ISSUE
I ~~~~~=-~~~ P~O~R ~ T~ A~= D-= E~X~E ~ R~C~I~S~ E __________~
Sports Medicine 13 (2): 108-115, 1992 0112-1642/92/0002-0108/$04.00/0 © Adis International Limited. All rights reserved. SPO'94
Exercise, Muscle Damage and Fatigue* H.-J. Appelt,1 J.M.C Soares 2 and J.A. R. Duarte 2 I Institute for Experimental Morphology, German Sports University, Cologne, Federal Republic of Germany 2 Department of Sport Biology, University of Porto, Portugal
Contents /0 /09
I IO /I/
/ /1 1/ J
Summary
Summary I. Muscle Damage After Prolonged Exercise 2. Muscle Damage After Eccentric Exercise 3. Delayed Onset of Muscle Soreness 4. Metabolic vs Mechanical Origin of Muscle Damage 5. Conclusions
Fatigue as a functional sign and muscle damage as a Slfuctural sign can be observed after prolonged exercise like marathon running or after strenuous exercise, especially with the involvement of eccentric contractions. For fatigue due to prolonged exercise, hypoxic conditions and the formation of free oxygen radicals seem to be of aetiological importance, resulting in an elevated lysosomal activity. Eccentric exercise of high intensity rather results in a mechanical stress to the fibres. Although these different mechanisms can be discerned experimentally, both result in similar impairments of muscle function. A good training status may attenuate the clinical signs of fatigue and muscle damage. The symptoms and events occurring during delayed onset of muscle soreness (DO MS) can be explained by a cascade of events following structural damage to muscle proteins.
A remarkable example of excessive physical exercise leading to fatigue has been described by Sjastram et al. (1987): a 46-year-old man ran 3529km over a period of nearly 7 weeks, which is equivalent to 1.7 marathon runs per day. Biopsies were obtained from vastus lateralis muscle 1 month before and immediately after the whole run. The
*
This article was presented at a Symposium on Fatigue in Sport and Exercise in November 1990 and updated by the author for publication in Sports Medicine.
muscle was infiltrated with inflammatory cells especially in the vicinity of small blood vessels, and an increased amount of connective tissue was found. 10 to 30% of the fibres contained central nuclei, a few necrotic fibres were seen, fibres being phagocytosed, and several regenerated fibres. The uneven staining pattern suggested the existence of architectural disturbances within the fibres. It was concluded from these findings that fibre size variations, and degenerated and regenerated fibres represented the picture of strong wear and tear due to
Exercise, Muscle Damage and Fatigue
the long run. Moreover, the occurrence of angular fibres and fibre type grouping suggested that repeated denervation and reinnervation of parts of the fibre populations had taken place. This description demonstrates that ultralong activity of muscles may lead to considerable changes in the muscle architecture, but, on the other hand, also illustrates the outstanding regeneration capacity of skeletal muscle. Considering the latter, it may be questioned whether skeletal muscle has structural fatigue limits at all. In this article, we describe the structural alterations occurring mainly as acute responses to repetitive or strenuous exercise, with some emphasis on the sensation of delayed onset of muscle soreness. Finally, an approach is presented discriminating between muscle damage of metabolic or mechanical origin.
1. Muscle Damage After Prolonged Exercise Competitive marathon running can lead to considerable disturbances in the contractile apparatus of gastrocnemius muscle (Hikida et al. 1983; Warhol et al. 1985), such as Z-band streaming, myofibrillar lysis and contracture bands. The pathological changes also extend to mitochondria, which show focal swelling and crystalline inclusions, as well as to the sarcolemma and to the sarcotubular system with dilatation and disruption. In accordance with earlier animal studies (Vihko et al. 1978a, 1979), there was evidence of cell necrosis and inflammatory response, but not fibre regeneration, 7 days after the race (Hikida et al. 1983). Another study (Warhol et al. 1985), however, failed to demonstrate evidence of severe fibre injuries or inflammation, but showed various signs of repair starting 1 week after the run; 10 weeks after, the muscle showed mostly a normal structure. The differences in these studies might be attributed to the different training history of the subjects (Salminen et al. 1984) and intensity of running since a protective effect against fibre damage may occur with training (Tiidus & Ianuzzo 1983). Different mechanisms may contribute to muscle
109
f
1.2
r-
E 1.0
E :; 0.8
r--
:! ~ 0.6
~ :2
0.4
os
0.2
.~
~ o
r-
r-
~
n o
2
3
5
7
10
r-
15
Days after exercise
Fig. 1. Activity of p-glucuronidase in m. vastus medialis in mice after exhaustive (9 hours) running. According to this key enzyme, the lysosomal activity is most pronounced 3 days after the exertion. Modified from Salminen & Vihko (1984).
damage after strenuous exercise of long duration. It is well documented that free Qxygen radicals are
generated as a side product of oxidative metabolism (Chance et al. 1979) which may initiate lipid peroxidation and thus cause cell injury (Del Maestro 1980). Muscle fibres, however, have scavenger systems against oxygen stress, which are improved by regular training (Salminen & Vihko 1983). On the other hand, autophagic response is assumed to play an important role in the development of muscle damage. Histochemical studies show that the activities of lysosomal acid hydrolases increase in injured muscle of exercised mice (Salminen & Vihko 1980; Vihko et al. 1978b) and that this increase is due to invading phagocytes rich in acid hydrolase and the increase oflysosomal enzymatic activities in surviving muscle fibres (Vihko et al. 1978a). The activity of ~-glucuronidase as a marker for lysosomal activity was reported to increase the first days following prolonged exertion of mice (fig. 1), reaching its maximum on the third postexercise day (Salminen & Vihko 1984). The corresponding structural changes documented in this study (Salminen & Vihko 1984) were early development of oedema followed by disarray of myofilaments, intrafibre autophagic vacuoles, occurrence of necrotic fibres and invasion of in-
Sports Medicine 13 (2) 1992
110
::J
flammatory cells. These results were interpreted as myopathies of ischaemic, rather than mechanical, origin, similar to those found in human skeletal muscle after temporary complete ischaemia and during intermittent claudication (Sjostrom et al. 1980, 1982) resembling a compression syndrome (Getzen & Carr 1967). In the early phase, hypoxic conditions may be a contributing factor (Decker et al. 1980). The events occuring during muscle damage after strenuous exercise have been described as typical features of muscle structure to be seen step by step during the first week after the exercise bout (Kuipers et al. 1983). However, strenuous exercise as such does not seem to induce autophagic uptake, since the occurrence of autophagic vacuoles was most frequent 3 to 7 days after exercise. It is also unlikely that autophagic response reflects an attempt to wall off damaged structures. The increased autophagic activity in surviving fibres during regeneration would rather simply reflect augmented degradation and turnover (Salminen & Vihko 1984).
