Simultaneous stiffness and force measurements reveal subtle injury to rabbit soleus muscles

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1 Molecular and Cellular Biochemistry 179: , Kluwer Academic Publishers. Printed in the Netherlands. 147 Simultaneous stiffness and force measurements reveal subtle injury to rabbit soleus muscles Robert J. Benz, 1 Jan Fridén 2 and Richard L. Lieber 1 1 Departments of Orthopaedics and Bioengineering, Biomedical Sciences Graduate Group, University of California and Veterans Administration Medical Centers, San Diego, CA, USA; 2 Department of Hand Surgery, Göteborg University, Sweden Abstract The time course of force generation and the time course of muscle stiffness were measured in rabbit soleus muscles during eccentric contraction to understand the underlying basis for the force loss in these muscles. Muscles were activated for 600 msec every 10 sec for 30 min. Soleus muscles contracting isometrically maintained constant tension throughout the treatment period, while muscles subjected to eccentric contraction rapidly dropped tension generation by 75% within the first few minutes and then an additional 10% by the end of 30 min. This indicated a dramatic loss in force-generating ability throughout the 30 min treatment period. To estimate the relative number of cross-bridges attached during the isometric force generation phase immediately preceding each eccentric contraction, stiffness was measured during a small stretch of a magnitude equal to 1.5% of the fiber length. Initially, muscle stiffness exceeded 1300 g/mm and, as eccentric treatment progressed, stiffness decreased to about 900 g/mm. Thus, while muscle stiffness decreased by only 30% over the 30 min treatment period, isometric force decreased by 85%. In isometrically activated muscles, stiffness remained constant throughout the treatment period. These data indicate that, while soleus muscles decreased their force generating capability significantly, there were a number of cross-bridges still attached that were not generating force. In summary, the loss of force generating capacity in the rabbit soleus muscle appears to be related to a fundamental change in myosin cross-bridge properties without the more dramatic morphological changes observed in other eccentric contraction models. These results are compared and contrasted with the observations made on muscles composed primarily of fast fibers.(moll Cell Biochem 179: , 1998) Key words: muscle injury, eccentric contractions, cytoskeleton, biomechanics, fiber types Introduction Muscle structural damage after forced lengthening (eccentric contraction, EC) has been observed in a number of different muscles and experimental models [1 4]. Eccentric muscle contractions are relatively common during normal movement when, for example, the quadriceps femoris are forced to lengthen as the foot strikes the ground. While the mechanism of injury is not completely understood, evidence exists that muscle injury can occur due to mechanical, metabolic and inflammatory factors (Table 1). We previously reported morphological abnormalities in rabbit tibialis anterior (TA) muscle fibers subjected to EC wherein fibers became rounded and enlarged to over four times their normal size [5]. All abnormal fibers were high in myofibrillar ATPase activity indicating a fast-contraction speed and were low in succinate dehydrogenase activity indicating low oxidative capacity. We later identified more subtle changes in these fibers that included loss of the desmin cytoskeletal protein [6], and inclusion of plasma fibronectin [7]. Immunohistochemical studies revealed that loss of desmin staining preceded the loss of cell membrane integrity [6] suggesting that desmin loss was required for cell membrane stability. Subsequently, it was shown that desmin loss proceeded extremely rapidly, with the first losses in desmin staining being observed in the extensor digitorum longus (EDL) muscle within 5 min of initiation of EC and within 15 min in the TA [8]. This led us to propose an injury mechanism in which EC resulted in the rapid Address for offprints: R.L. Lieber, Department of Orthopaedics, U.C. San Diego School of Medicine and V.A. Medical Center, 9500 Gilman Drive, La Jolla, CA , USA

2 148 Table 1. Mechanisms of injury to skeletal muscle Proposed mechanism System studied Time-course Reference Inflammation Human quadriceps Several days [35 37] Inflammation Rabbit TA Several days [38] Mechanical force Mouse EDL Min [19] Mechanical strain Rabbit TA Min [12] Mechanical stress Rat Soleus Min [4] Transmembrane stress mdx mouse EDL and diaphragm Min [31] Sarcomere popping Toad sartorius, rat vastus intermedius Sec [25, 26, 40] Cytoskeletal disruption Rabbit TA and EDL Min [8] Low fiber oxidative capacity Rabbit TA Min [3, 5] Not low fiber oxidative capacity Rabbit TA and EDL? [30] hydrolysis of cytoskeletal proteins such as desmin via intracellular activation of the calcium activated neutral protease, calpain. This proposal was consistent with the observation that