production in single contractions (Vjpt) were reduced to 42 % and 32 % of control

Transcription

1 Journal of Physiology (1993), 470, pp With 5 figures Printed in Great Britain POWER PRODUCTION AND WORKING CAPACITY OF RABBIT TIBIALIS ANTERIOR MUSCLES AFTER CHRONIC ELECTRICAL STIMULATION AT 10 Hz BY JONATHAN C. JARVIS From the Department of Human Anatomy and Cell Biology, The University of Liverpool, PO Box 147, Liverpool L69 3BX (Received 15 February 1993) SUMMARY 1. The muscles of the distal anterior compartment of the left hindlimb of rabbits were subjected to continuous indirect electrical stimulation at 10 Hz for periods of up to 12 weeks by means of an implantable stimulator. 2. The maximum shortening velocity (Vmax) and the velocity for maximum power production in single contractions (Vjpt) were reduced to 42 % and 32 % of control values respectively after 12 weeks of stimulation. The rate of change of these parameters was greatest between the second and sixth week of stimulation. These changes, it is suggested, reflect the documented time course of the replacement of fast with slow isoforms of myosin. 3. The reductions in force production and speed of the stimulated muscles combined to produce a marked, progressive decline in the maximum power produced in single contractions. After 8 weeks of stimulation, the maximum power output had fallen to less than 10% of the control value. 4. The fatigue resistance of the stimulated and control muscles was tested over several hours of cyclical shortening contractions designed to elicit an initial power output of 10 W kg-' with the muscles set to contract at Vopt. This level of work output represented about 1-6 % (control) and 25% (12-week-stimulated) of the absolute maximum power output achieved during single contractions. 5. Despite the large reduction in the maximum power output of single contractions, the stimulated muscles showed less than 10 % reduction in their power output during the fatigue tests over periods of up to 7 h. The control muscles showed a 70 % reduction over the same period. There was no difference in the fatigue resistance under this protocol between muscles stimulated for 2 weeks and those stimulated for longer periods. Transformation of myosin isoforms, which is known to occur later than 2 weeks after the start of stimulation, is not necessary for the induction of this degree of fatigue resistance. INTRODUCTION Mammalian skeletal muscle cells undergo a remarkable series of intracellular changes in response to chronic electrical stimulation of their motor nerves. Although the response to 10 Hz stimulation has been investigated intensively in terms of the MS 1222

