Length dependence of active force production in skeletal muscle

The sliding filament and cross-bridge theories of muscle contraction provide discrete predictions of the tetanic force-length relationship of skeletal muscle that have been tested experimentally. The active force generated by a maximally activated single fiber (with sarcomere length control) is maximal when the filament overlap is optimized and is proportionally decreased when overlap is diminished. The force-length relationship is a static property of skeletal muscle and, therefore, it does not predict the consequences of dynamic contractions. Changes in sarcomere length during muscle contraction result in modulation of the active force that is not necessarily predicted by the cross-bridge theory. The results of in vivo studies of the force-length relationship suggest that muscles that operate on the ascending limb of the force-length relationship typically function in stretch-shortening cycle contractions, and muscles that operate on the descending limb typically function in shorten-stretch cycle contractions. The joint moments produced by a muscle depend on the moment arm and the sarcomere length of the muscle. Moment arm magnitude also affects the excursion (length change) of a muscle for a given change in joint angle, and the number of sarcomeres arranged in series within a muscle fiber determines the sarcomere length change associated with a given excursion.

Cadence, power, and muscle activation in cycle ergometry

PURPOSE

Based on the resistance-rpm relationship for cycling, which is not unlike the force-velocity relationship of muscle, it is hypothesized that the cadence which requires the minimal muscle activation will be progressively higher as power output increases.

METHODS

To test this hypothesis, subjects were instrumented with surface electrodes placed over seven muscles that were considered to be important during cycling. Measurements were made while subjects cycled at 100, 200, 300, and 400 W at each cadence: 50, 60, 80, 100, and 120 rpm. These power outputs represented effort which was up to 32% of peak power output for these subjects.

RESULTS

When all seven muscles were averaged together, there was a proportional increase in EMG amplitude each cadence as power increased. A second-order polynomial equation fit the EMG:cadence results very well (r2 = 0.87- 0.996) for each power output. Optimal cadence (cadence with lowest amplitude of EMG for a given power output) increased with increases in power output: 57 +/- 3.1, 70 +/- 3.7, 86 +/- 7.6, and 99 +/- 4.0 rpm for 100, 200, 300, and 400 W, respectively.

CONCLUSION

The results confirm that the level of muscle activation varies with cadence at a given power output. The minimum EMG amplitude occurs at a progressively higher cadence as power output increases. These results have implications for the sense of effort and preferential use of higher cadences as power output is increased.

Force-frequency relationship and potentiation in mammalian skeletal muscle

Repetitive activation of a skeletal muscle results in potentiation of the twitch contractile response. Incompletely fused tetanic contractions similar to those evoked by voluntary activation may also be potentiated by prior activity. We aimed to investigate the role of stimulation frequency on the enhancement of unfused isometric contractions in rat medial gastrocnemius muscles in situ. Muscles set at optimal length were stimulated via the sciatic nerve with 50-micros duration supramaximal pulses. Trials consisted of 8 s of repetitive trains [5 pulses (quintuplets) 2 times per second or 2 pulses (doublets) 5 times per second] at 20, 40, 50, 60, 70, and 80 Hz. These stimulation frequencies represent a range over which voluntary activation would be expected to occur. When the frequency of stimulation was 20, 50, or 70 Hz, the peak active force (highest tension during a contraction - rest tension) of doublet contractions increased from 2.2 +/- 0.2, 4.1 +/- 0.4, and 4.3 +/- 0.5 to 3.1 +/- 0.3, 5.6 +/- 0.4, and 6.1 +/- 0.7 N, respectively. Corresponding measurements for quintuplet contractions increased from 2.2 +/- 0.2, 6.1 +/- 0.5, and 8.7 +/- 0.7 to 3.2 +/- 0.3, 7.3 +/- 0.6, and 9.0 +/- 0.7 N, respectively. Initial peak active force values were 27 +/- 1 and 61.5 +/- 5% of the maximal (tetanic) force for doublet and quintuplet contractions, respectively, at 80 Hz. With doublets, peak active force increased at all stimulation frequencies. With quintuplets, peak active force increased significantly for frequencies up to 60 Hz. Twitch enhancement at the end of the 8 s of repetitive stimulation was the same regardless of the pattern of stimulation during the 8 s, and twitch peak active force returned to prestimulation values by 5 min. These experiments confirm that activity-dependent potentiation is evident during repeated, incompletely fused tetanic contractions over a broad range of frequencies. This observation suggests that, during voluntary motor unit recruitment, derecruitment or decreased firing frequency would be necessary to achieve a fixed (submaximal) target force during repeated isometric contractions over this time period.

