Damage to different motor units from active lengthening of the medial gastrocnemius muscle of the cat

Delayed Impairment of Postural, Physical, and Muscular Functions Following Downhill Compared to Level Walking in Older People

Transient symptoms of muscle damage emanating from unaccustomed eccentric exercise can adversely affect muscle function and potentially increase the risk of falling for several days. Therefore, the aims of the present study were to investigate the shorter- and longer-lasting temporal characteristics of muscle fatigue and damage induced by level (i.e., concentrically biased contractions) or downhill (i.e., eccentrically biased contractions) walking on postural, physical, and muscular functions in older people. Nineteen participants were matched in pairs for sex, age and self-selected walking speed and allocated to a level (n = 10, age = 72.3 ± 2.9 years) or downhill (n = 9, age = 72.1 ± 2.2 years) walking group. Postural sway, muscle torque and power, physical function (5× and 60 s sit-to-stand; STS), and mobility (Timed-Up-and-Go; TUG) were evaluated at baseline (pre-exercise), 1 min, 15 min, 30 min, 24 h, and 48 h after 30 min of level (0% gradient) or downhill (−10% gradient) walking on a treadmill. Following downhill walking, postural sway (+66 to 256%), TUG (+29%), 60 s STS (+29%), five times STS (−25%) and concentric power (−33%) did not change at 1–30 min post exercise, but were significantly different (p < 0.05) at 24 and48 h post-exercise when compared to baseline (p < 0.05). Muscle torque decreased immediately after downhill walking and remained impaired at 48 h post-exercise (−27 to −38%). Immediately following level walking there was an increase in postural sway (+52 to +98%), slower TUG performance (+29%), fewer STS cycles in 60 s (−23%), slower time to reach five STS cycles (+20%) and impaired muscle torque (−23%) and power (−19%) which returned to baseline 30-min after exercise cessation (p > 0.05). These findings have established for the first time distinct impairment profiles between concentric and eccentric exercise. Muscle damage emanating from eccentrically biased exercise can lead to muscle weakness, postural instability and impaired physical function persisting for several days, possibly endangering older adult’s safety during activities of daily living by increasing the risk of falls.

The influence of capillarization on satellite cell pool expansion and activation following exercise-induced muscle damage in healthy young men

KEY POINTS

Skeletal muscle stem cells (satellite cells) play a crucial role in repair and remodelling of muscle in response to exercise. Satellite cells are in close spatial proximity to muscle capillaries and therefore may be influenced by them. In this study, we describe the activation and expansion of the satellite cell pool in response to eccentric contraction-induced muscle damage in individuals with significantly different levels of muscle capillarization. Individuals with greater capillarization and capacity for muscle perfusion demonstrated enhanced activation and/or expansion of the satellite cell pool allowing for an accelerated recovery of muscle function. These results provide insight into the critical relationship between muscle capillarization and satellite cells during skeletal muscle repair.

ABSTRACT

Factors that determine the skeletal muscle satellite cell (SC) response remain incompletely understood. It is known, however, that SC activation status is closely related to the anatomical relationship between SCs and muscle capillaries. We investigated the impact of muscle fibre capillarization on the expansion and activation status of SCs following a muscle-damaging exercise protocol in healthy young men. Twenty-nine young men (21 ± 0.5 years) performed 300 unilateral eccentric contractions (180 deg s^-1 ) of the knee extensors. Percutaneous muscle biopsies from the vastus lateralis and blood samples from the antecubital vein were taken prior to (Pre) exercise and at 6, 24, 72 and 96 h of post-exercise recovery. A comparison was made between subjects who had a relative low mixed muscle capillary-to-fibre perimeter exchange index (CFPE; Low group) and high mixed muscle CFPE index (High group) at baseline. Type I and type II muscle fibre size, myonuclear content, capillarization, and SC response were determined via immunohistochemistry. Overall, there was a significant correlation (r = 0.39; P < 0.05) between the expansion of SC content (change in total Pax7⁺ cells/100 myofibres) 24 h following eccentric exercise and mixed muscle CFPE index. There was a greater increase in activated SCs (MyoD⁺ /Pax7⁺ cells) in the High as compared to the Low CFPE group 72 h following eccentric exercise (P < 0.05). The current study provides further evidence that muscle fibre capillarization may play an important role in the activation and expansion of the SC pool during the process of skeletal muscle repair.

Early effects of eccentric contractions on muscle glucose uptake

Muscle-damaging eccentric exercise impairs muscle glucose uptake several hours to days after exercise. Little, however, is known about the acute effects of eccentric exercise on contraction- and insulin-induced glucose uptake. This study compares glucose uptake rates in the first hours following eccentric, concentric, and isometric contractions with and without insulin present. Isolated rat extensor digitorum longus muscles were exposed to either an eccentric, concentric, or isometric contraction protocol, and muscle contractions were induced by electric stimulation that was identical between contraction protocols. In eccentric and concentric modes, length changes of 0.6 or 1.2 mm were used during contractions. Both contraction- and insulin-induced glucose uptake were assessed immediately and 2 h after contractions. Glucose uptake increased significantly following all modes of contraction and was higher after eccentric contractions with a stretch of 1.2 mm compared with the remaining contraction groups when assessed immediately after contractions [eccentric (1.2 mm) > eccentric (0.6 mm), concentric (1.2 mm), concentric (0.6 mm), isometric > rest; P < 0.05]. After 2 h, contraction-induced glucose uptake was still higher than noncontracting levels, but with no difference between contraction modes. The presence of insulin increased glucose uptake markedly, but this response was blunted by, respectively, 39–51% and 29–36% ( P < 0.05) immediately and 2 h after eccentric contractions stretched 1.2 mm compared with concentric and isometric contractions. The contrasting early effects of eccentric contractions on contraction- and insulin-induced glucose uptake suggest that glucose uptake is impaired acutely following eccentric exercise because of reduced insulin responsiveness.

NEW & NOTEWORTHY This study shows that, in isolated rat muscle, muscle-damaging eccentric contractions result in a transient increase in contraction-induced glucose uptake compared with isometric and concentric contractions induced by identical muscle activation protocols. Furthermore, our results demonstrate that, in contrast, the insulin-stimulated glucose uptake is impaired immediately following muscle-damaging eccentric contractions.