2 158 J. C. JARVIS changes in isometric contractile properties, calcium handling mechanisms, cellular biochemistry and gene expression (Salmons & Henriksson, 1981; Pette & Vrbova, 1992; Ausoni, Gorza, Schiaffino, Gundersen & Lomo, 1990) the effects on the ability to produce external movement and work have not been addressed to the same extent. Several reports have highlighted the induction of extreme fatigue resistance in stimulated muscle from rabbits and rats (Salmons & Sreter, 1976; Hudlicka, Brown, Cotter, Smith & Vrbova', 1977; Brown, Henrikson & Salmons, 1989; Pette & Simoneau, 1990), but the tests were performed with fixed-length contractions. This study demonstrates how the working capacity of skeletal muscles is altered by chronic stimulation and highlights the important differences in the effects on the power output of single shortening contractions (which is decreased) and of long series of repeated shortening contractions (which is increased). The mechanical output of control and stimulated muscles was measured by means of a novel apparatus (Jarvis & Salmons, 1990) and a novel fatigue-testing procedure. The muscles were tested in a series of shortening contractions in which it was arranged for the initial power output to be 10 W kg-' of muscle. Since the mechanical properties of the experimental and control muscles were so different, the parameters of the testing procedure (period of activation, duty cycle, velocity of shortening) had also to be different for each muscle. It was considered that a functionally based fatigue test would be more informative than one in which the parameters were identical. METHODS Implantation and stimulation Miniature electronic stimulators were made to a published design (Jarvis & Salmons, 1991; Salmons & Jarvis, 1991). The devices were encapsulated in silicone rubber (Dow Corning 3140RTV) and were between 15 and 35 mm in diameter, dependent on the capacity of the lithium power source, and 8 mm thick. A Dacron mesh extended from the encapsulant to allow the device to be sutured in place. The output from the device was taken via multistranded stainless-steel wires with PVC insulation (Cooner Sales Company, Chatsworth, CA, USA) to loop-electrodes formed from the bared wire and a Dacron velour pad. The output from the implanted electronic pulse generator was gated by a photosensitive switch and could therefore be turned on or off remotely by light flashes through the skin. The devices used in the present study gave pulses of duration 0-2 ms and amplitude 3-2 V at a frequency of 10 Hz. This protocol provides supramaximal stimulation of all the motor neurones in the common peroneal nerve. The devices were sterilized in benzalkonium chloride (10 g -1) for 24 h and implanted into rabbits under fentanyl/fluanisone anaesthesia (Hypnorm: fentanyl citrate, mg ml-' and fluanisone, 10 mg ml-', Janssen Pharmaceutica, Grove, Wantage, Oxon; 0-3 ml kg-, I.M.) after premedication with atropine sulphate (Sigma Chemical Co. Ltd, UK; 3 mg kg-1) and diazepam (Roche Products Ltd, UK; 5 mg kg-) given subcutaneously. Full aseptic precautions were taken. The device was implanted under the abdominal skin and the leads taken subcutaneously to the lower hindlimb. One electrode was placed in a slip of the gastrocnemius muscle immediately beneath the common peroneal nerve approximately 10 mm proximal to the border of the lateral head of the gastrocnemius muscle. The other electrode was placed mm distally on the surface of the lateral head of the gastrocnemius muscle. The wounds were closed with Prolene sutures (Ethicon Ltd, Edinburgh) and the rabbit left to recover for 1 week before the stimulator was switched on. Stimulation of the common peronal nerve was then continuous for between 2 and 12 weeks until the terminal experiment. Terminal experiments Experiments were performed on twenty-four rabbits, divided into six experimental groups in which the left tibialis anterior muscle received different amounts of stimulation as follows: no