mitochondrial calcium levels (presumably reflecting cytoplasmic calcium levels) were elevated in eccentrically exercised muscles [9], and also that exercise results in calpain activation [10]. However, it is possible that the mechanism presented above is not generally applicable to all muscles. For example, while we had observed that injury within TA muscle fibers occurred primarily in fast fibers with low oxidative capacity [3, 5], Armstrong and colleagues observed injury primarily in slow muscle fiber types and muscles composed primarily of slow fibers in rats subjected to downhill running (i.e. exercise biased toward eccentric contractions, [11]). We interpreted this to reflect preferential recruitment of slow fibers and slow muscles during physiological activation in rats compared to supramaximal activation of the entire peripheral nerve in our rabbit eccentric exercise model. A second difference between our experimental results obtained from fast contracting rabbit muscle (i.e. TA and EDL) and those obtained by Armstrong and colleagues on the slow-contracting soleus muscle, was the mechanical cause of muscle injury itself. In the rabbit TA it was demonstrated that the magnitude of force decline was primarily related to the strain imposed upon the muscle rather than the stress [12]. Strain is defined as the relative muscle length change. Warren et al. [4] found that rat soleus muscle injury was best correlated with muscle stress rather than strain. Stress is defined as muscle force divided by muscle cross-sectional area. Thus, in the TA study, the relative length change was the most important determinant of injury while in the rat soleus muscle, force was the primary determinant. While the experimental paradigms used to vary stress and strain in the two studies differed, it is possible that different mechanisms of injury exist in predominantly fast muscles compared to predominantly slow muscles. Further, differences in the results obtained among studies may simply reflect species Fig. 1. Schematic representation of the biochemical cross-bridge cycle along with the associated mechanical events. Cycle begins where ATP binds to actinmyosin complex, displacing actin. Then, ATP is hydrolyzed as the myosin-atp complex. Actin then reassociates with the M ADP P i complex, displacing the reaction products and resulting in increased muscle stiffness. Finally, the attached cross-bridge generates force during power stroke. The current study addressed the steps in the cross-bridge cycle affected by eccentric contraction, shown by question marks. (Adapted from reference [39].)

3 149 differences. Armstrong and colleagues also identified another factor contributing to force loss after eccentric exercise, namely, impaired excitation-contraction coupling [13]. Since cross-bridges are known to cycle through multiple states (Fig. 1), these data implicate impaired cross-bridge attachment in the decreased force generating capacity of muscle after eccentric exercise. To attempt to resolve the various results obtained from different experimental models (i.e. fiber type specific injury, mechanical factor(s) responsible for injury, morphology of injury), we studied the effect of cyclic eccentric contraction imposed upon the slow-contracting rabbit soleus muscle to investigate the early events associated with eccentric contraction-induced injury. Specifically, our goal was to quantify the relative importance of [1] the relative number of attached cross-bridges and [2] the relative number of attached crossbridges failing to generate force (Fig 1, question marks) to the force drop observed after eccentric exercise. A brief version of this report appeared in abstract form [14]. Methods Surgical isolation of the soleus muscle The animal model used was the adult male New Zealand White rabbit weighing approximately 3.5 kg. Rabbits were induced (n = 16) with 5% halothane anesthesia and then maintained on halothane anesthesia (2%, 1 l/min) for the duration of the experiment ( 2.5 h). Heart and respiratory rates were manually monitored throughout testing. A 3.5 mm threaded Steinman pin was piaced in the distal femur and a second placed in the mid tibia. The animal was then placed prone into a sling with the hindlimbs free. The Steinman pins in the right hindlimb were then placed into mounting brackets and secured so that the hip and knee joints were near full extension. The dorsal aspect of the leg was incised from the ankle to the mid thigh. Skin flaps were raised along the length of the leg to create a reservoir for heated mineral oil. Using electrocautery to provide meticulous hemostasis, the leg musculature was dissected to isolate the soleus muscle. In rabbits, unlike cats and rats, the plantaris, soleus, and the medial and lateral gastrocnemius muscles possess a very adherent distal tendon. Thus, the dissection to free the soleus from the surrounding soft tissue without disturbing the arterial, venous, or nerve supply was difficult. The plantaris tendon was cut distally and carefully dissected free