A Moderate Supplementation of Native Whey Protein Promotes Better Muscle Training and Recovery Adaptations Than Standard Whey Protein - A 12-Week Electrical Stimulation and Plyometrics Training Study

The purpose of this study was to assess if native whey protein (NW) supplementation could promote recovery and training adaptations after an electrostimulation (ES) training program combined to plyometrics training. Participants were allocated into three groups, supplemented 5 days/week, either with 15 g of carbohydrates + 15 g of NW (n = 17), 15 g of carbohydrates + 15 g of standard whey protein (SW; n = 15), or placebo (PLA; 30 g of carbohydrates; n = 10), while undergoing a 12-week ES training program of the knee extensors. Concentric power (Pmax) was evaluated before, immediately after, as well as 30 min, 60 min, 24 h, and 48 h after the 1st, 4th and last ES training session. The maximal voluntary contraction torque (MVC), twitch amplitude, anatomical cross-sectional area (CSA) and maximal voluntary activation level (VA) were measured before (T0), and after 6 (T1) and 12 weeks of training (T2). P_max recovery kinetics differed between groups (p < 0.01). P_max started to recover at 30 min in NW, 24 h in SW and 48 h in PLA. Training adaptations also differed between groups: MVC increased between T0 and T2 in NW (+11.8%, p < 0.001) and SW (+7.1%, p < 0.05), but not PLA. Nevertheless, the adaptation kinetics differed: MVC increased in NW and SW between T0 and T1, but an additional gain was only observed between T1 and T2 in NW. VA declined at T1 and T2 in PLA (−3.9%, p < 0.05), at T2 in SW (−3.5%, p < 0.05), and was unchanged in NW. CSA increased, but did not differ between groups. These results suggest that NW could promote a faster recovery and neuromuscular adaptations after training than SW. However, the mechanisms underlying this effect remain to be identified.

Muscle fiber conduction velocity of the vastus medilais and lateralis muscle after eccentric exercise induced-muscle damage

Change in muscle fiber conduction velocity (MFCV) has been reported after eccentric exercise induces muscle fiber damage, most likely due to a change in membrane permeability of the injured fiber. The extent of damage to the muscle fiber depends on the morphological and architectural characteristics of the muscle fibers. Morphological and architectural characteristics of the VMO muscle fibers are different from VL muscle. Thus, it is expected that eccentric exercise of quadriceps muscle results in a non-uniform fiber damage within the VMO and VL muscle and, as a consequence, non-uniform changes in membrane excitability and conduction velocity. The aim of the study was to investigate MFCV of the VMO and VL muscles before and 24 h after eccentric exercise. Multichannel surface EMG signals were concurrently recorded from the right VMO and VL muscles of 15 healthy men during sustained isometric contractions at 50% of the maximal force. Maximal voluntary force significantly reduced after eccentric exercise with respect to the pre-exercise condition (P < 0.0001). MFCV decreased over time during the sustained contractions at faster rates when assessed 24 h after exercise (VMO = -26.1; VL = -20.1) with respect to the pre-exercise condition (VMO = -9.1; VL = -13.7, P < 0.0001). Moreover, VMO showed a greater rate of reduction in MFCV over sustained contraction (26.1 ± 10.7%) in comparison with VL muscle (20.1 ± 8.5%, P < 0.025) 24 h after eccentric exercise. The result indicates that eccentric exercise contributes to a larger reduction in MFCV within the VMO muscle as compared to the VL muscle. This may abolish the ability of VMO to counteract the lateral pull of the VL muscle during knee extension, thereby leaving the knee complex more vulnerable to injury.

Muscle damage produced by isometric contractions in human elbow flexors

Isometric exercise is often prescribed during rehabilitation from injury to maintain muscle condition and prevent disuse atrophy. However, such exercise can lead to muscle soreness and damage. Here we investigate which parameters of isometric contractions are responsible for the damage. Bouts of 30 repetitions of maximum voluntary contractions of elbow flexors in 38 subjects were carried out and peak force, soreness, and tenderness were measured before the exercise, immediately afterwards, at 2 h, and at 24 h postexercise. When one arm was held near the optimum angle for force generation (90°), the force it produced was greater by 28% than by the other arm held at a longer length (155°). However, despite the smaller contraction forces of the muscle held at the longer length, after the exercise it exhibited a greater fall in force that persisted out to 24 h (20% fall) and more delayed soreness than the muscle exercised at 90° (7% fall at 24 h). The result indicates a length dependence of the damage process for isometric contractions at maximum effort. In four additional experiments, evidence was provided that the damage occurred during the plateau of the contraction and not the rising or relaxation phases. The damage had a prompt onset and was cumulative, continuing for the duration of the contraction. We interpret our findings in terms of the nonuniform lengthening of sarcomeres during the plateau of the contractions and conclude that muscle damage from isometric exercise is minimized if carried out at lengths below the optimum, using half-maximum or smaller contractions. NEW & NOTEWORTHY Isometric exercise, where muscle contracts while the limb is held fixed, is often possible for individuals rehabilitating from injury and can help maintain muscle condition. Such exercise has been reported to cause some muscle damage and soreness. We confirm this and show that to minimize damage, exercising muscles should be held at shorter than the optimum length for force and carried out at half-maximum effort or less.

Differential Effects of Unilateral Concentric Vs. Eccentric Exercise on the Dominant and Nondominant Forearm Flexors

The purpose of this study was to compare the electromyographic (EMG) intensity patterns after unilateral concentric vs. eccentric exercise in the dominant (DOM) and nondominant (NONDOM) forearm flexors. Twenty-six men (mean ± SD: age, 24.0 ± 3.7 years) volunteered to perform a maximal isometric muscle action of the DOM and NONDOM forearm flexors before (PRE) and immediately after (POST) a series of maximal concentric isokinetic or maximal eccentric isokinetic muscle actions of the DOM forearm flexors. The concentric isokinetic and eccentric isokinetic muscle actions were performed on separate days that were randomly ordered. However, in both cases, the subjects performed 6 sets of 10 maximal muscle actions. A bipolar surface EMG signal was detected from the biceps brachii of the DOM and NONDOM limbs during the PRE and POST isometric muscle actions. The signals were then analyzed with a wavelet analysis, and the resulting intensity patterns were classified with a paired pattern classification procedure. The results indicated that the EMG intensity patterns could be correctly classified into their respective PRE vs. POST categories with an accuracy rate that was significantly better than random (20 of 26 patterns = 76.9% accuracy) but only for the DOM limb following the eccentric muscle actions. All other classifications were not significantly better than random. These findings indicated that eccentric exercise had a significant influence on the muscle activation pattern for the forearm flexors. It is possible that the muscle damage resulting from eccentric exercise affects muscle spindle or golgi tendon organ or both activity, thereby altering the muscle activation pattern.

