E-C coupling failure in mouse EDL muscle after in vivo eccentric contractions

Is recovery from muscle damage retarded by a subsequent bout of eccentric exercise inducing larger decreases in force?

The aim of this study was to investigate whether a subsequent bout of eccentric exercise inducing larger decreases in force than the initial bout would exacerbate muscle damage and retard recovery. Changes in indirect markers of muscle damage were measured over 14 days when 24 maximal eccentric actions of the elbow flexors were performed on days 1 (ECC1) and 7 (ECC2], with electrical stimulation superimposed percutaneously to the elbow flexors during maximal eccentric actions in ECC2. Maximal isometric force (MIF), range of motion (ROM), upper arm circumference, muscle soreness, B-mode ultrasound, and several muscle proteins in the blood were assessed before, immediately after and for 5 days after both bouts. Magnetic resonance Imaging (MRI) was assessed 4 days after both bouts. MIF decreased to 45% of the pre-exercise value immediately after ECC 1 and recovered to 59% by day 7 post-exercise. MIF decreased to 22% of pre-ECC1 value immediately after ECC2, but recovered to 105% of pre-ECC2 value 5 days following ECC2. Recovery of MIF and ROM was slightly retarded for 1-2 days after ECC2. However circumference, muscle soreness, and biochemical parameters did not increase following ECC2. There were no signs of additional damage in ultrasound and MRI after ECC2. It was concluded that a second bout of maximal eccentric exercise with electrical stimulation slightly retarded recovery of muscle function with minimal muscle damage.

Is eccentric exercise-induced torque decrease contraction type dependent?

PURPOSE

This study was designed to determine whether torque decrease following an acute eccentric exercise is contraction type dependent.

METHODS

Ten active males performed an exercise session consisting of five sets of ten maximal eccentric muscle actions of the elbow flexors. Before and immediately after the exercise, maximal voluntary eccentric (-60 degrees.s-1; Ecc60), isometric (0 degrees.s-1; Iso) and concentric (60 degrees.s-1; Con60 and 240 degrees.s-1; Con240) torque were measured. In order to distinguish central from peripheral factors involved in torque decrement, activation level (twitch interpolation technique), myoelectrical activity (RMS) of biceps brachii, as well as electrically evoked M-wave and peak twitch torque (Pt) were recorded.

RESULTS

The eccentric exercise induced a significant torque reduction (P < 0.01), whatever the muscular contraction type [mean (SD): -22.3 (8.1)% for Ecc60; -20.8 (11.2)% for Iso; -18.5 (6.1)% for Con60 and -12.5 (8.9)% for Con240]. Relative torque decrement was however significantly less for Con240 compared with Ecc60, Iso, and Con60 (P < 0.05). Torque decreases were associated with a reduction of both M-wave amplitude (P < 0.01) and Pt (P < 0.001), probably related to an impairment of the excitation-contraction coupling. Concurrently, activation level was reduced (P < 0.01), therefore indicating the occurrence of central fatigue, as also confirmed by RMS decreases for all the conditions (P < 0.05), except Con240.

DISCUSSION

An acute eccentric exercise induced a significant voluntary maximal torque reduction during eccentric, isometric, and concentric muscle actions ascribed to both peripheral and central failure of force production capacity. It can be concluded that eccentric exercise-induced torque decrease is not contraction type dependent.

Muscle damage and soreness after endurance exercise of the elbow flexors

PURPOSE

This study investigated changes in indirect markers of muscle damage after endurance exercise of the elbow flexors and compared the changes with those after maximal eccentric actions (Max-ECC) of the elbow flexors.

METHODS

Eighteen male students rhythmically lifted (1 s) and lowered (1 s) a light dumbbell (1.1-1.8 kg: 9% of MIF) in 60-180 degrees of elbow joint angle for 2 h (2-h Ex). Maximal isometric force (MIF), relaxed (RANG) and flexed elbow joint angles (FANG), upper-arm circumference (CIR), muscle soreness (SOR), B-mode ultrasound (US), and plasma creatine kinase (CK) activity were assessed before and immediately after, and up to 96 h after exercise.

RESULTS

All measures were altered significantly (P < 0.05) after 2-h Ex in a similar time course to Max-ECC; however, changes in RANG, FANG, CIR, US, and CK (peak: 356 +/- 121 IU.L-1) were significantly (P < 0.05) smaller compared with those after Max-ECC. SOR developed immediately after 2-h Ex and peaked 24-48 h after exercise. MIF dropped to 44.1% of the preexercise level, which was significantly (P < 0.05) lower than that after Max-ECC (58.1%), immediately postexercise. MIF recovered to 79.8% at 24 h, and 97.8% at 96 h postexercise, which was a significantly (P < 0.05) faster recovery compared with that of Max-ECC (73.1% at 96 h).

CONCLUSION

These results showed low-intensity continuous muscle contractions (3600 times) resulted in muscle damage; however, the magnitude of the muscle damage was less severe, and the recovery was faster compared with 12 maximal eccentric muscle actions.

Contraction-induced muscle damage is unaffected by vitamin E supplementation

PURPOSE

Vitamin E supplementation may confer a protective effect against eccentrically biased exercise-induced muscle damage through stabilization of the cell membrane and possibly via inhibition of free radical formation. Evidence supporting a protective role of vitamin E after contraction-induced muscle injury in humans is, however, inconsistent. The present study sought to determine the effect of vitamin E supplementation on indices of exercise-induced muscle damage and the postexercise inflammatory response after performance of repeated eccentric muscle contractions.

