Muscle damage induced by stretch-shortening cycle exercise

Monitoring exercise-induced muscle damage indicators and myoelectric activity during two weeks of knee extensor exercise training in young and old men

This study considered the effects of repeated bouts of short-term resistive exercise in old (age: 64.5±5.5 years; n = 10) and young men (age: 25.1±4.9 years; n = 10) who performed six knee extension exercise bouts over two weeks using various markers of exercise-induced muscle damage and electromyographic activity. We found that time-course changes in quadriceps isometric torque, creatine kinase activity, and muscle soreness in the two groups were similar. However, recovery in the acute torque deficit was mediated by more favourable electromyographic activity changes in the young group than in the older adults group. Muscle elastic energy storage and re-use assessed with dynamometry was selectively improved in the young group by the end of the protocol. Serum myoglobin concentration increased selectively in old group, and remained elevated with further bouts, suggesting higher sarcolemma vulnerability and less effective metabolic adaptation in the older adults, which, however, did not affect muscle contractility.

Effect of downhill walking on next-day muscle damage and glucose metabolism in healthy young subjects

This study aimed to investigate the effect of downhill walking on muscle damage and glucose metabolism in healthy subjects. All ten healthy young men and women (age, 24.0 ± 1.4 years) performed rest, uphill walking, and downhill walking trials. In the exercise trials, uphill (+ 5%) or downhill (- 5%) treadmill walking was performed at 6 km/h for 30 min. On the next day, muscle soreness was significantly higher in the downhill trial than in the uphill trial (P < 0.01). Respiratory metabolic performance did not differ between trials. However, carbohydrate oxidation was negatively correlated with plasma creatine kinase (r = - 0.41) and muscle soreness (r = - 0.47). Fasting blood glucose was significantly lower in the uphill trial than in the rest trial (P < 0.01) but not in the downhill trial. These observations suggest that downhill but not uphill walking causes mild delayed-onset muscle damage, which did not cause marked impairment in glucose metabolism. However, higher muscle damage responders might exhibit lower glucose metabolism.

Biomechanical response of skeletal muscle to eccentric contractions

The forced lengthening of an activated skeletal muscle has been termed an eccentric contraction (EC). This review highlights the mechanically unique nature of the EC and focuses on the specific disruption of proteins within the cell known as cytoskeletal proteins. The major intermediate filament cytoskeletal protein, desmin, has been the focus of work in this area because changes to desmin occur within minutes of ECs and because desmin has been shown to play both a mechanical and biologic role in a muscle's response to EC. It is hoped that these types of studies will assist in decreasing the incidence of muscle injury in athletes and facilitating the development of new therapies to treat muscle injuries.

Acute neuromechanical modifications and 24-h recovery in quadriceps muscle after maximal stretch-shortening cycle exercise

In the present study we investigated the acute and the delayed changes in corticospinal excitability and in the neuromechanical properties of the quadriceps muscle after maximal intensity stretch-shortening cycle exercise. Ten young males performed 150 jumps to provoke fatigue and muscle damage. Voluntary force, various electrically evoked force variables, and corticospinal excitability were measured at baseline, immediately (IP) and at 24 h post-exercise. Voluntary force, single twitch force, and low frequency force decreased at IP (p < 0.05) but recovered at 24 h, although mild soreness developed in the quadriceps. High frequency force, voluntary activation, and corticospinal excitability remained unchanged. However, vastus lateralis myoelectric activity increased from baseline to IP (p < 0.05). The jumps selectively induced low frequency peripheral fatigue, and central mechanisms did not mediate the acute loss of voluntary force. Because soreness developed at 24 h post-exercise, all force variables recovered, and vastus lateralis electric activity increased, we argue that a dual process of muscle damage, and early neural adaptation as a compensation mechanism took place after the maximal stretch-shortening cycle exercise.

An examination of respiratory and metabolic demands of alpine skiing

Background/Objective

To measure the cardiorespiratory and metabolic variables during the giant slalom (GS) skiing activity under actual race conditions using a mobile gas analyzer.

