Maximal Eccentric Contractions Research Articles

Induction and Assessment of Exertional Skeletal Muscle Damage in Humans

Contraction-induced muscle damage via voluntary eccentric (lengthening) contractions offers an excellent model for studying muscle adaptation and recovery in humans. Herein we discuss the design of an eccentric exercise protocol to induce damage in the quadriceps muscles, marked by changes in strength, soreness, and plasma creatine kinase levels. This method is simple, ethical, and widely applicable since it is performed in human participants and eliminates the interspecies translation of the results. Subjects perform 300 maximal eccentric contractions of the knee extensor muscles at a speed of 120°/sec on an isokinetic dynamometer. The extent of the damage is measurable using relatively non-invasive isokinetic and isometric measures of strength loss, soreness, and plasma creatine kinase levels over several days following the exercise. Therefore, its application can be directed to specific populations in an attempt to identify mechanisms for muscle adaptation and regeneration.

Induction and Assessment of Exertional Skeletal Muscle Damage in Humans.

Contraction-induced muscle damage via voluntary eccentric (lengthening) contractions offers an excellent model for studying muscle adaptation and recovery in humans. Herein we discuss the design of an eccentric exercise protocol to induce damage in the quadriceps muscles, marked by changes in strength, soreness, and plasma creatine kinase levels. This method is simple, ethical, and widely applicable since it is performed in human participants and eliminates the interspecies translation of the results. Subjects perform 300 maximal eccentric contractions of the knee extensor muscles at a speed of 120°/sec on an isokinetic dynamometer. The extent of the damage is measurable using relatively non-invasive isokinetic and isometric measures of strength loss, soreness, and plasma creatine kinase levels over several days following the exercise. Therefore, its application can be directed to specific populations in an attempt to identify mechanisms for muscle adaptation and regeneration.

Neuromuscular Changes and Damage after Isoload versus Isokinetic Eccentric Exercise.

This study compared the effects of isoload (IL) and isokinetic (IK) knee extensor eccentric exercises on changes in muscle damage and neuromuscular parameters to test the hypothesis that the changes would be different after IL and IK exercises. Twenty-two young men were paired based on their strength and placed in the IL (N = 11) or the IK (N = 11) group. The IL group performed 15 sets of 10 eccentric contractions with a 150% of predetermined one-repetition maximum load. The IK group performed 15 sets of several maximal eccentric contractions matched set by set for the total amount of work and mean angular velocity with the IL group. Muscle damage markers (voluntary isometric peak torque, muscle soreness, and creatine kinase activity) and neuromuscular variables (e.g., voluntary activation, H-reflex, M-wave, and evoked torque) were measured before, immediately after, and 24, 48, 72, and 96 h postexercise. Voluntary isometric peak torque decreased to the same extent (P = 0.94) in both groups immediately after (IL = -40.6% ± 13.8% vs IK = -42.4% ± 10.2%) to 96 h after the exercise (IL = -21.8% ± 28.5% vs IK = -26.7% ± 23.5%). Neither peak muscle soreness (IL = 48.1 ± 28.2 mm vs IK = 54.7 ± 28.9 mm, P = 0.57) nor creatine kinase activity (IL = 12,811 ± 22,654 U·L vs IK = 15,304 ± 24,739 U·L, P = 0.59) significantly differed between groups. H-reflex (IL = -23% vs IK = -35%) and M-wave (IL = -10% vs IK = -17%) significantly decreased immediately postexercise similarly between groups. The changes in muscle damage and neuromuscular function after the exercise are similar between IL and IK, suggesting that resistance modality has little effects on acute muscle responses.

Utility of 17-(allylamino)-17-demethoxygeldanamycin treatment for skeletal muscle injury.

Repeated eccentric contractions can injure skeletal muscle and result in functional deficits that take several weeks to fully recover. The 70-kDa heat shock protein (Hsp70) is a stress-inducible molecular chaperone that maintains protein quality and plays an integral role in the muscle's repair processes following injury. Here, we attempted to hasten this recovery by pharmacologically inducing Hsp70 expression in mouse skeletal muscle with 17-(allylamino)-17-demethoxygeldanamycin (17-AAG) (40mg/kg) both prior to and throughout the first 7days after an injurious bout of 150 maximal eccentric contractions. Hsp70 content in the injured skeletal muscle was strongly induced following the eccentric contractions and remained elevated over the next 7days as the muscle underwent repair. Treatment with 17-AAG increased Hsp70 content ∼fivefold; however, this was significantly less than that induced by the injury. Moreover, 17-AAG treatment did not recover the decrements to in vivo isometric torque production following the bout of eccentric contractions. Together, these findings demonstrate that although Hsp70 content was induced in the uninjured skeletal muscle, treatment of 17-AAG (40mg/kg) was not a preventive measure to either reduce the severity of skeletal muscle damage or enhance functional recovery following a bout of maximal eccentric contractions.

