Gordon L. Warren

Measurement Tools Used in the Study of Eccentric Contraction-Induced Injury

The objective of this review is to evaluate the measurement tools currently used in the study of eccentric contraction-induced muscle injury, with emphasis on their usefulness for quantifying the magnitude and duration of the injury and as indicators of muscle functional deficits. In studies in humans, it was concluded that measurements of maximal voluntary contraction torque and range of motion provide the best methods for quantifying muscle injury. Similarly, in animal studies, the in vitro measurement of electrically elicited force under isometric conditions was considered to be the best of the measurement tools currently in use.For future studies, more effort should be put into measuring other contractile parameters (e.g. force/torque-velocity and force/torque-length relationships, maximal shortening velocity and fatigue susceptibility) that may reflect injury-induced functional impairments. The use of histology, ratings of soreness and the measurement of blood or bath levels of myofibre proteins should be discouraged for purposes of quantifying muscle injury and/or functional impairment.

Mechanisms of Exercise-Induced Muscle Fibre Injury

SummaryExercise for which a skeletal muscle is not adequately conditioned results in focal sites of injury distributed within and among the fibres. Exercise with eccentric contractions is particularly damaging. The injury process can be hypothesised to occur in several stages. First, an initial phase serves to inaugurate the sequence. Hypotheses for the initial event can be categorised as either physical or metabolic in nature. We argue that the initial event is physical, that stresses imposed on sarcolemma by sarcomere length inhomogeneities occurring during eccentric contractions cause disruption of the normal permeability barrier provided by the cell membrane and basal lamina. This structural disturbance allows Ca++ to enter the fibre down its electrochemical gradient, precipitating the Ca++overload phase. If the breaks in the sarcolemma are relatively minor, the entering Ca++ may be adequately handled by ATPase pumps that sequester and extrude Ca++ from the cytoplasm (‘reversible’ injury). However, if the Ca++ influx overwhelms the Ca++ pumps and free cytosolic Ca++ concentration rises, the injury becomes ‘irreversible’. Elevations in intracellular Ca++ levels activate a number of Ca++-dependent proteolytic and phospholipolytic pathways that are indigenous to the muscle fibres, which respectively degrade structual and contractile proteins and membrane phospholipids; for instance, it has been demonstrated that elevation of intracellular Ca++ levels with Ca++ ionophores results in loss of creatine kinase activity from the fibres through activation of phospholipase A2 and subsequent production of leukotrienes. This autogenetic phase occurs prior to arrival of phagocytic cells, and continues during the inflammatory period when macrophages and other phagocytic cells are active at the damage site. The phagocytic phase is in evidence by 2 to 6 hours after the injury, and proceeds for several days. The regenerative phase then restores the muscle fibre to its normal condition. Repair of the muscle fibres appears to be complete; the fibres adapt during this process so that future bouts of exercise of similar type, intensity, and duration cause less injury to the muscle.

Excitation-contraction uncoupling: major role in contraction-induced muscle injury.

WARREN, G.L., C.P. INGALLS, D.A. LOWE, and R.B. ARMSTRONG. Excitation-contraction uncoupling: major role in contraction-induced muscle injury. Exerc. Sports Sci. Rev., Vol. 29, No. 2, pp. 82-87, 2001. The mechanisms that account for the strength loss after contraction-induced muscle injury remain controversial. We present data showing that (1) most of the early strength loss results from a failure of excitation-contraction coupling and (2) a slow loss of contractile protein in the days after injury prolongs the recovery time.

Mechanical factors in the initiation of eccentric contraction-induced injury in rat soleus muscle.