Methods

This study included 20 voluntary male alpine ski racers (mean age, 22.00 ± 1.45 years) who participated in international races. First, incremental running test was conducted to obtain volunteers' maximal oxygen consumption (VO_2max) values. Second, respiratory data were measured during their performance on the GS course. Before both GS performance and incremental running test and at 1 minute, 3 minutes, and 5 minutes after the tests, blood lactate concentration was measured.

Results

VO_2max values of the volunteers were 51.36 ± 2.68 mL/kg/min and they used 74.96% of this during their performance on the GS course. Their blood lactate concentrations reached the maximum level of 13.69 ± 2.06 mmol/L at the 5^th minute following the maximal exercise testing. After the GS performance, blood lactate values reached the maximum level of 10.13 ± 0.43 mmol/L at the 3^rd minute. While the maximum heart rate was 196.5 ± 4.3 bpm during the maximal exercise testing, it reached 201.7 ± 20 bpm during the GS performance.

Conclusion

It is observed that the GS race is a high-intensity activity and that high amount of anaerobic contribution is used by alpine ski racers during the GS race. By contrast, it is understood that the aerobic contribution is also at a considerable level during such an anaerobic activity as GS.

Muscle damage produced during a simulated badminton match in competitive male players

The purpose of the study was to assess the occurrence of muscle damage after a simulated badminton match and its influence on physical and haematological parameters. Sixteen competitive male badminton players participated in the study. Before and just after a 45-min simulated badminton match, maximal isometric force and badminton-specific running/movement velocity were measured to assess muscle fatigue. Blood samples were also obtained before and after the match. The badminton match did not affect maximal isometric force or badminton-specific velocity. Blood volume and plasma volume were significantly reduced during the match and consequently haematite, leucocyte, and platelet counts significantly increased. Blood myoglobin and creatine kinase concentrations increased from 26.5 ± 11.6 to 197.3 ± 70.2 µg·L(-1) and from 258.6 ± 192.2 to 466.0 ± 296.5 U·L(-1), respectively. In conclusion, a simulated badminton match modified haematological parameters of whole blood and serum blood that indicate the occurrence of muscle fibre damage. However, the level of muscle damage did not produce decreased muscle performance.

Effect of DHA on plasma fatty acid availability and oxidative stress during training season and football exercise

The aim was to determine the effects of a diet supplemented with 1.14 g per day of docosahexaenoic acid (DHA) for eight weeks on the plasma oxidative balance and anti-inflammatory markers after training and acute exercise. Fifteen volunteer male football players were randomly assigned to placebo or experimental and supplemented groups. Blood samples were taken under resting conditions at the beginning and after eight weeks of training under resting and post-exercise conditions. The experimental beverage increased the plasma DHA availability in non-esterified fatty acids (NEFAs) and triglyceride fatty acids (TGFAs) and increased the polyunsaturated fatty acid (PUFA) fraction of NEFAs but had no effects on the biomarkers for oxidative balance in plasma. During training, plasma protein markers of oxidative damage, the haemolysis degree and the antioxidant enzyme activities increased, but did not affect lipid oxidative damage. Training season and DHA influenced the circulating levels of prostaglandin E2 (PGE2). Acute exercise did not alter the basal levels of plasma markers for oxidative and nitrosative damage of proteins and lipids, and the antioxidant enzyme activities, although DHA-diet supplementation significantly increased the PGE2 in plasma after acute exercise. In conclusion, the training season and acute exercise, but not the DHA diet supplementation, altered the pattern of plasma oxidative damage, as the antioxidant system proved sufficient to prevent the oxidative damage induced by the acute exercise in well-trained footballers. The DHA-diet supplementation increased the prostaglandin PGE2 plasma evidencing anti-inflammatory effects of DHA to control inflammation after acute exercise.