from the underlying soleus and medial gastrocnemius muscles without injuring the soleus venous outflow. The distal medial gastrocnemius tendon was dissected free from the common distal tendon and the muscle was then freed from the adjacent soleus. Due to the extremely adherent nature of the muscle bellies of the soleus and the lateral gastrocnemius, the lateral gastrocnemius was myotomized proximal to the area where these two muscles become adherent. Neurotomies were then performed on the tibial nerve branches to the plantaris and the medial gastrocnemius. The soleus nerve supply traverses the lateral gastrocnemius muscle belly before entering the soleus and thus the proximal lateral gastrocnemius remained innervated. After adjusting the position of the servomotor, the proximal tip of the calcaneous bone was cut and the Achilles tendonbone complex was attached to the motor arm. The muscle was kept at its resting length until testing began. The distal portion of the tibial nerve was then dissected free from its surrounding connective tissue and a nerve stimulator cuff electrode was positioned around the nerve with care taken not to mechanically traumatize the nerve. Eccentric exercise model Eccentric contractions were imposed upon the soleus muscle by the servomotor (Cambridge Technology Model 310, Cambridge, MA) attached to the isolated distal tendon. Great care was taken to minimize system compliance, ensuring that the imposed deformation pattern was taken up by the muscle itself, and not the apparatus or the external tendon. This is more difficult with the soleus compared to the TA since the entire distal soleus tendon extends proximally into the muscle. System compliance, including the transducer, was 1.7 µm/g. Pre-warmed (37 C) light mineral oil provided a bath which covered the entire soleus muscle-tendon complex and tibial nerve. A temperature probe attached to a heat lamp was then placed in the mineral oil bath to maintain the temperature at 37 C using a servo temperature controller (YSI Model 73A, Yellow Springs, OH). Under computer control, muscle length was adjusted to the length at which twitch tension was maximum (L o ) and contractile properties were determined prior to experimental treatment (see below). For the soleus muscle, L o occurred at a length where significant passive tension was present. Typically, passive tension was 150 g prior to imposing eccentric contractions (in contrast to the TA for which passive tension was only 50 g). Contractile properties measured included: time-to-peak twitch tension (TPT), the rate of rise of twitch and tetanic tension (dp/dt), twitch half-relaxation time (HRT), maximum twitch tension (P t ), passive force in response to 25% strain at 63% fiber lengths/second (L f /s) and contractile tension at the following stimulation frequencies: 5, 25, 50, 75, and 100 Hz. Maximum tetanic tension (P o ) was defined as the tension measured while stimulating at 100 Hz (the peak of the force-frequency relationship). Measurements of L o were made on each muscle

4 150 from the tendinous origin to the most distal muscle fiber insertion. Based on previous architectural studies [15], muscle fiber length (L f ) was calculated for each muscle as 0.25*L o, where 0.25 represents the L f :L o ratio. Mechanical testing protocol The deformation pattern imposed upon the soleus muscle was designed to measure force-generating capacity, measure muscle stiffness, and finally to induce eccentric injury (Fig. 2A). Muscle stiffness was estimated by imposing a small rapid length change upon the activated, isometrically contracting muscle (Fig. 2B). Stiffness measurements have been used to estimate the relative number of cross-bridges attached [16]. This method originally assumed that myofilaments themselves were fairly inextensible which has since been shown not to be true [17]. However, if stiffness is measured at the same fiber length, the method will yield reliable information. In a typical contraction cycle, the muscle was activated for a total of 600 msec. The muscle was allowed to develop isometric tension (Fig. 2A) for 230 msec which was used to estimate force generating capacity on a contraction-bycontraction basis. Then, a small ( 1.5 % L f, completed in 5 7 min) length change was imposed upon the isometrically contracting muscle to estimate the relative number of attached crossbridges (Fig. 2B). The nominal magnitude of this deformation was 0.3 mm (80 mm muscle length 0.25 L f /L o ratio 0.015). The passive force change corresponding to this small length change was negligible. After a 70 msec delay to permit cross-bridge equilibrium to return (i.e. for the rates of crossbridge attachment and detachment to stabilize), the muscle was strained approximately 25% L f. All contractile records were stored in real-time and post-processed using the Mathematica Software Package (Wolfram Research Inc., Champaign, IL, USA). Thus, the mechanical parameters measured during the 30 min eccentric exercise period included isometric force (prior to stretch), stiffness