1. Mechanical factor(s) associated with the initiation of eccentric contraction‐induced muscle injury were investigated in isolated rat soleus muscles (n = 180; 42 protocols with 4‐6 muscles per protocol). Five eccentric contractions were performed with 4 min between contractions. Three levels of peak eccentric contraction force (100, 125 and 150% of pre‐injury maximal isometric tetanic tension, P0), length change (0.1, 0.2 and 0.3 muscle length, L0) and lengthening velocity (0.5, 1.0 and 1.5 L0/s) were utilized. Force was varied with stimulation frequency (10‐150 Hz). The eccentric contractions were initiated at muscle lengths of 0.85 or 0.90 L0. Following the fifth eccentric contraction, the muscle was incubated in Krebs‐Ringer buffer for 60 min. Peak isometric twitch tension (PT), P0, maximal rate of tension development (+ dP/dt), maximal rate of relaxation (‐dP/dt), and creatine kinase (CK) release were measured prior to the five eccentric contractions and at 15 min intervals during the incubation period. Total muscle [Ca2+] was measured after 60 min incubation. 2. The mean (+/‐ S.E.M.) initial decline in P0 for the muscles performing the most injurious protocol was 13.6 +/‐ 4.8% (n = 6); P0 in control muscles immediately following performance of five isometric contractions was elevated 1.2 +/‐ 1.0% (n = 8). These means were different at probability, p = 0.005. Mean [ATP] in muscles immediately following the isometric control and most injurious protocols, respectively, were 16.30 +/‐ 1.49 and 19.84 +/‐ 1.38 mumol/g dry wt (p = 0.229). 3. Decrements in P0, PT, +dP/dt, and ‐dP/dt immediately after the injury protocol were related most closely to the peak forces produced during the eccentric contractions; greater initial declines in P0, +dP/dt and ‐dP/dt were also observed at higher lengthening velocities independent of peak force. Slow declines in P0 and ‐dP/dt during the 60 min incubation following the injury protocol were greatest for muscles performing contractions at the longer initial length. CK release was independent of all mechanical factors with the exception of lengthening velocity. CK activity at 45 and 60 min into the incubation period was greater for muscles lengthened at the highest velocity used (1.5 L0/s). Mean total muscle [Ca2+] for muscles performing the eccentric contractions was elevated by 38% over isometric control muscles but the elevation was unrelated to any of the four mechanical factors. 4. These data support the hypothesis that eccentric contraction‐induced injury is initiated by mechanical factors, with muscle tension playing the dominant role.(ABSTRACT TRUNCATED AT 400 WORDS)

Excitation failure in eccentric contraction-induced injury of mouse soleus muscle.

1. Histological evidence suggests that the force deficit associated with eccentric contraction‐induced muscle injury is due to structural damage to contractile elements within the muscle fibre. Alternatively, the force deficit could be explained by an inability to activate the contractile proteins. It was the objective of this study to investigate the latter possibility. 2. Mouse soleus muscles were isolated, placed in an oxygenated Krebs‐Ringer buffer at 37 degrees C, and baseline measurements were made. The muscle then performed one of three contraction protocols: (1) twenty eccentric (n = 10 muscles); (2) ten eccentric (n = 12); or (3) twenty isometric (n = 10) contractions. At the end of the injury protocol, measurements were made during performance of a passive stretch, twitch and tetanus. Next, force was recorded during exposure of the muscle to buffer containing 50 mM caffeine. 3. Decrements in maximal isometric tetanic force (P0) observed for muscles in the twenty eccentric, ten eccentric, and twenty isometric contraction protocols were 42.6 +/‐ 4.2, 20.0 +/‐ 2.3 and 3.9 +/‐ 2.4%, respectively. However, the caffeine‐elicited forces in muscles from the three protocols were not different when corrected for initial differences in P0 (64.9 +/‐ 1.3, 64.2 +/‐ 2.1 and 68.9 +/‐ 2.5% of pre‐injury P0). The peak caffeine‐elicited force was 118.4 +/‐ 8.6% of post‐injury P0 for the muscles in the twenty eccentric contraction protocol, which was significantly different from that observed for the other protocols (71.8‐80.2% post‐injury P0). These findings indicate that the force deficit in this muscle injury model results from a failure of the excitation process at some step prior to calcium (Ca2+) release by the sarcoplasmic reticulum. 4. In an attempt to locate the site of failure, intracellular measurements were made in injured muscles to test whether injury to the sarcolemma might have resulted in a shift of the resting membrane potential of the muscle fibre. However, microelectrode measurements of resting membrane potential for muscles in the twenty eccentric contraction protocol (‐74.4 +/‐ 0.6 mV) were not different from muscles in the twenty isometric contraction protocol (‐73.4 +/‐ 1.0 mV). These data suggest that membrane resting conductances were normal and are compatible with the idea that the ability of the injured fibres to conduct action potentials was probably not impaired.

