Maximal Lengthening Contractions Research Articles

Exosomes Isolated From Platelet-Rich Plasma and Mesenchymal Stem Cells Promote Recovery of Function After Muscle Injury

Background: Clinical use of platelet-rich plasma (PRP) and mesenchymal stem cells (MSCs) has gained momentum as treatment for muscle injuries. Exosomes, or small cell–derived vesicles, could be helpful if they could deliver the same or better physiological effect without cell transplantation into the muscle. Hypothesis: Local delivery of exosomes derived from PRP (PRP-exos) or MSCs (MSC-exos) to injured muscles hastens recovery of contractile function. Study Design: Controlled laboratory study. Methods: In a rat model, platelets were isolated from blood, and MSCs were isolated from bone marrow and expanded in culture; exosomes from both were isolated through ultracentrifugation. The tibialis anterior muscles were injured in vivo using maximal lengthening contractions. Muscles were injected with PRP-exos or MSC-exos (immediately after injury and 5 and 10 days after injury); controls received an equal volume of saline. Histological and biochemical analysis was performed on tissues for all groups. Results: Injury resulted in a significant loss of maximal isometric torque (66% ± 3%) that gradually recovered over 2 weeks. Both PRP-exos and MSC-exos accelerated recovery, with similar faster recovery of contractile function over the saline-treated group at 5, 10, and 15 days after injury (P < .001). A significant increase in centrally nucleated fibers was seen with both types of exosome groups by day 15 (P < .01). Genes involved in skeletal muscle regeneration were modulated by different exosomes. Muscles treated with PRP-exos had increased expression of Myogenin gene (P < .05), whereas muscles treated with MSC-exos had reduced expression of TGF-β (P < .05) at 10 days after muscle injury. Conclusion: Exosomes derived from PRP or MSCs can facilitate recovery after a muscle strain injury in a small-animal model likely because of factors that can modulate inflammation, fibrosis, and myogenesis. Clinical Relevance: Given their small size, low immunogenicity, and ease with which they can be obtained, exosomes could represent a novel therapy for many orthopaedic ailments.

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

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

Neurophysiological responses and adaptation following repeated bouts of maximal lengthening contractions in young and older adults

A bout of maximal lengthening contractions is known to produce muscle damage, but confers protection against subsequent damaging bouts, with both tending to be lower in older adults. Neural factors contribute to this adaptation, but the role of the corticospinal pathway remains unclear. Twelve young (27 ± 5 yr) and 11 older adults (66 ± 4 yr) performed two bouts of 60 maximal lengthening dorsiflexions 2 weeks apart. Neuromuscular responses were measured preexercise, immediately postexercise, and at 24 and 72 h following both bouts. The initial bout resulted in prolonged reductions in maximal voluntary torque (MVC; immediately postexercise onward, P < 0.001) and increased creatine kinase (from 24 h onward, P = 0.001), with both responses being attenuated following the second bout (P < 0.015), demonstrating adaptation. Smaller reductions in MVC following both bouts occurred in older adults (P = 0.005). Intracortical facilitation showed no changes (P ≥ 0.245). Motor-evoked potentials increased 24 and 72 h postexercise in young (P ≤ 0.038). Torque variability (P ≤ 0.041) and H-reflex size (P = 0.024) increased, while short-interval intracortical inhibition (SICI; P = 0.019) and the silent period duration (SP) decreased (P = 0.001) in both groups immediately postexercise. The SP decrease was smaller following the second bout (P = 0.021), and there was an association between the change in SICI and reduction in MVC 24 h postexercise in young adults (R = -0.47, P = 0.036). Changes in neurophysiological responses were mostly limited to immediately postexercise, suggesting a modest role in adaptation. In young adults, neural inhibitory changes are linked to the extent of MVC reduction, possibly mediated by the muscle damage-related afferent feedback. Older adults incurred less muscle damage, which has implications for exercise prescription.NEW & NOTEWORTHY This is the first study to have collectively assessed the role of corticospinal, spinal, and intracortical activity in muscle damage attenuation following repeated bouts of exercise in young and older adults. Lower levels of muscle damage in older adults are not related to their neurophysiological responses. Neural inhibition transiently changed, which might be related to the extent of muscle damage; however, the role of processes along the corticospinal pathway in the adaptive response is limited.