Protective effect by maximal isometric contractions against maximal eccentric exercise-induced muscle damage of the knee extensors

ABSTRACTThis study investigated whether maximal voluntary isometric contractions (MVIC) performed before maximal eccentric contractions (MaxEC) would attenuate muscle damage of the knee extensors. Untrained men were placed to an experimental group that performed 6 sets of 10 MVIC at 90° knee flexion 2 weeks before 6 sets of 10 MaxEC or a control group that performed MaxEC only (n = 13/group). Changes in muscle damage markers were assessed before to 5 days after each exercise. Small but significant changes in maximal voluntary concentric contraction torque, range of motion (ROM) and plasma creatine kinase (CK) activity were evident at immediately to 2 days post-MVIC (p < 0.05), but other variables (e.g. thigh girth, myoglobin concentration, B-mode echo intensity) did not change significantly. Changes in all variables after MaxEC were smaller (p < 0.05) by 45% (soreness)–67% (CK) for the experimental than the control group. These results suggest that MVIC conferred potent protective effect against MaxEC-induced muscle damage.

Stretching of Active Muscle Elicits Chronic Changes in Multiple Strain Risk Factors.

The muscle stretch intensity imposed during "flexibility" training influences the magnitude of joint range of motion (ROM) adaptation. Thus, stretching while the muscle is voluntarily activated was hypothesized to provide a greater stimulus than passive stretching. The effect of a 6-wk program of stretch imposed on an isometrically contracting muscle (i.e., qualitatively similar to isokinetic eccentric training) on muscle-tendon mechanics was therefore studied in 13 healthy human volunteers. Before and after the training program, dorsiflexion ROM, passive joint moment, and maximal isometric plantarflexor moment were recorded on an isokinetic dynamometer. Simultaneous real-time motion analysis and ultrasound imaging recorded gastrocnemius medialis muscle and Achilles tendon elongation. Training was performed twice weekly and consisted of five sets of 12 maximal isokinetic eccentric contractions at 10°·s. Significant increases (P < 0.01) in ROM (92.7% [14.7°]), peak passive moment (i.e., stretch tolerance; 136.2%), area under the passive moment curve (i.e., energy storage; 302.6%), and maximal isometric plantarflexor moment (51.3%) were observed after training. Although no change in the slope of the passive moment curve (muscle-tendon stiffness) was detected (-1.5%, P > 0.05), a significant increase in tendon stiffness (31.2%, P < 0.01) and a decrease in passive muscle stiffness (-14.6%, P < 0.05) were observed. The substantial positive adaptation in multiple functional and physiological variables that are cited within the primary etiology of muscle strain injury, including strength, ROM, muscle stiffness, and maximal energy storage, indicate that the stretching of active muscle might influence injury risk in addition to muscle function. The lack of change in muscle-tendon stiffness simultaneous with significant increases in tendon stiffness and decreases in passive muscle stiffness indicates that tissue-specific effects were elicited.

Vastus Intermedius And Vastus Lateralis Fascicle Length Changes During Maximal Eccentric Knee Extensor Contractions