Creatine supplementation does not alter neuromuscular recovery after eccentric exercise

INTRODUCTION

The purpose of this study was to investigate the effects of creatine (CR) supplementation on recovery after eccentric exercise (ECC).

METHODS

Fourteen men were assigned randomly to ingest 0.3 g/kg of CR or placebo (PL) before and during recovery (48 hours) from 6 sets of 8 repetitions of ECC. Maximal voluntary contraction (MVC), voluntary activation (VA), muscle thickness (MT), electromyography (EMG), contractile properties, and soreness were assessed.

RESULTS

MVC, evoked twitch torque, and rate of torque development decreased for both groups immediately after ECC and recovered at 48 hours. MT increased and remained elevated at 48 hours for both groups. Soreness increased similarly for both groups. EMG activation was higher for CR versus PL only at 48 hours. There were no group differences for torque, total work, or fatigue index during ECC.

CONCLUSIONS

CR supplementation before and during recovery from ECC had no effect on strength, voluntary activation, or indicators of muscle damage. Muscle Nerve 54: 487-495, 2016.

Type of Ground Surface during Plyometric Training Affects the Severity of Exercise-Induced Muscle Damage

The purpose of this study was to compare the changes in the symptoms of exercise-induced muscle damage from a bout of plyometric exercise (PE; 10 × 10 vertical jumps) performed in aquatic, sand and firm conditions. Twenty-four healthy college-aged men were randomly assigned to one of three groups: Aquatic (AG, n = 8), Sand (SG, n = 8) and Firm (FG, n = 8). The AG performed PE in an aquatic setting with a depth of ~130 cm. The SG performed PE on a dry sand surface at a depth of 20 cm, and the FG performed PE on a 10-cm-thick wooden surface. Plasma creatine kinase (CK) activity, delayed onset muscle soreness (DOMS), knee range of motion (KROM), maximal isometric voluntary contraction (MIVC) of the knee extensors, vertical jump (VJ) and 10-m sprint were measured before and 24, 48 and 72 h after the PE. Compared to baseline values, FG showed significantly (p < 0.05) greater changes in CK, DOMS, and VJ at 24 until 48 h. The MIVC decreased significantly for the SG and FG at 24 until 48 h post-exercise in comparison to the pre-exercise values. There were no significant (p > 0.05) time or group by time interactions in KROM. In the 10-m sprint, all the treatment groups showed significant (p < 0.05) changes compared to pre-exercise values at 24 h, and there were no significant (p > 0.05) differences between groups. The results indicate that PE in an aquatic setting and on a sand surface induces less muscle damage than on a firm surface. Therefore, training in aquatic conditions and on sand may be beneficial for the improvement of performance, with a concurrently lower risk of muscle damage and soreness.

Acute and delayed peripheral and central neuromuscular alterations induced by a short and intense downhill trail run

Downhill sections are highly strenuous likely contributing to the development of neuromuscular fatigue in trail running. Our purpose was to investigate the consequences of an intense downhill trail run (DTR) on peripheral and central neuromuscular fatigue at knee extensors (KE) and plantar flexors (PF). Twenty-three runners performed a 6.5-km DTR (1264-m altitude drop) as fast as possible. The electromyographic activity of vastus lateralis (VL) and gastrocnemius lateralis (GL) was continuously recorded. Neuromuscular functions were assessed Pre-, Post-, and 2-day Post-DTR (Post2d). Maximal voluntary torques decreased Post (∼ -19% for KE, ∼ -25% for PF) and Post2d (∼ -9% for KE, ∼ -10% for PF). Both central and peripheral dysfunctions were observed. Decreased KE and PF voluntary activation (VA), evoked forces, VL M-wave amplitude, and KE low-frequency fatigue were observed at Post. Changes in VL M-wave amplitude were negatively correlated to VL activity during DTR. Changes in PF twitch force and VA were negatively correlated to GL activity during DTR. The acute KE VA deficit was about a third of that reported after ultramarathons, although peripheral alterations were similar. The prolonged force loss seems to be mainly associated to VA deficit likely induced by the delayed inflammatory response to DTR-induced ultrastructural muscle damage.

Enhanced Glycogen Storage of a Subcellular Hot Spot in Human Skeletal Muscle during Early Recovery from Eccentric Contractions

Unaccustomed eccentric exercise is accompanied by muscle damage and impaired glucose uptake and glycogen synthesis during subsequent recovery. Recently, it was shown that the role and regulation of glycogen in skeletal muscle are dependent on its subcellular localization, and that glycogen synthesis, as described by the product of glycogen particle size and number, is dependent on the time course of recovery after exercise and carbohydrate availability. In the present study, we investigated the subcellular distribution of glycogen in fibers with high (type I) and low (type II) mitochondrial content during post-exercise recovery from eccentric contractions. Analysis was completed on five male subjects performing an exercise bout consisting of 15 x 10 maximal eccentric contractions. Carbohydrate-rich drinks were subsequently ingested throughout a 48 h recovery period and muscle biopsies for analysis included time points 3, 24 and 48 h post exercise from the exercising leg, whereas biopsies corresponding to prior to and at 48 h after the exercise bout were collected from the non-exercising, control leg. Quantitative imaging by transmission electron microscopy revealed an early (post 3 and 24 h) enhanced storage of intramyofibrillar glycogen (defined as glycogen particles located within the myofibrils) of type I fibers, which was associated with an increase in the number of particles. In contrast, late in recovery (post 48 h), intermyofibrillar, intramyofibrillar and subsarcolemmal glycogen in both type I and II fibers were lower in the exercise leg compared with the control leg, and this was associated with a smaller size of the glycogen particles. We conclude that in the carbohydrate-supplemented state, the effect of eccentric contractions on glycogen metabolism depends on the subcellular localization, muscle fiber’s oxidative capacity, and the time course of recovery. The early enhanced storage of intramyofibrillar glycogen after the eccentric contractions may entail important implications for muscle function and fatigue resistance.