HSP72 expression is specific to skeletal muscle contraction type.

Exercise is capable of inducing the cellular stress response and increasing skeletal muscle heat shock protein (HSP) content. HSPs function as molecular chaperones and play roles in facilitating protein folding thereby contributing to muscle proteostasis. To determine the relationship between muscle contraction types, muscle damage, and HSP content, one tibialis anterior (TA) muscle from male Sprague-Dawley rats (n = 5/group) was electrically stimulated while actively lengthening (LC), shortening (SC), or remaining to stagnate (IC) for 15 repetitions (3 sets of five). Two additional LC groups underwent 5 and 10 repetitions. Maximal tetanic tension (MTT) was recorded prior to (pre) and at 5min after (post) the last contraction. Twenty-four hours after stimulation, TA muscles were removed, processed, and assessed for damage and for HSP25 and HSP72 content. Post-MTT was significantly decreased following 15 LCs, (24%; p < 0.05) but not following 15 SCs or 15 ICs. Post-MTT was also decreased by 8% (p < 0.05), and 18% (p < 0.05) for muscles subjected to 5 and 10 LCs, respectively. HSP72 content increased after all LCs conditions but not following ICs or SCs. HSP25 content remained unchanged following all contractions. Similarly, muscle damage was observed only after LCs and not after other contraction types. In conclusion, muscle HSP72 content can be increased with as few as 5 maximal lengthening contractions and appears to be related to muscle damage. This may have important implications for muscle rehabilitation and exercise training programs.

The Effects of Quercetin Supplementation on Eccentric Exercise-Induced Muscle Damage.

The aim of the present investigation was to test the hypothesis that quercetin (Q) may prevent the strength loss and neuromuscular impairment associated with eccentric exercise-induced muscle damage (EEIMD). Twelve young men (26.1 ± 3.1 years) ingested either Q (1000 mg/day) or placebo (PLA) for 14 days using a randomized, double-blind, crossover study design. Participants completed a comprehensive neuromuscular (NM) evaluation before, during and after an eccentric protocol able to induce a severe muscle damage (10 sets of 10 maximal lengthening contractions). The NM evaluation comprised maximal voluntary isometric contraction (MVIC) and force–velocity relationship assessments with simultaneous recording of electromyographic signals (EMG) from the elbow flexor muscles. Soreness, resting arm angle, arm circumference, plasma creatine kinase (CK) and lactate dehydrogenase (LDH) were also assessed. Q supplementation significantly increased the isometric strength recorded during MVIC compared to baseline (+4.7%, p < 0.05). Moreover, the torque and muscle fiber conduction velocity (MFCV) decay recorded during the eccentric exercise was significant lower in Q compared to PLA. Immediately after the EEIMD, isometric strength, the force–velocity relationship and MFCV were significantly lower when participants were given PLA rather than Q. Fourteen days of Q supplementation seems able to attenuate the severity of muscle weakness caused by eccentric-induced myofibrillar disruption and sarcolemmal action potential propagation impairment.

Enhanced Corticospinal Excitability and Volitional Drive in Response to Shortening and Lengthening Strength Training and Changes Following Detraining.

There is a limited understanding of the neurological adaptations responsible for changes in strength following shortening and lengthening resistance training and subsequent detraining. The aim of the study was to investigate differences in corticospinal and spinal responses to resistance training of the tibialis anterior muscle between shortening or lengthening muscle contractions for 4 weeks and after 2 weeks of detraining. Thirty-one untrained individuals were assigned to either shortening or lengthening isokinetic resistance training (4 weeks, 3 days/weeks) or a non-training control group. Transcranial magnetic stimulation and peripheral nerve stimulation (PNS) were used to assess corticospinal and spinal changes, respectively, at pre-, mid-, post-resistance training and post detraining. Greater increases changes (P < 0.01) in MVC were found from the respective muscle contraction training. Motor evoked potentials (expressed relative to background EMG) significantly increased in lengthening resistance training group under contraction intensities ranging from 25 to 80% of the shortening and lengthening contraction intensity (P < 0.01). In the shortening resistance training group increases were only seen at 50 and 80% of both contraction type. Volitional drive (V-wave) showed a greater increase following lengthening resistance training (57%) during maximal lengthening contractions compared to maximal shortening contractions following shortening resistance training (23%; P < 0.001). During the detraining period MVC and V-wave did not change (P > 0.05), although MEP amplitude decreased during the detraining period (P < 0.01). No changes in H-reflex were found pre to post resistance training or post detraining. Modulation in V-wave appeared to be contraction specific, whereby greatest increases occurred following lengthening resistance training. Strength and volitional drive is maintained following 2 weeks detraining, however corticospinal excitability appears to decrease when the training stimulus is withdrawn.