Mechanical, hormonal, and hypertrophic adaptations to 10 weeks of eccentric and stretch-shortening cycle exercise training in old males

The growth promoting effects of eccentric (ECC) contractions are well documented but it is unknown if the rate of stretch per se plays a role in such muscular responses in healthy aging human skeletal muscle. We tested the hypothesis that exercise training of the quadriceps muscle with low rate ECC and high rate ECC contractions in the form of stretch-shortening cycles (SSCs) but at equal total mechanical work would produce rate-specific adaptations in healthy old males age 60-70. Both training programs produced similar improvements in maximal voluntary isometric (6%) and ECC torque (23%) and stretch-shortening cycle function (reduced contraction duration [24%] and enhanced elastic energy storage [12%]) (p<0.05). The rate of torque development increased 30% only after SSC exercise (p<0.05). Resting testosterone and cortisol levels were unchanged but after each program the acute exercise-induced cortisol levels were 12-15% lower (p<0.05). Both programs increased quadriceps size 2.5% (p<0.05). It is concluded that both ECC and SSC exercise training produces favorable adaptations in healthy old males' quadriceps muscle. Although the rate of muscle tension during the SSC vs. ECC contractions was about 4-fold greater, the total mechanical work seems to regulate the hypetrophic, hormonal, and most of the mechanical adaptations. However, SSC exercise was uniquely effective in improving a key deficiency of aging muscle, i.e., its ability to produce force rapidly.

Running Economy During a Simulated 60-km Trial

The effect of a prolonged running trial on the energy cost of running (Cr) during a 60-km ultramarathon simulation at the pace of a 100-km competition was investigated in 13 men (40.8 ± 5.6 y, 70.7 ± 5.5 kg, 177.5 ± 4.5 cm) and 5 women (40.4 ± 2.3 y, 53.7 ± 4.4 kg, 162.4 ± 4.8 cm) who participated in a 60-km trial consisting of 3 consecutive 20-km laps. Oxygen uptake (VO2) at steady state was determined at constant speed before the test and at the end of each lap; stride length (SL) and frequency and contact time were measured at the same time points; serum creatine kinase (S-CPK) was measured before and at the end of the test. Cr in J · kg−1 · m−1, as calculated from VO2ss and respiratory-exchange ratio, did not increase with distance. SL significantly decreased with distance. The net increase in S-CPK was linearly related with the percentage increase of Cr observed during the trial. It is concluded that, in spite of increased S-CPK, this effort was not able to elicit any peripheral or central fatigue or biomechanical adaptation leading to any modification of Cr.

Role of oxidative stress in impaired insulin signaling associated with exercise-induced muscle damage

Skeletal muscle is a major tissue that utilizes blood glucose. A single bout of exercise improves glucose uptake in skeletal muscle through insulin-dependent and insulin-independent signal transduction mechanisms. However, glucose utilization is decreased in muscle damage induced by acute, unaccustomed, or eccentric exercise. The decrease in glucose utilization is caused by decreased insulin-stimulated glucose uptake in damaged muscles with inhibition of the membrane translocation of glucose transporter 4 through phosphatidyl 3-kinase/Akt signaling. In addition to inflammatory cytokines, reactive oxygen species including 4-hydroxy-2-nonenal and peroxynitrate can induce degradation or inactivation of signaling proteins through posttranslational modification, thereby resulting in a disturbance in insulin signal transduction. In contrast, treatment with factors that attenuate oxidative stress in damaged muscle suppresses the impairment of insulin sensitivity. Muscle-damaging exercise may thus lead to decreased endurance capacity and muscle fatigue in exercise, and it may decrease the efficiency of exercise therapy for metabolic improvement.

Dynamic contractility and efficiency impairments in stretch-shortening cycle are stretch-load-dependent after training-induced muscle damage

To determine the acute task and stretch-load dependency of neuromuscular impairments after muscle-damaging exercises, we examined the magnitude of strength deficits in isometric and stretch-shortening cycle (SSC) contractions after a single bout of exercise. Ten trained men performed 90 unilateral isokinetic eccentric-concentric knee extensions on a dynamometer. Plasma creatine kinase activity, muscle soreness, maximal isometric torque, short-range stiffness, and peak torque in the eccentric phase of the SSC contraction at 3 stretch-loads (120, 150, and 180 J) were determined in the quadriceps before and 24 hours after exercise. During SSC, positive mechanical work and efficiency were also calculated. Creatine kinase and soreness increased at 24 hours (p < 0.05). In each of the 3 stretch-load conditions, muscle damage affected short-range stiffness less than isometric and peak SSC torque (p < 0.05), providing evidence for a selective impairment in contractile function after muscle damage. With greater SSC stretch-load peak, SSC torque deficit increased linearly, whereas short-range stiffness deficit was unaffected. Efficiency declined only at the 180-J condition (p < 0.05) as a result of decreased positive work (p < 0.05). It was concluded that intense exercise produced microtrauma in the muscle, and a selective loss of force generating capacity, which suggests greater damage to the contractile machinery. Practitioners may expect greater acute impairment of force generation in movements that use large loads in their daily training drills. However, altered knee flexion strategy during SSC may compensate for the force deficit, preserving mechanical efficiency at smaller stretch-loads.