METHODS

Young healthy men performed a bout of 240 maximal isokinetic eccentric muscle contractions (0.52 rad.s-1) after being supplemented for 30 d with either vitamin E (N = 9; 1200 IU.d-1) or placebo (N = 7; safflower oil).

RESULTS

Measurements of torque (isometric and concentric) decreased (P < 0.05) below preexercise values immediately post- and at 48 h post-exercise. Biopsies taken 24 h postexercise showed a significant increase in the amount of extensive Z-band disruption (P < 0.01); however, neither the torque deficit nor the extent of Z-band disruption were affected by vitamin E. Exercise resulted in increased macrophage cell infiltration (P = 0.05) into muscle, which was also unaffected by vitamin E. Serum CK also increased as a result of the exercise (P < 0.05) with no effect of vitamin E.

CONCLUSION

We conclude that vitamin E supplementation (30 d at 1200 IU.d-1), which resulted in a 2.8-fold higher serum vitamin E concentration (P < 0.01), had no affect on indices of contraction-induced muscle damage nor inflammation (macrophage infiltration) as a result of eccentrically biased muscle contractions.

Development of T-tubular vacuoles in eccentrically damaged mouse muscle fibres

Single fibres were dissected from mouse flexor digitorum brevis muscles and subjected to a protocol of eccentric stretches consisting of ten tetani each with a 40 % stretch. Ten minutes later the fibres showed a reduced force, a shift in the peak of the force-length relation and a steepening of the force-frequency relation. Addition of the fluorescent dye sulforhodamine B to the extracellular space enabled the T-tubular system to be visualized. In unstimulated fibres and fibres subjected to 10 isometric tetani, the T-tubules were clearly delineated. Sulforhodamine B diffused out of the T-tubules with a half-time of 18 +/- 1 s. Following the eccentric protocol, vacuoles connected to the T-tubules were detected in six out of seven fibres. Sulforhodamine B diffused out of the vacuoles of eccentrically damaged fibres extremely slowly with a half-time of 6.3 +/- 2.4 min and diffused out of the T-tubules with a half-time of 39 +/- 4 s. Vacuole production was eliminated by application of 1 mM ouabain to the muscle during the eccentric protocol. On removal of the ouabain, vacuoles appeared over a period of 1 h and were more numerous and more widely distributed than in the absence of ouabain. We propose that T-tubules are liable to rupture during eccentric contraction probably because of the relative movement associated with the inhomogeneity of sarcomere lengths. Such rupture raises intracellular sodium and when the sodium is pumped from the cell by the sodium pump, the volume load of Na(+) and water exceeds the capacity of the T-tubules and causes vacuole production. The damage to the T-tubules may underlie a number of the functional changes that occur in eccentrically damaged muscle fibres.

Silver ions induce Ca2+ release from the SR in vitro by acting on the Ca2+ release channel and the Ca2+ pump

Silver nitrate (AgNO3) is a sulfhydryl oxidizing agent that induces a biphasic Ca2+ release from isolated sarcoplasmic reticulum (SR) vesicles by presumably oxidizing critical sulfhydryl groups in the Ca2+ release channel (CRC), causing the channel to open. To further examine the effects of AgNO3 on the CRC and the Ca2+-ATPase, Ca2+ release was measured in muscle homogenates prepared from rat hindlimb muscle using indo 1. Cyclopiazonic acid (CPA) and ruthenium red (RR) were used to inhibit the Ca2+-ATPase and block the CRC, respectively, before inducing Ca2+ release with both AgNO3 and 4-chloro-m-cresol (4-CMC), a releasing agent specific for the CRC. With AgNO3 and CPA, the early rapid rate of release (phase 1) was increased (P < 0.05) by 42% (314 +/- 5 vs. 446 +/- 39 micromol x g protein(-1) x min(-1)), whereas the slower, more prolonged rate of release (phase 2) was decreased (P < 0.05) by 72% (267 +/- 39 vs. 74 +/- 7.7 micromol x g protein(-1) x min(-1)). RR, in combination with AgNO3, had no effect on phase 1 (P > 0.05) (314 +/- 51 vs. 334 +/- 43 micromol x g protein(-1) x min(-1)) and decreased phase 2 (P < 0.05) by 65% (245 +/- 34 vs. 105 +/- 8.2 micromol x g protein(-1) x min(-1)). With 4-CMC, CPA had no effect (P > 0.05) on either phase 1 or 2. With addition of RR, phase 1 was reduced (P < 0.05) by 59% (2,468 +/- 279 vs. 1,004 +/- 87 micromol x g protein(-1) x min(-1)), and RR completely blocked phase 2. Both AgNO3 and 4-CMC fully inhibited Ca2+-ATPase activity measured in homogenates. These findings indicate that AgNO3, but not 4-CMC, induces Ca2+ release by acting on both the CRC and the Ca2+-ATPase.