Understanding fascicle behaviors during muscle contraction is essential for our understanding of muscle function. Several studies have examined fascicle behaviors during contractions of different muscles, however little is known about vastus intermedius (VI) fascicle behavior during eccentric contractions. The VI fascicles originate directly on the femur, but those of vastus lateralis (VL) originate on a compliant aponeurosis. Thus, it is assumed that fascicle behavior is different between VI and VL during eccentric knee extensor contractions. PURPOSE: This study compared VI and VL fascicle length changes during maximal eccentric knee extensor contractions using ultrasonography. METHODS: Thirteen healthy men (age: 27.1 ± 3.5 y, height: 176.1 ± 6.1 cm, body mass: 76.0 ± 7.9 kg) performed passive knee flexions and maximal eccentric knee extensor contractions through the same knee joint range of motion from 35° to 105° (0° = full extension) at an angular velocity of 30°/s. B-mode ultrasound images of VI and VL were simultaneously recorded at 30 Hz using two serially-arranged ultrasound probes (11 cm imaging window) placed on the skin surface over VL. VI and VL fascicle lengths at knee joint angles of 40° and 100° were measured from the images, and the magnitude of fascicle lengthening from 40° to 100° was calculated. RESULTS: VI and VL fascicle lengths at 40° were 92 ± 13 mm and 99 ± 8 mm during passive knee flexion, and 72 ± 7 mm and 75 ± 8 mm during eccentric contraction, respectively. VI and VL fascicles were lengthened similarly during passive knee flexion (VI: 36 ± 5 mm, VL: 33 ± 9 mm), but VI fascicles were lengthened more (35 ± 9 mm) than VL fascicles (24 ± 5 mm) during eccentric contraction (P<0.05). The magnitude of fascicle lengthening was similar between passive knee flexion and eccentric contraction for VI, but was smaller during eccentric contraction than passive knee flexion for VL (P<0.05). CONCLUSIONS: These results show that VI fascicles are lengthened more than VL fascicles during eccentric contractions. This may suggest that either VI is less activated or VI receives greater mechanical stress during eccentric contraction when compared with VL. Further study is necessary to investigate how the different fascicle behaviors affect the magnitude of muscle damage induced by eccentric contractions between the muscles.

The Contribution of the Neuromuscular System in the Repeated Bout Effect

Maximal eccentric contractions induce an immediate strength loss that results from skeletal muscle damage, the symptoms of which can persist for several days after the exercise insult. The repeated bout effect (RBE) refers to the acute adaptive response precipitated from a single bout of eccentric contractions that result in an attenuated damage response on subsequent, potentially damaging, bouts of exercise. The mechanisms for this adaptive response are thought to be, at least in part, from neural adaptations.

Time-Course of Neuromuscular Changes during and after Maximal Eccentric Contractions.

This study tested the relationship between the magnitude of muscle damage and both central and peripheral modulations during and after eccentric contractions of plantar flexors. Eleven participants performed 10 sets of 30 maximal eccentric contractions of the plantar flexors at 45°·s−1. Maximal voluntary torque, evoked torque (peripheral component) and voluntary activation (central component) were assessed before, during, immediately after (POST) and 48 h after (48 h) the eccentric exercise. Voluntary eccentric torque progressively decreased (up to −36%) concomitantly to a significant alteration of evoked torque (up to −34%) and voluntary activation (up to −13%) during the exercise. Voluntary isometric torque (−48 ± 7%), evoked torque (−41 ± 14%) and voluntary activation (−13 ± 11%) decreased at POST, but only voluntary isometric torque (−19 ± 6%) and evoked torque (−10 ± 18%) remained depressed at 48 h. Neither changes in voluntary activation nor evoked torque during the exercise were related to the magnitude of muscle damage markers, but the evoked torque decrement at 48 h was significantly correlated with the changes in voluntary activation (r = −0.71) and evoked torque (r = 0.77) at POST. Our findings show that neuromuscular responses observed during eccentric contractions were not associated with muscle damage. Conversely, central and peripheral impairments observed immediately after the exercise reflect the long-lasting reduction in force-generating capacity.

Increases in M-wave latency of biceps brachii after elbow flexor eccentric contractions in women

Eccentric contractions (ECCs) induce muscle damage that is indicated by prolonged loss of muscle function and delayed onset muscle soreness. It is possible that ECCs affect motor nerves, and this may contribute to the prolonged decreases in force generating capability. The present study investigated the hypothesis that M-wave latency of biceps brachii would be increased after maximal elbow flexor ECCs resulting in prolonged loss of muscle strength. Fifteen women performed exercise consisting of 60 maximal ECCs of the elbow flexors using their non-dominant arm. M-wave latency was assessed by the time taken from electrical stimulation applied to the Erb's point to the onset of M-wave of the biceps brachii before, immediately after, and 1-4days after exercise. Maximal voluntaryisometric contraction (MVC) torque, range of motion (ROM) and muscle soreness using a numerical rating scale were also assessed before and after exercise. Prolonged decreases in MVC torque (1-4days post-exercise: -54 to -15%) and ROM (1-2days: -32 to -22%), and increased muscle soreness (peak: 4.2 out of 10) were evident after exercise (p<0.05). The M-wave latency increased (p<0.01) from 5.8±1.0ms before exercise to 6.5±1.7ms at 1day and 7.2±1.5ms at 2days after exercise for the exercised arm only. No significant changes in M-wave amplitude were evident after exercise. The increased M-wave latency did not fully explain the prolonged decreases in MVC torque after eccentric exercise, but may indicate reversible motor nerve impairment.