Fluctuations in the skeletal muscle power-velocity relationship and interferon-γ after a muscle-damaging event in humans

Background

Skeletal muscle power is velocity-dependent under constant load conditions. Interferon (IFN)-γ is an inflammatory cytokine that regulates skeletal muscle recovery following insult in experimental animals. It is unknown if the power-velocity relationship and IFN-γ are modulated after a muscle-damaging event in humans. Therefore, the purpose of this study was to identify the power-velocity relationship and circulating IFN-γ concentration responses to a muscle-damaging event in humans.

Methods

Nine healthy males participated in this study. Each subject had one leg randomly assigned as the control leg. The other leg served as the treatment leg and performed an intense-stretch-shortening cycling (SSC) exercise protocol to induce muscle damage. To measure muscle damage and the power-velocity relationship, unilateral peak isometric force and power output (forces and velocities) measurements were performed prior to, immediately after, and during the days following the SSC protocol. The circulating IFN-γ concentrations were measured in serum samples obtained prior to, immediately after, and during the days following the SSC protocol. Statistical significance of single-leg isometric force and power output data were assessed using a two-way (time and leg treatment) analysis of variance (ANOVA) with repeated measures, followed by a Tukey’s honestly significant difference (HSD) to test multiple pairwise comparisons. The statistical significance of the IFN-γ data were assessed using a one-way (time) ANOVA with repeated measures, followed by a Tukey’s HSD to test multiple pairwise comparisons.

Results

In the treatment leg, significant (P < 0.05) peak isometric force deficits occurred immediately and persisted several days after the SSC protocol, thereby identifying muscle damage-induced weakness. During muscle weakness in the treatment leg, peak power was significantly (P < 0.05) depressed and the velocities at peak power were significantly (P < 0.05) slower. Interestingly, circulating IFN-γ concentrations decreased at 2 and 3 days after compared to those immediately following the SSC protocol.

Conclusion

We conclude that the velocity to achieve a compromised peak power is reduced, and speculatively, the circulating IFN-γ excursion could be influential on the recovery of skeletal muscle after a muscle-damaging event in humans.

Cytokine and satellite cell responses to muscle damage: interpretation and possible confounding factors in human studies

It is plausible that multiple muscle biopsies following a muscle damaging intervention can exacerbate the inflammatory and subsequent satellite cell responses. To elucidate confounding effects of muscle biopsy procedure on satellite cell number, indirect markers of damage and the inflammatory response following acute downhill running (DHR) were investigated. 10 healthy male participant were divided into a non-exercising control (n = 4) and DHR (12 × 5min bouts, 10 % decline at 85 % VO(2)max) (n = 6) group. Blood samples were taken pre, post and every 24 h for 9 days. Serum was analysed for creatine kinase (CK), myoglobin (Mb), lactate dehydrogenase (LDH), TNF-α, IL-6 and IL-10. Muscle biopsies taken on days 1 and 2 post intervention from opposing legs were analysed for Pax7(+) satellite cells. In the DHR group, Mb (536 ± 277 ng mL(-1)), IL-6 (12.6 ± 4.7 pg mL(-1)) and IL-10 (27.3 ± 11.5 pg mL(-1)) peaked immediately post DHR, while CK (2651 ± 1911 U L(-1)), LDH (202 ± 47 U L(-1)) and TNF-α (25.1 ± 8.7 pg mL(-1)) peaked on day 1. A 30 % increase in Pax7(+) satellite cells on day 1 in the DHR group was no longer apparent on day 2. H&E staining show evidence of phagocytosis in the DHR group. No significant changes over time were observed in the control group for any of the variables measured. Events observed in the DHR group were as a result of the intervention protocol and subsequent muscle damage. The relationship between SC proliferation and pro-inflammatory cytokine release appears to be complex since the IL-6/IL-10 response time differs significantly from the TNF-α response.