Caffeine ingestion and performance of a 1,500-metre swim

The purpose of this study was to evaluate the potential ergogenic benefit of caffeine in the performance of a 1,500-meter swim. Caffeine (6 mg.kg-1) or placebo was administered 2-1/2 hrs prior to the swim trial in a double-blind crossover design. Caffeine resulted in a significantly lower perceived exertion for 100-m warm-up swims. Subjects swam significantly (p < 0.05) faster with caffeine (20:58.8 +/- 0:36.4, mean +/- SEM) than without (21:21.8 +/- 0:38). Plasma potassium was significantly lower prior to the swim with caffeine, and blood glucose was higher after that swim. Caffeine provides an ergogenic benefit for a 1,500-meter swim, an event that is completed in less than 25 min. Lower plasma potassium concentration prior to exercise and higher blood glucose following the trial suggest that electrolyte balance and glucose availability may be important aspects of the ergogenic effects of caffeine.

Force-interval relations of twitches and cold contractures in rat cardiac trabeculae. Effect of ryanodine

The twitch force (Ft)-interval relation of cardiac muscle reflects recovery of calcium release from the sarcoplasmic reticulum (SR). The calcium content of the SR is thought to be reflected by force developed during a contracture (Fc), induced by rapid cooling to near 0 degrees C. In right ventricular trabeculae of rat, under control conditions, the Ft-interval relation consisted of recovery of Ft to steady state (early recovery), followed by a secondary increase of Ft up to a maximum at an interval of approximately 100 seconds (rest potentiation) and a decline of Ft at intervals greater than 100 seconds (rest depression). The mechanisms that may underlie recovery of force after the last twitch at short intervals are 1) time-dependent transport of Ca2+ from the uptake compartment of the SR to the release compartment, 2) recovery of slow inward Ca2+ current during the action potential, and 3) recovery of the Ca2+ release channels in the SR. The Fc-interval relation was similar to the Ft-interval relation in that both a rest potentiation and a rest depression phase were present. However, at short interstimulus intervals (less than 1 second), Fc was independent of time, suggesting that the mechanism underlying early recovery was bypassed. Ryanodine (0.1-10 nM) reduced rest potentiation in a dose-dependent manner and accelerated rest depression of both Ft and Fc. At high ryanodine concentration, a significant Fc could only be induced after short intervals. Significant acceleration of rest depression was observed at low ryanodine concentrations, when Ft at intervals of 5 seconds was kept constant by increasing the stimulus frequency of [Ca2+]o, suggesting that the ryanodine effect was enhanced by increased [Ca2+]i. Ryanodine also increased the rate of decay of postextrasystolic potentiation in a dose-dependent manner. A significant effect was observed in 10 nM ryanodine. The twitch was not prolonged by ryanodine at these concentrations. These results suggest that the small magnitude of the twitch at short intervals is due to the finite time required by SR Ca2+ release channels to fully recover after a twitch. Furthermore, the results offer support for the hypothesis that ryanodine (in the nanomolar range) promotes Ca2+ leak from the SR in a dose-dependent manner and thereby limits Ca2+ accumulation during the interstimulus interval. Therefore, it may be expected that the negative inotropic effect of ryanodine is due to the SR Ca2+ depletion, and it is not necessary to postulate that ryanodine "blocks" the Ca2+ release channels in the SR.