IL-6 RESPONSES TO GLYCAEMIC INDEX DURING RECOVERY FROM EXERCISE

Purpose: This study examined the effect of meal with different glycaemic index (GI) on plasma IL-6 concentration and glucose metabolism after maximal lengthening contractions of the knee extensors. Using a cross-over design, Material : 10 healthy males completed 5 sets of 10 lengthening (eccentric) contractions at 120% 1 repetition-maximum. Subjects were randomized to consume the GI beverage (high-GI, low-GI (15% weight per volume; 3 g/kg BM) or placebo in three times within 10 min following exercise, and again at 50 and 110 min during recovery time. Blood samples were collected before exercise and after 0.60, 180 min and 24 h of recovery. Results: Concentration of plasma IL-6 in HGI group was less than LGI and Pla groups. IL-6 tended to significantly increase after exercise in recovery time in 3 groups (all P &lt; 0.05), except for 24 hours (P = 1.00), furthermore there was significant difference for IL-6 between placebo and high glycemic groups in 3hours after exercise (P=.016). Concentration of serum CK in HGI group was less than LGI and Pla groups, CK was significantly elevated at all times points during recovery in 3 groups (all P &lt; 0.05), except for 1 hour after exercise in HGI group (P = 0.31), but there was no significant difference for CK between groups. Conclusion: In summary, consuming HGI carbohydrate during recovery from exercise attenuate plasma IL-6 concentration.

Similar increases in muscle size and strength in young men after training with maximal shortening or lengthening contractions when matched for total work

Training exclusively with eccentric (lengthening) contractions can result in greater muscular adaptations than training with concentric (shortening) contractions. We aimed to determine whether training-induced increases in muscle size and strength differed between muscles performing maximal lengthening (LC) or maximal shortening (SC) contractions when total external work is equivalent. Nine healthy young males completed a 9-week isokinetic (0.79rad/s) resistance training program of the elbow flexors whereby they performed LC with one arm and an equivalent volume of total external work with the contralateral arm as SC. Training increased isometric peak torque for both LC (~10%) and SC (~20%) with no difference (P=0.14) between conditions. There were also similar increases in isokinetic peak torque at both slow (0.79rad/s) and fast (5.24rad/s) shortening and lengthening peak torque for both LC (~8-10%) and SC (~9-20%). Training increased work per repetition similarly for both LC (~17%) and SC (~22%), in spite of ~40% greater work per repetition with LC. The increase in muscle cross-sectional area with training was also similar (P=0.37) between LC (~6.5%) and SC (~4.6%). We conclude that increases in muscle size and strength with short-term unilateral resistance training are unrelated to muscle contraction type when matched for both exercise intensity (i.e. maximal contractions) and total external work.

The repeated-bout effect: influence on biceps brachii oxygenation and myoelectrical activity

This study investigated biceps brachii oxygenation and myoelectrical activity during and following maximal eccentric exercise to better understand the repeated-bout effect. Ten men performed two bouts of eccentric exercise (ECC1, ECC2), consisting of 10 sets of 6 maximal lengthening contractions of the elbow flexors separated by 4 wk. Tissue oxygenation index minimum amplitude (TOI(min)), mean and maximum total hemoglobin volume by near-infrared spectroscopy, torque, and surface electromyography root mean square (EMG(RMS)) during exercise were compared between ECC1 and ECC2. Changes in maximal voluntary isometric contraction (MVC) torque, range of motion, plasma creatine kinase activity, muscle soreness, TOI(min), and EMG(RMS) during sustained (10-s) and 30-repeated isometric contraction tasks at 30% (same absolute force) and 100% MVC (same relative force) for 4 days postexercise were compared between ECC1 and ECC2. No significant differences between ECC1 and ECC2 were evident for changes in torque, TOI(min), mean total hemoglobin volume, maximum total hemoglobin volume, and EMG(RMS) during exercise. Smaller (P < 0.05) changes and faster recovery of muscle damage markers were evident following ECC2 than ECC1. During 30% MVC tasks, TOI(min) did not change, but EMG(RMS) increased 1-4 days following ECC1 and ECC2. During 100% MVC tasks, EMG(RMS) did not change, but torque and TOI(min) decreased 1-4 days following ECC1 and ECC2. TOI(min) during 100% MVC tasks and EMG(RMS) during 30% MVC tasks recovered faster (P < 0.05) following ECC2 than ECC1. We conclude that the repeated-bout effect cannot be explained by altered muscle activation or metabolic/hemodynamic changes, and the faster recovery in muscle oxygenation and activation was mainly due to faster recovery of force.