The effect of sports specialization on musculus quadriceps function after exercise-induced muscle damage

The primary aim of the present study was to examine the effect of eccentric exercise-induced (100 submaximal eccentric contractions at an angular velocity of 60° s⁻¹, with 20-s rest intervals) muscle damage on peripheral and central fatigue of quadriceps muscle in well-trained long-distance runners, sprint runners, volleyball players, and untrained subjects. We found that (i) indirect symptoms of exercise-induced muscle damage (prolonged decrease in maximal voluntary contraction, isokinetic concentric torque, and electrically induced (20 Hz) torque) were most evident in untrained subjects, while there were no significant differences in changes of muscle soreness and plasma creatine kinase 48 h after eccentric exercise between athletes and untrained subjects; (ii) low-frequency fatigue was greater in untrained subjects and volleyball players than in sprint runners and long-distance runners; (iii) in all subjects, electrically induced (100 Hz) torque decreased significantly by about 20%, while central activation ratio decreased significantly by about 8% in untrained subjects and sprint runners, and by about 3%-5% in long-distance runners and volleyball players. Thus, trained subjects showed greater resistance to exercise-induced muscle damage for most markers, and long-distance runners had no advantage over sprint runners or volleyball players.

Testosterone stimulates myoglobin expression in different muscles of the mouse

The regulation of energy metabolism is one of the major functions of steroid hormones. This study was performed to explore whether testosterone can regulate the aerobic capacity of skeletal muscles via myoglobin expression. To study this, changes in testosterone level were quantified, and the level of myoglobin protein was analyzed using Western blot in mice subjected to 6 weeks of training (T) or testosterone administration (A). Both treatments significantly increased the plasma testosterone level when compared to the untrained (U) or control (C) group. Training induced a significant increase in the myoglobin content in gastrocnemius and plantaris muscles (287 and 83%, respectively). Testosterone administration increased myoglobin concentration in plantaris (183%) but not in gastrocnemius. In extensor digitorum longus muscle the protein content decreased slightly after exercise, but increased 78% after testosterone administration. In soleus and rectus femoris muscles the myoglobin content was unchanged after both treatments. The data show that testosterone and training have differential effects on the concentration of myoglobin in some, but not all muscles. This may have an influence on the aerobic capacity in mouse skeletal muscles. The data demonstrated that both testosterone administration and training induced an increase in plasma testosterone level. However, the effects of the treatments on the myoglobin concentration differ.

Force deficits and breakage rates after single lengthening contractions of single fast fibers from unconditioned and conditioned muscles of young and old rats

The deficit in force generation is a measure of the magnitude of damage to sarcomeres caused by lengthening contractions of either single fibers or whole muscles. In addition, permeabilized single fibers may suffer breakages. Our goal was to understand the interaction between breakages and force deficits in "young" and "old" permeabilized single fibers from control muscles of young and old rats and "conditioned" fibers from muscles that completed a 6-wk program of in vivo lengthening contractions. Following single lengthening contractions of old-control fibers compared with young-control fibers, the twofold greater force deficits at a 10% strain support the concept of an age-related increase in the susceptibility of fibers to mechanical damage. In addition, the much higher breakage rates for old fibers at all strains tested indicate an increase with aging in the number of fibers at risk of being severely injured during any given stretch. Following the 6-wk program of lengthening contractions, young-conditioned fibers and old-conditioned fibers were not different with respect to force deficit or the frequency of breakages. A potential mechanism for the increased resistance to stretch-induced damage of old-conditioned fibers is that, through intracellular damage and subsequent degeneration and regeneration, weaker sarcomeres were replaced by stronger sarcomeres. These data indicate that, despite the association of high fiber breakage rates and large force deficits with aging, the detrimental characteristics of old fibers were improved by a conditioning program that altered both sarcomeric characteristics as well as the overall structural integrity of the fibers.