Temperature dependency of force loss and Ca(2+) homeostasis in mouse EDL muscle after eccentric contractions

The goals of this study were first to determine the effect of temperature on the force loss that results from eccentric contractions in mouse extensor digitorum longus (EDL) muscles and then to evaluate a potential role for altered Ca(2+) homeostasis explaining the greater isometric force loss observed at the higher temperatures. Isolated muscles performed five eccentric or five isometric contractions at either 15, 20, 25, 30, 33.5, or 37 degrees C. Isometric force loss, caffeine-induced force, lactate dehydrogenase (LDH) release, muscle accumulation of (45)Ca(2+) from the bathing medium, sarcoplasmic reticulum (SR) Ca(2+) uptake, and resting muscle fiber free cytosolic Ca(2+) concentration ([Ca(2+)](i)) were measured. The isometric force loss after eccentric contractions increased progressively as temperature rose; at 15 degrees C, there was no significant loss of force, but at 37 degrees C, there was a 30-39% loss of force. After eccentric contractions, caffeine-induced force was not affected by temperature nor was it different from that of control muscles at any temperature. Loss of cell membrane integrity and subsequent influx of extracellular Ca(2+) as indicated by LDH release and muscle (45)Ca(2+) accumulation, respectively, were minimal over the 15-25 degrees C range, but both increased as an exponential function of temperature between 30 and 37 degrees C. SR Ca(2+) uptake showed no impairment as temperature increased, and the eccentric contraction-induced rise in resting fiber [Ca(2+)](i) was unaffected by temperature over the 15-25 degrees C range. In conclusion, the isometric force loss after eccentric contractions is temperature dependent, but the temperature dependency does not appear to be readily explainable by alterations in Ca(2+) homeostasis.

Force deficits by stretches of activated muscles with constant or increasing velocity

PURPOSE

Force deficits produced by constant (CV) versus increasing velocity (IV) stretches of rat plantar flexor muscles at low and high levels of nerve activation were studied.

METHODS

Twenty repeated stretches were imposed on isometric contractions by ankle rotation from 90 degrees to 40 degrees at 300 degrees.s(-1) and at 3000 degrees.s(-2) during 80-Hz (CV80 and IV80) and 20-Hz stimulation (CV20 and IV20). Rest periods between contractions were 3 min. Isometric and peak stretch forces during the stretch protocols and force-frequency relationships before and 1 h after the stretch protocols were measured.

RESULTS

Peak stretch forces were similar for IV80-CV80 and for IV20-CV20 rats but were lower for IV20-CV20 than for IV80-CV80 rats throughout the stretch protocol. At the end of the stretch protocol, isometric force deficits were similar for IV80 (49.9 +/- 2.1%) and CV80 (54.5 +/- 2.5%) and for IV20 (16.4 +/- 2.8%) and CV20 (15.8 +/- 1.9%) but lower for IV20-CV20 rats. In contrast, for all groups, deficits in peak stretch force were similar at the end of the stretch protocol (IV80: 35.0 +/- 1.8%, CV80: 32.3 +/- 2.2%, IV20: 26.8 +/- 3.6%, CV20: 28.0 +/- 2.0%). After 1 h, isometric force deficits were similar for either IV80-CV80 or IV20-CV20 at 5, 10, 20, 40, 60, and 80 Hz stimulation but were lower for IV20-CV20.

CONCLUSIONS

Variation in velocity of ankle rotation with similar peak stretch forces did not influence the amount of stretch-induced force deficits. High peak stretch forces produced greater isometric force deficits than low peak stretch forces, but the relative loss in peak stretch force was not force dependent. Different mechanisms may account for isometric force deficits and peak stretch force deficits caused by repeated stretches of activated skeletal muscles.

Exercise-induced muscle damage and the potential protective role of estrogen

Exercise-induced muscle damage is a well documented phenomenon that often follows unaccustomed and sustained metabolically demanding activities. This is a well researched, but poorly understood area, including the actual mechanisms involved in the muscle damage and repair cycle. An integrated model of muscle damage has been proposed by Armstrong and is generally accepted. A more recent aspect of exercise-induced muscle damage to be investigated is the potential of estrogen to have a protective effect against skeletal muscle damage. Estrogen has been demonstrated to have a potent antioxidant capacity that plays a protective role in cardiac muscle, but whether this antioxidant capacity has the ability to protect skeletal muscle is not fully understood. In both human and rat studies, females have been shown to have lower creatine kinase (CK) activity following both eccentric and sustained exercise compared with males. As CK is often used as an indirect marker of muscle damage, it has been suggested that female muscle may sustain less damage. However, these findings may be more indicative of the membrane stabilising effect of estrogen as some studies have shown no histological differences in male and female muscle following a damaging protocol. More recently, investigations into the potential effect of estrogen on muscle damage have explored the possible role that estrogen may play in the inflammatory response following muscle damage. In light of these studies, it may be suggested that if estrogen inhibits the vital inflammatory response process associated with the muscle damage and repair cycle, it has a negative role in restoring normal muscle function after muscle damage has occurred. This review is presented in two sections: firstly, the processes involved in the muscle damage and repair cycle are reviewed; and secondly, the possible effects that estrogen has upon these processes and muscle damage in general is discussed. The muscle damage and repair cycle is presented within a model, with particular emphasis on areas that are important to understanding the potential effect that estrogen has upon these processes.

Muscle impairment occurs rapidly and precedes inflammatory cell accumulation after mechanical loading

Modified muscle use can result in muscle atrophy and impairment. We tested whether inflammatory cell concentrations correlate temporally with muscle impairment during modified loading periods. Rat hindlimbs were unloaded for 10 days followed by reloading. The density of neutrophils and ED1+ macrophages was significantly increased by 16.5- and 9.8-fold, respectively, after 1 day of reloading. ED2+ macrophage concentration was not significantly increased until 3 days of reloading. Maximal isometric tetanic tension (P(o); N/cm2) decreased during hindlimb suspension (HS), which was followed by a second drop in P(o) after 2 h of reloading. This latter loss in muscle force was uncoupled with the significant elevation in muscle inflammatory cell concentrations. Experiments where HS soleus muscles were incubated with caffeine revealed that at least 40% of the P(o) decrement at 2 h could be associated with a loss of efficiency of the excitation-contraction (E-C) coupling process. These data suggest that an important mechanism for the early loss in force is the inability to activate the contractile machinery likely caused by a failure in the E-C coupling process during the reloading period.

