systematic review, therefore, was to detail the medium- to long-term adaptations of different types of isometric training on morphological, neurological, and performance variables. Exploration of the relevant subject matter was performed through MEDLINE, PubMed, SPORTDiscus, and CINAHL databases. English, full-text, peer-reviewed journal articles and unpublished doctoral dissertations investigating medium- to long-term (≥3weeks) adaptations to isometric training in humans were identified. These studies were evaluated further for methodological quality. Twenty-six research outputs were reviewed. Isometric training at longer muscle lengths (0.86%-1.69%/week, ES=0.03-0.09/week) produced greater muscular hypertrophy when compared to equal volumes of shorter muscle length training (0.08%-0.83%/week, ES=-0.003 to 0.07/week). Ballistic intent resulted in greater neuromuscular activation (1.04%-10.5%/week, ES=0.02-0.31/week vs 1.64%-5.53%/week, ES=0.03-0.20/week) and rapid force production (1.2%-13.4%/week, ES=0.05-0.61/week vs 1.01%-8.13%/week, ES=0.06-0.22/week). Substantial improvements in muscular hypertrophy and maximal force production were reported regardless of training intensity. High-intensity (≥70%) contractions are required for improving tendon structure and function. Additionally, long muscle length training results in greater transference to dynamic performance. Despite relatively few studies meeting the inclusion criteria, this review provides practitioners with insight into which isometric training variables (eg, joint angle, intensity, intent) to manipulate to achieve desired morphological and neuromuscular adaptations.