Effect of Slow-Velocity Lengthening Contractions on Muscle Damage Induced by Fast-Velocity Lengthening Contractions

This study tested the hypothesis that the first exercise bout consisting of slow-velocity (30° · s(-1)) maximal lengthening contractions would not affect muscle damage in a subsequent bout consisting of fast-velocity (210° · s(-1)) lengthening contractions. Eighteen men were randomly assigned into either a repeated bout group (n = 10) or control group (n = 8). The repeated bout group performed 2 bouts of exercise consisting of 210 maximal lengthening contractions of the elbow flexors separated by 14 days at a velocity of 30° · s(-1) for the first and 210° · s(-1) for the second bout. The control group performed a single bout of the fast-velocity exercise. Changes in maximal isometric strength, range of motion (ROM), upper-arm circumference, muscle thickness, muscle soreness, serum creatine kinase, and lactate dehydrogenase activities were measured before, immediately after, and 24 to 96 hours after exercise. The repeated bout group showed significantly (p < 0.05) smaller changes in all criterion measures except for muscle soreness after the fast-velocity exercise compared with the control group. A significant (p < 0.05) difference was evident only for ROM between the slow- and fast-velocity bouts of the repeated bout group. These results suggest that slow-velocity exercise reduced muscle damage induced by fast-velocity exercise, although the reduction was not large.

Diffusion Tensor MRI to Assess Damage in Healthy and Dystrophic Skeletal Muscle after Lengthening Contractions

The purpose of this study was to determine if variables calculated from diffusion tensor imaging (DTI) would serve as a reliable marker of damage after a muscle strain injury in dystrophic (mdx) and wild type (WT) mice. Unilateral injury to the tibialis anterior muscle (TA) was induced in vivo by 10 maximal lengthening contractions. High resolution T1- and T2-weighted structural MRI, including T2 mapping and spin echo DTI was acquired on a 7T small animal MRI system. Injury was confirmed by a significant loss of isometric torque (85% in mdx versus 42% in WT). Greater increases in apparent diffusion coefficient (ADC), axial, and radial diffusivity (AD and RD) of the injured muscle were present in the mdx mice versus controls. These changes were paralleled by decreases in fractional anisotropy (FA). Additionally, T2 was increased in the mdx mice, but the spatial extent of the changes was less than those in the DTI parameters. The data suggest that DTI is an accurate indicator of muscle injury, even at early time points where the MR signal changes are dominated by local edema.

Comparison between maximal lengthening and shortening contractions for biceps brachii muscle oxygenation and hemodynamics

Eccentric contractions (ECC) require lower systemic oxygen (O2) and induce greater symptoms of muscle damage than concentric contractions (CON); however, it is not known if local muscle oxygenation is lower in ECC than CON during and following exercise. This study compared between ECC and CON for changes in biceps brachii muscle oxygenation [tissue oxygenation index (TOI)] and hemodynamics [total hemoglobin volume (tHb)=oxygenated-Hb+deoxygenated-Hb], determined by near-infrared spectroscopy over 10 sets of 6 maximal contractions of the elbow flexors of 10 healthy subjects. This study also compared between ECC and CON for changes in TOI and tHb during a 10-s sustained and 30-repeated maximal isometric contraction (MVC) task measured immediately before and after and 1-3 days following exercise. The torque integral during ECC was greater (P<0.05) than that during CON by approximately 30%, and the decrease in TOI was smaller (P<0.05) by approximately 50% during ECC than CON. Increases in tHb during the relaxation phases were smaller (P<0.05) by approximately 100% for ECC than CON; however, the decreases in tHb during the contraction phases were not significantly different between sessions. These results suggest that ECC utilizes a lower muscle O2 relative to O2 supply compared with CON. Following exercise, greater (P<0.05) decreases in MVC strength and increases in plasma creatine kinase activity and muscle soreness were evident 1-3 days after ECC than CON. Torque integral, TOI, and tHb during the sustained and repeated MVC tasks decreased (P<0.01) only after ECC, suggesting that muscle O2 demand relative to O2 supply during the isometric tasks was decreased after ECC. This could mainly be due to a lower maximal muscle mass activated as a consequence of muscle damage; however, an increase in O2 supply due to microcirculation dysfunction and/or inflammatory vasodilatory responses after ECC is recognized.