What mechanisms contribute to the strength loss that occurs during and in the recovery from skeletal muscle injury?

In the workplace or on the athletic field, muscle strength can be decreased by 50% or more following performance of a relatively few high-force, eccentric contractions. The strength loss can be prolonged, taking a month or more for complete recovery. It is important to understand the cause(s) of the strength loss so we can develop means of preventing or attenuating this loss. The cellular-level mechanisms explaining the loss of strength following contraction-induced muscle injury remain controversial. The traditional thought is that initial strength loss is due solely to damage to force-bearing structures within the muscle, as evidenced by histopathology. In addition, inflammation in the days following injury is commonly thought to exacerbate the strength loss. We present data to the contrary. Recent data show that most of the early strength loss results from a failure of excitation-contraction coupling processes and that a slow loss of contractile protein in the days following injury prolongs the time for recovery.

Responses of human elbow flexor muscles to electrically stimulated forced lengthening exercise

This study developed an electrical stimulation model for human elbow flexors to examine eccentric exercise-induced muscle damage and adaptation. Male students (n=17) were randomly placed into one of two groups; isometric (ES-ISO, n=8) and eccentric (ES-ECC, n=9). The elbow joint was fixed at 90 degrees (1.57 rad) and the elbow flexors stimulated percutaneously by an electronic muscle stimulator for 5 s through two electrodes placed over the muscles for ES-ISO. In ES-ECC, the muscles were stimulated similarly to the ES-ISO, but the elbow joint was forcibly extended from an elbow flexed (90 degrees 1.57 rad) to a full-extended position (180 degrees, 3.14 rad) in 3 s. Maximal voluntary isometric force, range of motion, upper arm circumference, muscle thickness by ultrasonography, muscle soreness, plasma creatine kinase and aspartate aminotransferase activities were assessed before and for 4 days after exercise. ES-ECC produced significantly larger changes in all criterion measures compared with ES-ISO (P < 0.01). These findings confirmed that eccentric muscle actions induced muscle damage, but isometric contractions resulted in little or no damage. Six subjects from the ES-ECC group repeated the same eccentric exercise (ECC2) 2 weeks after the first bout (ECC1), and changes in the criterion measures were compared between the bouts. Changes in all criterion measures after ECC2 were significantly smaller than ECC1 (P < 0.01). These results suggest that the first eccentric exercise produced a protective effect against muscle damage in the subsequent eccentric exercise bout, which does not involve adaptations in the central nervous system.

Effect of eccentric contraction-induced injury on force and intracellular pH in rat skeletal muscles

The effect of eccentric contraction on force generation and intracellular pH (pH(i)) regulation was investigated in rat soleus muscle. Eccentric muscle damage was induced by stretching muscle bundles by 30% of the optimal length for a series of 10 tetani. After eccentric contractions, there was reduction in force at all stimulation frequencies and a greater reduction in relative force at low-stimulus frequencies. There was also a shift of optimal length to longer lengths. pH(i) was measured with a pH-sensitive probe, 2',7'-bis-(2-carboxyethyl)-5(6)-carboxyfluorescein AM. pH(i) regulation was studied by inducing an acute acid load with the removal of 20-40 mM ammonium chloride, and the rate of pH(i) recovery was monitored. The acid extrusion rate was obtained by multiplying the rate of pH(i) recovery by the buffering power. The resting pH(i) after eccentric contractions was more acidic, and the rate of recovery from acid load post-eccentric contractions was slower than that from postisometric controls. This is further supported by the slower acid extrusion rate. Amiloride slowed the recovery from an acid load in control experiments. Because the Na(+)/H(+) exchanger is the dominant mechanism for the recovery of pH(i), this suggests that the impairment in the ability of the muscle to regulate pH(i) after eccentric contractions is caused by decreased activity of the Na(+)/H(+) exchanger.

Muscle damage from eccentric exercise: mechanism, mechanical signs, adaptation and clinical applications

In eccentric exercise the contracting muscle is forcibly lengthened; in concentric exercise it shortens. While concentric contractions initiate movements, eccentric contractions slow or stop them. A unique feature of eccentric exercise is that untrained subjects become stiff and sore the day afterwards because of damage to muscle fibres. This review considers two possible initial events as responsible for the subsequent damage, damage to the excitation-contraction coupling system and disruption at the level of the sarcomeres. Other changes seen after eccentric exercise, a fall in active tension, shift in optimum length for active tension, and rise in passive tension, are seen, on balance, to favour sarcomere disruption as the starting point for the damage. As well as damage to muscle fibres there is evidence of disturbance of muscle sense organs and of proprioception. A second period of exercise, a week after the first, produces much less damage. This is the result of an adaptation process. One proposed mechanism for the adaptation is an increase in sarcomere number in muscle fibres. This leads to a secondary shift in the muscle's optimum length for active tension. The ability of muscle to rapidly adapt following the damage from eccentric exercise raises the possibility of clinical applications of mild eccentric exercise, such as for protecting a muscle against more major injuries.

