Roger G. Eston

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.

Exercise-Induced Muscle Damage and Potential Mechanisms for the Repeated Bout Effect

Unfamiliar, predominantly eccentric exercise, frequently results in muscle damage. A repeated bout of similar eccentric exercise results in less damage and is referred to as the ‘repeated bout effect’. Despite numerous studies that have clearly demonstrated the repeated bout effect, there is little consensus as to the actual mechanism. In general, the adaptation has been attributed to neural, connective tissue or cellular adaptations. Other possible mechanisms include, adaptation in excitation-contraction coupling or adaptation in the inflammatory response.The ‘neural theory’ predicts that the initial damage is a result of high stress on a relatively small number of active fast-twitch fibres. For the repeated bout, an increase in motor unit activation and/or a shift to slow-twitch fibre activation distributes the contractile stress over a larger number of active fibres. Although eccentric training results in marked increases in motor unit activation, specific adaptations to a single bout of eccentric exercise have not been examined.The ‘connective tissue theory’ predicts that muscle damage occurs when the noncontractile connective tissue elements are disrupted and myofibrillar integrity is lost. Indirect evidence suggests that remodelling of the intermediate filaments and/or increased intramuscular connective tissue are responsible for the repeated bout effect.The ‘cellular theory’ predicts that muscle damage is the result of irreversible sarcomere strain during eccentric contractions. Sarcomere lengths are thought to be highly non-uniform during eccentric contractions, with some sarcomeres stretched beyond myofilament overlap. Loss of contractile integrity results in sarcomere strain and is seen as the initial stage of damage. Some data suggest that an increase in the number of sarcomeres connected in series, following an initial bout, reduces sarcomere strain during a repeated bout and limits the subsequent damage.It is unlikely that one theory can explain all of the various observations of the repeated bout effect found in the literature. That the phenomenon occurs in electrically stimulated contractions in an animal model precludes an exclusive neural adaptation. Connective tissue and cellular adaptations are unlikely explanations when the repeated bout effect is demonstrated prior to full recovery, and when the fact that the initial bout does not have to cause appreciable damage in order to provide a protective effect is considered. It is possible that the repeated bout effect occurs through the interaction of various neural, connective tissue and cellular factors that are dependent on the particulars of the eccentric exercise bout and the specific muscle groups involved.

Muscle soreness, swelling, stiffness and strength loss after intense eccentric exercise.

High-intensity eccentric contractions induce performance decrements and delayed onset muscle soreness. The purpose of this investigation was to study the magnitude and time course of such decrements and their interrelationships in 26 young women of mean(s.d.) age 21.4(3.3) years. Subjects performed 70 maximal eccentric contractions of the elbow flexors on a pulley system, specially designed for the study. The non-exercised arm acted as the control. Measures of soreness, tenderness, swelling (SW), relaxed elbow joint angle (RANG) and isometric strength (STR) were taken before exercise, immediately after exercise (AE), analysis of variance and at 24-h intervals for 11 days. There were significant (P < 0.01, analysis of variance) changes in all factors. Peak effects were observed between 24 and 96 h AE. With the exception of STR, which remained lower (P < 0.01), all variables returned to baseline levels by day 11. A non-significant correlation between pain and STR indicated that pain was not a major factor in strength loss. Also, although no pain was evident, RANG was decreased immediately AE. There was no relationship between SW, RANG and pain. The prolonged nature of these symptoms indicates that repair to damaged soft tissue is a slow process. Strength loss is considered particularly important as it continues when protective pain and tenderness have disappeared. This has implications for the therapeutic management of patients with myopathologies and those receiving eccentric exercise for rehabilitation.