Cytokine Responses to Carbohydrate Ingestion During Recovery from Exercise-Induced Muscle Injury

We investigated the effect of carbohydrate ingestion after maximal lengthening contractions of the knee extensors on circulating concentrations of myocellular proteins and cytokines, and cytokine mRNA expression in muscle. Using a cross-over design, 10 healthy males completed 5 sets of 10 lengthening (eccentric) contractions (unilateral leg press) at 120% 1 repetition-maximum. Subjects were randomized to consume a carbohydrate drink (15% weight per volume; 3 g/kg BM) for 3 h after exercise using one leg, or a placebo drink after exercise using the contralateral leg on another day. Blood samples (10 mL) were collected before exercise and after 0, 30, 60, 90, 120, 150, and 180 min of recovery. Muscle biopsies (vastus lateralis) were collected before exercise and after 3 h of recovery. Following carbohydrate ingestion, serum concentrations of glucose (30-90 min and at 150 min) and insulin (30-180 min) increased (P < 0.05) above pre-exercise values. Serum myoglobin concentration increased ( approximately 250%; P < 0.05) after both trials. In contrast, serum cytokine concentrations were unchanged throughout recovery in both trials. Muscle mRNA expression for IL-8 (6.4-fold), MCP-1 (4.7-fold), and IL-6 (7.3-fold) increased substantially after carbohydrate ingestion. TNF-alpha mRNA expression did not change after either trial. Carbohydrate ingestion during early recovery from exercise-induced muscle injury may promote proinflammatory reactions within skeletal muscle.

Assessment of Skeletal Muscle Damage in Mice using Diffusion Tensor MRI

Diagnosis of acute muscle strains is still typically made based on physical exam and patient history, but muscle injuries can be detected with magnetic resonance imaging (MRI). Conventionally, muscle strains are revealed best by T2-weighted MRI, which optimizes contrast between edema and muscular tissue. PURPOSE: To determine if variables calculated from diffusion tensor imaging (DTI) would serve as an earlier and more sensitive marker of damage after a muscle strain injury in dystrophic (mdx) and control mice. We hypothesize that DTI biomarkers will provide a more informative assessment of muscle injury, and these measures will further elucidate the increased susceptibility to injury of the mdx model. METHODS: Unilateral injury to the tibialis anterior muscle (TA) was induced by 15 maximal lengthening contractions in mdx mice (n=3) and control mice (n=3). High resolution T1 and T2-weighted structural MRI (100μm × 100μm × 750 μm) including T2 mapping and spin echo DTI (150μm × 150μm × 750 μm × 12 directions) were acquired on a 7T MRI system. T2 and diffusion tensor parameters (ie apparent diffusion coefficient [ADC], fractional anisotropy [FA], axial and radial diffusivity [AD and RD]) were calculated at each pixel location. TA imaging was compared to functional changes and HE staining and immunofluorescent labeling of TA cross-sections. RESULTS: Injury was confirmed by a significant loss of isometric torque (85% in mdx vs. 42% in controls). Parameter values were calculated as the percent increase in relation to the contralateral, uninjured TA of each animal. Greater increase in ADC, AD, and RD of the injured muscle was present in the mdx mice vs. controls when compared at proximal, medial, and distal locations of the muscle. These changes were paralleled by decreases in FA. On average, changes in DTI parameters were 1.8× greater in the mdx mice vs. controls. In comparison, changes in T2 did not indicate clear differences between mdx and controls. CONCLUSIONS: These data suggest that DTI may be a better indicator of muscle injury, even at early time points where the MR signal changes are dominated by local edema. Supported by grants from the Muscular Dystrophy association (MDA, grant 4278) and NIH-NIAMS (K01AR053235) to RML