Severity of contraction-induced injury is affected by velocity only during stretches of large strain

Our purpose was to investigate the effect of velocity of stretch on contraction-induced injury to whole skeletal muscles. Single stretches provide an effective method for studying factors that initiate contraction-induced injury. We tested the null hypothesis that the severity of injury is not dependent on the velocity of the stretch. From the plateau of maximum isometric contractions, extensor digitorum longus muscles of mice were administered single stretches in situ of 30--50% strain relative to muscle fiber length (L(f)) at rates of 1--16 L(f)/s. The magnitude of injury was represented by the isometric force deficit 1--10 min after the stretch. Although the null hypothesis was not supported because the force deficit was affected by velocity (r(2) = 0.09), the effect was relatively weak and was not significant except at the largest strain. Velocity had no effect on peak or average force or work input, factors established to have significant relationships with the force deficit. Velocity may play a minor role in contraction-induced injury, but its importance is negligible relative to that of strain.

Eccentric exercise-induced morphological changes in the membrane systems involved in excitation-contraction coupling in rat skeletal muscle

1. Physiological evidence suggests that excitation-contraction (E-C) coupling failure results from eccentric contraction-induced muscle injury because of structural and morphological damage to membrane systems directly associated with the E-C coupling processes within skeletal muscle fibres. In this study using rats, we observed the ultrastructural features of the membrane systems of fast-twitch (FT) and slow-twitch (ST) muscle fibres involved in E-C coupling following level and downhill running exercise. Our aim was to find out whether mechanically mediated events following eccentric exercise caused disorder in the membrane systems involved in E-C coupling, and how soon after exercise such disorder occurred. We also compared the morphological changes of the membrane systems between ST and FT muscle fibres within the same muscles. 2. Single muscle fibres were dissected from triceps brachii muscles of male Fischer 344 rats after level or downhill (16 deg decline) motor-driven treadmill running (18 m min(-1), 5 min running with 2 min rest interval, 18 bouts). All single muscle fibres were histochemically classified into ST or FT fibres. The membrane systems were visualized using Ca(2+)-K(3)Fe(CN)(6)-OsO(4) techniques, and observed by high voltage electron microscopy (120-200 kV). 3. There were four obvious ultrastructural changes in the arrangement of the transverse (t)-tubules and the disposition of triads after the downhill running exercise: (1) an increase in the number of longitudinal segments of the t-tubule network, (2) changes in the direction and disposition of triads, (3) the appearance of caveolar clusters, and (4) the appearance of pentads and heptads (close apposition of two or three t-tubule elements with three or four elements of terminal cisternae of the sarcoplasmic reticulum). The caveolar clusters appeared almost exclusively in the ST fibres immediately after downhill running exercise and again 16 h later. The pentads and heptads appeared almost exclusively in the FT fibres, and their numbers increased dramatically 2-3 days after the downhill running exercise. 4. The eccentric exercise led to the formation of abnormal membrane systems involved in E-C coupling processes. These systems have unique morphological features, which differ between ST and FT fibres, even within the same skeletal muscle, and the damage appears to be concentrated in the FT fibres. These observations also support the idea that eccentric exercise- induced E-C coupling failure is due to physical and chemical disruption of the membrane systems involved in the E-C coupling process in skeletal muscle.

Changes in passive tension of muscle in humans and animals after eccentric exercise

1. This is a report of experiments on ankle extensor muscles of human subjects and a parallel series on the medial gastrocnemius of the anaesthetised cat, investigating the origin of the rise in passive tension after a period of eccentric exercise. 2. Subjects exercised their triceps surae of one leg eccentrically by walking backwards on an inclined, forward-moving treadmill. Concentric exercise required walking forwards on a backwards-moving treadmill. For all subjects the other leg acted as a control. 3. Immediately after both eccentric and concentric exercise there was a significant drop in peak active torque, but only after eccentric exercise was this accompanied by a shift in optimum angle for torque generation and a rise in passive torque. In the eccentrically exercised group some swelling and soreness developed but not until 24 h post-exercise. 4. In the animal experiments the contracting muscle was stretched by 6 mm at 50 mm s(-1) over a length range symmetrical about the optimum length for tension generation. Measurements of passive tension were made before and after the eccentric contractions, using small stretches to a range of muscle lengths, or with large stretches covering the full physiological range. 5. After 150 eccentric contractions, passive tension was significantly elevated over most of the range of lengths. Measurements of work absorption during stretch-release cycles showed significant increases after the contractions. 6. It is suggested that the rise in passive tension in both human and animal muscles after eccentric contractions is the result of development of injury contractures in damaged muscle fibres.

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

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.

Sarcoplasmic reticulum function and muscle contractile character following fatiguing exercise in humans

1. This study examined the alterations in calcium release, calcium uptake and calcium ATPase activity of skeletal muscle sarcoplasmic reticulum in response to a bout of intense dynamic knee extensor exercise, and the relationship between these changes and alterations in muscle contractile characteristics in the human quadriceps. 2. In biopsy samples taken from the vastus lateralis, sarcoplasmic reticulum calcium release and calcium uptake were significantly depressed (P < 0.01 and 0.05, respectively) immediately following the exercise with no alteration in the sarcoplasmic reticulum Ca2+-ATPase activity. 3. A 33 % reduction in the maximum voluntary isometric torque was found following the exercise, with reduced torques from electrically evoked isometric contractions at low frequencies of stimulation (10 and 20 Hz) but not at higher frequencies (50 and 100 Hz). 4. The depressed calcium release was correlated (P < 0.05) with a decreased ratio of torques generated at 20:50 Hz, indicating an involvement in low frequency fatigue; however, no correlations between the muscle relaxation times or rates of change of torque and calcium uptake were observed.