1. Muscle Damage After Eccentric Exercise Eccentric exercise requires lower energy costs than concentric exercise (Abbott et al. 1952; Rail 1985), probably because fewer motor units are activated for a given load (Bigland-Ritchie & Woods 1976), but leads to higher tension per cross-sectional area of active skeletal muscle fibres (Davies & White 1981). Typically, eccentric work is performed during downhill running as opposed to uphill running, i.e. when the muscle is being stretched with simultaneous contraction. Such types of work have been systematically compared in animals (Armstrong et al. 1983a). Both uphill and downhill running resulted in elevations in plasma enzymes (CK, LDH) immediately after exercise; elevated levels were recorded only in the eccentric group 2 to 3 days after the exertion (fig. 2). An increase of the glucose-6-phosphate dehydrogenase was observed in the muscles of both groups 1 to 3 days following exercise, being most
o Downhill • Level
:? 600
% 500 E
.s 400
f aI
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5 200 aI
~ 100
aI
~
O~--r---~--.---'---,,---r---.--~ C
0
6
12
24
36
48
72
96
Hours after exercise Fig.2. Plasma creatine kinase (CK) activity in rats after level and downhill running for 30 minutes. Note the second peak after downhill running (eccentric exercise). Modified from Armstrong et a!. (I 983a). C = control
pronounced for the downhill runners. This was always associated with accumulation of mononuclear cells in the muscles. The histological examination revealed focal disruption of the banding pattern of the muscle fibres, and later fibre necrosis and regeneration (Armstrong et al. 1983a). When subjecting untrained volunteers to eccentric bicycle work, similar fibre damage was found at the ultrastructural level (O'Reilly et al. 1987). However, in these untrained subjects, the pathological changes were still persistent and no signs of regneration were visible 10 days after the exercise bout. Since glycogen was still depleted in the fibres, it was concluded that the alterations in muscle structure were related to the degree of energy stores repletion. This could lead to the assumption that fibre damage after intense exercise with eccentric involvement is mainly due to metabolic exhaustion as a sign offatigue and not primarily to mechanical stress. If this was true, similar histological pictures might be observed after all types of exhaustive exercise. However, the muscles of sprinters who had to perform 20 repetitions of 25 seconds on a treadmill corresponding to 86% of their personal best 200m times showed contrary results (Friden et al. 1988); the majority of the fine structural alterations occurred in type lIB fibres, while this fibre type was least glycogen depleted. The preferential tearing of the myofibrils in type lIB fibres may be explained
Exercise, Muscle Damage and Fatigue
111
was faster after the second and third bouts (Newham et al. 1987). It was concluded that the first bout of exercise had caused damage and destruction to a population of susceptible fibres, possibly those near their end of life cycle. The time between the exercise bouts would then have been sufficient for regeneration, and a fibre population of high mechanical resistance would have been available afterwards.
3. Delayed Onset of Muscle Soreness Fig. 3. Light micrograph of a muscle biopsy from a bodybuilder who suffered from rhabdomyolysis. Note intrafibre oedema, hypercontraction areas, and deterioration of myofibrils. Original magnification x 63.
by the fact that their cytoskeleton is less developed and that they have the narrowest Z-band of all fibre types (Payne et al. 1975). The most severe pathological alteration of muscle tissue due to overuse is known as rhabdomyolysis. It may be accompanied by myoglobinuria which can lead to acute renal failure. Hageloch et al. (1988) described a case of severe rhabdomyolysis in a bodybuilder who had selfadministered anabolic steroids together with a strenuous training programme. Serum myoglobin amounted to over 5 000 ,.,.gjL and CK was elevated up to 53900 U/L. The muscle morphology was dramatically altered (fig. 3). It appears from animal experiments that training can reduce the magnitude of pathological alterations which occur after eccentric exercise (Schwane & Armstrong 1983). This has also been documented in humans. Three exercise sessions with maximal eccentric contractions for 20 minutes were performed, each spaced 2 weeks apart. Muscle pain, strength and contractile properties and plasma CK were studied before and after each exercise bout. Muscle tenderness was greatest after the first bout and thereafter progressively decreased; very high plasma CK levels occurred after the first bout, but the other bouts did not affect the plasma CK (fig. 4); the recovery rate of both strength (fig. 5) and force-frequency characteristics
Delayed onset of muscle soreness (OOMS) is the sensation of pain and discomfort in skeletal muscle that occurs after unaccustomed muscular exertion. The soreness increases in intensity in the first 24 to 48 hours after the exercise, remaining symptomatic for 1 to 2 days; 5 days after exercise it is gone (Arm strong 1984). Sore muscles are often described as being stiff or tender, and their ability to produce force is reduced (Evans et al. 1990; Yates & Armbruster 1990). It is clear that DOMS results from overuse of muscles, where the muscle has to produce forces of a greater magnitude or over a longer time than usual. Intensity seems to be the more important determinant than duration (Tiidus & Ianuzzo 1983). Already the early study by Hough (1902) showed that the occurrence of delayed pain was directly related to the peak forces developed and 100
~
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:::- 60
i
3los
40
c:
32 ~ 20
~
(,) e
oc..., ~~~~~~~~~
o
2
4
6
8
11
14 16
29 31 33
Fig. 4 Plasma creatine kinase (CK) levels after repetitive bouts of eccentric exercise on days I, 15 and 29. Note that only the first training session led to a marked increase. Modified from Newham et al. (1987).