Posttetanic potentiation and skeletal muscle fatigue: interactions with caffeine

The purpose of this study was to determine the interaction of three factors that modify twitch contraction amplitude in the rat gastrocnemius muscle in situ: posttetanic potentiation, fatigue, and caffeine. Posttetanic (200 Hz for 1 s) twitch responses were observed before and after 15 Hz stimulation for 6 min (group FS), injection of caffeine (75 mg/kg dissolved in saline, group NC), a combination of both repetitive stimulation and caffeine injection (group FC), or no treatment (group NS). Developed tension increased significantly with posttetanic potentiation and caffeine injection and these potentiating factors were additive (group NC). Repetitive stimulation attenuated the twitch response and the fatigued muscle was still responsive to the potentiating factors. Posttetanic potentiation was accomplished primarily by a significant increase in the peak rate of force development whereas caffeine potentiation and fatigue were effected with a proportional change in contraction time. It seems likely that the mechanism of posttetanic potentiation is not the same as the mechanism of caffeine-induced potentiation. Caffeine-induced potentiation is known to be related to increased release of calcium. Because changes in contraction time with fatigue were opposite to those associated with caffeine potentiation, it is proposed that the attenuated twitch response in fatigue results from reduced release of calcium.

Myosin light chain phosphorylation and posttetanic potentiation in fatigued skeletal muscle

Myosin light chain (P-LC) phosphorylation, which is thought to be the principle mechanism for twitch potentiation in skeletal muscle, is significantly decreased during staircase in fatigued muscle. Attenuated phosphorylation of P-LC could be due to either depressed Ca2+ transients in fatigue, or to some inhibitory influence of contractile activity on myosin light chain kinase (MLCK). Tetanic stimulation, which would presumably result in maximal activation of MLCK, could be used to evaluate these potential mechanisms. P-LC phosphorylation and twitch developed tension (DT) were assessed at 20 and 120 s following a tetanic contraction in either rested or fatigued rat gastrocnemius muscle in situ. P-LC phosphorylation was significantly lower in fatigued muscles (39.7 +/- 3.2% vs 54.8 +/- 3. 5%, 20 s after a 2-s tetanic contraction), while posttetanic potentiation (PTP) was similar in fatigued (189.1 +/- 6.5%) versus rested muscle (169.5 +/- 2.6%). Tetanic DT was reduced following the fatigue protocol and, thus, the assumption that the MLCK system was fully activated by Ca2+ may not be valid. The potentiation-phosphorylation relationships were linear for both rested and fatigued muscles; however this relationship was shifted markedly leftward in fatigued muscles. It appears that during PTP, equivalent potentiation is attained with correspondingly lower levels of P-LC phosphorylation in fatigued muscle. This enhanced relative potentiation for a given level of phosphorylation could be expected if Ca2+ transients were attenuated in the fatigued muscle. However the results do not rule out the possibility that other factors contribute to potentiation under these circumstances.

Human skeletal muscle fibre types and force: velocity properties

It has been reported that there is a relationship between power output and fibre type distribution in mixed muscle. The strength of this relationship is greater in the range of 3-8 rad.s-1 during knee extension compared to slower or faster angular knee extensor speeds. A mathematical model of the force: velocity properties of muscle with various combinations of fast- and slow-twitch fibres may provide insight into why specific velocities may give better predictions of fibre type distribution. In this paper, a mathematical model of the force:velocity relationship for mixed muscle is presented. This model demonstrates that peak power and optimal velocity should be predictive of fibre distribution and that the greatest fibre type discrimination in human knee extensor muscles should occur with measurement of power output at an angular velocity just greater than 7 rad.s-1. Measurements of torque:angular velocity relationships for knee extension on an isokinetic dynamometer and fibre type distribution in biopsies of vastus lateralis muscles were made on 31 subjects. Peak power and optimal velocity were determined in three ways: (1) direct measurement, (2) linear regression, and (3) fitting to the Hill equation. Estimation of peak power and optimal velocity using the Hill equation gave the best correlation with fibre type distribution (r < 0.5 for peak power or optimal velocity and percentage of fast-twitch fibres). The results of this study confirm that prediction of fibre type distribution is facilitated by measurement of peak power at optimal velocity and that fitting of the data to the Hill equation is a suitable method for evaluation of these parameters.