Muscle mechanical characteristics in fatigue and recovery from a marathon race in highly trained runners

The aim of the present study was to examine muscle mechanical characteristics before and after a marathon race. Eight elite runners underwent a pre-test 1 week before the marathon and post-tests 30 min, two and five-day-post-marathon. Actual marathon race performance was 2:34:40 +/- 0:04:13. Energy expenditure at marathon pace (EE(Mpace)) was elevated 4% post-marathon (pre: 4,465 +/- 91 vs. post 4,638 +/- 91 J kg bodyweight(-1) km(-1), P < 0.05), but was lowered by 6 and 9.5% two- and five-day-post-marathon compared to EE(Mpace) pre-marathon. Countermovement jump (CMJ) power decreased 13% post-marathon (pre: 21.5 +/- 0.9 vs. post: 18.9 +/- 1.2 W kg(-1); P < 0.05) and remained depressed two- (18%) and five-day (12%) post-marathon. CMJ force was unaltered across all four tests occasions. Knee extensor and plantar flexor maximal voluntary contraction (MVC) decreased from 176.6 +/- 9.5 to 136.7 +/- 16.8 Nm and 144.9 +/- 8.7 to 119.2 +/- 15.1 Nm post-marathon corresponding to 22 and 17%, respectively (P < 0.05). No significant changes were detected in evoked contractile parameters, except a 25% increase in force at 5 Hz, and low frequency fatigue was not observed. In conclusion, leg muscle power decreased acutely post-marathon race and recovered very slowly. The post-marathon increase in EE(Mpace) might be attributed to a reduction in stretch shortening cycle efficiency. Finally, since MVC was reduced after the marathon race without any marked changes in evoked muscle contractile properties, the strength fatigue experienced by the subjects in this study seems to be related to central rather than peripheral mechanisms.

Review of exercise-induced muscle injury: relevance for athletic populations

Exercise-induced skeletal muscle injury is well understood as the product of unfamiliar or strenuous physical activity. Eccentric or lengthening actions are primarily responsible for inducing injury, which subsequently leads to a variety of signs and symptoms. Although significant research supports this finding, most observations are specific to untrained individuals. In addition, many protocols designed both to induce muscle injury and assess performance following the injury are dissimilar from those utilized by physically trained individuals or are impractical in relation to athletic performance or both. Therefore, difficulty may arise when extrapolating information, from the available literature and applying the findings to athletic populations. This review addresses the efficacy of applying our current understanding of exercise-induced skeletal muscle injury to a physically trained population as well as highlights concerns that require future investigation.

Confiabilidade do teste de salto vertical com 4 séries de 15 segundos

OBJETIVO: O objetivo deste estudo foi verificar a confiabilidade do teste e reteste do teste salto vertical com quatro séries de 15 segundos (TSVI). MÉTODO: Dezoito atletas do sexo masculino, divididos em 11 handebolistas (25,74 ± 4,71 anos; 85,84 ± 7,63kg; 182,14 ± 3,46cm) e sete basquetebolistas (18,60 ± 0,77 anos; 83,32 ± 10,02kg; 188,14 ± 5,76cm) foram os voluntários desse estudo. As variáveis estudadas para o teste e reteste foram o pico de potência (PP), potência média (PM), índice de fadiga (IF). Os desempenhos dessas variáveis foram mensurados através do teste de salto vertical com quatro séries de 15 segundos com 10 segundos de recuperação entre as séries. O tratamento estatístico foi realizado através da técnica descritiva e do coeficiente de correlação intraclasse (CCI). RESULTADOS: Os resultados demonstraram um alto CCI nas medidas repetidas em dias diferentes para todas as variáveis: PP (R = 0,992; p = 0,0360); PM (R = 0,993; p = 0,0107) e IF (R = 0,981; p = 0,0556); além disso, indicaram altos coeficientes de correlações entre teste e reteste para os indicadores de qualidade nas medidas da técnica de salto vertical com contramovimento sem auxílio dos membros superiores (CMJ) (R = 0,991; p = 0,0800), nos números de saltos em um trabalho de 15 e 60 segundos (NSV15s, R = 0,936; p = 0,0062 e NSV60s, R = 0,978; p = 0,0139) e na altura saltada, em um trabalho de 15 e 60 segundos (SV15s, R = 0,993; p = 0,0467; e SV60s, R = 0,988; p = 0,0014). CONCLUSÃO: A análise dos dados aponta para a existência de uma medida confiável do TSVI na estimativa da resistência de força explosiva através das variáveis PM e IF.