Sports Medicine 13 (2) 1992
112
to the rate of force development in rhythmic contractions, but not to the rate of fatigue. Experimental studies done over the century after Hough's description have generally supported his contentions (for refs. see Armstrong 1984). The easiest way of producing DOMS is to force the muscles to perform eccentric contractions (Asmussen 1956; Edwards et al. 1981; Schwane et al. 1983a). It seems probable that the increased tension per unit area of muscle could cause mechanical disruption of structural elements in the muscle fibres themselves (Armstrong et al. 1983a; Friden et al. 1981; Newham et al. 1983) or in the connective tissue that is in series with the contractile elements (Abraham 1977; Tullson & Armstrong 1981), or both. The concept that metabolic waste products may be responsible for DOMS (Asmussen 1956) does not hold anymore for the following reasons: exercise involving eccentric contractions requires relatively low energy expenditure; for a given load, eccentric work requires lower oxygen consumption and produces less lactate than concentric work (Armstrong et al. 1983b; Bonde-Petersen et al. 1972; Schwane et al. 1983). Armstrong (1984) has proposed a possible cascade of events for the development of DOMS based on the assumption that high tensions in muscle cause structural injury: high mechanical forces, particularly eccentric exercise, cause disruption of 100
•
After first bout
D After second bout
80
~
~40
u
> ::2:
20 0
0
2
3
4
6
8
7
Days after exercise
Fig. 5. Changes in maximum voluntary contraction force (MVC) after 2 bouts of eccentric exercise. Note improved recovery after the second bout of exercise. Modified from Newham et al. (I987).
structural proteins in muscle fibres and connective tissue. Structural damage to the sarcolemma are accompanied by a net influx of Ca++ from the interstitium into the muscle fibre, where the mitochondria can accumulate the ion, which inhibits cellular respiration. Consecutively proteolytic enzymes may be activated degrading structural components of the contractile apparatus. The progressive deterioration of the sarcolemma would be accompanied by diffusion of intracellular components into the interstitium and plasma, where they would attract monocytes that convert to macrophages in the areas of injury. Further accumulation of histamine and kinins in the interstitium resulting from phagocytosis and cellular necrosis as well as elevated pressure from tissue oedema could then activate the nociceptors and result in the sensation of DOMS.
4. Metabolic vs Mechanical Origin of Muscle Damage Considering the structural changes in skeletal muscle with prolonged exercise, as marathon running, and eccentric exercise, one gets the impression that these conditions might represent different aspects of a common pathological entity of reversible muscle damage. While for eccentric exercise muscle damage is most probably mechanically induced due to high tensions in single fibres during contraction, the autophagic response seems to play the major role in fibre damage after endurance exercise due to leakage of enzymes and probably also metabolic exhaustion. Both result in a breakdown of muscle fibres which eventually will regenerate. To comparatively study these mechanisms, 2 of us (JS and JD) have subjected rats to different running protocols: both groups ran on a treadmill for 1 hour, the eccentric exercise group with a negative slope of -16 at 60% ofthe maximal speed the animals were able to run downhill, and endurance exercise group without slope at 80% of their maximal speed; the absolute speed was the same for both groups (lOOOm per hour). The animals were killed immediately after exercise or 48 hours later, re0
113
Exercise, Muscle Damage and Fatigue
_
Immediately after 48
Injured fibres
hours after
Distensions
Loss of
cross-
striation
PAS light
Lysosomes
Fig. 6. Structural changes to muscle fibres after I hour of level or downhill running: relative occurrence of injured fibres, sarcomeric distensions, loss of cross striation, glycogen-depleted (PAS light) fibres, and lysosomes in muscle samples of murine soleus muscle immediately and 48 hours after exercise.
spectively, and their soleus muscles were studied in the light and electron microscope. The following results were obtained (fig. 6): The muscles subjected to endurance exercise contained many fibres which were glycogen-depleted and which showed vacuolisation indicating the presence of lysosomes. This, however, was not the case in the group that had to run downhill. These differences clearly indicate the metabolically exhaustive character of the endurance exercise protocol. Alterations of the contractile structures fall into 2 typical patterns: (a) enlargement of the isotropic and anisotropic bands of the myofibrils, giving the impression of structural damage due to high ten-
sions (at the ultrastructural level also characterised by Z-band disruption); or (b) complete loss of the typical striation pattern as seen in longitudinal sections. In the endurance group, 16% of the fibres showed alterations of the first type but virtually none of the second type immediately after exercise. In contrast, 48 hours after the exercise bout, the histological picture was reversed; distension of the sarcomeres was only rarely seen, but 15% of the fibres showed focally a complete loss of the striation pattern. For the downhill runners, 33% of the fibres showed disruptions within the sarcomeres immediately after exercise, but the striation pattern was still visible, which supports the assumption of a great mechanical stress during downhill running. 48 hours after the exercise, alterations in the striation pattern (in the sense of disruption) were still visible in 15% of the fibres, but in 29% of the fibres the striation pattern had disappeared focally. The high incidence of disruptions of the myofibrils in the muscles of the downhill runners can be attributed to mechanical stress, since they have especially been observed immediately after eccentric exercise; the mechanical aetiology is further supported by the absence of glycogen-depleted fibres. In contrast, the changes in striation pattern observed in the endurance group predominantly occurred in those fibres which also were glycogen depleted. This suggests primarily a metabolic aetiology of the breakdown of contractile material, also supported by the high percentage of fibres which showed complete loss of sarcomeric organisation. In this case the myofibrillar disruptions and breakdown may have been induced by autophagic activity of the fibres.
5. Conclusions Muscle damage may occur especially in subjects who perform unaccustomed types of exercise, who suddenly increase the magnitude and/or intensity of training, or who engage particularly in eccentric exercise. Fatigue and damage may be judged by several laboratory parameters like plasma activities of muscle enzymes, morphology of muscle
114
biopsies (not recommended because of its invasive character), or most simply by clinical symptoms like pain and muscle tenderness. Although both metabolic and mechanical origins of muscle damage can be discerned, they represent a common entity with regard to their practical importance. Since skeletal muscle has a remarkable ability for regeneration, chronic effects of overuse can probably be ignored. However, the muscular system should be given a sufficient time for regeneration after strenuous training sessions.