Differential susceptibility on myosin heavy chain isoform following eccentric-induced muscle damage

Based on myosin heavy chain (MHC) isoform, human skeletal muscle fibers can be categorized into three fiber types, type I, IIa, IIx fibers, and each fiber type has different characteristics. Typical characteristics are difference in force production, shortening velocity, and fatigue resistance. When the muscle is contract and stretched by a force that is greater than the force generated by the muscle, contraction-induced muscle damage frequently occurs. Several experimental models involving both human and animal have considered the susceptibility of different muscle fiber type and part of muscles to eccentric induced muscle damage. General consensus is a greater susceptibility of fast-twitch fiber or type II fiber to damage following eccentric contractions. However, the results from these previous efforts were not enough to conclude the susceptibility between each individual fiber types after eccentric contraction. Therefore, the purpose of this review is to explore detail limitation and interpretation of previous studies, and review the recent study that eliminated the prior limitations, such as strain magnitude, lengthening velocity, fiber type heterogeneity, and muscle architecture issue.

A model for creating a single stretch injury in murine biarticular muscle

Background

We developed a single stretch injury model to create damage near the musculotendinous junction (MTJ) of the gastrocnemius muscle in mice. Our hypothesis was that magnitude of muscle injury could be controlled by stepped shortening of the Achilles tendon (AT) prior to a lengthening contraction. Increased shortening would result in a greater isometric torque deficit and morphological damage 24 hours post-injury.

Methods

Sixteen mice were randomly assigned to sham or injury predicated on stepped increases in AT shortening. The AT was exposed and placed in a customized stainless steel roller-clamp system to achieve a specific level of shortening; 0 mm (resting length), 0.7 mm or 1.4 mm. Plantar flexors were stimulated to tetany with a needle electrode and then actively lengthened at 450°/sec from neutral to 75° of dorsiflexion. Passive and isometric torques were measured pre- and immediately post-injury. Isometric torque was measured again 24 h post-injury. Peak isokinetic torque was recorded during eccentric injury.

Results

Injury resulted in decreased passive and immediate absolute isometric torque only when induced with AT shortening. The percentage of pre-injury isometric torque was significantly lower in the AT shortened groups immediately and 24 h post-injury, but was unaffected by the level of shortening. Relative isometric torque deficits were noted in the 0 mm group only 24 h post-injury. Peak isokinetic torque during injury was similar in all groups. Histological evaluation 24 h post-injury revealed increased morphological damage near the MTJ in the AT shortened groups.

Conclusion

Single stretch with AT shortening created morphological damage near the MTJ and isometric torque deficits immediately and 24 h post-injury, but the magnitude of damage could not be titrated with stepped increases in AT shortening. This model provides an opportunity to utilize transgenic mice in order to elucidate inflammatory mediators that promote regeneration and inhibit fibrosis in order to optimize therapeutic interventions for complete functional recovery.

Motor unit activity after eccentric exercise and muscle damage in humans

It is well known that unaccustomed eccentric exercise leads to muscle damage and soreness, which can produce long-lasting effects on muscle function. How this muscle damage influences muscle activation is poorly understood. The purpose of this brief review is to highlight the effect of eccentric exercise on the activation of muscle by the nervous system, by examining the change in motor unit activity obtained from surface electromyography (EMG) and intramuscular recordings. Previous research shows that eccentric exercise produces unusual changes in the EMG–force relation that influences motor performance during isometric, shortening and lengthening muscle contractions and during fatiguing tasks. When examining the effect of eccentric exercise at the single motor unit level, there are substantial changes in recruitment thresholds, discharge rates, motor unit conduction velocities and synchronization, which can last for up to 1 week after eccentric exercise. Examining the time course of these changes suggests that the increased submaximal EMG after eccentric exercise most likely occurs through a decrease in motor unit conduction velocity and an increase in motor unit activity related to antagonist muscle coactivation and low-frequency fatigue. Furthermore, there is a commonly held view that eccentric exercise produces preferential damage to high-threshold motor units, but the evidence for this in humans is limited. Further research is needed to establish whether there is preferential damage to high-threshold motor units after eccentric exercise in humans, preferably by linking changes in motor unit activity with estimates of motor unit size using selective intramuscular recording techniques.

Physiological responses to an intensified period of rugby league competition

This study investigated the physiological responses to an intensified period of rugby league competition and the subsequent impact on match performance. The participants were 7 rugby league players competing in an international student tournament. The tournament involved three 80-minute games over a 5-day period, with 48 hours between each match. Baseline measures of upper and lower body neuromuscular functions via a plyometric press-up (PP) and countermovement jump (CMJ), respectively (peak power and peak force were measured), blood creatine kinase (CK), and perceptions of well-being were assessed with a questionnaire. These measures were repeated every morning of the competition; neuromuscular fatigue and CK were additionally assessed within 2 hours after the cessation of each game. During each match, player movements were recorded via global positioning system units. There were meaningful reductions in upper (effect size [ES] = -0.55) and lower body (ES = -0.73) neuromuscular functions, and perceptual well-being (ES = -1.56) and increases in blood CK (ES = 2.32) after game 1. These changes increased in magnitude as the competition progressed. There were large reductions in the relative distance covered in high-speed running (ES = -1.49) and maximal accelerations (ES = -0.85) during game 3. Additionally, moderate reductions in the percentage of successful tackles completed were observed during game 3 (ES = -0.59). Collectively, these results demonstrate that during an intensified period of rugby league competition, characterized by only 48 hours between matches, fatigue will accumulate. This cumulative fatigue may compromise high-intensity match activities such as high-speed running, accelerations, and tackling. Furthermore, CMJs and PPs appear to be sensitive measures for monitoring neuromuscular function in rugby league players.