The impact of damaging exercise on electromechanical delay in biceps brachii

Electromechanical delay (EMD) can be a useful tool in providing information on muscle function changes following a number of exercise interventions. The purpose of this investigation was to examine the effect of damaging exercise on EMD. Twelve volunteers completed 45 maximal lengthening contractions of the elbow flexors. Indices of muscle damage (maximum voluntary isometric torque and isokinetic torque (60 s −1 and 210 s −1), creatine kinase (CK), muscle soreness (DOMS)) were recorded pre-exercise, 48 h and 96 h post-exercise; electromyography was recorded from the belly of biceps brachii and EMD was determined during each contraction condition. Significant time effects were observed for all damage indices ( P < 0.05). Whilst muscle function was close to full recovery at 96 h, both CK and DOMS were significantly ( P < 0.001) elevated above baseline. EMD was significantly greater at 96 h for all contraction conditions ( P ⩽ 0.017), but not at 48 h. Damaging exercise induces alterations in EMD beyond the apparent recovery of maximal voluntary contractions, which is likely to be a reflection of the changes in post-synaptic events. These data have applications for the use of EMD as a tool for exercise prescription and monitoring recovery following strenuous damaging exercise.

Maximal lengthening contractions induce different signaling responses in the type I and type II fibers of human skeletal muscle

The molecular mechanisms by which resistance exercise enlarges muscle mass, particularly the mass of fast-twitch type II fibers, are likely to involve enhanced phosphorylation/activation of key enzymes regulating protein synthesis. The hypothesis is that resistance exercise influences the phosphorylation of such key signaling proteins to a greater extent in type II than in type I fibers. Six recreationally active male subjects performed four sets of six maximal lengthening contractions with one leg. Muscle biopsies were taken from the vastus lateralis before and immediately after exercise and following 1 and 2 h of recovery. Samples were freeze-dried, and individual muscle fibers were dissected out and identified as type I or type II after staining for myosin ATPase. Phosphorylation of p70(S6k) on Thr(389) and S6 in type II fibers was increased three-to fourfold and six- to ninefold (P < 0.05), respectively, 1 and 2 h after exercise, whereas phosphorylation in type I fibers remained unchanged. Phosphorylation of Akt, mammalian target of rapamycin (mTOR) and AMP-activated protein kinase (AMPK) was unaltered in both fiber types, whereas that of eukaryotic elongation factor 2 (eEF2) was attenuated 20-45% (P < 0.05) in type II fibers during recovery. Phosphorylation of ERK1/2 was elevated six- to sevenfold (P < 0.05) immediately after exercise, and p38 MAPK phosphorylation was increased three- to fourfold (P < 0.05) for as long as 1 h after exercise in both types of fibers, although the level was markedly higher in type II fibers (P < 0.05). In conclusion, the elevation of p70(S6k) and the reduction of eEF2 phosphorylation in the type II fibers following resistance exercise suggest stimulation of protein synthesis, which may contribute to a more pronounced enlargement of these fibers. Our findings also suggest that p70(S6k) is activated, at least in part, via pathways not involving Akt-mTOR and MAPK.

Neural control of shortening and lengthening contractions: influence of task constraints

Although the performance capabilities of muscle differ during shortening and lengthening contractions, realization of these differences during functional tasks depends on the characteristics of the activation signal discharged from the spinal cord. Fundamentally, the control strategy must differ during the two anisometric contractions due to the lesser force that each motor unit exerts during a shortening contraction and the greater difficulty associated with decreasing force to match a prescribed trajectory during a lengthening contraction. The activation characteristics of motor units during submaximal contractions depend on the details of the task being performed. Indexes of the strategy encoded in the descending command, such as coactivation of antagonist muscles and motor unit synchronization, indicate differences in cortical output for the two types of anisometric contractions. Furthermore, the augmented feedback from peripheral sensory receptors during lengthening contractions appears to be suppressed by centrally and peripherally mediated presynaptic inhibition of Ia afferents, which may also explain the depression of voluntary activation that occurs during maximal lengthening contractions. Although modulation of the activation during shortening and lengthening contractions involves both supraspinal and spinal mechanisms, the association with differences in performance cannot be determined without more careful attention to the details of the task.

