R. B. Armstrong

HINDLIMB MUSCLE FIBER POPULATIONS OF FIVE MAMMALS

The fiber type profiles of hindlimb muscles in the Hartley guinea pig, Sprague-Dawley rat, cat, Galago senegalensis (lesser bushbaby) and Nycticebus coucang (slow loris) were estimated histochemically. Fibers were classified as fast oxidative glycolytic, fast glycolytic or slow oxidative according to their myosin adenosine triphosphatase, α-glycerophosphate dehydrogenase and reduced nicotinamide adenine dinucleotide diaphorase activities. It was found that the soleus and vastus intermedius muscles had the highest proportion of slow oxidative fibers in all five species, demonstrating the constancy of muscle fiber profiles dependent upon anatomical position and functional utilization. The tensor fascia latae and white vastus lateralis of the guinea pig were mostly fast glycolytic, while the red vastus lateralis of the guinea pig consisted of predominantly fast oxidative glycolytic fibers. The majority of muscles investigated in these five mammals were heterogeneous, having a wide range of percentages of the three fiber types.

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.

Initial events in exercise-induced muscular injury.

Immediately following unaccustomed exercise, particularly that with eccentric contractions, there is evidence of injury to skeletal muscle fibers: a) disruption of the normal myofilament structures in some sarcomeres, observable with both light and electron microscope and b) loss of intramuscular proteins (e.g., creatine kinase enzymes) into the plasma, indicating damage to sarcolemma. This pathology is probably responsible for the temporary reductions in muscle force and delayed-onset soreness that can occur following eccentric exercise. The mechanisms underlying this injury are not known, although loss of intracellular Ca2+ homeostasis could play a primary role. In other experimental muscle injury models, elevated [Ca2+]i appears to cause release of muscle enzymes through activation of phospholipase A2, which in turn could induce injury to sarcolemma through production of leukotrienes and prostaglandins, through free O2 radical formation (in the subsequent lipoxygenase and cyclooxygenase reactions), and/or through release of detergent lysophospholipids. On the other hand, the mechanism responsible for the rapid damage to myofibrils caused by increased [Ca2+]i is unknown. Regardless of the cause(s), the initial and early events in the injury process are autogenetic; i.e., they are indigenous to the muscle cells and occur before phagocytic cells enter the injury site.

Delayed-onset muscular soreness and plasma CPK and LDH activities after downhill running.

We tested the hypothesis that running down an incline, during which muscles primarily perform eccentric contractions, causes greater delayed-onset muscle soreness and greater increases in plasma enzyme activities than does running on the level, during which muscles perform similar amounts of concentric and eccentric contractions. Subjective sensations of muscular soreness and plasma activities of CPK and LDH were assessed in seven subjects at 0, 24, 48, and 72 h after 45 min of running (one time on the level and a second time down a 10% incline). Following downhill running (57% of VO2max), significant delayed-onset soreness was experienced in gluteal, quadricep, anterior leg, and posterior leg muscles, and plasma CPK (but not LDH) activity was significantly increased (351% at 24 h). In contrast, following level running (78% of VO2max), no statistically significant soreness occurred in any muscle group, and plasma CPK and LDH activities were not elevated. Thus, our results generally support the hypothesis. Secondarily, we investigated whether delayed-onset soreness with downhill running is accompanied by increases in peripheral white blood cell counts suggestive of inflammation. No such association was observed. We suggest that both delayed-onset muscular soreness and plasma enzyme activities are affected by structural changes in muscle tissue resulting from eccentric contractions.

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)

Design of the mammalian respiratory system. VI. Distribution of mitochondria and capillaries in various muscles

The variability of structures supporting tissue oxygen transport (capillaries) and oxygen consumption (mitochondria) was analyzed in skeletal muscles of wildebeest and dik-dik. Regional differences in mitochondria and capillary densities within individual muscles were found for M. semitendinosus (twofold) but not for M. longissimus dorsi and diaphragm. Comparing 20 different muscles from both animals, the volume density of mitochondria in the muscle fibers [Vv(mt,f)] was significantly higher in diaphragm (10-12%) and varied considerably (1-6%) in the other muscles. The relation between Vv(mt,f) and the number of capillaries per cross-sectional fiber area NA(c,f) showed great variability. In glycolytic fibers Vv(mt,f) was typically low (1%) whereas in oxidative fibers it ranged from 5-15%. No systematic trend was found for the packing of cristae in subsarcolemmal and interfibrillar mitochondria from both types of fibers in large and small animals.

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.

Brain norepinephrine and metabolites after treadmill training and wheel running in rats.

Regional changes in concentrations of brain norepinephrine [NE] and its metabolites after chronic exercise have not been described for exercise protocols not confounded by other stressors. We examined levels of [NE], 3-methoxy-4-hydroxyphenylglycol [MHPG], and 3,4-dihydroxyphenylglycol [DHPG] in the frontal cortex, hippocampus, pons-medulla, and spinal cord after 8 wk of exercise. Male Sprague-Dawley rats (N = 36) were randomly assigned to three conditions: 1) 24-h access to activity wheel running (WR), 2) treadmill running (TR) at 0 degrees incline for 1 h.d-1 at 25-30 m.min-1, or 3) a sedentary control group (C). Levels (nmol.g-1) of [NE], [MHPG], and [DHPG] were assayed by high performance liquid chromatography with electrochemical detection. Planned contrasts (P < 0.05) indicated that exercise training increased succinate dehydrogenase activity (mmol cytochrome C reduced.min-1.g-1 wet weight) in soleus muscle for TR compared with WR or C. [NE] was higher in the pons-medulla and spinal cord for both TR and WR compared with C. [DHPG] was higher in the pons-medulla for TR compared with C, and [MHPG] was higher in the frontal cortex and in the hippocampus for TR compared with C. Our results suggest that treadmill exercise training is accompanied by brain noradrenergic adaptations consistent with increased metabolism of NE in areas containing NE cell bodies and ascending terminals, whereas treadmill running and wheel running are accompanied by increases in levels of NE in the areas of NE cell bodies and the spinal cord, independently of an exercise training effect.