Changes in kinematic variables at various muscle lengths of human elbow flexors following eccentric exercise

Exercise-induced muscle damage causes a disproportionally larger drop in maximal force when measured at short versus optimal or long muscle lengths, resulting in a shift of the length (angle)-force relationship towards longer lengths. However, little attention has been given to the potential effect of this shift on the rate of force development (RFD) and isotonic function at different muscle lengths. This study examined RFD at various elbow angles and kinematic variables at two different ranges of elbow flexion, so as to include mainly the ascending (S condition) or the descending limb (L condition) of the angle-force curve, following eccentric exercise. Seven male volunteers performed an eccentric exercise protocol with the elbow flexors, which caused significant changes in indicators of muscle damage (P < 0.05-0.001). Optimum angle for force generation was significantly shifted towards longer elbow flexors lengths post exercise (P < 0.05-0.01). RFD was significantly decreased at all the angles tested but no differences were revealed between angles (P < 0.05-0.001). The kinematic variables measured were also significantly impaired following eccentric damage (P < 0.05-0.001). Maximal isotonic force showed a greater impairment in the S condition, however no significant differences between the S and L condition were found in maximal angular velocity (MAV) and time, angle and isotonic force needed to achieve MAV. These results suggest that impairment of RFD following muscle damage is not muscle-length dependent and the rightward shift of the angle-force curve is not the determinant of the decline in either RFD or the isotonic performance at the different ranges of the elbow flexion movement.

Power loss is greater following lengthening contractions in old versus young women

Compared with isometric and dynamic velocity-constrained (isokinetic) tasks, less is known regarding velocity-dependent (isotonic) muscle power and recovery in older adults following repeated fatiguing lengthening contractions. We investigated voluntary and evoked neuromuscular properties of the dorsiflexors in nine old (68.3 ± 6.1 years) and nine young women (25.1 ± 1.3 years) during and following 150 lengthening contractions for up to 30 min of recovery. At baseline, the old were ~21% weaker for maximum isometric voluntary contraction (MVC) torque (P < 0.05), ~21% slower for peak loaded shortening velocity (P < 0.05), and ~39% less powerful compared with the young (P < 0.05). Following the task, MVC torque was depressed equally (~28%) for both groups (P < 0.05), but power was reduced ~19% in the old and only ~8% in the young (P < 0.05). Both measures remained depressed during the 30-min recovery period. Peak twitch torque (P (t)) was ~50% lower in the old at task termination, whereas the young were unchanged. However, by 5 min of recovery, P (t) was reduced similarly (~50%) in both groups, and neither recovered by 30 min. The old were affected more by low-frequency torque depression than the young, as shown by the ~40% and ~20% decreases in the stimulated 10:50 Hz ratio at task termination respectively, whereas both groups were affected similarly (~50%) 5 min into recovery, and neither recovered by 30 min. Thus, the coexistence of fatigue and muscle damage induced by the repetitive lengthening contractions impaired excitation-contraction coupling and cross-bridge function to a greater extent in the old, leading to a more pronounced initial loss of power than the young for up to 10 min following the exercise However, power remained blunted in both groups during the 30-min recovery period. These results indicate that older women are more susceptible to power loss than young following lengthening contractions, likely owing to a greater impairment in calcium kinetics.

Neuromuscular dysfunction with the experimental arm acting as its own reference following eccentric and isometric exercise

Eccentric exercise has been extensively used as a model to study muscle damage-induced neuromuscular impairment, adopting mainly a bilateral matching task between the reference (unexercised) arm and the indicator (exercised) arm. However, little attention has been given to the muscle proprioceptive function when the exercised arm acts as its own reference. This study investigated muscle proprioception and motor control, with the arm acting both as reference and indicator, following eccentric exercise and compared them with those observed after isometric exercise. Fourteen young male volunteers were equally divided into two groups and performed an eccentric or isometric exercise protocol with the elbow flexors of the non-dominant arm on an isokinetic dynamometer. Both exercise protocols induced significant changes in indicators of muscle damage, that is, muscle soreness, range of motion and maximal isometric force post-exercise (p < 0.05-0.001), and neuromuscular function was similarly affected following both protocols. Perception of force was impaired over the 4-day post-exercise period (p < 0.001), with the applied force being systematically overestimated. Perception of joint position was significantly disturbed (i.e., target angle was underestimated) only at one elbow angle on day 4 post-exercise (p < 0.05). The misjudgements and disturbed motor output observed when the exercised arm acted as its own reference concur with the view that they could be a result of a mismatch between the central motor command and an impaired motor control after muscle damage.

Delayed recovery of velocity-dependent power loss following eccentric actions of the ankle dorsiflexors

Unaccustomed eccentric exercise has been shown to impair muscle function, although little is known regarding this impairment on muscle power. The purpose of this study was to investigate changes in neuromuscular properties of the ankle dorsiflexors during and after an eccentric contraction task and throughout recovery in 21 (10 men, 11 women) recreationally active young adults (25.8+/-2.3 yr). All subjects performed 5 sets of 30 eccentric contractions at 80% of maximum isometric voluntary contraction (MVC) torque. Data were recorded at baseline, during the fatigue task, and for 30 min of recovery. There were no significant sex differences for all fatigue measures; thus data were pooled. After the fatigue task, MVC torque declined by 28% (P<0.05) and did not recover fully, and voluntary activation of the dorsiflexors, as assessed by the interpolated twitch technique, was near maximal (>99%) during and after the fatigue task (P>0.05). Peak twitch torque was reduced by 21% at 2 min of recovery and progressively decreased to 35% by 30 min (P<0.05). Low-frequency torque depression (10-to-50 Hz ratio) was present at 30 s of recovery, increased to 51% by 10 min, and did not recover fully (P<0.05). Velocity-dependent concentric power was reduced by 8% immediately after task termination and did not recover fully within 30 min (P<0.05). The main findings of an incomplete recovery of MVC torque, low-frequency torque depression, and shortening velocity indicate the presence of muscle damage, which may have altered excitation-contraction coupling and cross-bridge kinetics and reduced the number of functional sarcomeres in series, ultimately leading to velocity-dependent power loss.