3 MUSCLE MECHANICS AFTER CHRONIC ACTIVATION stimulation, unoperated rabbits (n = 5); days (n = 4); days (n = 4); days (n = 3); days (n = 5); days (n = 3). The terminal experiments were performed under urethane (250 g l-1; 500 mg kg-1) and pentobarbitone sodium (Sagatal, May and Baker Ltd, 30 mg kg-') anaesthesia, the drugs being administered via a cannula in the ear vein. Supplementary doses of pentobarbitone sodium were given throughout the experiment as necessary. The trachea was cannulated. Flap electrodes (Barnard, Barnard, Jarvis & Lai, 1986) were implanted around the common peroneal nerves. In the stimulated leg, the flap electrode was placed proximal to the chronically implanted electrodes, which were left in place. The nerve was cut proximal to all electrodes. The muscle layer was then closed with Prolene sutures and the skin reapposed. The nerve was thus maintained in a natural environment and its threshold for supramaximal stimulation remained low and steady for up to 13 h. The tibialis anterior muscles were then exposed and markers placed at the most proximal point of origin (a fine drill into the tibia) and at the distal myotendinous junction (a black thread). The muscle length was measured between these markers with the femur vertical, the tibia horizontal and the foot fully plantarflexed, fully dorsiflexed and at 90 deg to the tibia. The tibialis anterior tendon was then cut below the retinaculum and threaded through a miniature titanium alloy clamp, leaving a minimum of unclamped tendon. The rabbit was placed on a heavy myograph table and the legs clamped in C-clamps at the knee (on the head of the femur) and at the ankle (on the medial and lateral malleoli); this fixation was performed under deep anaesthesia. The blood supply appeared to be unhindered and a stable nerve-muscle preparation was obtained; this has been used successfully in the present experiments for periods of up to 10 h. The muscle temperature was maintained near 37 C by means of heating lamps. Measurements were made simultaneously on the left and right muscles by means of two independent ergometer systems (Jarvis & Salmons, 1990). The tendon clamps were attached via non-compliant carbon fibre-epoxy links to the force transducers of the ergometers. The ergometers were mounted on rack and pinion mechanisms so that the length of the muscles could be adjusted. In addition, the muscle attachments could be moved at a constant velocity, allowing the muscles to shorten at a controlled rate during a period of activation. Both force and displacement signals were displayed on digital storage oscilloscopes and could be plotted from these monitors on an X-Y recorder. The signals were also fed to the A-D converter of a PC-based capture and analysis system (Microlink, Biodata, Manchester). The sampling frequency was 1 khz for each channel and the data were stored on disk. The muscles were activated at a range of frequencies (1-200 Hz) over a series of lengths and the force records captured. The muscles were then set at the length which corresponded to the beginning of the sharp rise in passive tension with increasing length. This length was usually about 2 mm shorter than Lo (the length for maximum active twitch tension) but the muscles invariably produced more than 95% of maximum tetanic tension in this position. This initial setting was chosen to minimize the release of stored elastic energy which would have obscured the force records during shortening contractions from longer lengths (Jarvis & Salmons, 1990). Force-velocity curves were obtained by the method of isovelocity release over the full range of shortening velocities of the muscles. The muscles were activated at 100 Hz in each contraction and 2 min were allowed for recovery between contractions. The muscles were released from the isometric condition at a time after the start of nerve stimulation chosen so that there was little readjustment of the elastic components of the muscle in the period immediately following release (see Hill, 1970; examples of force and displacement records produced by this method are given in Jarvis & Salmons, 1990). The protocol has the same purpose as the commonly used method of discharging the series elastic component by imposing a very rapid step reduction in length; that is to ensure that the series elastic component is extended just enough before the constant velocity ramp to support the load that the contractile component produces at the chosen velocity of shortening. After several pilot experiments, a timing protocol was established that gave appropriate delays between the activation of the muscle and the opening of the solenoid-operated valves that controlled the movement of the ergometer. The force measurement was not made at the identical muscle length for each velocity (see Fig. 5 in Jarvis & Salmons, 1990). During the fastest releases, the muscle was at a length at which, under isometric conditions, it would have produced about 60 % of the maximum isometric force. For this reason, the value for Vmax obtained by this method would be expected to be lower than that obtained with the use of isotonic contractions. Since the ergometers were able to move at more than 400 mm s-1, it was possible to obtain records at all velocities of shortening between zero and Vmax, the velocity at which the muscles produced no external force. This velocity was determined with care by adjusting the time of release 159

4 160 J. C. JARVIS so that the shape of the active force recorded during release was little different from that during a passive release without stimulation of the nerve. Records were taken at eighteen to twenty-five different velocities of shortening (Fig. ID). All the data points were used for curve fitting (see below). The resting and active force and displacement records were analysed using a digital analysis package (Biodata, Microlink, Manchester), and the force-velocity and power-velocity curves plotted. In every case, the force values used for analysis were calculated as active force minus resting force. The velocity for maximum power (VJ'pt) was estimated from the plotted curves during the experiment and the ergometers were set to give releases at Vo,pt for each muscle. The amount of work performed by the muscles in a single contraction over the full range of movement of the ergometers (19-5 mm, or approximately 30% Lo) was then calculated as the area under the force-time curve (active-passive) multiplied by the velocity. The period of activation was set to correspond to the time taken for the full excursion. The muscle mass was then estimated as 0O09 % of the body mass for control muscles (mean + S.E.M '002, n = 21) and from a documented time course similar to Fig. 1A of the reduction of mass with 10 Hz stimulation for the experimental muscles. The repeat frequency of the cyclic contractions that made up the fatigue test was then calculated for each muscle to produce an initial power output of 10 W kg-' wet weight. The power output was monitored over time by capturing active and passive force and displacement records at intervals during the test. The mean duration of the tests described here was 3-8 h, but several tests extended to 7 h. A fatigue index was also defined for comparison between muscles as the ratio of the power output after 1 h of cyclic contractions to the power output at the start of the test. The actual power output per kilogram used in the following analyses was calculated using the measured muscle mass, rather than that estimated during the experiment. The isometric recordings were analysed after the experiment from the graphical display of the digital system. Cursors were placed by eye and the force read directly. The time to peak of the isometric twitch was measured from the first point of deviation from the resting force level. The time taken for the twitch force to fall to half its peak active level (half-relaxation time) was also measured. The untreated force-velocity data were fitted to the Hill equation, V = b(po -P)/(P +a), where V is the velocity, P0 is the maximum tetanic force, P is the force and a and b are constants, with the use of a Marquardt algorithm. Data forvj,,ax',v'pt and the Hill constants in the figures are taken from the fitted curve, which was not constrained to pass through any specific data points. The estimates of P0 from the fitted curve were only slightly lower than the highest measured tension (ratio 0-93 ± 0-06). RESULTS Student's t test was used to test for differences in muscle properties among the experimental groups. No significant differences were found in any parameter between the muscles from the unoperated rabbits and the unstimulated muscle in the operated rabbits. The figure legends each contain the mean and standard error for the relevant parameter calculated from all the unstimulated muscles in the study. Since the interanimal variation is so much greater than the side-to-side variation in individual animals (Al-Amood, Buller & Pope, 1973), the results have been presented as means +S.E.M. of the left: right (stimulated: unstimulated) ratios in individual rabbits. Muscle mass Chronic stimulation produced a rapid loss of muscle mass in the first 2 weeks, which then continued at a slower rate up to 12 weeks (Fig. 1A). The final weight of the muscle was of the control value. A similar degree of atrophy has been noted in skeletal muscle stimulated for up to one year (Mayne, Anderson, Hammond, Eisenberg, Stephenson & Salmons, 1991) and probably represents a new equilibrium