References Abbott BC, Bigland B, Ritchie JM. The physiological cost of negative work. Journal of Physiology (London) 117: 380-390, 1952 Abraham WM. Factors in delayed muscle soreness. Medicine and Science in Sports 9: 11-20, 1977 Armstrong RB. Mechanisms of exercise-induced delayed onset muscular soreness: a brief review. Medicine and Science in Sports and Exercise 16: 529-538, 1984 Armstrong RB, Laughlin MH, Rome L, Taylor CR. Metabolism of rats running up and down an incline. Journal of Applied Physiology 55: 518-521, 1983b Armstrong RB, Ogilvie RW, Schwane JA. Eccentric exerciseinduced injury to rat skeletal muscle. Journal of Applied Physiology 54: 80-93, 1983a Asmussen E. Observations on experimental muscular soreness. Acta Rheumatologica Scandinavica 2: 109-116, 1956 Bigland-Ritchie B, Woods JJ. Integrated electromyogram and oxygen uptake during positive and negative work. Journal of Physiology (London) 260: 267-277, 1976 Bonde-Petersen F, Knuttgen HG, Henriksson J. Muscle metabolism during exercise with concentric and eccentric contractions. Journal of Applied Physiology 33: 792-795, 1972 Chance B, Sies H, Boveris A. Hydroperoxide metabolism in mammalian organs. Physiological Reviews 59: 527-605, 1979 Davies CTM, White JM. Muscle weakness following eccentric work in man. Pfhigers Archiv 392: 166-171, 1981 Decker RS, Poole AR, Crie JS, Dingle JT, Wildenthal K. Lysosomal alterations in hypoxic and reoxygenated hearts. n. Immunohistochemical and biochemical changes in cathepsin D. American Journal of Pathology 98: 445-456, 1980 Del Maestro RF. An approach to free radicals in medicine and biology. Acta Physiologica Scandinavica 492: 153-168, 1980 Edwards RHT, Mills KR, Newham DJ. Measurement of severity and distribution of experimental muscle tenderness. Journal of Physiology (London) 317: I P-2P, 1981 Evans DT, Smith LL, Chenier TC, Israel RG. McCammon MR, et al. Changes in peak torque, limb volume, and delayed onset muscle soreness following repetitive eccentric contractions. Abstract. International Journal of Sports Medicine 11: 403, 1990 Firden J, Seger J, Ekblom B. Sublethal muscle fibre injuries after high-tension anaerobic exercise. European Journal of Applied Physiology 57: 360-368, 1988 Friden J, Sjiistriim M, Ekblom B. A morphological study of delayed muscle soreness. Experientia 37: 506-507, 1981
Sports Medicine 13 (2) 1992
Getzen LC, Carr III JE. Etiology of anterior tibial compartment syndrome. Surgery, Gynecology and Obstetrics 125: 347-350, 1967 Hageloch W, Appell HJ, Weicker H. Rhabdomyolyse bei Bodybuilder unter Anabolika-Einnahme. Sportverletzung Sportschaden 2: 122-125, 1988 Hikida RS, Staron RS, Hagerman FS, Sherman WM, Costill DL. Muscle fiber necrosis associated with human marathon runners. Journal of Neurological Science 59: 185-203, 1983 Hough T. Ergographic studies in muscular soreness. American Journal of Physiology 7: 76-92, 1902 Kuipers H, Drukker J, Frederick PM, Geurten P, von Kranenburg G. Muscle degeneration after exercise in rats. International Journal of Sports Medicine 4: 45-51, 1983 Newham DJ, Jones DA, Clarkson PM. Repeated high-force eccentric exercise: effects on muscle pain and damage. Journal of Applied Physiology 63: 1381-1386, 1987 Newham DJ, McPhail G, Mills KB, Edwards RHT. Ultrastructural changes after concentric and eccentric contractions of human muscle. Journal of Neurological Science 61: 109-122, 1983 O'Reilly KP, Warhol MJ, Fielding RA, Frontera WR, Meredith CN, et al. Eccentric exercise-induced muscle damage impairs muscle glycogen repletion. Journal of Applied Physiology 63: 252-256, 1987 Payne CM, Stem LZ, Curless RG, Hannapel LK. Ultrastructural fiber typing in normal and diseased human muscle. Journal of Neurological Science 25: 88-108, 1975 Rail JA. Energetic aspects of skeletal muscle contraction: implication of fiber types. Exercise and Sports Science Reviews 13: 33-74, 1985 Salminen A, Vihko V, Acid proteolytic capacity in mouse cardiac and skeletal muscles after prolonged submaximal exercise. Pfliigers Archiv 389: 17-20, 1980 Salminen A, Hongisto K, Vihko V. Lysosomal changes related to exercise injuries and training-induced protection in mouse skeletal muscle. Acta Physiologica Scandinavica 120: 15-20, 1984 Salminen A, Vihko V. Endurance training reduces the susceptibility of mouse skeletal muscle to lipid peroxidation in vitro. Acta Physiologica Scandinavica 117: 109-113, 1983 Salminen A, Vihko V. Autophagic response to strenuous exercise in mouse skeletal muscle fibers. Virchows Archiv (Cell Pathology) 45: 97-106, 1984 Schwane JA, Armstrong RB. Effect of training on skeletal muscle injury from downhill running in rats. Journal of Applied Physiology 55: 969-975, 1983 Schwane JA, Johnson SR, Vandenakker CB, Armstrong RB. Delayed-onset muscular soreness and plasma CPK and LPH activities after downhill running. Medicine and Science in Sports and Exercise 15: 51-56, 1983a Schwane JA, Watrous BG, Johnson SR, Armstrong RB. Is lactic acid related to delayed-onset muscle soreness? Physician and Sportsmedicine 11: 124-131, 1983 Sjiistriim M, Angquist KA, Rais O. Intermittent claudication and muscle fiber fine structcure: correlation between clinical and morphological data. Ultrastructural Pathology I: 309-326, 1980 Sjiistriim M, Friden J, Ekblom B. Endurance, what is it? Muscle morphology after an extremely long distance run. Acta Physiologica Scandinavica 130: 513-520, 1987 Sjiistriim M, Neglen P, Friden J, EkliifB. Human skeletal muscle metabolism and morphology after temporary incomplete is-
Exercise, Muscle Damage and Fatigue
chaemia. European Journal of Clinical Investigation 12: 69-79, 1982 Tiidus PM, Ianuzzo CD. Effects of intensity and duration ofmuscular exercise on delayed soreness and serum enzyme activities. Medicine and Science in Sports and Exercise 15: 461-465, 1983 Tullson P, Armstrong RB. Muscle hexose monophosphate shunt activity following exercise. Experientia 37: 1311-1312, 1981 Vihko V, Rantamaki J, Salminen A. Exhaustive physical exercise and acid hydrolase activity in mouse skeletal muscle. Histochemistry 57: 237-249, 1978a Vihko V, Salminen A, Rantamaki J. Acid hydrolase activity in red and white skeletal muscle of mice during a two-week period folowing exhausting exercise. Pfliigers Archiv 378: 99-106, 1978b
115
Vihko V, Salminen A, Rantamaki J. Exhaustive exercise, endurance training, and acid hydrolase activity in mouse skeletal muscle. Journal of Applied Physiology 47: 43-50, 1979 Warhol MJ, Siegel AJ, Evans WJ, Silverman LM, Skeletal muscle injury and repair in marathon runners after competition. American Journal of Pathology 118: 331-338, 1985 Yates JW, Armbruster WJ. Concentric and eccentric strength loss and recovery folowing exercised-induced muscle soreness. Abstract. International Journal of Sports Medicine 11: 403, 1990
Correspondence and reprints: Prof. Dr H.-J. Appel/, Institute for Experimental Morphology, German Sports University, P.O. Box 450327,0-5000 Cologne 41, Federal Republic of Germany.