The physiological effects of IGF-1 (class 1:Ea transgene) over-expression on exercise-induced damage and adaptation in dystrophic muscles of mdx mice

Duchenne muscular dystrophy (DMD) is a genetic disorder in which muscle weakness and fragility contribute to ongoing muscle degeneration. Although exercise-induced muscle damage is associated with adaptation that protects normal muscle from further damage, exploiting this process to protect dystrophic muscle has been avoided for fear of inducing excessive muscle degeneration. However, muscle-specific over-expression of the class 1:Ea isoform of insulin-like growth factor-1 (IGF-1) reduces myofibre necrosis in dystrophic mdx mice (a model for DMD) and, therefore, may enhance the adaptation process in response to eccentric exercise. To test this hypothesis, we evaluated the effect of transgenic class 1:Ea IGF-1 over-expression on the susceptibility to muscle damage and subsequent adaptation in 12-week-old dystrophic mdx and non-dystrophic control mice. Experiments were conducted in vivo using a custom-built isokinetic mouse dynamometer to measure the deficit in joint torque (indicating muscle damage) after 20 maximal lengthening (eccentric) contractions. Adaptation to this damaging exercise was evaluated by repeating the protocol 7 days after the initial exercise. The over-expression of IGF-1 significantly increased the normalised joint torque in non-dystrophic mice and appeared to ameliorate the muscle weakness in dystrophic mice. All mice displayed a marked reduction in the susceptibility to muscle damage on day 7; however, this adaptation was unaffected by IGF-1, showing that IGF-1 does not protect the dystrophic muscles of adult mdx mice against damage resulting from maximal lengthening contractions.

Comparison between old and young men for responses to fast velocity maximal lengthening contractions of the elbow flexors

Controversy exists concerning the susceptibility of elderly individuals to eccentric exercise-induced muscle damage. This study investigated the hypothesis that muscle damage induced by fast maximal lengthening contractions would be greater for old than young men. Ten old (64 +/- 4 years) and young (25 +/- 6 years) men performed 30 maximal voluntary lengthening contractions of the elbow flexors at an angular velocity of 210 degrees s(-1). Prior to exercise, no significant differences were evident between groups for criterion measures. A significant (P < 0.05) group x time interaction was found only for isometric strength and muscle soreness with the old group showing significantly slower recovery of strength and less development of soreness compared with the young group. These results did not support the hypothesis that old men would be more susceptible to muscle damage, but confirmed a previous study reporting that recovery of muscle strength was slower for old than young individuals.

Effect of Lengthening Contraction Velocity on Muscle Damage of the Elbow Flexors

This study investigated the effect of lengthening contraction velocity on exercise-induced muscle damage. Sixteen men were placed into two groups performing either 30 (N = 8) or 210 (N = 8) maximal lengthening contractions of the elbow flexors on an isokinetic dynamometer. Dominant and nondominant arms were randomly assigned for a slow-velocity (S: 30 degrees.s(-1)) or a fast-velocity (F: 210 degrees.s(-1)) exercise separated by 14 d. Maximal voluntary strength of isometric contractions (iMVC) and isokinetic concentric contractions (cMVC), range of motion (ROM), upper-arm circumference, muscle soreness, and serum creatine kinase (CK) activity were measured before, immediately after, and 1-120 h after exercise. Changes in these measures over time were compared by a two-way repeated-measures ANOVA to examine the effect of velocity in the same number of contractions (S30 vs F30; S210 vs F210) or the effect of contraction number at the same velocity (S30 vs S210; F30 vs F210). A significant (P < 0.05) interaction effect was evident only for iMVC between S30 and F30, but it was evident for iMVC, cMVC, ROM, and CK between S210 and F210. Changes in most of the measures were significantly (P < 0.05) smaller after 30 contractions (S30 and F30) than after 210 contractions (S210 and F210). These results suggest that the effect of contraction velocity on the magnitude of muscle damage after 30 contractions is minor; however, when 210 lengthening contractions were performed, the effect of contraction velocity became conspicuous. It is concluded that fast-velocity lengthening contractions are likely to induce greater muscle damage than slow-velocity contractions; however, muscle fatigue seems to be a confounding factor for the velocity effect.