5 MUSCLE MECHANICS AFTER CHRONIC ACTIVATION 161 state in which protein synthesis and degradation are balanced under continuing stimulation at 10 Hz. Muscle length Stimulation resulted in a shortening of the muscle-tendon complex, which resulted in a restriction of plantarflexion of the foot. In terminal procedures under anaesthesia A B * G 1.0 i ii 0.8 r 0.8 i 'E0.4 * D C i C.-~~~~~~~~~~~~~~~~u oo20 I 0.4- a)c Icia O Velocity (mm s-1) Fig. 1. The time course of the changes in muscle wet weight (A); maximum tetanic tension (B); maximum tension per estimated cross-sectional area (C), after chronic electrical stimulation. The points are plotted as the means+s. E.M. of the ratio of the values of the stimulated (left, L) muscles to the unstimulated (right, R) muscles in individual rabbits. The unstimulated control values were g (n = 21) (A), N (n = 28) (B) and knm-2 (C); D shows the data points and the fitted curves (see text) from force-velocity experiments on a rabbit tibialis anterior muscle that had been stimulated for 8 weeks (@) and its contralateral unstimulated control muscle (0). after 8 weeks of stimulation, the foot could be passively plantarflexed to about 110 deg from the tibia, with the femur and tibia at 90 deg. On the unstimulated side, the foot could readily be plantarflexed to 180 deg. The muscle tissue itself was