Exercise-induced homeostatic perturbations provoked by singles tennis match play with reference to development of fatigue

This review addresses metabolic, neural, mechanical and thermal alterations during tennis match play with special focus on associations with fatigue. Several studies have provided a link between fatigue and the impairment of tennis skills proficiency. A tennis player's ability to maintain skilled on-court performance and/or optimal muscle function during a demanding match can be compromised as a result of several homeostatic perturbations, for example hypoglycaemia, muscle damage and hyperthermia. Accordingly, an important physiological requirement to succeed at competitive level might be the player's ability to resist fatigue. However, research evidence on this topic is limited and it is unclear to what extent players experience fatigue during high-level tennis match play and what the physiological mechanisms are that are likely to contribute to the deterioration in performance.

The sarcomeric M-band during development and in disease

The C-terminus of connectin/titin at the M-band of the sarcomere interacts with several structural as well as potential signalling proteins. One of these is myomesin, which can also bind to myosin and has been suggested to function as an integral structural linker of the thick filaments into the sarcomere. Recent evidence that myomesin possesses the ability to form antiparallel dimers via its C-terminal domain has prompted us to propose a novel three-dimensional model for the sarcomeric M-band. A splice variant of myomesin, termed EH-myomesin, contains an additional segment that has disordered conformation and functions as an entropic spring. It is expressed in a subset of muscle types that are characterised by a broader operational range and are more resistant to damage caused by eccentric contraction. In addition, it is also re-expressed in dilated cardiomyopathy. DRAL/FHL-2 is another protein that interacts with the M-band portion of connectin/titin and which probably functions as an adaptor for the compartmentalisation of metabolic enzymes. Together these results suggest that the M-band is crucial for sarcomere function and maintenance and that its molecular composition can be adapted to divergent physiological needs in different muscle types, which may help to cope with pathological alterations.

Altering the Length-Tension Relationship with Eccentric Exercise

The effects of eccentric exercise on muscle injury prevention and athletic performance are emerging areas of interest to researchers. Of particular interest are the adaptations that occur after a single bout, or multiple bouts of eccentric exercise. It has been established that after certain types of eccentric exercise, the optimum length of tension development in muscle can be shifted to longer muscle lengths. Altering the length-tension relationship can have a profound influence on human movements. It is thought that the length-tension relationship is influenced by the structural makeup of muscle. However, the mechanism responsible for the shift in optimum length is not readily agreed upon. Despite the conflict, several studies have reported a shift in optimum length after eccentric exercise. Unfortunately, very few of these studies have been randomised, controlled training studies, and the duration of the shift has not yet been established. Nonetheless, this adaptation may result in greater structural stability at longer muscle lengths and consequently may have interesting implications for injury prevention and athletic performance. Both contentions remain relatively unexplored and provide the focus of this review.

The shift in muscle's length-tension relation after exercise attributed to increased series compliance

Eccentric exercise can produce damage to muscle fibres. Here damage indicators are measured in the medial gastrocnemius muscle of the anaesthetised cat after eccentric contractions on the descending limb of the muscle's length-tension relation, compared with eccentric contractions on the ascending limb and concentric contractions on the descending limb. One damage indicator is a shift of the optimum length for peak active tension, in the direction of longer muscle lengths. The shift has been attributed to an increase in muscle compliance. It is a corollary of a current theory for the mechanism of the damage. With the intention of seeking further support for the theory, in these experiments we test the idea that other damage indicators, specifically the fall in twitch:tetanus ratio and in muscle force are due, in part, to such an increase in compliance. This was tested in an undamaged muscle by insertion of a compliant spring (0.19 mm N(-1)) in series with the muscle. This led to a fall in tetanic tension by 17%, a shift in optimum length of 1.7 mm in the direction of longer muscle lengths and a fall in twitch tetanus ratio by 15%. The fall in tension is postulated to be due to development of non-uniform sarcomere lengths within muscle fibres. It is concluded that after a series of eccentric contractions of a muscle, the fall in force is the result of a number of interdependent factors, not all of which are a direct consequence of the damage process.

Stretch-induced force deficits in murine extensor digitorum longus muscles after cardiotoxin injection

A leftward shift in a muscle's length-tension relationship is thought to impair myofilament overlap. We hypothesized that left-shifted muscles would incur greater eccentric contraction-induced damage compared to controls. We evaluated contractile properties and force deficits in regenerating murine extensor digitorum longus (EDL) muscles 7, 14, and 21 days after cardiotoxin (CTX) injection. Specific tension recovered to control values by 21 days. CTX-injected muscles demonstrated left-shifted length-tension curves and incurred greater contraction-induced force deficits than controls (P < 0.001) on day 7. We speculate that increased contraction-induced damage in 7-day CTX-injected muscles results from changes in myofilament overlap that occurs during early regeneration.

Soleus muscle force following downhill running in ovariectomized rats treated with estrogen

The ovariectomized (OVX) rat model was used to investigate the effects of estrogen treatment on soleus muscle functionality in situ following muscle injury induced by downhill running. Fifty immature, 24- to 26-d-old, OVX rats were randomly assigned to 5 separate experimental groups: sedentary controls (OVX-Sed), placebo-treated and studied immediately after exercise (OVX-Pb0), placebo-treated and studied 72 h after exercise (OVX-Pb72), estradiol-treated and studied immediately after exercise (OVX-Ed0), and estradiol-treated and studied 72 h after exercise (OVX-Ed72). At the age of 9 weeks, under ketamine and xylazine anesthesia i.p., the rats were subcutaneously implanted with either placebo or 17beta-estradiol-impregnated pellets (0.05 mg/pellet, 3 week release). Treatment with 17beta-estradiol increased the estradiol plasma levels in OVX animals to those normally seen during the proestrous cycle of normal animals. Three weeks after the implantation the rats were subjected to a 90 min intermittent downhill running protocol. Our results indicate that the exercise protocol used in the study induced injury in the soleus muscle, as it was detected by the significant reduction in unfused (stimulation at 10, 20, and 40 Hz) and maximal (Po) tetanic force, as well as the decreased ability of the soleus muscle to maintain tension (stimulation at 40 Hz for 3 min) in OVX-Pb0 and OVX-Pb72 placebo-treated animals subjected to downhill running (injured muscles) as compared with OVX-Sed control rats (uninjured muscle). Estradiol replacement in OVX rats partially protected the soleus muscle from the injury normally induced by downhill running. As compared with the OVX-Pb0 and OVX-Pb72 placebo-treated groups, the soleus muscles of OVX-Ed0 and OVX-Ed72 estradiol-treated rats were capable of producing significantly greater unfused tetanic force and had an increased ability to maintain tension after fatigue. However, estrogen at the dose administered did not prevent the decrease in maximal tetanic force. We postulate that the protective effect of estrogens on muscle strength may be related to the ability of estrogen hormones to attenuate the E--C coupling failure and (or) the disorganization of the contractile apparatus associated with eccentric exercise through a mechanism or mechanisms yet to be fully understood.