Criterion-Referenced Standards for Youth Health-Related Fitness Tests: A Tutorial

A new development in the testing of physical fitness of youth is the use of criterion-referenced standards (CRS). Although three national youth health-related physical fitness (HRPF) tests currently have CRS, a detailed description of the procedures used in their development has not been published nor have the standards been validated. Consequently, the scientific basis of these standards has been questioned. The purposes of this tutorial are (a) to discuss briefly issues related to the development of CRS for HRPF tests, (b) to provide a detailed description of procedures used in development of mile run/walk test CRS as an example, and (c) to illustrate how these standards can be validated. The objective is to stimulate discussion and critical evaluation of CRS for youth HRPF tests.

Decreased EMG median frequency during a second bout of eccentric contractions

PURPOSE Others have reported preferential recruitment of fast motor units in muscles during performance of eccentric contractions and there is evidence that fast muscle fibers are more susceptible to eccentric contraction-induced injury. We tested the hypothesis that during a second bout of maximal eccentric contractions 1 wk after the first, there would be a reduction in the electromyographic (EMG) median frequency (MF) with minimal change in the EMG root-mean-square (RMS), indicating greater reliance on slower motor units. This could provide an explanation for the enhanced resistance to eccentric contraction-induced injury after a single bout of eccentric exercise. METHODS Human subjects performed 50 maximal voluntary eccentric (N = 10) or concentric (N = 10) contractions of the anterior crural muscles on two occasions separated by 1 wk. To determine whether MF changes during the second bout could be a consequence of injury to fibers in fast motor units, the anterior crural muscles of mice were electrically stimulated to perform 50 maximal eccentric (N = 10) or concentric (N = 9) contractions on two occasions separated by 1 wk. In both the humans and mice, torque production and tibialis anterior muscle RMS and MF were measured during the two exercise bouts. RESULTS In human tibialis anterior muscle, MF was 30% lower (P < 0.01) during the second eccentric bout although RMS was the same. In the mice, RMS and MF were unchanged at any time after the first eccentric bout despite torque deficits similar to those observed in the humans. CONCLUSIONS The data indicate that with repetition of maximal voluntary eccentric contractions, there is an increased activation of slow motor units and a concomitant decrease in activation of fast units.

Effect of caffeine ingestion on muscular strength and endurance: a meta-analysis.

PURPOSE Our objective was to perform a systematic review and meta-analysis of the research literature assessing the effect of caffeine ingestion on maximal voluntary contraction (MVC) strength and muscular endurance. METHODS Thirty-four relevant studies between 1939 and 2008 were included in the meta-analyses of caffeines effects on MVC strength (n = 27 studies) and muscular endurance (n = 23 studies). Effect sizes (ES) were calculated as the standardized mean difference and meta-analyses were completed using a random-effects model. RESULTS Overall, caffeine ingestion was found to result in a small beneficial effect on MVC strength (overall ES = 0.19, P = 0.0003). However, caffeine appears to improve MVC strength primarily in the knee extensors (i.e., by approximately 7%, ES = 0.37) and not in other muscle groups such as the forearm or the knee flexors. In an attempt to offer a physiological mechanism behind caffeines ability to improve MVC strength, a meta-analysis was run on ES from nine studies that measured percent muscle activation during MVC in trials comparing caffeine versus placebo; the overall ES (0.67) was highly significant (P = 0.00008) and of moderate to large size, thus implicating an effect of caffeine on the CNS. Caffeine ingestion was also found to exert a small beneficial effect on muscular endurance (overall ES = 0.28, P = 0.00005). However, it appears caffeine improves muscular endurance only when it is assessed using open (i.e., by approximately 18%, ES = 0.37) and not fixed end point tests. CONCLUSIONS Overall, caffeine ingestion improves MVC strength and muscular endurance. The effect on strength appears exclusively in the knee extensors, and the effect on muscular endurance appears only detectable with open end point tests.