6 162 J. C. JARVIS reduced in length by about 10% after 12 weeks. The left: right ratio for the length at full passive extension after 12 weeks was for the control group and in the stimulated group (P < 0-01). The same ratios with the tibia and femur held at 90 deg were and (P < 0-01). The length for maximum twitch tension (Lo) also decreased by about 10% after weeks of stimulation. The left: right ratio was for control and after weeks of stimulation (P < 0 001). In one animal, biopsies were taken of the 8-week-stimulated and unstimulated muscles by tying a small sample to a balsa stick while the muscles were held at Lo, The samples were embedded in resin and longitudinal sections were prepared for the measurement of sarcomere length. From each muscle ten fibres were chosen randomly and the number of sarcomeres along a mm eyepiece graticule scale were counted. The mean sarcomere length was calculated and was not significantly different for the two muscles (stimulated, ,sm; control, ,um). The shortening of the muscle must therefore have been accompanied by a loss of sarcomeres. Maximum tetanic tension (PO) Po showed a marked, highly significant reduction over the first 4 weeks of stimulation, which then settled to about 40% of control after 6 weeks (Fig. 1B). The loss of muscle tension was greater than the reduction in physiological cross-section (mass/length, assuming a density of unity), particularly in the period between 2 and 6 weeks. The specific tension (Fig. 1 C) reached a minimum after 4 weeks and then recovered to % of control after 12 weeks. The fall in tension above that which can be explained by changes in cross-sectional area may be due to a genuine difference in the maximum specific tension of the stimulated (slow) fibres (Eddinger & Moss, 1987). Isometric twitch properties The time course of changes in the time to peak of the isometric twitch due to electrical stimulation has been documented in a number of previous papers. Salmons & Vrbova' (1969) presented data up to 6 weeks of stimulation, at which time the properties appeared still to be changing. Eerbeek, Kernell & Verhey (1984) used a variety of tonic patterns of stimulation and presented a time course similar to that shown in Fig. 2A, but the maximum changes were 2-15-fold after 8 weeks; in the present study changes reached 385-fold at 12 weeks. The tibialis anterior muscle became as slow as the soleus muscle by 6 weeks of stimulation and continued to become slower. After 12 weeks the contraction time was ms, which may be compared with ms for the rabbit soleus (Salmons & Vrbova', 1969). Salmons & Sreter (1976) reported a contraction time of 64-9 ms (3-8 times the value for the contralateral control) for one tibialis anterior muscle stimulated at 10 Hz for 20 weeks. Pette, Muller, Leisner & Vrbova' (1976) reported contraction times of 84 ms for one 8-week stimulated muscle and 62 ms for one 9-week stimulated muscle. Half-relaxation time This parameter showed a very similar time course to that of the contraction time, with an equivalent 3*6-fold change after 12 weeks (Fig. 2A).

7 MUSCLE MECHANICS AFTER CHRONIC ACTIVATION 163 Force-velocity relationship It was necessary to calculate and plot the force-velocity data during the experiments in order to set the parameters of the fatigue test. An example of the data obtained from a tibialis anterior muscle stimulated for 8 weeks, together with the A B XE 1! 0 T; E i I~~ [ 1. U ~04 CC @8-1-2 o D E ~ ~~~~~~~~~~~~~~~~0.0 -J D F o0,6 0.48stimulated(efteLkusls of teustimulationrgt,r ( Weekls ofsindviualratio ts C.) aig. Th0 2 Thrve teo the changs. in time to pea(0andthaelaxa time(oa ero bars ntmuae omitd ofaed onrlvluswr isome-tri 200 e ms(otaintmen28ad withor exprb); 24g822+8T90 tms (oreo h hneek(adhalf-relaxation n iet e58±orbar omi23e) (C);h 1s49±0r08twitsh (A 2); (D)u. elct fshreig V. ) time,n=2)(;393±16ls n=25 (B); contraatera ontrohil munscle, is'give in, Fig. maidumthetunloadednshortening velocityfo andimthepvelocitydforimaximumipowercproductionsvot wereatheefre estoimaletediycand drainglatcurvetohe dt points.aeplte aftethemexpersiment,the datao wertevalusofanlyed byheaunstioflanedunbiase sxtimuolation., theponturve-plottedna caure-fttnthoedumean technique.0-7m +tendmd (compraredowith,n toftherastiomofthe ourexprimnta value of the exdtrapoelation,the curve-fittingweprodureteondwede toerovereestimated Vma iandth