Is the force-length relationship a useful indicator of contractile element damage following eccentric exercise?

Eccentric exercise has been shown to have a measurable effect on the force-length relationship (FLR), as peak force is shifted to longer muscle lengths following exercise. Recently, this shift in the FLR has been proposed as a "simple, reliable indicator" for assessing contractile element damage following eccentric exercise. However, eccentric exercise causes fatigue and damage, and there is evidence that fatigue alone may also cause a shift in the FLR. The purpose of this paper was to assess the role of fatigue on the FLR (as measured by a torque-joint angle relationship) following isometric and eccentric exercise in the New Zealand white (NZW) rabbit. Six NZW rabbits were divided into two groups for eccentric or isometric contractions of the hindlimb dorsiflexor muscles. Pre- and post-exercise torque-joint angle relationships were measured, and the shift from the pre- to the post-exercise relationship was measured as the change in joint angle at which peak torque was produced. Eccentric exercise resulted in a rightward shift of seven degrees; isometric exercise, which is thought to not cause damage, resulted in a shift of four degrees. Furthermore, torque production was reduced to a greater extent at short compared to long muscle lengths for the eccentric and isometric exercise, resulting in a post-exercise torque-joint angle relationship that was altered in shape. We conclude from these results, that the shift in peak torque may not be a simple and reliable indicator of muscle damage, but is caused by a combination of damage and post-exercise fatigue.

The magnitude of muscle strain does not influence serial sarcomere number adaptations following eccentric exercise

It is generally accepted that eccentric exercise, when performed by a muscle that is unaccustomed to that type of contraction, results in a delayed onset of muscle soreness (DOMS). A prolonged exposure to eccentric exercise leads to the disappearance of the signs and symptoms associated with DOMS, which has been referred to as the repeated bout effect (RBE). Although the mechanisms underlying the RBE remain unclear, several mechanisms have been proposed, including the serial sarcomere number addition following exercise induced muscle damage. In the traditional DOMS and RBE protocols, muscle injury has been treated as a global parameter, with muscle force and strain assumed to be uniform throughout the muscle. To assess the effects of muscle-tendon unit strain, fiber strain, torque and injury on serial sarcomere number adaptations, three groups of New Zealand White (NZW) rabbits were subjected to chronic repetitive eccentric exercise bouts of the ankle dorsiflexors for 6 weeks. These eccentric exercise protocols consisted of identical muscle tendon unit (MTU) strain, but other mechanical factors were systematically altered. Following chronic eccentric exercise, serial sarcomere number adaptations were not identical between the three eccentric exercise protocols, and serial sarcomere number adaptations were not uniform across all regions of the muscle. Peak torque and relaxation fiber strain were the best predictors of serial sarcomere number across all three protocols. Therefore, MTU strain does not appear to be the primary cause for sarcomerogenesis, and differential adaptations within the muscle may be explained by the nonuniform architecture of the muscle, resulting in differential local fiber strains.

The effects of exercise-induced muscle damage on maximal intensity intermittent exercise performance

Exercise-induced muscle damage (EIMD) is a common occurrence following activities with a high eccentric component. Alterations to the torque-velocity relationship following EIMD would appear to have serious implications for athletic performance, particularly as they relate to impairment of maximal intensity exercise. However, this has been studied infrequently. The purpose of this study was to assess the effects of EIMD on maximal intermittent sprint performance. Ten male participants (age 22.4 +/- 3.2 years, height 178.6 +/- 5.2 cm, mass 80.6 +/- 10.7 kg) performed 10 x 6 s cycle ergometer sprints, interspersed with 24 s recovery against a load corresponding to 0.10 kp/kg and 10 x 10 m sprints from a standing start, each with 12 s active (walking) recovery. All variables were measured immediately before and at 30 min, 24, 48 and 72 h following a plyometric exercise protocol comprising of 10 x 10 maximal counter movement jumps. Repeated measures ANOVA showed significant changes over time (all P<0.05) for perceived soreness, plasma creatine kinase activity (CK), peak power output (PPO), sprint time and rate of fatigue. Soreness was significantly higher (P<0.01) than baseline values at all time intervals (3.1, 4.9, 5.5 and 3.2 at 30 min, 24, 48 and 72 h, respectively). CK was significantly elevated (P<0.05) at 24 h (239 IU/l) and 48 h (245 IU/l) compared to baseline (151 IU/l). PPO was significantly lower (P<0.05) than baseline (1,054 W) at all time intervals (888, 946, 852 and 895 W, at 30 min, 24, 48 and 72 h, respectively). The rate of fatigue over the ten cycling sprints was reduced compared to baseline, with the greatest reduction of 48% occurring at 48 h (P<0.01). This was largely attributed to the lower PPO in the initial repetitions, resulting in a lower starting point for the rate of fatigue. Values returned to normal at 72 h. Sprint times over 10 m were higher (P<0.05) at 30 min, 24 h and 48 h compared to baseline (1.96 s) with values corresponding to 2.01, 2.02 and 2.01 at 30 min, 24 h and 48 h, respectively. Values returned to baseline by 72 h. The results provide further evidence that, following a plyometric, muscle-damaging exercise protocol, the ability of the muscle to generate power is reduced for at least 3 days. This is also manifested by a small, but statistically significant reduction in very short-term (approximately 2 s) intermittent sprint running performance. These findings have implications for appropriate training strategies in multiple sprint sports.