Validation of the Genea Accelerometer

PURPOSE The study aims were: 1) to assess the technical reliability and validity of the GENEA using a mechanical shaker; 2) to perform a GENEA value calibration to develop thresholds for sedentary and light-, moderate-, and vigorous-intensity physical activity; and 3) to compare the intensity classification of the GENEA with two widely used accelerometers. METHODS A total of 47 GENEA accelerometers were attached to a shaker and vertically accelerated, generating 15 conditions of varying acceleration and/or frequency. Reliability was calculated using SD and intrainstrument and interinstrument coefficients of variation, whereas validity was assessed using Pearson correlation with the shaker acceleration as the criterion. Next, 60 adults wore a GENEA on each wrist and on the waist (alongside an ActiGraph and RT3 accelerometer) while completing 10-12 activity tasks. A portable metabolic gas analyzer provided the criterion measure of physical activity. Analyses involved the use of Pearson correlations to establish criterion and concurrent validity and receiver operating characteristic curves to establish intensity cut points. RESULTS The GENEA demonstrated excellent technical reliability (CVintra = 1.4%, CVinter = 2.1%) and validity (r = 0.98, P < 0.001) using the mechanical shaker. The GENEA demonstrated excellent criterion validity using VO2 as the criterion (left wrist, r = 0.86; right wrist, r = 0.83; waist, r = 0.87), on par with the waist-worn ActiGraph and RT3. The GENEA demonstrated excellent concurrent validity compared with the ActiGraph (r = 0.92) and the RT3 (r = 0.97). The waist-worn GENEA had the greatest classification accuracy (area under the receiver operating characteristic curve (AUC) = 0.95), followed by the left (AUC = 0.93) and then the right wrist (AUC = 0.90). The accuracy of the waist-worn GENEA was virtually identical with that of the ActiGraph (AUC = 0.94) and RT3 (AUC = 0.95). CONCLUSION The GENEA is a reliable and valid measurement tool capable of classifying the intensity of physical activity in adults.

Effects of cold water immersion on the symptoms of exercise-induced muscle damage

Cryotherapy is an effective treatment for acute sports injury to soft tissue, although the effect of cryotherapy on exercise-induced muscle damage is unclear. The aim of this study was to assess the effects of cold water immersion on the symptoms of exercise-induced muscle damage following strenuous eccentric exercise. After performing a bout of damage-inducing eccentric exercise (eight sets of five maximal reciprocal contractions at 0.58 rad x s(-1)) of the elbow flexors on an isokinetic dynamometer, 15 females aged 22.0+/-2.0 years (mean +/- s) were allocated to a control group (no treatment, n = 7) or a cryotherapy group (n = 8). Subjects in the cryotherapy group immersed their exercised arm in cold water (15 degrees C) for 15 min immediately after eccentric exercise and then every 12 h for 15 min for a total of seven sessions. Muscle tenderness, plasma creatine kinase activity, relaxed elbow angle, isometric strength and swelling (upper arm circumference) were measured immediately before and for 3 days after eccentric exercise. Analysis of variance revealed significant (P < 0.05) main effects for time for all variables, with increases in muscle tenderness, creatine kinase activity and upper arm circumference, and decreases in isometric strength and relaxed elbow angle. There were significant interactions (P<0.05) of group x time for relaxed elbow angle and creatine kinase activity. Relaxed elbow angle was greater and creatine kinase activity lower for the cryotherapy group than the controls on days 2 and 3 following the eccentric exercise. We conclude that although cold water immersion may reduce muscle stiffness and the amount of post-exercise damage after strenuous eccentric activity, there appears to be no effect on the perception of tenderness and strength loss, which is characteristic after this form of activity.

Measurement of physical activity in children with particular reference to the use of heart rate and pedometry.

SummaryUnderstanding the progression of physical activity behaviour from childhood to adulthood requires a valid, reliable and practical method of assessing activity levels which is appropriate for use in large groups. The measurement of physical activity in large scale research projects requires a method which is low in cost, agreeable to the study volunteer and accurate. Self-report can be used to determine adult activity patterns, but children lack the cognitive ability to recall details about their activity patterns. Heart rate telemetry has been used to estimate daily activity in children as a sole criterion and to validate commercial accelerometers. However, heart rate is an indirect estimate of physical activity which makes assumptions based on the linear relationship between heart rate and oxygen uptake. It is sensitive to emotional stress and body position, and takes longer to reach resting levels after physical exertion compared with oxygen uptake. It also lags behind movement, particularly as children’s physical activity is spasmodic or intermittent in nature. One alternative is the pedometer. Many early studies reported that the pedometer is inaccurate and unreliable in measuring distance or counting steps. While reasonably accurate at mid range speeds, the accuracy of the pedometer decreases in very slow walking or very fast walking or running. However, more recent studies have examined the efficacy of using pedometers to assess daily or weekly activity patterns as a whole, and these have produced more promising results. In this regard, the pedometer has a number of advantages. It is very cheap, objective and does not interfere with daily activities and is therefore appropriate for use in population studies. Commercial accelerometers with a time-sampling mechanism offer further potential and could be used to provide a picture of the pattern of children’s activity. As it has been observed that prolonged activity periods are not typically associated with childhood behaviour patterns, the use of a threshold value for ‘aerobic’ training stimulus is not appropriate as a cut-off value for physical activity. Instead, there is evidence to suggest that the total activity data measured by pedometers over limited periods of time may be more appropriate to assess how active children are.