8 164 stimulated muscles because of the large curvature of the fitted hyperbolae, and to underestimate Vmax for the control muscles because of the near-linearity of the relationship between force and velocity at low velocities. The ratios fitted:extrapolated were 1x09+0*04 for the stimulated muscles and for the control muscles i- i ' 1.0-0) m :08 E~~~~~~~~ x~ Fig. 3. The time course of the changes in maximum power in single contractions per gram of muscle tissue after chronic stimulation. The points are plotted as the means + S.E.M. of the ratio of the values of the stimulated (left) muscles to the unstimulated (right) muscles in individual rabbits. The unstimulated control value was W kg-' (n = 21). The unloaded shortening velocity expressed in muscle lengths per second did not change significantly between 0 and 4 weeks of stimulation (Fig. 2B). This -was in spite of a twofold change in the contraction time (Fig. 2A), a reduction to 40% of control in the tetanic tension (Fig. IB) and a significant fall in the velocity for maximum power (Fig. 2D). The curvature of the fitted hyperbolae changed markedly, however, and this is reflected in the sharp fall in the ratio of the Hill constant 'a' to the maximum tetanic tension (Fig. 2C; Woledge, 1968). Between 4 and 6 weeks of stimulation the left:right ratio for maximum shortening velocity fell significantly from to J. C. JARVIS (P <0-01). The maximum shortening velocity was then stable at about 1-6 lengths per second from 6 to 12 weeks of stimulation (Fig. 2B). The velocity for maximum power production in single contractions (Vopt, Fig. 2D) showed a pattern of response to stimulation very similar to that described above for Vmax. After 12 weeks this variable had fallen to of control. The experimental estimates for Vopt were closer to the value obtained from the fitted curve than for Vmax. The fitted: estimated ratio was '02 for the stimulated muscles and for the control muscles. Maximum power The maximum power was calculated from the force-velocity relationship and should therefore be thought of only as the maximum power attainable briefly in a single tetanus. This variable fell sharply over the first 6 weeks of stimulation as a consequence of the reductions in speed and force. Thereafter the maximum power did

9 MUSCLE MECHANICS AFTER CHRONIC ACTIVATION not change up to 12 weeks of stimulation. The final level was only of control. This represents a fall from to W for individual muscles. Part of this reduction was due to the loss of mass, and therefore force-generating capacity, but even in terms ofw kg-' of muscle the reduction was still to '07 of control, from to W kg-' (Fig. 3). 165.~10 Em EE 0mE *E. * o i I I I Time (min) Fig. 4. The power production of a 2-week-stimulated tibialis anterior muscle (@) and its contralateral unstimulated muscle (0) during a 5 h fatigue test of repeated shortening contractions, with the protocol described in the text. *, the result of a similar test on an 8-week-stimulated muscle. Fatigue properties Two examples are given of fatigue tests in which it was arranged for stimulated and control muscles to shorten actively at Vopt with a contraction repeat frequency that gave an initial work output of about 10 W kg-1. Figure 4 shows that after only 2 weeks of stimulation the left muscle (@) in this rabbit was able to maintain a power output of about 13 W kg-' over 5 h, while the right, unstimulated muscle (0) showed progressive fatigue. In this particular experiment, the initial working rate represented 8-1 % of the maximum power output in a single tetanus for the stimulated muscle and 4-5 % for the control muscle. Figure 4 also shows a similar test for a tibialis anterior muscle after 8 weeks of stimulation (*). In this case the initial working rate of 10 W kg-' represented 20% of the maximum power output in a single contraction because of the loss of force and speed. Nevertheless, the stimulated muscle was still much better able to maintain this level of working than a control muscle. Fatigue resistance to this absolute power requirement was clearly induced early in the course of continuous stimulation at 10 Hz. The fatigue data for all the experimental muscles are summarized in Fig. 5, in which the power output after 1 h of cyclic contractions is plotted as a percentage of the power output at the start of the test, arranged to be close to 10 W kg-'. Most of the stimulated muscles showed negligible reduction of power output, whereas the control muscles showed a decline to about 50 % of the initial level. Direct stimulation at 80 V did not overcome this loss of contractile function and we may therefore conclude that the fatigue was not due to a reduction in the efficiency of neuromuscular transmission.