I ~~~~~=-~~~ P~O~R ~ T~ A~= D-= E~X~E ~ R~C~I~S~ E __________~
Sports Medicine 13 (2): 108-115, 1992 0112-1642/92/0002-0108/$04.00/0 © Adis International Limited. All rights reserved. SPO'94
Exercise, Muscle Damage and Fatigue* H.-J. Appelt,1 J.M.C Soares 2 and J.A. R. Duarte 2 I Institute for Experimental Morphology, German Sports University, Cologne, Federal Republic of Germany 2 Department of Sport Biology, University of Porto, Portugal
Contents /0 /09
I IO /I/
/ /1 1/ J
Summary
Summary I. Muscle Damage After Prolonged Exercise 2. Muscle Damage After Eccentric Exercise 3. Delayed Onset of Muscle Soreness 4. Metabolic vs Mechanical Origin of Muscle Damage 5. Conclusions
Fatigue as a functional sign and muscle damage as a Slfuctural sign can be observed after prolonged exercise like marathon running or after strenuous exercise, especially with the involvement of eccentric contractions. For fatigue due to prolonged exercise, hypoxic conditions and the formation of free oxygen radicals seem to be of aetiological importance, resulting in an elevated lysosomal activity. Eccentric exercise of high intensity rather results in a mechanical stress to the fibres. Although these different mechanisms can be discerned experimentally, both result in similar impairments of muscle function. A good training status may attenuate the clinical signs of fatigue and muscle damage. The symptoms and events occurring during delayed onset of muscle soreness (DO MS) can be explained by a cascade of events following structural damage to muscle proteins.
A remarkable example of excessive physical exercise leading to fatigue has been described by Sjastram et al. (1987): a 46-year-old man ran 3529km over a period of nearly 7 weeks, which is equivalent to 1.7 marathon runs per day. Biopsies were obtained from vastus lateralis muscle 1 month before and immediately after the whole run. The
*
This article was presented at a Symposium on Fatigue in Sport and Exercise in November 1990 and updated by the author for publication in Sports Medicine.
muscle was infiltrated with inflammatory cells especially in the vicinity of small blood vessels, and an increased amount of connective tissue was found. 10 to 30% of the fibres contained central nuclei, a few necrotic fibres were seen, fibres being phagocytosed, and several regenerated fibres. The uneven staining pattern suggested the existence of architectural disturbances within the fibres. It was concluded from these findings that fibre size variations, and degenerated and regenerated fibres represented the picture of strong wear and tear due to
Exercise, Muscle Damage and Fatigue
the long run. Moreover, the occurrence of angular fibres and fibre type grouping suggested that repeated denervation and reinnervation of parts of the fibre populations had taken place. This description demonstrates that ultralong activity of muscles may lead to considerable changes in the muscle architecture, but, on the other hand, also illustrates the outstanding regeneration capacity of skeletal muscle. Considering the latter, it may be questioned whether skeletal muscle has structural fatigue limits at all. In this article, we describe the structural alterations occurring mainly as acute responses to repetitive or strenuous exercise, with some emphasis on the sensation of delayed onset of muscle soreness. Finally, an approach is presented discriminating between muscle damage of metabolic or mechanical origin.
1. Muscle Damage After Prolonged Exercise Competitive marathon running can lead to considerable disturbances in the contractile apparatus of gastrocnemius muscle (Hikida et al. 1983; Warhol et al. 1985), such as Z-band streaming, myofibrillar lysis and contracture bands. The pathological changes also extend to mitochondria, which show focal swelling and crystalline inclusions, as well as to the sarcolemma and to the sarcotubular system with dilatation and disruption. In accordance with earlier animal studies (Vihko et al. 1978a, 1979), there was evidence of cell necrosis and inflammatory response, but not fibre regeneration, 7 days after the race (Hikida et al. 1983). Another study (Warhol et al. 1985), however, failed to demonstrate evidence of severe fibre injuries or inflammation, but showed various signs of repair starting 1 week after the run; 10 weeks after, the muscle showed mostly a normal structure. The differences in these studies might be attributed to the different training history of the subjects (Salminen et al. 1984) and intensity of running since a protective effect against fibre damage may occur with training (Tiidus & Ianuzzo 1983). Different mechanisms may contribute to muscle
109
f
1.2
r-
E 1.0
E :; 0.8
r--
:! ~ 0.6
~ :2
0.4
os
0.2
.~
~ o
r-
r-
~
n o
2
3
5
7
10
r-
15
Days after exercise
Fig. 1. Activity of p-glucuronidase in m. vastus medialis in mice after exhaustive (9 hours) running. According to this key enzyme, the lysosomal activity is most pronounced 3 days after the exertion. Modified from Salminen & Vihko (1984).
damage after strenuous exercise of long duration. It is well documented that free Qxygen radicals are
generated as a side product of oxidative metabolism (Chance et al. 1979) which may initiate lipid peroxidation and thus cause cell injury (Del Maestro 1980). Muscle fibres, however, have scavenger systems against oxygen stress, which are improved by regular training (Salminen & Vihko 1983). On the other hand, autophagic response is assumed to play an important role in the development of muscle damage. Histochemical studies show that the activities of lysosomal acid hydrolases increase in injured muscle of exercised mice (Salminen & Vihko 1980; Vihko et al. 1978b) and that this increase is due to invading phagocytes rich in acid hydrolase and the increase oflysosomal enzymatic activities in surviving muscle fibres (Vihko et al. 1978a). The activity of ~-glucuronidase as a marker for lysosomal activity was reported to increase the first days following prolonged exertion of mice (fig. 1), reaching its maximum on the third postexercise day (Salminen & Vihko 1984). The corresponding structural changes documented in this study (Salminen & Vihko 1984) were early development of oedema followed by disarray of myofilaments, intrafibre autophagic vacuoles, occurrence of necrotic fibres and invasion of in-
Sports Medicine 13 (2) 1992
110
::J
flammatory cells. These results were interpreted as myopathies of ischaemic, rather than mechanical, origin, similar to those found in human skeletal muscle after temporary complete ischaemia and during intermittent claudication (Sjostrom et al. 1980, 1982) resembling a compression syndrome (Getzen & Carr 1967). In the early phase, hypoxic conditions may be a contributing factor (Decker et al. 1980). The events occuring during muscle damage after strenuous exercise have been described as typical features of muscle structure to be seen step by step during the first week after the exercise bout (Kuipers et al. 1983). However, strenuous exercise as such does not seem to induce autophagic uptake, since the occurrence of autophagic vacuoles was most frequent 3 to 7 days after exercise. It is also unlikely that autophagic response reflects an attempt to wall off damaged structures. The increased autophagic activity in surviving fibres during regeneration would rather simply reflect augmented degradation and turnover (Salminen & Vihko 1984).