during isometric contraction and peak muscle force during eccentric contraction. Contractions were imposed at 10 sec intervals to minimize effects of fatigue (Fig. 2A). To ensure that muscles were electrically activated throughout the experimental period, intramuscular electromyogram (EMG) was measured using needle electrodes (Grass platinum subdermal electrodes, Type E2, Grass Instrument Division, West Warwick, RI, USA) and was found not to vary over the 30 min treatment period in any group receiving stimulation (data not shown). Significant attenuation of the electromyogram was observed when stimulation interval was decreased below 5 sec or at stimulation frequencies greater than 80 Hz, indicating muscle excitation failure. Control groups included in the experimental design consisted of [1] a group receiving only passive stretch of 25% L f, [2] a shamoperated group in which the soleus was exposed for the same experimental duration ( 2.5 h) as the treated group, and [3] a group receiving isometric exercise of the same duration but without any length perturbations except for stiffness measurements which themselves induced no injury (see below). Immunohistochemistry At the end of the 30 min treatment period, animals were sacrificed and the soleus muscle removed. A segment from the muscle midportion (approximately 1 cm) was excised. Care was taken to sample from the same region of each muscle. The samples were placed with their fibers perpendicular to the surface of a piece of cardboard, and frozen in isopentane cooled by liquid nitrogen ( 159 C) and stored at 80 C for subsequent analysis. Cross-sections (8 µm), taken from the mid-portion of the tissue blocks, were cut on a cryostat at 25 C (Reichert Jung, 2800 Frigocut, Austria). Serial sections were stained with haematoxylin and eosin for routine histological analysis, and individual fibers typed as fast or slow using commercially available antibodies (Novocastro Laboratories Ltd., distributed by Vector Laboratories, Burlingame, CA, USA). Five micron thick cryosections were also stained with antibodies against laminin (rabbit antiserum) to visualize the fiber s basal lamina, fibronectin (monoclonal mouse antibody reacting with the cellular and plasma forms) to detect intracellular deposits indicating membrane lesions, desmin (monoclonal mouse antihuman clone D33 [Dako A/S, Glostrum, Denmark] and monoclonal mouse anti-pig clone DER11 [Novocastra Laboratories Inc., Newcastle, England]). Antibody binding was visualized by the indirect peroxidase-antiperoxidase techrtique (Dakopatts, Copenhagen, Denmark) with appropriate negative controls and blocking of endogenous peroxidase activity as previously described [18]. Statistical methods To determine whether a significant difference between the four experimental groups was obtained, percentage change in P o was compared between groups using a 1 4 one-way ANOVA (Abacus Concepts Inc., Berkeley, CA, USA). Posthoc comparison between the four groups was made using Fischer s Least Squared Differences (LSD) test which compensates for multiple paired comparisons. Significance level was set to α = 0.05 and statistical power (1-β) was approximately 85%.

5 151 Fig. 2. (A) Soleus muscle force (upper trace) and motor movement (lower trace) during a single eccentric contraction cycle. Soleus muscle is isometrically activated (region 1), subjected to a small length change to estimate stiffness (region 2) and then eccentrically activated (region 3). This process takes 600 msec and is repeated every 10 sec for 30 min. Using this approach, it is possible to measure the relative number of attached crossbridges compared to those attached cross-bridges that actually generate force. (B) High-resolution view of the stiffness pulse measurement method used at the end of the early isometric contraction phase shown in region 2 of (A). A small length change of 0.26 mm is completed in about 7 msec and results in a 496 tension increase. Thus, for this contraction, soleus muscle stiffness was 1910 g/mm. These data were acquired for every contraction and are plotted in Figs. 5 and 6. Cartoons associated with each trace represent the portion of the overall trace shown in the upper panel of Fig. 2A during which the high-speed trace is presented. Results Morphological alterations after eccentric contraction In contrast to the dramatic morphological and immunohistochemical changes that result from cyclic eccentric contraction of the TA, the soleus muscle showed no such abnormalities (Fig. 3). For example, the TA subjected to approximately the same amount of treatment demonstrated dramatic loss of the cytoskeletal protein desmin (Fig. 3A). As previously reported [6, 8], this protein loss is rapid and precedes cellular disruption. The soleus muscle fibers, on the other hand, demonstrate uniform and complete staining with the desmin antibody (Fig. 3B). In fact, the eccentrically exercised soleus muscles were indistinguishable from either isometrically exercised, passively stretched, or shamoperated soleus muscles.