Are the Responses to Resistance Training Different Between the Preferred and Nonpreferred Limbs?

Humans preferentially recruit limbs to functionally perform a range of daily tasks, which may lead to performance asymmetries. Because initial training status plays an important role in the rate of progression during resistance training, could asymmetries between the preferred and nonpreferred limbs lead to different magnitudes of strengthening during a resistance training program? This issue motivated this study, in which 12 healthy and physically active men completed a 4-week control period followed by a 12-week isokinetic resistance training program, performed twice a week, including 3-5 sets of 10 maximal eccentric contractions for each limb. Every 4 weeks, knee extensor peak torques at concentric, isometric, and eccentric tests were measured using an isokinetic dynamometer and the sum of quadriceps muscle thickness was determined by ultrasound images. Before training, concentric peak torque was similar between limbs but isometric and eccentric peak torques were significantly smaller in the nonpreferred compared with the preferred limb (4.9 and 5.8%, respectively). Bilateral strength symmetry remained constant throughout the training period for concentric tests. For eccentric and isometric tests, symmetry was reached at the fourth and eighth training weeks, respectively. After 12 weeks, between-limb percent nonsignificant differences were -0.62% for isometric and -1.93% for eccentric tests. The sum of knee extensor muscle thickness had similar values before training and presented similar changes throughout the study for both the preferred and the nonpreferred limbs. In conclusion, the nonpreferred limb presents higher strength gain than the preferred limb at the initial phase of an isokinetic resistance training program, and this increased strength gain is not associated with muscle hypertrophy.

Muscle force loss and soreness subsequent to maximal eccentric contractions depend on the amount of fascicle strain invivo.

Defining the origins of muscle injury has important rehabilitation and exercise applications. However, current knowledge of muscle damage mechanics in human remains unclear invivo. This study aimed to determine the relationships between muscle-tendon unit mechanics during maximal eccentric contractions and the extent of subsequent functional impairments induced by muscle damage. The length of the muscle-tendon unit, fascicles and tendinous tissues was continuously measured on the gastrocnemius medialis using ultrasonography, in time with torque, during 10 sets of 30 maximal eccentric contractions of plantar flexors at 45°s(-1) , in seventeen participants. Muscle-tendon unit, fascicles and tendinous tissues were stretched up to 4.44±0.33cm, 2.31±0.64cm and 1.92±0.61cm respectively. Fascicle stretch length, lengthening amplitude and negative fascicle work beyond slack length were significantly correlated with the force decrease 48h post-exercise (r=0.51, 0.47 and 0.68, respectively; P<0.05). This study demonstrates that the strain applied to human muscle fibres during eccentric contractions strongly influences the magnitude of muscle damage invivo. Achilles tendon compliance decreases the amount of strain, while architectural gear ratio may moderately contribute to attenuating muscle fascicle lengthening and hence muscle damage. Further studies are necessary to explore the impact of various types of task to fully understand the contribution of muscle-tendon interactions during active lengthening to muscle damage.

Undesirable effects of the eccentric isokinetic contractions

Even though sub-maximal eccentric exercises do not entail specific risks, the eccentric contractions at maximal intensity could increase the risk of injury in comparison with the other contraction modes. Based on the length-tension relationship, the maximal eccentric contraction allows to develop the highest torque (for instance on the isokinetic dynamometer), particularly in the elongated position of the muscle groups. Therefore, the injury risk is increased for the muscle, tendon and joint structures. In spite of specific safety mechanics inherent to the isokinetic tool, the risk of injury must be taken into consideration by the clinician. A quadriceps injury was reported in a professional soccer player subjected to eccentric testing of this muscle group at the occasion of a possible transfer. Do we consider such a testing as relevant? After surgical repair of the ACL, we refer to the possible risk for the graft in the case of maximal eccentric contraction of the quadriceps. Complaints frequently reported during eccentric contractions of the shoulder rotators are sometimes incompatible with testing, because of safety and measurement validity. Such an eccentric testing should concern mainly specialized healthy athletes involved in overhead gestures. The isokinetic dynamometer is also used for eccentric training and not only for testing. Strengthening in eccentric allows frequently improving very efficiently the maximal performance for this contraction mode, yet the improvement extent for the other modes remains controversial. On the other hand, intense and unusual eccentric exercises are known to induce Delayed Onset Muscle Soreness (DOMS). Although functional consequences of DOMS are transitory, they could disturb the rehabilitation and/or training process. Clinician and trainers must be aware of this side effect in order to amend their eccentric strengthening interventions.