1. Muscle Damage After Eccentric Exercise Eccentric exercise requires lower energy costs than concentric exercise (Abbott et al. 1952; Rail 1985), probably because fewer motor units are activated for a given load (Bigland-Ritchie & Woods 1976), but leads to higher tension per cross-sectional area of active skeletal muscle fibres (Davies & White 1981). Typically, eccentric work is performed during downhill running as opposed to uphill running, i.e. when the muscle is being stretched with simultaneous contraction. Such types of work have been systematically compared in animals (Armstrong et al. 1983a). Both uphill and downhill running resulted in elevations in plasma enzymes (CK, LDH) immediately after exercise; elevated levels were recorded only in the eccentric group 2 to 3 days after the exertion (fig. 2). An increase of the glucose-6-phosphate dehydrogenase was observed in the muscles of both groups 1 to 3 days following exercise, being most
o Downhill • Level
:? 600
% 500 E
.s 400
f aI
300
5 200 aI
~ 100
aI
~
O~--r---~--.---'---,,---r---.--~ C
0
6
12
24
36
48
72
96
Hours after exercise Fig.2. Plasma creatine kinase (CK) activity in rats after level and downhill running for 30 minutes. Note the second peak after downhill running (eccentric exercise). Modified from Armstrong et a!. (I 983a). C = control
pronounced for the downhill runners. This was always associated with accumulation of mononuclear cells in the muscles. The histological examination revealed focal disruption of the banding pattern of the muscle fibres, and later fibre necrosis and regeneration (Armstrong et al. 1983a). When subjecting untrained volunteers to eccentric bicycle work, similar fibre damage was found at the ultrastructural level (O'Reilly et al. 1987). However, in these untrained subjects, the pathological changes were still persistent and no signs of regneration were visible 10 days after the exercise bout. Since glycogen was still depleted in the fibres, it was concluded that the alterations in muscle structure were related to the degree of energy stores repletion. This could lead to the assumption that fibre damage after intense exercise with eccentric involvement is mainly due to metabolic exhaustion as a sign offatigue and not primarily to mechanical stress. If this was true, similar histological pictures might be observed after all types of exhaustive exercise. However, the muscles of sprinters who had to perform 20 repetitions of 25 seconds on a treadmill corresponding to 86% of their personal best 200m times showed contrary results (Friden et al. 1988); the majority of the fine structural alterations occurred in type lIB fibres, while this fibre type was least glycogen depleted. The preferential tearing of the myofibrils in type lIB fibres may be explained
Exercise, Muscle Damage and Fatigue
111
was faster after the second and third bouts (Newham et al. 1987). It was concluded that the first bout of exercise had caused damage and destruction to a population of susceptible fibres, possibly those near their end of life cycle. The time between the exercise bouts would then have been sufficient for regeneration, and a fibre population of high mechanical resistance would have been available afterwards.
3. Delayed Onset of Muscle Soreness Fig. 3. Light micrograph of a muscle biopsy from a bodybuilder who suffered from rhabdomyolysis. Note intrafibre oedema, hypercontraction areas, and deterioration of myofibrils. Original magnification x 63.
by the fact that their cytoskeleton is less developed and that they have the narrowest Z-band of all fibre types (Payne et al. 1975). The most severe pathological alteration of muscle tissue due to overuse is known as rhabdomyolysis. It may be accompanied by myoglobinuria which can lead to acute renal failure. Hageloch et al. (1988) described a case of severe rhabdomyolysis in a bodybuilder who had selfadministered anabolic steroids together with a strenuous training programme. Serum myoglobin amounted to over 5 000 ,.,.gjL and CK was elevated up to 53900 U/L. The muscle morphology was dramatically altered (fig. 3). It appears from animal experiments that training can reduce the magnitude of pathological alterations which occur after eccentric exercise (Schwane & Armstrong 1983). This has also been documented in humans. Three exercise sessions with maximal eccentric contractions for 20 minutes were performed, each spaced 2 weeks apart. Muscle pain, strength and contractile properties and plasma CK were studied before and after each exercise bout. Muscle tenderness was greatest after the first bout and thereafter progressively decreased; very high plasma CK levels occurred after the first bout, but the other bouts did not affect the plasma CK (fig. 4); the recovery rate of both strength (fig. 5) and force-frequency characteristics
Delayed onset of muscle soreness (OOMS) is the sensation of pain and discomfort in skeletal muscle that occurs after unaccustomed muscular exertion. The soreness increases in intensity in the first 24 to 48 hours after the exercise, remaining symptomatic for 1 to 2 days; 5 days after exercise it is gone (Arm strong 1984). Sore muscles are often described as being stiff or tender, and their ability to produce force is reduced (Evans et al. 1990; Yates & Armbruster 1990). It is clear that DOMS results from overuse of muscles, where the muscle has to produce forces of a greater magnitude or over a longer time than usual. Intensity seems to be the more important determinant than duration (Tiidus & Ianuzzo 1983). Already the early study by Hough (1902) showed that the occurrence of delayed pain was directly related to the peak forces developed and 100
~
E 80
-li.
:::- 60
i
3los
40
c:
32 ~ 20
~
(,) e
oc..., ~~~~~~~~~
o
2
4
6
8
11
14 16
29 31 33
Fig. 4 Plasma creatine kinase (CK) levels after repetitive bouts of eccentric exercise on days I, 15 and 29. Note that only the first training session led to a marked increase. Modified from Newham et al. (1987).