Fiber-type susceptibility to eccentric contraction-induced damage of hindlimb-unloaded rat AL muscles

Slow oxidative (SO) fibers of the adductor longus (AL) were predominantly damaged during voluntary reloading of hindlimb unloaded (HU) rats and appeared explainable by preferential SO fiber recruitment. The present study assessed damage after eliminating the variable of voluntary recruitment by tetanically activating all fibers in situ through the motor nerve while applying eccentric (lengthening) or isometric contractions. Muscles were aldehyde fixed and resin embedded, and semithin sections were cut. Sarcomere lesions were quantified in toluidine blue-stained sections. Fibers were typed in serial sections immunostained with antifast myosin and antitotal myosin (which highlights slow fibers). Both isometric and eccentric paradigms caused fatigue. Lesions occurred only in eccentrically contracted control and HU muscles. Fatigue did not cause lesions. HU increased damage because lesioned- fiber percentages within fiber types and lesion sizes were greater than control. Fast oxidative glycolytic (FOG) fibers were predominantly damaged. In no case did damaged SO fibers predominate. Thus, when FOG, SO, and hybrid fibers are actively lengthened in chronically unloaded muscle, FOG fibers are intrinsically more susceptible to damage than SO fibers. Damaged hybrid-fiber proportions ranged between these extremes.

Eccentric muscle damage: mechanisms of early reduction of force

Pain and weakness are prominent symptoms which occur after a delay in muscles which have been stretched during contraction (eccentric contraction). These symptoms are particularly severe when the exercise is unaccustomed and when the stretch occurs in muscles on the descending limb of the force-length relation, i.e. at long muscle lengths. It is known that sarcomeres are potentially unstable on the descending limb and it has been proposed by Morgan that uncontrolled elongation of some sarcomeres occurs during eccentric contractions on the descending limb. In this article, the evidence that this mechanism leads to the reduced force is considered. If overextended sarcomeres persist after the eccentric exercise it will cause a shift in the peak of the force-length curve. There is also evidence that in some types of muscle, excitation-contraction coupling is impaired and contributes to the muscle weakness. Cytoskeletal proteins stabilize the sarcomeric structure and may be injured either by the overextended sarcomeres or by activation of proteases. The potential of these mechanisms to contribute to the effects of muscle training and to the symptoms of muscle disease, such as muscular dystrophy, is considered.

Generation of tension by skinned fibers and intact skeletal muscles from desmin-deficient mice

We have investigated the physiological role of desmin in skeletal muscle by measuring isometric tension generated in skinned fibres and intact skeletal muscles from desmin knock-out (DES-KO) mice. About 80% of skinned single extensor digitorum longus (EDL) fibres from adult DES-KO mice generated tensions close to that of wild-type (WT) controls. Weights and maximum tensions of intact EDL but not of soleus (SOL) muscles were lowered in DES-KO mice. Repeated contractions with stretch did not affect subsequent isometric tension in EDL muscles of DES-KO mice. Tension during high frequency fatigue (HFF) declined faster and this deficiency was compensated in DES-KO EDL muscles by 5 mM caffeine which had no influence on HFF in WT EDL. Furthermore, caffeine evoked twitch potentiation was higher in DES-KO than in WT muscles. We conclude that desmin is not essential for acute tensile strength but rather for optimal activation of intact myofibres during E-C coupling.

Impact of muscle injury and accompanying inflammatory response on thermoregulation during exercise in the heat

This study examined whether muscle injury and the accompanying inflammatory responses alter thermoregulation during subsequent exercise-heat stress. Sixteen subjects performed 50 min of treadmill exercise (45-50% maximal O(2) consumption) in a hot room (40 degrees C, 20% relative humidity) before and at select times after eccentric upper body (UBE) and/or eccentric lower body (LBE) exercise. In experiment 1, eight subjects performed treadmill exercise before and 6, 25, and 30 h after UBE and then 6, 25, and 30 h after LBE. In experiment 2, eight subjects performed treadmill exercise before and 2, 7, and 26 h after LBE only. UBE and LBE produced marked soreness and significantly elevated creatine kinase levels (P < 0.05), but only LBE increased (P < 0.05) interleukin-6 levels. In experiment 1, core temperatures before and during exercise-heat stress were similar for control and after UBE, but some evidence for higher core temperatures was found after LBE. In experiment 2, core temperatures during exercise-heat stress were 0.2-0.3 degrees C (P < 0.05) above control values at 2 and 7 h after LBE. The added thermal strain after LBE (P < 0.05) was associated with higher metabolic rate (r = 0.70 and 0.68 at 2 and 6-7 h, respectively) but was not related (P > 0.05) to muscle soreness (r = 0.47 at 6-7 h), plasma interleukin-6 (r = 0.35 at 6-7 h), or peak creatine kinase levels (r = 0.22). Local sweating responses (threshold core temperature and slope) were not altered by UBE or LBE. The results suggest that profuse muscle injury can increase body core temperature during exercise-heat stress and that the added heat storage cannot be attributed solely to increased heat production.