Leg stiffness modulation during exhaustive stretch-shortening cycle exercise

The present study examined the effects of muscle activity modulation on leg stiffness during an exhaustive stretch-shortening cycle (SSC) exercise in eight male subjects. Reaction force, electromyography (EMG) of the soleus (Sol), gastrocnemius (Ga) and vastus lateralis (VL) muscles and sledge seat position were recorded during the SSC exercise, consisting of 100 maximal intermittent drop jumps followed by a continuous submaximal jumping until exhaustion, on a sledge apparatus. Metabolic loading was determined by measuring blood lactate (La). No change was found in leg stiffness during the maximal jumps, whereas the subsequent submaximal jumping induced a significant reduction by 27+/-12% (P<0.05). Leg stiffness was closely related to the EMG ratio between the braking and push-off phases in Sol (r=0.81, P<0.05) and particularly in Ga (r=0.98, P<0.001) (but not in VL, r=0.64, NS) at the end of the submaximal jumping. Furthermore, the post-exercise La was significantly associated with the EMG ratio at the end of the submaximal jumping in Sol (r=-0.88, P<0.01) and Ga (r=-0.98, P<0.001). These results indicate that activity modulation between the braking and push-off phases in the triceps surae muscle, particularly in Ga, plays an important role in leg stiffness adjustments during fatiguing SSC exercise. It is suggested that efficient activity modulation (i.e. high EMG ratio) of the triceps surae muscle during an intensive fatiguing SSC exercise may postpone the exhaustion and development of metabolic fatigue.

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.

Evaluation of muscle damage after a rugby match with special reference to tackle plays

OBJECTIVE

To investigate blood indices of muscle damage after a competitive rugby match.

METHODS

Fifteen elite amateur rugby players volunteered to participate (mean (SE) age 26.6 (0.7) years, height 179.8 (1.0) cm, weight 87.4 (2.2) kg, and VO2MAX 58.5 (1.2) ml/kg/min). The study was conducted after two competitive matches during the 1999-2000 season. Plasma concentrations of lactate, potassium (K+), sodium (Na+), and myoglobin, and the activity of creatine kinase were measured before and after the matches. In addition, the number of tackles by and on each subject and the average duration of the work and rest periods were analysed using video recordings of the matches.

RESULTS

Myoglobin concentration and creatine kinase activity showed appreciable transient increases after the match. Peak values for myoglobin concentration (980 (166) microg/l) and creatine kinase activity (1081 (159) U/l) were observed 45 minutes and 24 hours after the match respectively. Positive and significant correlations were observed between the number of tackles and both peak myoglobin concentration (r=0.85, p<0.01; n=14) and peak creatine kinase activity (r=0.92, p<0.01; n=14). Plasma lactate and K+ concentrations also showed appreciable increases after the match, whereas plasma Na+ concentration showed a gradual decrease. The mean duration of the work and rest periods were 21.5 (2.2) and 24.3 (3.1) seconds respectively.

CONCLUSIONS

The rugby matches resulted in serious structural damage to the muscles, the extent of which was highly dependent on the number of tackles.