Dissociation of force production from MHC and actin contents in muscles injured by eccentric contractions

The primary purpose of this study was to determine the relationship between myosin heavy chain (MHC) and actin contents and maximum isometric tetanic force (Po) in mouse extensor digitorum longus (EDL) muscles following eccentric contraction-induced injury. Po and protein contents were measured in injured (n=80) and contralateral control (n = 80) EDL muscles at the following time points after in vivo injury: sham, 0, 0.25, 1, 3, 5, 14, and 28 days. Po was reduced by 37 ± 2.3% to 49 ± 3.8% (p ≤ 0.05), while MHC and actin contents were unaltered from 0 to 3 days after injury. Whereas Po partially recovered between 3 and 5 days (from −49 ± 3.8% to −35 ± 3.6%), MHC and actin contents in the injured muscles declined by 19 ± 4.9% and 20 ± 5.3%, respectively, by 5 days compared with control muscles. Decrements in Po were similar to the reductions in MHC and actin contents at 14 (∼24%) and 28 (∼11%) days. Evaluation of myofibrillar and soluble protein fractions indicated significant reductions in the content of major proteins at 5 and 14 days. Immunoblots of heat shock protein 72 revealed elevations starting at 0.25 days, peaking during 1–3 days, and declining after 5days. These findings indicate that decreased contractile protein content is not related to the initial decrease in Po. However, decreased MHC and actin contents could account for 58% of the Po reduction at 5 days, and for nearly all the decrements in Po from 14 to 28 days.

Uncoupling of in vivo torque production from EMG in mouse muscles injured by eccentric contractions

1 The main objective of this study was to determine whether eccentric contraction‐induced muscle injury causes impaired plasmalemmal action potential conduction, which could explain the injury‐induced excitation‐contraction coupling failure. Mice were chronically implanted with stimulating electrodes on the left common peroneal nerve and with electromyographic (EMG) electrodes on the left tibialis anterior (TA) muscle. The left anterior crural muscles of anaesthetized mice were stimulated to perform 150 eccentric (ECC) (n= 12 mice) or 150 concentric (CON) (n= 11 mice) contractions. Isometric torque, EMG root mean square (RMS) and M‐wave mean and median frequencies were measured before, immediately after, and at 1, 3, 5 and 14 days after the protocols. In parallel experiments, nicotinic acetylcholine receptor (AChR) concentration was measured in TA muscles to determine whether the excitation failure elicited a denervation‐like response. 2 Immediately after the ECC protocol, torque was reduced by 47–89%, while RMS was reduced by 9–21%; the RMS decrement was not different from that observed for the CON protocol, which did not elicit large torque deficits. One day later, both ECC and CON RMS had returned to baseline values and did not change over the next 2 weeks. However, torque production by the ECC group showed a slow recovery over that time and was still depressed by 12–30% after 2 weeks. M‐wave mean and median frequencies were not affected by performance of either protocol. 3 AChR concentration was elevated by 79 and 368% at 3 and 5 days, respectively, after the ECC protocol; AChR concentration had returned to control levels 2 weeks after the protocol. At the time of peak AChR concentration in the ECC protocol muscles (i.e. 5 days), AChR concentration in CON protocol muscles was not different from the control level. 4 In conclusion, these data demonstrate no major role for impaired plasmalemmal action potential conduction in the excitation‐contraction coupling failure induced by eccentric contractions. Additionally, a muscle injured by eccentric contractions shows a response in AChR concentration similar to a transiently denervated muscle.