Sports Medicine 13 (2) 1992
112
to the rate of force development in rhythmic contractions, but not to the rate of fatigue. Experimental studies done over the century after Hough's description have generally supported his contentions (for refs. see Armstrong 1984). The easiest way of producing DOMS is to force the muscles to perform eccentric contractions (Asmussen 1956; Edwards et al. 1981; Schwane et al. 1983a). It seems probable that the increased tension per unit area of muscle could cause mechanical disruption of structural elements in the muscle fibres themselves (Armstrong et al. 1983a; Friden et al. 1981; Newham et al. 1983) or in the connective tissue that is in series with the contractile elements (Abraham 1977; Tullson & Armstrong 1981), or both. The concept that metabolic waste products may be responsible for DOMS (Asmussen 1956) does not hold anymore for the following reasons: exercise involving eccentric contractions requires relatively low energy expenditure; for a given load, eccentric work requires lower oxygen consumption and produces less lactate than concentric work (Armstrong et al. 1983b; Bonde-Petersen et al. 1972; Schwane et al. 1983). Armstrong (1984) has proposed a possible cascade of events for the development of DOMS based on the assumption that high tensions in muscle cause structural injury: high mechanical forces, particularly eccentric exercise, cause disruption of 100
•
After first bout
D After second bout
80
~
~40
u
> ::2:
20 0
0
2
3
4
6
8
7
Days after exercise
Fig. 5. Changes in maximum voluntary contraction force (MVC) after 2 bouts of eccentric exercise. Note improved recovery after the second bout of exercise. Modified from Newham et al. (I987).
structural proteins in muscle fibres and connective tissue. Structural damage to the sarcolemma are accompanied by a net influx of Ca++ from the interstitium into the muscle fibre, where the mitochondria can accumulate the ion, which inhibits cellular respiration. Consecutively proteolytic enzymes may be activated degrading structural components of the contractile apparatus. The progressive deterioration of the sarcolemma would be accompanied by diffusion of intracellular components into the interstitium and plasma, where they would attract monocytes that convert to macrophages in the areas of injury. Further accumulation of histamine and kinins in the interstitium resulting from phagocytosis and cellular necrosis as well as elevated pressure from tissue oedema could then activate the nociceptors and result in the sensation of DOMS.
4. Metabolic vs Mechanical Origin of Muscle Damage Considering the structural changes in skeletal muscle with prolonged exercise, as marathon running, and eccentric exercise, one gets the impression that these conditions might represent different aspects of a common pathological entity of reversible muscle damage. While for eccentric exercise muscle damage is most probably mechanically induced due to high tensions in single fibres during contraction, the autophagic response seems to play the major role in fibre damage after endurance exercise due to leakage of enzymes and probably also metabolic exhaustion. Both result in a breakdown of muscle fibres which eventually will regenerate. To comparatively study these mechanisms, 2 of us (JS and JD) have subjected rats to different running protocols: both groups ran on a treadmill for 1 hour, the eccentric exercise group with a negative slope of -16 at 60% ofthe maximal speed the animals were able to run downhill, and endurance exercise group without slope at 80% of their maximal speed; the absolute speed was the same for both groups (lOOOm per hour). The animals were killed immediately after exercise or 48 hours later, re0
113
Exercise, Muscle Damage and Fatigue
_
Immediately after 48
Injured fibres
hours after
Distensions
Loss of
cross-
striation
PAS light
Lysosomes
Fig. 6. Structural changes to muscle fibres after I hour of level or downhill running: relative occurrence of injured fibres, sarcomeric distensions, loss of cross striation, glycogen-depleted (PAS light) fibres, and lysosomes in muscle samples of murine soleus muscle immediately and 48 hours after exercise.
spectively, and their soleus muscles were studied in the light and electron microscope. The following results were obtained (fig. 6): The muscles subjected to endurance exercise contained many fibres which were glycogen-depleted and which showed vacuolisation indicating the presence of lysosomes. This, however, was not the case in the group that had to run downhill. These differences clearly indicate the metabolically exhaustive character of the endurance exercise protocol. Alterations of the contractile structures fall into 2 typical patterns: (a) enlargement of the isotropic and anisotropic bands of the myofibrils, giving the impression of structural damage due to high ten-
sions (at the ultrastructural level also characterised by Z-band disruption); or (b) complete loss of the typical striation pattern as seen in longitudinal sections. In the endurance group, 16% of the fibres showed alterations of the first type but virtually none of the second type immediately after exercise. In contrast, 48 hours after the exercise bout, the histological picture was reversed; distension of the sarcomeres was only rarely seen, but 15% of the fibres showed focally a complete loss of the striation pattern. For the downhill runners, 33% of the fibres showed disruptions within the sarcomeres immediately after exercise, but the striation pattern was still visible, which supports the assumption of a great mechanical stress during downhill running. 48 hours after the exercise, alterations in the striation pattern (in the sense of disruption) were still visible in 15% of the fibres, but in 29% of the fibres the striation pattern had disappeared focally. The high incidence of disruptions of the myofibrils in the muscles of the downhill runners can be attributed to mechanical stress, since they have especially been observed immediately after eccentric exercise; the mechanical aetiology is further supported by the absence of glycogen-depleted fibres. In contrast, the changes in striation pattern observed in the endurance group predominantly occurred in those fibres which also were glycogen depleted. This suggests primarily a metabolic aetiology of the breakdown of contractile material, also supported by the high percentage of fibres which showed complete loss of sarcomeric organisation. In this case the myofibrillar disruptions and breakdown may have been induced by autophagic activity of the fibres.
5. Conclusions Muscle damage may occur especially in subjects who perform unaccustomed types of exercise, who suddenly increase the magnitude and/or intensity of training, or who engage particularly in eccentric exercise. Fatigue and damage may be judged by several laboratory parameters like plasma activities of muscle enzymes, morphology of muscle
114
biopsies (not recommended because of its invasive character), or most simply by clinical symptoms like pain and muscle tenderness. Although both metabolic and mechanical origins of muscle damage can be discerned, they represent a common entity with regard to their practical importance. Since skeletal muscle has a remarkable ability for regeneration, chronic effects of overuse can probably be ignored. However, the muscular system should be given a sufficient time for regeneration after strenuous training sessions.