The effect of exercise-induced muscle damage on isometric and dynamic knee extensor strength and vertical jump performance.

In this study, we assessed the effect of exercise-induced muscle damage on knee extensor muscle strength during isometric, concentric and eccentric actions at 1.57 rad · s -1 and vertical jump performance under conditions of squat jump, countermovement jump and drop jump. The eight participants (5 males, 3 females) were aged 29.5 - 7.1 years (mean - s ). These variables, together with plasma creatine kinase (CK), were measured before, 1 h after and 1, 2, 3, 4 and 7 days after a bout of muscle damaging exercise: 100 barbell squats (10 sets 2 10 repetitions at 70% body mass load). Strength was reduced for 4 days ( P ≪ 0.05) but no significant differences ( P > 0.05) were apparent in the magnitude or rate of recovery of strength between isometric, concentric and eccentric muscle actions. The overall decline in vertical jump performance was dependent on jump method: squat jump performance was affected to a greater extent than countermovement (91.6 - 1.1% vs 95.2 - 1.3% of pre-exercise values, P ≪ 0.05) and drop jump (95.2 - 1.4%, P ≪ 0.05) performance. Creatine kinase was elevated ( P ≪ 0.05) above baseline 1 h after exercise, peaked on day 1 and remained significantly elevated on days 2 and 3. Strength loss after exercise-induced muscle damage was independent of the muscle action being performed. However, the impairment of muscle function was attenuated when the stretch-shortening cycle was used in vertical jumping performance.

Reliability of ratings of perceived effort regulation of exercise intensity.

This study assessed the reliability of ratings of perceived exertion (RPE) for the prescription of exercise intensity during cycling. Ten healthy men (21 to 62 years) and six women (25 to 50 years) attended the laboratory four times, 5 to 7 days apart. On the first visit each subjects maximal oxygen uptake was measured and, subsequently, they cycled at constant work rates based on their perception of ratings 9, 13 and 17 of the Borg 6-20 scale by regulating the resistance of the ergometer without reference to the instrument display panel. Analysis of variance revealed no significant between-trial differences with regard to oxygen uptake (VO2) or heart rate for men or women. The relative exercise intensities corresponding to the 3 ratings of exertion did not differ between men and women. Between-trial correlations for VO2 were highest for visits two and three at RPE levels 9 and 13 (r = 0.83 and r = 0.94) and consistently high (r = 0.92 or greater) for the three trials at RPE17. These data suggest that RPE is a useful frame of reference for the regulation of high levels of exercise intensity in healthy men and women. Small amounts of practice with the scale improve its applicability at lower exercise levels.

The Role of Passive Muscle Stiffness in Symptoms of Exercise-Induced Muscle Damage