Strength loss after eccentric contractions is unaffected by creatine supplementation

This study's objective was to determine whether 14 days of dietary creatine supplementation preceding an injurious bout of eccentric contractions affect the in vivo strength loss of mouse anterior crural muscles. Three groups of nine mice each were fed a meal diet for 14 days, one group at each of three levels of creatine supplementation (i.e., 0, 0.5, and 1% creatine). Electrically stimulated concentric, isometric, and eccentric contraction torques produced about the ankle were measured both before and after a bout of 150 eccentric contractions. Tibialis anterior muscle creatine concentration was significantly increased by the supplementation, being 12% higher in the mice fed the 1% creatine diet compared with control mice. After the bout of eccentric contractions, the reductions in torque (i.e., 46-58%) were similar for the isometric contraction, all eccentric contractions, and the slow (i.e., </=200 (o)/s) concentric contractions; above 200 (o)/s, the percent reduction in concentric torque increased progressively to 85-88% at 1,000-1,200 (o)/s. However, there was no effect of creatine supplementation on the isometric torque loss or on the torque loss at any eccentric or concentric angular velocity (P >/= 0.62). In conclusion, a moderate increase in muscle creatine concentration induced by dietary supplementation in mice does not affect the strength loss after eccentric contractions.

Performance of plantar flexor muscles with eccentric and isometric contractions in intact rats

PURPOSE

To examine the changes in performance of active plantar flexor muscles of rats by controlled dorsiflexion (i.e., stretching of muscles) at two angular velocities.

METHODS

Repeated stretches (30) at two velocities of ankle rotation [slow stretch (0.87 rads x s(-1) (i.e., 50 degrees x (s-1))), fast stretch (10.47 rad x s(-1) (i.e., 600 degrees x s-1))] were superimposed on maximally active muscles from an ankle position of 1.57 rad to 0.70 rad (i.e., from 90 degrees to 40 degrees). Repeated isometric contractions (30) of the same duration (1,900 ms) and rest interval (3 min) were performed at 1.13 rad (i.e., 65 degrees). Performance was assessed by measuring the isometric torque at ankle positions of 1.57 and 0.70 rad, work during concentric contractions [range of motion 1.22 rad (i.e., 70 degrees)], and the time to produce 50% of the maximal isometric torque.

RESULTS

Thirty isometric contractions resulted in a linear reduction in torque (total deficit of 13.8% at 1.57 rad), whereas for slow and fast stretches, half of the total, nonlinear deficit at 1.57 rad (about 30%) was completed after six stretches. Increases in half contraction times were larger for stretches than for isometric contractions. Reductions in isometric torque were greater at an ankle position of 1.57 rad than at 0.70 rad. One hour of rest after the repeated stretches and isometric contractions did not restore muscle performance.

CONCLUSIONS

Isometric contractions of skeletal muscle can create a torque deficit which is much less than that after stretches. Repeated fast and slow stretches resulted in similar torque deficits which did not recover after a rest period of 1 h.

Attenuation of force deficit after lengthening contractions in soleus muscle from trained rats

The purposes of this study were 1) to determine the extent to which endurance training reduces the functional deficit induced by lengthening contractions in the soleus (Sol) muscle and 2) to determine whether young and old rats training at a comparable relative exercise intensity would demonstrate a similar protective effect from lengthening-contraction-induced injury. Young (3-mo-old) and old (23-mo-old) male Fischer 344 rats were randomly assigned to either a control or exercise training group [young control (YC), old control (OC), young trained (YT), old trained (OT)]. Exercise training consisted of 10 wk of treadmill running (15% grade, 45 min/day, and 5 days/wk) such that by the end of training the young and old rats were exercising at 27 and 15 m/min, respectively. After training, contractile properties of the Sol muscle were measured in vitro at 26 degrees C. The percent decrease in maximal isometric specific force (P(o)) was determined after a series of 20 lengthening contractions (20% strain from optimal muscle length, 1 contraction every 5 s). After the lengthening-contraction protocol, Sol muscle P(o) was decreased by approximately 26% (19.6 vs. 14.6 N/cm(2)) and 28% (14.8 vs. 9.6 N/cm(2)) in the YC and OC rats, respectively. After exercise training, the reduction in P(o) was significantly (P < 0.05) attenuated to a similar degree ( approximately 13%) in both YT rats (18.7 vs. 16.2 N/cm(2)) and OT rats (15.8 vs. 13.7 N/cm(2)). It is concluded that exercise training attenuates the force deficit after repeated lengthening contractions to a comparable extent in young and old rats training at a similar exercise intensity.

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.

Neuromuscular disturbance outlasts other symptoms of exercise-induced muscle damage

This study examined the biochemical, immunological, functional, and neuromuscular responses associated with exercise-induced muscle damage in the quadriceps of untrained men. Muscle damage and soreness was elicited with maximal concentric/eccentric muscle actions at 0.53 rads s(-1). Significant (P<0.05) soreness was evident 1, 2, and 3 days following muscle insult, while plasma creatine kinase, a marker of muscle damage, was elevated 3 and 5 days post-insult. Plasma interleukin-Ibeta was significantly increased within 5 min, and remained elevated 1, 2, 5, and 7 days post-insult. Maximal isometric quadriceps function was impaired (P<0. 05) for 5 days following muscle challenge. Maximal isokinetic performance at 1.09 rads s(-1) was diminished (P<0.05) for 2 days post-insult; no significant decrements at 3.14 rads s(-1) were noted. Average electrical activation (iEMG) of the quadriceps was unaltered, but iEMG activity of the rectus femoris - where soreness was focused - was significantly increased. Neuromuscular efficiency (torque/iEMG) was compromised throughout the 10-day post-insult period investigated. While other symptoms of exercise-induced muscle damage dissipate within 7 days, neuromuscular perturbation persists for at least 10 days.