6 152 Fig. 4. Soleus muscle force (relative to maximum isometric tetanic tension) for each of the three experimental groups. Peak force is highest for the eccentric group, followed by the passive stretch and isometric groups. Schematic diagram illustrates the parameter plotted from the tension record. Dotted line on schematic represents zero tension. Individual data points were obtained every 10 sec but only selected points are plotted for clarity. Standard error bars are shown for a single time period for each group for clarity and approximate those measured over the 30 min treatment period. Fig 3. Cross-sections of extensor digitorum longus (A) and soleus muscles (B) labeled with antibodies against desmin. Note that the soleus muscle appears normal compared to the EDL muscle that demonstrates structural abnormalities. An EDL fiber lacking desmin is shown (arrow) as well as a fiber that has been digested by inflammatory cells (arrowhead). This EDL muscle was processed from a previous study (6) in which anterior compartment muscles were eccentrically exercised for 30 min and then permitted 2 days of recovery. Contractile records during treatment During the eccentric contraction bout, individual contractile records showed three characteristic phases (Fig. 2A). During the first 230 msec of stimulation, soleus muscle generated force isometrically, as no length change was permitted. Then, a very rapid 1.5% L f length change was imposed upon the muscle to estimate the relative number of attached crossbridges. These small, rapid length changes resulted in relatively large force increases within the muscle (on the order of 500 g) due to the high muscle stiffness (Fig. 2B). The muscle was then permitted 70 msec for the crossbridge population to restabilize and then the eccentric length change of 25% L f imposed upon the muscle over 150 msec. After a 10 second rest, the entire length-force perturbation was repeated for a total of 180 eccentric contractions over 30 min. From records such as those shown in Fig. 2, three parameters were measured for each eccentric contraction: peak force, isometric force-generating capacity immediately before the eccentric stretch, and stiffness immediately before the eccentric stretch. First, we documented the peak force measured over the 30 min treatment period during eccentric contraction (Fig. 4, circles), isometric contraction (Fig. 4, triangles), or passive stretch (Fig. 4, squares). To compensate for differences between experimental animals in terms of their size, tension was normalized to the maximum isometric tension generating capability of that muscle. Across all experimental groups, maximum tetanic tension was 2,171 ± 285 g (mean ± SD, n = 32). In the case of isometric contractions, muscles generated tensions of approximately 80% P o at the 40 Hz stimulation frequency and showed no significant change throughout the 30 min treatment period (p > 0.4). In the case of passive stretch, muscles sustained tensions of about 120% P o initially which then rapidly declined to approximately P o within the first few minutes of cyclic stretch and subsequently further decreased to about 80% P o over the remaining 25 min. As expected, the eccentric treatment resulted in the largest initial force of about 200% P o which decayed relatively rapidly to about 120% P o with a time constant of about 7 min. Next, to estimate the muscle s isometric contractile ability on a contraction-by-contraction basis, we measured the isometric force generating-capability of the soleus muscle prior to the stretch. While, as mentioned above, the normal soleus muscle activated isometrically at 40 Hz maintained a tension level of about 80% P o throughout the treatment period (Fig. 4), the muscle subjected to eccentric activation rapidly dropped from a tension generating level of 80% P o (1777 ± 111 g) to 60% P o (1396 ± 129 g) within the first few minutes to less than 50% P o (1011 ± 136 g) at the end of 30 min (Fig. 5). This indicates a dramatic loss in force-generating ability at 40 Hz throughout the 30 min treatment period.

7 153 Fig. 5. Time-course of isometric tension prior to eccentric stretch during the 30 min eccentric exercise period. Soleus muscle force generating capacity decreases by from about 80 to 50% P o during the eccentric exercise period. Schematic diagram illustrates the tension parameter plotted. Fig. 6. Time-course of soleus muscle stiffness during the 30 min eccentric exercise period. Soleus muscle stiffness decreases by about 20% during the eccentric exercise period but did not change significantly during isometric exercise. These data were obtained using the methodology illustrated in Fig. 2B. To estimate the relative number of cross-bridges attached during the isometric force generation phase of each contraction, the stiffness measured during the 1.5% L f stretch was calculated (Fig. 6). Initialiy, muscle stiffness exceeded 1300 g/mm, but as eccentric treatment progressed, stiffness decreased to 900 g/mm. Force loss after treatment As an estimate of the magnitude of muscle injury that occurred after each treatment type, maximum tetanic tension (P o ) was compared prior to treatment and 1 h after each of the treatment protocols (Fig. 7A). There was no significant difference between the sham operated controls, isometric contraction group and passive stretch group in terms of tetanic tension change with relatively modest tetanic tension drops of 3.5, 5.4 and 8.0% respectively (p > 0.6). The muscles subjected to eccentric contraction, on the other hand experienced a significant tension decrease of 23.3% which was significantly greater than any other group (p < 0.01). Despite the large tetanic tension decrease, there was no significant change in twitch tension between groups (p > 0.8) or between time periods (i.e. pre- vs. post-treatment, p > 0.2, Table 2; Fig. 7B) revealed by two-way ANOVA. Other twitch parameters demonstrated changes when compared to values obtained prior to treatment, but were not unique to the eccentric contraction and thus, probably do not represent injury per se. For example, significant increases in twitch time-to-peak tension (p < 0.01) and half-relaxation time (p < 0.01 ) were observed for all but the control and passive stretch groups (Table 2). Mechanical basis of force drop during eccentric contraction To understand the mechanical basis of the force decline after eccentric contraction we compared the change in soleus muscle Table 2. Soleus muscle contractile properties Contractile property measured Control Eccentric contraction Isometric contraction Passive stretch Tetanic tension rise rate (P o /sec) pre ± ± ± ± 1.5 post ± ± ± ± 1.5 Time-to-peak tension (msec) pre ± ± ± ± 2.1 post ± ± 2.8* 68.5 ± 6.9* 54.1 ± 4.0 Twitch tension (g) pre ± ± ± ± 67.8 post ± ± ± ± 38.9 Twitch half-relaxation time (msec) pre ± ± ± ± 15.9 post ± ± ± ± 16.5 *Values listed are sample means± SEM measured before (pre-) and after (post-) one of the treatments listed. n = 4 per group.