We examined whether passive stiffness of an eccentrically exercising muscle group affects the subsequent symptoms of muscle damage. Passive hamstring muscle stiffness was measured during an instrumented straight-leg-raise stretch in 20 subjects (11 men and 9 women) who were subsequently classified as “stiff” (N 7), “normal” (N 6), or “compliant” (N 7). Passive stiffness was 78% higher in the stiff subjects (36.2 3.3 N m rad 1) compared with the compliant subjects (20.3 1.8 N m rad 1). Subjects then performed six sets of 10 isokinetic (2.6 rad s 1) submaximal (60% maximal voluntary contraction) eccentric actions of the hamstring muscle group. Symptoms of muscle damage were documented by changes in isometric hamstring muscle strength, pain, muscle tenderness, and creatine kinase activity on the following 3 days. Strength loss, pain, muscle tenderness, and creatine kinase activity were significantly greater in the stiff compared with the compliant subjects on the days after eccentric exercise. Greater symptoms of muscle damage in subjects with stiffer hamstring muscles are consistent with the sarcomere strain theory of muscle damage. The present study provides experimental evidence of an association between flexibility and muscle injury. Muscle stiffness and its clinical correlate, static flexibility, are risk factors for more severe symptoms of muscle damage after eccentric exercise.

The effect of type of physical activity measure on the relationship between body fatness and habitual physical activity in children: a meta-analysis

Background: The relationship between activity levels and body fat in children is unclear, despite a large number of studies. The issue is clouded by the wide variety of methods used to assess childrens activity levels. It is important to assess whether the type of activity measure influences the fatness-activity relationship. This is a first step to uncovering the role of modifying variables such as gender, age, maturity, etc. Primary objective: This study uses meta-analytic procedures to synthesize the results of such studies and to assess whether the type of activity measure used has an effect on the strength of the relationship observed. Methods and procedures: Fifty studies were located that satisfied the inclusion criteria. Seventy-eight per cent of the studies showed a negative relationship, 18% no relationship and 4% a positive relationship between physical activity and body fatness. Data were analysed using the meta-analytic procedures described by Rosenthal (Meta-analytic Procedures for Social Research, Sage, 1991). Main outcomes and results: The mean effect size indicated a small to moderate, inverse relationship (r =-0.16). Mean effect sizes differed significantly (F(3,5,2) 8.04, p < 0.001) according to the activity measure used: questionnaire, r =-0.14; motion counters, r =-0.18; observation, r =-0.39; heart rate (HR), r = 0.00. Observation measures elicited a significantly stronger relationship with body fat than did questionnaire or heart rate measures (p < 0.05). However, there was no significant difference between the effect sizes elicited by observation and motion counters. Correlational analyses revealed no effect of age group or gender on the strength of the relationship between fatness and activity. Conclusions: This meta-analysis suggests there is a small to moderate relationship between body fat and activity in children. It is important to note, however, that the size of the relationship depends on the activity measure used. It is therefore recommended that direct measures of movement, such as observation or motion counter methods, are used to assess the relationship of activity levels with health.BACKGROUND The relationship between activity levels and body fat in children is unclear, despite a large number of studies. The issue is clouded by the wide variety of methods used to assess childrens activity levels. It is important to assess whether the type of activity measure influences the fatness-activity relationship. This is a first step to uncovering the role of modifying variables such as gender, age, maturity, etc. PRIMARY OBJECTIVE This study uses meta-analytic procedures to synthesize the results of such studies and to assess whether the type of activity measure used has an effect on the strength of the relationship observed. METHODS AND PROCEDURES Fifty studies were located that satisfied the inclusion criteria. Seventy-eight per cent of the studies showed a negative relationship, 18% no relationship and 4% a positive relationship between physical activity and body fatness. Data were analysed using the meta-analytic procedures described by Rosenthal (Meta-analytic Procedures for Social Research, Sage, 1991). MAIN OUTCOMES AND RESULTS The mean effect size indicated a small to moderate, inverse relationship (r = -0.16). Mean effect sizes differed significantly (F(3,52) = 8.04, p < 0.001) according to the activity measure used: questionnaire, r = -0.14; motion counters, r = -0.18; observation, r = -0.39; heart rate (HR), r = 0.00. Observation measures elicited a significantly stronger relationship with body fat than did questionnaire or heart rate measures (p < 0.05). However, there was no significant difference between the effect sizes elicited by observation and motion counters. Correlational analyses revealed no effect of age group or gender on the strength of the relationship between fatness and activity. CONCLUSIONS This meta-analysis suggests there is a small to moderate relationship between body fat and activity in children. It is important to note, however, that the size of the relationship depends on the activity measure used. It is therefore recommended that direct measures of movement, such as observation or motion counter methods, are used to assess the relationship of activity levels with health.