Alterations in sarcoplasmic reticulum function in female vastus lateralis with eccentric exercise

This study examined the alterations in sarcoplasmic reticulum (SR) Ca2+ sequestration function in homogenates during eccentric exercise and recovery and following additional eccentric exercise, and correlated these alterations with changes in force output. Eight healthy, untrained females, aged 20-25 years, cycled for a total of 60 min on an eccentric cycle ergometer (30 min at 66+/-3% VO2 peak and 30 min at 76+/-3% VO2 peak, determined during concentric exercise). Biopsies (extracted from the vastus lateralis) were taken before and after the exercise as well as on days 2, 6 and prior to and following identical exercise on day 14. Ca2+-uptake (nmol/min/mg protein) was unaffected (p > 0.05) following the first session of eccentric exercise; however, by day 2 a depression in uptake (p < 0.05) was observed which persisted throughout the remainder of the experiment. Maximal Ca2+-ATPase activity (nmol/min/mg protein) was elevated (p < 0.05) immediately following the first exercise session, remained elevated through day 2 and returned to pre-exercise levels by day 6 of recovery and increased again by day 14. No changes in either Ca2+-ATPase activity or Ca2+-uptake were observed with exercise on day 14. Both eccentric sessions, performed on days 0 and 14, resulted in similar depressions in force (p < 0.05) immediately following exercise. By day 2 force had recovered to pre-exercise levels. The results demonstrate that a prolonged alteration in SR Ca2+-uptake occurs following eccentric work that is unaccompanied by parallel changes in either SR Ca2+-ATPase activity or mechanical performance.

Early events in stretch-induced muscle damage

Unaccustomed exercise involving stretch of active muscle at long length causes an immediate loss of tension-generating capacity, a shift of optimum length, and changes in excitation-contraction coupling. Eventually, fiber damage may be observed, resulting in pain and tenderness. The subject of this review is the early stage in this process, particularly the cause of the immediate drop in tension. There is strong evidence pointing to sarcomere length instabilities and nonuniformities as important contributors to these changes. The evidence includes the influence of initial length, electron microscopy of rapidly fixed active fibers, the shift in optimum length in single fibers, and the effects of training on sacomere numbers. Experiments using Ca(2+)-sensitive dyes clearly show changes in excitiation-contraction coupling, but cross-species comparisons indicate that these are not always able to explain the consequences seen. We conclude that sarcomere length instabilities provide the most comprehensive explanation of the early consequences of eccentric exercise.

Intracellular Ca2+ transients in mouse soleus muscle after hindlimb unloading and reloading

The objective of this study was to determine whether altered intracellular Ca(2+) handling contributes to the specific force loss in the soleus muscle after unloading and/or subsequent reloading of mouse hindlimbs. Three groups of female ICR mice were studied: 1) unloaded mice (n = 11) that were hindlimb suspended for 14 days, 2) reloaded mice (n = 10) that were returned to their cages for 1 day after 14 days of hindlimb suspension, and 3) control mice (n = 10) that had normal cage activity. Maximum isometric tetanic force (P(o)) was determined in the soleus muscle from the left hindlimb, and resting free cytosolic Ca(2+) concentration ([Ca(2+)](i)), tetanic [Ca(2+)](i), and 4-chloro-m-cresol-induced [Ca(2+)](i) were measured in the contralateral soleus muscle by confocal laser scanning microscopy. Unloading and reloading increased resting [Ca(2+)](i) above control by 36% and 24%, respectively. Although unloading reduced P(o) and specific force by 58% and 24%, respectively, compared with control mice, there was no difference in tetanic [Ca(2+)](i). P(o), specific force, and tetanic [Ca(2+)](i) were reduced by 58%, 23%, and 23%, respectively, in the reloaded animals compared with control mice; however, tetanic [Ca(2+)](i) was not different between unloaded and reloaded mice. These data indicate that although hindlimb suspension results in disturbed intracellular Ca(2+) homeostasis, changes in tetanic [Ca(2+)](i) do not contribute to force deficits. Compared with unloading, 24 h of physiological reloading in the mouse do not result in further changes in maximal strength or tetanic [Ca(2+)](i).

Swelling of sarcoplasmic reticulum in the periphery of muscle fibres after isometric contractions in rat semimembranosus lateralis muscle

The decline in isometric force, swelling of sarcoplasmic reticulum and loss of desmin was measured in semimembranosus lateralis muscle of male Wistar rats immediately after a short series of brief (500 ms) maximal isometric contractions. For the active muscle, the series ended below (protocol A) and just over muscle optimum length (protocol AA). In one protocol, the muscle remained passive and was extended to lengths just over muscle optimum length (protocol P). After all experimental protocols, no loss of desmin was observed and sarcomere appearance was normal. Protocol A produced swelling (87%) of the sarcoplasmic reticulum but no decline in isometric force. Protocol AA produced larger swelling (147%) of the sarcoplasmic reticulum and an isometric force decline (<49%) at short muscle lengths. Swelling of sarcoplasmic reticulum was observed mainly in the periphery of muscle fibres. Protocol P did not result in swelling of the sarcoplasmic reticulum and isometric force decline. It is concluded that swelling of the sarcoplasmic reticulum in the periphery of muscle fibres after brief maximal isometric contractions is associated with muscle force and not muscle length.

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.