8 154 A isometric force generating capacity (Fig. 5) to soleus muscle stiffness measured just prior to the eccentric stretch, an estimate of the relative number of attached cross-bridges (Fig. 6). These two parameters, when plotted on a normalized scale relative to their initial value, demonstrate a differential response to eccentric exercise (Fig. 8). While muscle stiffness decreased only by about 30% over the 30 min treatment period, isometric force decreased by 85%. These data indicate that, while the muscle was generating much less force, cross-bridges were still being activated and attached but were simply not entering the force-producing state. Muscles activated isometrically showed no significant force (Fig. 4, triangles) or stiffness (Fig. 6, filled circles) change over time (p > 0.4) indicating that the stiffness and stimulating protocols caused no muscle injury. B Fig. 7. (A) Decrease in maximum isometric tension measured 1 h after 30 min of treatment. Four groups include sham-operated controls, isometric contraction, passive stretch and eccentric contraction. No difference was observed between any groups except the eccentric exercise group (p < 0.01). Data were obtained while stimulating the motor nerve at 100 Hz. (B) Twitch contractile records from eccentrically exercised (top) and isometrically exercised (bottom) soleus muscles. Solid line: pre-exercise. Dotted line: postexercise. Discussion The purpose of this study was to investigate the mechanical and structural basis of the force loss after eccentric contraction in a skeletal muscle with predominantly slow muscle fibers, the rabbit soleus. Previous studies on this muscle in rats and mice documented the force decrement that occurs after repeated eccentric contraction [4, 19]. The previous studies also revealed qualitative differences compared to rabbit tibialis anterior (TA) experiments in terms of the mechanical factors responsible for injury. While strain was the dominant factor responsible for injury in fast rabbit muscle, stress was the factor most related to injury in slow rat muscle. We had also documented a rapid loss of the desmin cytoskeletal protein within TA and extensor digitorum longus (EDL) muscles suggesting that the earliest events associated with eccentric contraction involved the cytoskeleton [8]. This injury was primarily associated with fast fibers of low oxidative capacity. The current study represents a follow-up to these studies not only to permit the interspecies comparison but to measure the contractile response of a muscle consisting primarily of type 1 (slow oxidative) and type 2A (fast-oxidative-glycolytic) muscle fibers. To our surprise, 30 min of cyclic eccentric contraction imposed upon soleus muscles resulted in no obvious morphological changes (Fig. 3B) such as the highly enlarged, rounded fibers that we had observed previously in the TA [5]. In addition, the desmin loss that was dramatic and occurred rapidly in both the TA and EDL [8] and persisted for several days after the injury bout [20] was not observed in any rabbit soleus muscle fibers. Yet, in spite of this lack of obvious structural disruption, a significant loss in muscle isometric force generating capacity was observed, similar in magnitude to that observed by others (Fig. 7). It is intriguing that there is such obvious structural disruption observed in fast animal and human skeletal muscles with so much less structural disruption observed in slow

9 155 Fig. 8. Comparison of the time-course of normalized isometric force measured just prior to eccentric stretch (filled circles) and normalized muscle stiffness (open circles) measured during eccentric contraction. Note that stiffness decreased by only 25% during eccentric contraction while isometric force decreased by over 85%. These data indicate that eccentric exercise is accompanied by an increasing number of cross-bridges that attach but do not enter the force generating state (c.f. Fig. 9). soleus muscles. Part of the reason for this difference may be the fact that soleus muscles are chronically activated during lengthening and thus are already adapted to eccentric exercise. This phenomenon has been most thoroughly studied in cat locomotion where the soleus muscle has been shown to be almost maximally activated during normal locomotion [21] and to experience rapid lengthening during the pawstrike phase (E 2 ) when the animal weight is supported by the activated ankle extensors [22]. The eccentric phase is followed by a muscle shortening phase during toe-off when ankle extensors propel the animal forward (E 3 ). This stretchshortening cycle has been proposed as an energy storage strategy whereby strain energy stored within the muscle-tendon unit is released during muscle shortening [23]. At the muscle fiber level, the sarcomere response to eccentric loading is less clear. This is because the length change imposed upon the muscle-tendon unit may be differentially absorbed by muscle and tendon. Hoffer et al. [24] clearly demonstrated this phenomenon in cat medial gastrocnemius (MG) muscles as, during the E 2 locomotion phase, while the muscle-tendon unit was actively stretched, MG muscle fibers shortened or remained nearly isometric while the tendon lengthened significantly. Further studies are required to understand the nature of the adaptation that occurs in muscles due to eccentric exercise. Morgan and colleagues proposed that the adaptation is simply an increase in sarcomere number sufficient to ensure that