References Abbott BC, Bigland B, Ritchie JM. The physiological cost of negative work. Journal of Physiology (London) 117: 380-390, 1952 Abraham WM. Factors in delayed muscle soreness. Medicine and Science in Sports 9: 11-20, 1977 Armstrong RB. Mechanisms of exercise-induced delayed onset muscular soreness: a brief review. Medicine and Science in Sports and Exercise 16: 529-538, 1984 Armstrong RB, Laughlin MH, Rome L, Taylor CR. Metabolism of rats running up and down an incline. Journal of Applied Physiology 55: 518-521, 1983b Armstrong RB, Ogilvie RW, Schwane JA. Eccentric exerciseinduced injury to rat skeletal muscle. Journal of Applied Physiology 54: 80-93, 1983a Asmussen E. Observations on experimental muscular soreness. Acta Rheumatologica Scandinavica 2: 109-116, 1956 Bigland-Ritchie B, Woods JJ. Integrated electromyogram and oxygen uptake during positive and negative work. Journal of Physiology (London) 260: 267-277, 1976 Bonde-Petersen F, Knuttgen HG, Henriksson J. Muscle metabolism during exercise with concentric and eccentric contractions. Journal of Applied Physiology 33: 792-795, 1972 Chance B, Sies H, Boveris A. Hydroperoxide metabolism in mammalian organs. Physiological Reviews 59: 527-605, 1979 Davies CTM, White JM. Muscle weakness following eccentric work in man. Pfhigers Archiv 392: 166-171, 1981 Decker RS, Poole AR, Crie JS, Dingle JT, Wildenthal K. Lysosomal alterations in hypoxic and reoxygenated hearts. n. Immunohistochemical and biochemical changes in cathepsin D. American Journal of Pathology 98: 445-456, 1980 Del Maestro RF. An approach to free radicals in medicine and biology. Acta Physiologica Scandinavica 492: 153-168, 1980 Edwards RHT, Mills KR, Newham DJ. Measurement of severity and distribution of experimental muscle tenderness. Journal of Physiology (London) 317: I P-2P, 1981 Evans DT, Smith LL, Chenier TC, Israel RG. McCammon MR, et al. Changes in peak torque, limb volume, and delayed onset muscle soreness following repetitive eccentric contractions. Abstract. International Journal of Sports Medicine 11: 403, 1990 Firden J, Seger J, Ekblom B. Sublethal muscle fibre injuries after high-tension anaerobic exercise. European Journal of Applied Physiology 57: 360-368, 1988 Friden J, Sjiistriim M, Ekblom B. A morphological study of delayed muscle soreness. Experientia 37: 506-507, 1981
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Getzen LC, Carr III JE. Etiology of anterior tibial compartment syndrome. Surgery, Gynecology and Obstetrics 125: 347-350, 1967 Hageloch W, Appell HJ, Weicker H. Rhabdomyolyse bei Bodybuilder unter Anabolika-Einnahme. Sportverletzung Sportschaden 2: 122-125, 1988 Hikida RS, Staron RS, Hagerman FS, Sherman WM, Costill DL. Muscle fiber necrosis associated with human marathon runners. Journal of Neurological Science 59: 185-203, 1983 Hough T. Ergographic studies in muscular soreness. American Journal of Physiology 7: 76-92, 1902 Kuipers H, Drukker J, Frederick PM, Geurten P, von Kranenburg G. Muscle degeneration after exercise in rats. International Journal of Sports Medicine 4: 45-51, 1983 Newham DJ, Jones DA, Clarkson PM. Repeated high-force eccentric exercise: effects on muscle pain and damage. Journal of Applied Physiology 63: 1381-1386, 1987 Newham DJ, McPhail G, Mills KB, Edwards RHT. Ultrastructural changes after concentric and eccentric contractions of human muscle. Journal of Neurological Science 61: 109-122, 1983 O'Reilly KP, Warhol MJ, Fielding RA, Frontera WR, Meredith CN, et al. Eccentric exercise-induced muscle damage impairs muscle glycogen repletion. Journal of Applied Physiology 63: 252-256, 1987 Payne CM, Stem LZ, Curless RG, Hannapel LK. Ultrastructural fiber typing in normal and diseased human muscle. Journal of Neurological Science 25: 88-108, 1975 Rail JA. Energetic aspects of skeletal muscle contraction: implication of fiber types. Exercise and Sports Science Reviews 13: 33-74, 1985 Salminen A, Vihko V, Acid proteolytic capacity in mouse cardiac and skeletal muscles after prolonged submaximal exercise. Pfliigers Archiv 389: 17-20, 1980 Salminen A, Hongisto K, Vihko V. Lysosomal changes related to exercise injuries and training-induced protection in mouse skeletal muscle. Acta Physiologica Scandinavica 120: 15-20, 1984 Salminen A, Vihko V. Endurance training reduces the susceptibility of mouse skeletal muscle to lipid peroxidation in vitro. Acta Physiologica Scandinavica 117: 109-113, 1983 Salminen A, Vihko V. Autophagic response to strenuous exercise in mouse skeletal muscle fibers. Virchows Archiv (Cell Pathology) 45: 97-106, 1984 Schwane JA, Armstrong RB. Effect of training on skeletal muscle injury from downhill running in rats. Journal of Applied Physiology 55: 969-975, 1983 Schwane JA, Johnson SR, Vandenakker CB, Armstrong RB. Delayed-onset muscular soreness and plasma CPK and LPH activities after downhill running. Medicine and Science in Sports and Exercise 15: 51-56, 1983a Schwane JA, Watrous BG, Johnson SR, Armstrong RB. Is lactic acid related to delayed-onset muscle soreness? Physician and Sportsmedicine 11: 124-131, 1983 Sjiistriim M, Angquist KA, Rais O. Intermittent claudication and muscle fiber fine structcure: correlation between clinical and morphological data. Ultrastructural Pathology I: 309-326, 1980 Sjiistriim M, Friden J, Ekblom B. Endurance, what is it? Muscle morphology after an extremely long distance run. Acta Physiologica Scandinavica 130: 513-520, 1987 Sjiistriim M, Neglen P, Friden J, EkliifB. Human skeletal muscle metabolism and morphology after temporary incomplete is-
Exercise, Muscle Damage and Fatigue
chaemia. European Journal of Clinical Investigation 12: 69-79, 1982 Tiidus PM, Ianuzzo CD. Effects of intensity and duration ofmuscular exercise on delayed soreness and serum enzyme activities. Medicine and Science in Sports and Exercise 15: 461-465, 1983 Tullson P, Armstrong RB. Muscle hexose monophosphate shunt activity following exercise. Experientia 37: 1311-1312, 1981 Vihko V, Rantamaki J, Salminen A. Exhaustive physical exercise and acid hydrolase activity in mouse skeletal muscle. Histochemistry 57: 237-249, 1978a Vihko V, Salminen A, Rantamaki J. Acid hydrolase activity in red and white skeletal muscle of mice during a two-week period folowing exhausting exercise. Pfliigers Archiv 378: 99-106, 1978b
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Vihko V, Salminen A, Rantamaki J. Exhaustive exercise, endurance training, and acid hydrolase activity in mouse skeletal muscle. Journal of Applied Physiology 47: 43-50, 1979 Warhol MJ, Siegel AJ, Evans WJ, Silverman LM, Skeletal muscle injury and repair in marathon runners after competition. American Journal of Pathology 118: 331-338, 1985 Yates JW, Armbruster WJ. Concentric and eccentric strength loss and recovery folowing exercised-induced muscle soreness. Abstract. International Journal of Sports Medicine 11: 403, 1990
Correspondence and reprints: Prof. Dr H.-J. Appel/, Institute for Experimental Morphology, German Sports University, P.O. Box 450327,0-5000 Cologne 41, Federal Republic of Germany.