The management of hamstring injury—Part 1: Issues in diagnosis

Hamstring injuries are the most prevalent muscle injury in sports involving rapid acceleration and maximum speed running. Injury typically occurs in an acute manner through an eccentric mechanism at the terminal stages of the swing phase of gait. Biceps femoris is most commonly injured. Re-injury rates are high and management is a challenge given the complex multi-factorial aetiology. The high rates of hamstring injury and re-injury may result from a lack of high-quality research into the aetiological factors underlying injury. Re-injury may also result from inaccuracy in diagnosis that results from the potential multi-factorial causes of these conditions. Inaccuracy in diagnosis could lead to multiple potential diagnoses that may result in the implementation of variable management protocols. Whilst potentially useful, such variability may also lead to the implementation of sub-optimal management strategies. Previous hamstring injury is the most recognized risk factor for injury, which indicates that future research should be directed at preventative measures. Much anecdotal and indirect evidence exists to suggest that several non-local factors contribute to injury, which may be addressed through the application of manual therapy. However, this connection has been neglected in previous research and literature. This paper will explore and speculate on this potential connection and offer some new contributive factors for hamstring injury management. This first paper of a two part series on hamstring injury will explore diagnostic issues relevant to hamstring injury and the second will investigate various established and speculative management approaches.

Popping sarcomere hypothesis explains stretch-induced muscle damage

1. Exercise that involves stretching a muscle while active cause microscopic areas of damage, delayed onset muscle soreness and adaptation to withstand subsequent similar exercise. 2. Longer muscle lengths are associated with greater damage and recent animal experiments show that it is the length relative to optimum that determines the damage. 3. In humans, walking down stairs, taking two at a time, increases the length of the muscle during the lengthening and increases the delayed onset muscle soreness. 4. The observed pattern of damage is consistent with explanations based on sarcomere length instabilities. 5. The pattern of adaptation is consistent with the number of sarcomeres in series in a muscle being modulated by exercise, especially the range of muscle lengths over which eccentric exercise regularly occurs.

Force matching errors following eccentric exercise

During eccentric exercise contracting muscles are forcibly lengthened, to act as a brake to control motion of the body. A consequence of eccentric exercise is damage to muscle fibres. It has been reported that following the damage there is disturbance to proprioception, in particular, the senses of force and limb position. Force sense was tested in an isometric force-matching task using the elbow flexor muscles of both arms before and after the muscles in one arm had performed 50 eccentric contractions at a strength of 30% of a maximum voluntary contraction (MVC). The exercise led to an immediate reduction of about 40%, in the force generated during an MVC followed by a slow recovery over the next four days, and to the development of delayed onset muscle soreness (DOMS) lasting about the same time. After the exercise, even though participants believed they were making an accurate match, they made large matching errors, in a direction where the exercised arm developed less force than the unexercised arm. This was true whichever arm was used to generate the reference forces, which were in a range of 5-30% of the reference arm's MVC, with visual feedback of the reference arm's force levels provided to the participant. The errors were correlated with the fall in MVC following the exercise, suggesting that participants were not matching force, but the subjective effort needed to generate the force: the same effort producing less force in a muscle weakened by eccentric exercise. The errors were, however, larger than predicted from the measured reduction in MVC, suggesting that factors other than effort might also be contributing. One factor may be DOMS. To test this idea, force matches were done in the presence of pain, induced in unexercised muscles by injection of hypertonic (5%) saline or by the application of noxious heat to the skin over the muscle. Both procedures led to errors in the same direction as those seen after eccentric exercise.

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

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

M-band: a safeguard for sarcomere stability?

The sarcomere of striated muscle is a very efficient machine transforming chemical energy into movement. However, a wrong distribution of the generated forces may lead to self-destruction of the engine itself. A well-known example for this is eccentric contraction (elongation of the sarcomere in the activated state), which damages sarcomeric structure and leads to a reduced muscle performance. The goal of this review is to discuss the involvement of different cytoskeletal systems, in particular the M-band filaments, in the mechanisms that provide stability during sarcomeric contraction. The M-band is the transverse structure in the center of the sarcomeric A-band, which is responsible both for the regular packing of thick filaments and for the uniform distribution of the tension over the myosin filament lattice in the activated sarcomere. Although some proteins from the Ig-superfamily, like myomesin and M-protein, are the major candidates for the role of M-band bridges, the exact molecular organisation of the M-band is not clear. However, the protein composition of the M-band seems to modulate the mechanical characteristics of the thick filament lattice, in particular its stiffness, adjusting it to the specific demands in different muscle types. The special M-band design in slow fibers might be part of structural adaptations, favouring sarcomere stability for a continuous contractile activity over a broad working range. In conclusion, we discuss why the interference with M-band structure might have fatal consequences for the integrity of the working sarcomere.

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

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

Low-frequency depression of tension in the cat gastrocnemius muscle after eccentric exercise

Subjecting a muscle to a series of eccentric contractions in which the contracting muscle is lengthened results in a number of changes in its mechanical properties. These include a fall in isometric tension that is particularly pronounced during low-frequency stimulation, a phenomenon known as low-frequency depression (LFD). Reports of LFD have not taken into account the shift in optimum length for active tension generation to longer muscle lengths that takes place after eccentric contractions. Given the length dependence of the stimulation frequency-tension curve, we tested the hypothesis that the change in this relationship after eccentric exercise is due to the shift in optimum length. We measured LFD by recording tension in response to a linearly increasing rate of stimulation of the nerve to medial gastrocnemius of anesthetized cats, over the range 0-100 pulses per second. Tension responses were measured before and after 50 eccentric contractions consisting of 6-mm stretches starting at 3 mm below optimum length and finishing at 3 mm above it. An index of LFD was derived from the tension responses to ramp stimulation. It was found that LFD after the eccentric contractions was partly, but not entirely, due to changes in the muscle's optimum length. An additional factor was the effect of fatigue. These observations led to the conclusion that the muscle length dependence of LFD was reduced by eccentric contractions. All of this means that after eccentric exercise the tension deficit at low rates of muscle activation is likely to be less severe than first thought.