sarcomeres operate on the ascending limb of their lengthtension curves [25]. Experimental support for such a proposition was presented for the rat vastus intermedius muscle subjected to 1 3 weeks of downhill running [26] but further experimentation is required to determine whether this is a general adaptive strategy used by all muscles. If the muscle was not clearly injured due to eccentric exercise, why was the force decreased to such a great extent ( 25%, Fig. 7)? Part of the answer probably lies in the loss of myofibrillar excitation accompanying neural stimulation. We measured muscle EMG activity and found no significant change during the treatment time, provided the stimulation interval was greater than 5 sec. However, even if EMG did not change, the coupling between the electrical activation signal and calcium release may still have been attenuated. This possibility was demonstrated by Warren et al. who measured maximal muscle force in mouse soleus muscles subjected to eccentric contraction using electrical stimulation and then due to activation in the presence of caffeine which is known to potentiate calcium release from intramuscular stores [13]. They found that almost all of the force loss could be recovered simply by raising myofibrillar calcium concentration and thus concluded that excitation-contraction coupling failure was the cause of the force loss that followed eccentric exercise. In this study, we further probed the condition of the myosin-based force generators within soleus muscles during eccentric contraction. In skeletal muscle, cross-bridge attachment precedes force generation (Fig. 1 ) which has led to cross-bridge models involving multiple states [27, 28]. By measuring the instantaneous stiffness accompanying the isometric contraction immediately prior to eccentric stretch,

10 156 Fig.9. Schematic depiction of cross-bridges attached and generating force early in the eccentric exercise bout (left panel) compared to cross-bridges that attach but do not enter the force-generating state later in the exercise bout (right panel). Heavily stippled heads represent cross-bridges attaching and generating force while lightly stippled heads represent cross-bridges only attaching but not entering the force-generating state. we obtained an estimate of the relative number of attached cross-bridges during the 30 min treatment period. We found that, while isometric force prior to stretch dropped by over 85% during the eccentric exercise, instantaneous stiffness dropped by only 30%. In other words, the fraction of crossbridges that entered the force-generating state decreased dramatically during the eccentric exercise (Fig. 9). The structural basis for such a change is not currently known but would probably not be detected using the light microscopic methods in this study. Further studies are now underway to investigate such structural changes at the ultrastructural level. Mechanisms of muscle injury Obviously, a definitive explanation for the mechanism of skeletal muscle injury cannot yet be given, but several contributing causes have been identified (Table 1). The earliest events associate with muscle injury are probably mechanical in nature. While we previously proposed that muscle injury might be mediated by metabolic factors, this clearly cannot apply to all muscles. In the TA, morphological disruption was observed only in muscle fibers with large diameter, high ATPase activity, and low oxidative capacity (i.e. so-called FG fiber types, [3, 5, 29]). We then hypothesized that the low oxidative capacity of these fibers predisposed them to injury since, as the fibers fatigued, crossbridges would lose the ability to detach, thus rendering them mechanically stiff. This stiffness would result in mechanical injury on subsequent stretches. The idea of a predisposition to injury by fibers with low oxidative capacity was tested explicitly by training muscles using electrical stimulation training for a period of three weeks to increase oxidative capacity prior to eccentric injury [30]. Electrical stimulation resulted in significant training of both the EDL and TA muscles with EDL citrate synthase (CS) activity increasing over 65% and TA CS activity increasing by 10% (p < 0.05). In addition TA capillary density increased from 839 ± 56 mm 2 to 1026 ± 71 mm 2 (p < 0.05) and capillary:fiber ratio increased from 1.3 ± 0.1 to 1.6 ± 0.1 (p < 0.05). In spite of this significant training effect, no differences were observed between stimulation trained and control groups for maximum dorsiflexion torque, maximum tetanic tension, or number of injured muscle fibers as indicated by loss in desmin staining. Additionally, no significant correlation was observed between CS activity and maximum tetanic tension for either the TA or EDL muscle (p > 0.3). We concluded that low muscle fiber oxidative capacity did not, in fact, predispose muscle fibers to eccentric contraction induced injury and did not explain the selective FG fiber damage observed after eccentric exercise. It is possible that selective FG fiber injury within the TA was actually related to their size rather than their oxidative capacity. Support for this concept was presented by Petrof et al. who demonstrated more injury to muscles composed of large fibers compared to those composed of smaller fibers in mouse muscles lacking the dystrophin protein [31]. They argued that larger fibers, with a smaller surface-tovolume ratio would experience higher transmembrane stresses resulting