Differences of activation pattern and damage in elbow flexor muscle after isokinetic eccentric contractions

BACKGROUND: Eccentric contractions (ECC)-induced muscle damage and activation pattern are not fully understood. OBJECTIVE: To investigate the activation level and damage in each muscle and site after ECC. METHODS: Eleven men performed 30 isokinetic ECC of the elbow flexors. Maximal voluntary isometric contraction (MVC) torque, range of motion of the elbow joint (ROM), delayed onset muscle soreness (DOMS), transverse relaxation time (T2), and cross-sectional area (CSA) were measured immediately before, immediately after, and 1, 3, 5 days after ECC. RESULTS: MVC and ROM significantly decreased while DOMS significantly increased until 3 days after ECC. The CSA of the brachialis significantly increased after ECC. The T2 in biceps brachii and middle and distal brachialis increased significantly at post ( p< 0.01) day 1 (middle; p< 0.01, distal; biceps brachii, p< 0.05, brachialis, p< 0.01), and day 3 (middle; p< 0.01, distal; biceps brachii, p< 0.05, brachialis, p< 0.01). In addition, after 5 days only the distal brachialis has shown a significant increase ( p< 0.01). CONCLUSIONS: Following maximal ECC the activation pattern and muscle damage differ among the elbow flexors depending upon the individual muscle and site of recording.

Low-intensity eccentric contractions of the knee extensors and flexors protect against muscle damage.

This study investigated the magnitude and duration of the protective effect of low-intensity eccentric contractions (LowEC) against damage induced by maximal eccentric contractions (MaxEC) of the knee flexors (KF) and extensors (KE). Young men were assigned to 8 experimental groups and 2 control groups (n = 13/group); the experimental groups performed LowEC of KF or KE 2 days (2d), 1 week (1wk), 2 weeks (2wk), or 3 weeks (3wk) before MaxEC, while the control groups performed MaxEC of KF or KE without LowEC. The 2d, 1wk, 2wk, and 3wk groups performed 30 LowEC of KF or 60 LowEC of KE with a load of 10% of maximal voluntary isometric contraction strength on a resistance-training machine, and all groups performed 30 MaxEC of KF or 60 MaxEC of KE on an isokinetic dynamometer. Several muscle damage markers were measured from before to 2 days after exercise (LowEC) or from before to 5 days after exercise (MaxEC). No significant changes in any variables were evident after LowEC. The changes in all variables after MaxEC were smaller (P < 0.05) for the 2d and 1wk groups (e.g., peak creatine kinase activity: 1002 ± 501 IU/L; peak muscle soreness: 13 ± 5 mm) than for the control group (peak creatine kinase activity: 3005 ± 983 IU/L; peak muscle soreness 28 ± 6 mm) for both KE and KF. There were no significant differences between the 2d and 1wk groups or among the 2wk, 3wk, and control groups. These results show that LowEC provided 30%-66% protection against damage induced by MaxEC of KF and KE, and the protective effect lasted 1 week.

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.

Associated decrements in rate of force development and neural drive after maximal eccentric exercise.

The present study investigated the changes in contractile rate of force development (RFD) and the neural drive following a single bout of eccentric exercise. Twenty-four subjects performed 15 × 10 maximal isokinetic eccentric knee extensor contractions. Prior to and at 24, 48, 72, 96, and 168 h during post-exercise recovery, isometric RFD (30, 50 100, and 200 ms), normalized RFD [1/6,1/2, and 2/3 of maximal voluntary contraction (MVC)] and rate of electromyography rise (RER; 30, 50, and 75 ms) were measured. RFD decreased by 28-42% peaking at 48 h (P < 0.01-P < 0.001) and remained depressed at 168 h (P < 0.05). Normalized RFD at 2/3 of MVC decreased by 22-39% (P < 0.01), peaked at 72 h and returned to baseline at 168 h. These changes in RFD were associated with a decrease in RER at 48 h-96 h (P < 0.05-P < 0.001). Accumulated changes (area under curve) revealed a greater relative decrease in accumulated RFD at 100 ms by -2727 ± 309 (%h; P < 0.05) and 200 ms by -3035 ± 271 (%h; P < 0.001) compared with MVC, which decreased, by -1956 ± 234 (%h). In conclusion, RFD and RER are both markedly reduced following a bout of maximal eccentric exercise. This association suggests that exercise-induced decrements in RFD can, in part, be explained decrements in neural drive.