10 166 J. C. JARVIS 100 i f *E Fig. 5. The power production of control and stimulated muscles after 1 h of contractions during the fatigue test described in the text. The power output is expressed as a percentage of that at the start of the test. DISCUSSION The adaptive response to chronic stimulation at 10 Hz consists of changes in many of the subsystems of the cell, which proceed at different rates and to different extents. While changes in mass, isometric contractile speed and tetanic force have been reported, their combined effect on the force-velocity relationship and the power output of stimulated muscles has not previously been demonstrated. This study provides a complete time course of the profound changes in the dynamic properties of such muscles over 3 months of stimulation. The 12-week-stimulated muscles were slower than control 'slow-twitch' soleus muscles in their production of isometric force (Salmons & Vrbova', 1969), and at least as slow in their maximum velocity of shortening (1P57 lengths per second compared with 1P8 for soleus, estimated from Sreter, Luff & Gergely, 1975). The control rabbit soleus muscle contains about 5 % of fast type 2 fibres. By every criterion used to date, rabbit tibialis anterior muscle stimulated at 10 Hz for more than 8-10 weeks appears to be composed exclusively of type 1 fibres and the properties of such muscles might therefore be considered representative of that fibre type. Although the dynamic properties of single identified type 1 fibres have been measured (Eddinger & Moss, 1987), the differences in methodology (low temperature, skinning) preclude direct comparison with the whole-muscle values. The degree of slowing was similar to that measured by Al-Amood et al. (1973) in cat flexor digitorum longus motor units stimulated for 8 weeks. In the present study the maximum power output for control tibialis anterior muscle was 290 W kg-', which fell to about 50 W kg-' after 12 weeks of stimulation. These values are in agreement with published data for naturally occurring fast and slow muscles. Faulkner, Claflin, Brooks & Burton (1991) have recorded figures for maximum power at 35 "C of W kg-' in bundles of fibres from human latissimus dorsi muscle, which contains approximately equal numbers of fast and

11 MUSCLE MECHANICS AFTER CHRONIC ACTIVATION slow fibres. The same group has published figures for mouse extensor digitorum longus and soleus muscle of 221 and 89 W kg-' respectively (Brooks, Faulkner & McCubbrey, 1990). The results of Marechal & Beckers-Bleukx (1991) for the soleus muscle of the mouse ranged from 15 to 90 W kg-' at 20 'C. The data for maximum muscle power output is therefore consistent between experimental studies and between species. The figures are, furthermore, of the same order as those estimated from the power output of whole organisms. The maximum voluntary power output of a man, for example, is about 2 kw (Wilkie, 1959). Assuming that this power is produced from about 30 kg of muscle, and that the muscles spend equal times contracting and relaxing, the maximum power output in single contractions would be about 70 W kg-'. While the maximum power output in single muscle contractions is an important factor in the locomotor performance of animals, this level cannot be sustained. In order to investigate what level of mechanical power output can be generated indefinitely, a test consisting of repeated contraction and relaxation cycles must be used. In any fatigue test consisting of shortening contractions certain parameters have to be fixed. Brooks & Faukner (1991) chose to set the length over which a contraction was allowed (10% fibre length), and the velocity of shortening. The duration of each contraction was therefore fixed. They then varied the duty cycle to find the sustainable power, defined as the level at which a further decrease in the resting time between contractions produced no extra average power. We made no attempt in this study to assess the maximum sustainable power. Instead, the duty cycle was fixed for each muscle so as to elicit an initial power output of 10 W kg-'. Brooks & Faulkner (1991) found that the sustainable power available from a slow mouse muscle (soleus, W kg-') was lower than that available from a fast muscle (extensor digitorum longus, W kg-') when both muscles worked at their respective velocities for maximum power production. In the present study the slowest muscles tested (12-week-stimulated tibialis anterior muscles) were able to sustain 10 W kg-' indefinitely; the control fast muscle fatigued rapidly at this work level. It is interesting to note that 10 W kg-' was close to the absolute maximum power output for the 12-week-stimulated muscles: the duty cycle was about 50% and the muscles were so slow that a further decrease of this parameter would not have allowed the muscles to relax completely between contractions. It is possible that 12-week-stimulated muscle is more fatigue resistant than naturally occurring slow muscle. Certainly after 8 weeks the oxidative capacity is elevated above that of control soleus muscle (Henriksson et al. 1986). Fatigue resistance has traditionally been defined as the ability to maintain isometric force production. The present results show that when fatigue resistance is defined as the ability to perform a specific amount of work per second per kilogram of muscle, fast-twitch muscle whose oxidative capacity has been increased by training is just as fatigue resistant as slow muscle. It appears, however, that the adaptation achieved after 2 weeks of stimulation, although it imparts considerable fatigue resistance to the muscle, does not remove the intracellular cues which continue to drive the muscle fibres further towards the slow end of the fibre-type spectrum. By 12 weeks the muscle fibres had reached a new state of equilibrium; there was little change in any parameter between 8 and 12 weeks. Since fibres in the 167

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