Dynamic force responses of skeletal muscle during stretch-shortening cycles

Muscle damage due to stretch-shortening cycles (i.e., cyclic eccentric/concentric muscle actions) is one of the major concerns in sports and occupational related activities. Mechanical responses of whole muscle have been associated with damage in neural motor units, in connective tissues, and the force generation mechanism. The objective of this study was to introduce a new method to quantify the real-time changes in skeletal muscle forces of rats during injurious stretch-shortening cycles. Male Sprague Dawley rats ( n=24) were selected for use in this study. The dorsi flexor muscle group was exposed to either 150 stretch-shortening cycles ( n=12) or 15 isometric contractions ( n=12) in vivo using a dynamometer and electrical stimulation. Muscle damage after exposure to stretch-shortening cycles was verified by the non-recoverable force deficit at 48 h and the presence of myofiber necrosis. Variations of the dynamic forces during stretch-shortening cycles were analyzed by decomposing the dynamic force signature into peak force ( F(peak)), minimum force ( F(min)), average force ( F(mean)), and cyclic force ( F(a)). After the 15th set of stretch-shortening cycles, the decrease in the stretch-shortening parameters, F(peak), F(min), F(mean), and F(a), was 50% ( P<0.0001), 26% ( P=0.0055), 68% ( P<0.0001), and 50% ( P<0.0001), respectively. Our results showed that both isometric contractions and stretch-shortening cycles induce a reduction in the isometric force. However, the force reduction induced by isometric contractions fully recovered after a break of 48 h while that induced by stretch-shortening cycles did not. Histopathologic assessment of the tibialis anterior exposed to stretch-shortening cycles showed significant myofiber degeneration and necrosis with associated inflammation, while muscles exposed to isometric contractions showed no myofiber degeneration and necrosis, and limited inflammation. Our results suggest that muscle damage can be identified by the non-recoverable isometric force decrement and also by the variations in the dynamic force signature during stretch-shortening cycles.

The scientific basis for high-intensity interval training: optimising training programmes and maximising performance in highly trained endurance athletes

While the physiological adaptations that occur following endurance training in previously sedentary and recreationally active individuals are relatively well understood, the adaptations to training in already highly trained endurance athletes remain unclear. While significant improvements in endurance performance and corresponding physiological markers are evident following submaximal endurance training in sedentary and recreationally active groups, an additional increase in submaximal training (i.e. volume) in highly trained individuals does not appear to further enhance either endurance performance or associated physiological variables [e.g. peak oxygen uptake (VO2peak), oxidative enzyme activity]. It seems that, for athletes who are already trained, improvements in endurance performance can be achieved only through high-intensity interval training (HIT). The limited research which has examined changes in muscle enzyme activity in highly trained athletes, following HIT, has revealed no change in oxidative or glycolytic enzyme activity, despite significant improvements in endurance performance (p < 0.05). Instead, an increase in skeletal muscle buffering capacity may be one mechanism responsible for an improvement in endurance performance. Changes in plasma volume, stroke volume, as well as muscle cation pumps, myoglobin, capillary density and fibre type characteristics have yet to be investigated in response to HIT with the highly trained athlete. Information relating to HIT programme optimisation in endurance athletes is also very sparse. Preliminary work using the velocity at which VO2max is achieved (V(max)) as the interval intensity, and fractions (50 to 75%) of the time to exhaustion at V(max) (T(max)) as the interval duration has been successful in eliciting improvements in performance in long-distance runners. However, V(max) and T(max) have not been used with cyclists. Instead, HIT programme optimisation research in cyclists has revealed that repeated supramaximal sprinting may be equally effective as more traditional HIT programmes for eliciting improvements in endurance performance. Further examination of the biochemical and physiological adaptations which accompany different HIT programmes, as well as investigation into the optimal HIT programme for eliciting performance enhancements in highly trained athletes is required.

Skeletal muscle fatigue in long-distance runners, sprinters and untrained men after repeated drop jumps performed at maximal intensity

One hundred drop jumps were performed at maximal intensity every 20 s in 12 untrained subjects (UT), 9 sprinters (S) and 10 long-distance runners (LDR). Muscle contraction force (P20, P50) induced by percutaneous electrical stimulation (20 Hz and 50 Hz, respectively) as well as maximal voluntary contraction force and the height of vertical jumps performed in different ways decreased (P<0.05) and was not restored to the initial value 20 min post exercise. There was a marked increase in low frequency fatigue (LFF) in all the groups studied as substantiated by a significant decrease in the ratio of P20/P50 immediately after exercise as well as 20 min post exercise compared to pre exercise values (P<0.05). However, low frequency fatigue was similar in UT, S and LDR. The jump height of the sprinters during counter-movement jump and drop jump at 90 degrees decreased to a smaller extent compared to jumps performed by LDR and UT. Muscle pain did not differ between UT, S and LDR at 24 h post exercise. The present data indicate that endurance training status as well as prevalence of muscle fibres of the slow type does not decrease muscle resistance to LFF nor accelerate the recovery of muscle contraction force following maximal, intermittent stretch-shortening cycle exercise.