in greater mechanical disruption. It is interesting to note that, in the rabbit soleus muscle, the type 1 fibers are larger than the type 2A fibers so we were interested to see if they demonstrated the majority of the structural disruption. This would be a good test of the relative importance of oxidative capacity compared to fiber size in rendering a fiber susceptible to injury. As it turned out, neither fiber type showed significant morphological abnormality so this question remains open. We previously proposed an injury mechanism related to the loss of desmin staining in skeletal muscle [8]. We speculated that the rapid changes in desmin immunostaining pointed to mechanisms involving muscle fiber membrane disruption and/or subsequent proteolysis of the cytoskeletal network due to mechanical strain. An attractive candidate for the proteolytic mechanism was the calcium activated protease, calpain, which is present in skeletal muscle [32] and for which desmin is a substrate [10]. The mechanism of action of calpain requires raised intracellular calcium ion ([Ca 2+ ] i ) concentration. While there is, as yet, no direct evidence for such an increase, Duan et al. [9] demonstrated an increase in the mitochondrial calcium concentration in muscles subjected

11 157 to an exercise protocol biased toward eccentric contraction. Since mitochondrial calcium concentration indirectly reflects cytoplasmic [Ca 2+ ] i, the results of Duan et al. might be construed as indirect evidence for increased [Ca 2+ ] i. This observation, in conjunction with our earlier demonstration that muscle fiber strain was the mechanical factor that most strongly influenced the magnitude of muscle injury [12] lead to the following hypothesis regarding the early mechanism of eccentric contraction induced muscle damage (Fig. 10): (a) muscle fiber strain results in an increased [Ca 2+ ] i. Such an increase may be due to calcium influx via strain activated channels [33], by disruption of the intracellular stores of calcium in the sarcoplasmic reticulum, or by disruption of the T-system or sarcolemma (Fig. 10A). This may be related to sarcomere popping, proposed as a damage mechanism during eccentric contraction [25]. (b) Following the increased [Ca 2+ ] i, calpain activation results in selective hydrolysis or disruption of the intermediate filament network (Fig. 10B). It has been demonstrated that desmin is a substrate for calpain while actin and myosin are not [34]. This could explain the selective desmin loss in sections that still demonstrate regular arrangements of contractile and metabolic proteins. (c) Finally, after the intermediate filament network has been altered due to proteolysis or conformational changes, the myofibrillar apparatus is disrupted on repeated muscle activation and unable to develop normal tension (Fig. 10C). While this mechanism does not explain the changes that occur in the soleus muscle, it is possible that other cytoskeletal proteins that we have not studied are involved in the soleus. It is interesting to note that, in a rat downhill running model, which biases muscles toward eccentric contraction, Belcastro and colleagues demonstrated large and significant increases in calpain activity in the fast plantaris and vastus lateralis muscles but minimal changes in the soleus muscle (Belcastro, 1996, pers. comm.). After initial injury, evidence exists that inflammation further results in tissue deterioration [35 38]. In animal models where the time-course of muscle strength is measured, injury due to the presumed inflammation is nearly equivalent to the injury caused by the mechanical injury itself [38]. In fact, while antiinflammatory medication may attenuate the inflammation-mediated tissue injury, evidence exists that the longterm muscle function is compromised and thus may be required for appropriate cellular remodeling after injury. Fig. 10. Schematic depiction of eccentric contraction-induced muscle damage. Calcium ions are represented as filled dots. (A) muscle fiber strain results in an increased [Ca 2+ ] i. (B) Increased [Ca 2+ ] i leads to calpain activation and selective hydrolysis or disruption of the intermediate filament network. (C) After intermediate filament network damage, the myofibrillar apparatus is disrupted on repeated muscle activation (figure used with permission from Ref. [8]). on the toes during jumping, or lowering the forefoot after heel strike. Eccentric contractions, by their nature, serve a braking role. By keeping muscle stiffness relatively high, these muscles would resist stretch in a fashion that is disproportionate to the amount of force they could generate. This could be analogous to putting the foot on the brake and backing off of the accelerator. Physiological significance of cross-bridge stiffness Of what benefit could it be to a muscle to permit cross-bridges to remain attached but not able to generate force? We speculate that, by maintaining muscle stiffness, even in the absence of force generation, these muscles are better able to serve a braking role under conditions that rely on eccentric contraction such as knee flexion during heel strike, landing Acknowledgements This work was supported by the Veterans Administration, NIH Grant AR40050, Swedish National Centre for Research in Sports, the Lundberg Foundation and the Medical Faculty at the University of Umeå. We thank Tina Patel and Dr. Gordon Lutz for critical reading of the manuscript.

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