Adaptation of local muscle blood flow and surface electromyography to repeated bouts of eccentric exercise.

The aim of this randomized controlled crossover study was to investigate the effect of a bout of unaccustomed eccentric exercise (ECC) followed by a consecutive bout of the same intensity on local muscle blood flow, amplitude, and frequency of the electromyographic (EMG) signal from the exercised tibialis anterior muscle. Sixteen healthy male participants (age, 25.7 (0.6) years; body mass index 24.8 (1) kg·m(-2) participated in this study. Two identical bouts of high-intensity ECC were performed on the tibialis anterior muscle 7 days apart. Control sessions involving no exercise were performed 4 weeks either before or after the exercise sessions. Changes in local total blood flow [ΔtHb], EMG root mean square, and median power frequency were recorded during isometric maximum voluntary contraction of ankle dorsiflexion. Measurements were performed before, immediately after, and the day after both ECCs (ECC1 and ECC2). The participants rested quietly in a chair in the control session. Eccentric exercise 1 led to a significant decrease in [ΔtHb] on the day after (p ≤ 0.05), whereas ECC2 did not. Median power frequency decreased significantly in ECC2 compared with ECC1 (p < 0.01). Root mean square was unchanged in all the instants. The present study showed that adaptation is depicted in the local muscle blood flow and the frequency contents of the EMG after an unaccustomed ECC inducing muscle soreness. These alterations provide a potential mechanism for a rapid adaptation, which decreases susceptibility of the muscle to develop further soreness in the subsequent ECC bout.

Reduced muscle lengthening during eccentric contractions as a mechanism underpinning the repeated-bout effect.

This study investigated biceps brachii distal myotendinous junction (MTJ) displacement during maximal eccentric elbow flexor contractions to test the hypothesis that muscle length change would be smaller (less MTJ displacement) during the second than the first exercise bout. Ten untrained men performed two eccentric exercise bouts (ECC1 and ECC2) with the same arm consisting of 10 sets of six maximal isokinetic (60°/s) eccentric elbow flexor contractions separated by 4 wk. Biceps brachii distal MTJ displacement was assessed using B-mode ultrasonography, and changes in the displacement (muscle length change) from the start to the end of each contraction during each set and over 10 sets were compared between bouts by two-way repeated-measures ANOVA. Several indirect muscle damage markers were also measured and compared between bouts by two-way repeated-measures ANOVA. The magnitude of MTJ displacement (average of six contractions) increased from set 1 (8.2 ± 4.7 mm) to set 10 (16.4 ± 4.7 mm) during ECC1 (P < 0.05), but no significant changes over sets were evident during ECC2 (set 1: 8.5 ± 4.0 mm; set 10: 9.3 ± 3.1 mm). Changes in maximal voluntary isometric contraction strength, range of motion, muscle thickness, ultrasound echo intensity, serum creatine kinase activity, and muscle soreness (visual analog scale) were smaller (P < 0.05) following ECC2 than ECC1, showing less damage in the repeated bout. These results indicate that the magnitude of muscle lengthening was less during the second than the first eccentric exercise bout, which appears to be a mechanism underpinning the repeated-bout effect.

History-dependent force, angular velocity and muscular endurance in ACTN3 genotypes.

This study aimed at determining the influence of the ACTN3 R577X polymorphism on muscle strength and muscle endurance in non-athletic young men. 266 healthy young men were included in this study. Each subject performed maximal isometric, concentric and eccentric contractions of the knee extensor muscles on an isokinetic dynamometer. Force depression, force enhancement and the fatigue index were derived from these data. In addition, handgrip strength, squat jump (SJ) and counter movement jump (CMJ) height were obtained. Our group included 83 RR (31 %), 131 RX (49 %) and 52 XX (20 %) individuals. The muscle bone cross-sectional area of the thigh was 5 % higher in RR compared to XX individuals (P = 0.033). RR genotypes showed 6 % higher handgrip strength compared to the XX group (P = 0.047). They also jumped 5 % higher in both the SJ and CMJ tests (P = 0.029; P = 0.031). No differences were found in force depression, force enhancement, isometric or eccentric strength. The relative concentric knee torque at 200°/s and at 300°/s was 7 and 8 % higher in RR compared to XX genotypes, respectively (P = 0.049; P = 0.048). Also, the fatigue index was found to be 4 % lower in XX genotypes (P = 0.037). Our findings are in agreement with the higher prevalence of the RR genotype in power-oriented activities. The better fatigue index of XX genotypes may be beneficial in endurance-type activities.