Length-dependent potentiation and myosin light chain phosphorylation in rat gastrocnemius muscle

Changes in muscle length affect the degree of staircase potentiation in skeletal muscle, but the mechanism by which this occurs is unknown. In this study, we tested the hypothesis that length-dependent change in staircase is modulated by phosphorylation of the myosin regulatory light chains (RLC), since this is believed to be the main mechanism of potentiation. In situ isometric contractile responses of rat gastrocnemius muscle during 10 s of repetitive stimulation at 10 Hz were analyzed at optimal length (Lo), Lo - 10%, and Lo + 10%. The degree of enhancement of developed tension during 10 s of repetitive stimulation was observed to be length dependent, with increases of 118.5 +/- 7.8, 63.1 +/- 3.9, and 45.6 +/- 4.1% (means +/- SE) at Lo - 10%, Lo, and Lo + 10%, respectively. Staircase was accompanied by increases in the average rate of force development of 105.6 +/- 7.7, 55.6 +/- 4.1, and 37.2 +/- 4.4% for Lo - 10%, Lo, and Lo + 10%, respectively. RLC phosphorylation after 10 s of 10-Hz stimulation was higher than under resting conditions but not different among Lo - 10% (40 +/- 3.5%), Lo (35 +/- 3.5%), and Lo + 10% (41 +/- 3.5%). This study shows that there is a length dependence of staircase potentiation in mammalian skeletal muscle that may not be directly modulated by RLC phosphorylation. Interaction of RLC phosphorylation with length-dependent changes in Ca2+ release and intermyofilament spacing may explain these observations.

Norepinephrine increases canine skeletal muscle VO2 during recovery

The purpose of this study was to investigate the effect of norepinephrine on the rate of O2 uptake (VO2) of the denervated canine gastrocnemius-plantaris muscle group in situ. In seven experiments, VO2 and developed tension were measured without and with norepinephrine infusion during rest, during contractions at 1.0 Hz, and during recovery. In six additional experiments with two consecutive rest-contraction-recovery periods, no norepinephrine was given during the second sequence. During rest, VO2 was increased by norepinephrine. Changes in VO2 during contractions were small, but the arteriovenous O2 content difference was significantly greater during norepinephrine infusion. The most significant finding was that net recovery VO2 was increased 40% by norepinephrine infusion. The ratio of net recovery VO2 to the VO2 during the preceding contraction period was significantly increased from 0.78 +/- 0.07 (mean +/- SEM) to 1.23 +/- 0.07 (mean +/- SEM) by norepinephrine infusion. This increase in net recovery VO2 could be due to either a direct metabolic effect of norepinephrine during recovery or to hypoxia during the preceding contractions. In either case, the data indicated that norepinephrine can produce large increases in muscle recovery VO2; this supports the notion that catecholamines may make a significant contribution to post-exercise recovery VO2.

Staircase in mammalian muscle without light chain phosphorylation

In disuse atrophied skeletal muscle, the staircase response is virtually absent and light chain phosphorylation does not occur. The purpose of the present study was to determine if staircase could be restored in atrophied muscle with continued absence of myosin light chain phosphorylation, by reducing what appears to be an otherwise enhanced calcium release. Control (untreated) and sham-operated female Sprague-Dawley rats were compared with animals after 2 weeks of complete inactivity induced by tetrodotoxin (TTX) application to the left sciatic nerve. In situ isometric contractile responses of rat gastrocnemius muscle were analyzed before and after administration of dantrolene sodium (DS), a drug which is known to inhibit Ca2+ release in skeletal muscle. Twitch active force (AF) was attenuated by DS from 2.2 +/- 0.2 N, 2.7 +/- 0.1 N and 2.4 +/- 0.2 N to 0.77 +/- 0.2 N, 1.05 +/- 0.1 N and 1.01 +/- 0.2 N in TTX (N = 5), sham (N = 11) and control (N = 7) muscles, respectively. Following dantrolene treatment, 10 s of 10-Hz stimulation increased AF to 1.32 +/- 0.2 N, 1.52 +/- 0.1 N and 1.45 +/- 0.2 N for the TTX, sham and control groups, respectively, demonstrating a positive staircase response. Regulatory light chain (R-LC) phosphorylation was lower for TTX-treated (5.5 +/- 5.5%) than for control (26.1 +/- 5.3%) and sham (20.0 +/- 5%) groups. There was no significant change from resting levels for any of the groups after DS treatment (P = 0.88). This study shows that treatment with dantrolene permits staircase in atrophied muscle as well as control muscle, by a mechanism which appears to be independent of R-LC phosphorylation.

Attenuation of myosin light chain phosphorylation and posttetanic potentiation in atrophied skeletal muscle

Previously we have demonstrated that the absence of staircase potentiation in atrophied rat gastrocnemius muscle is accompanied by a virtual absence of phosphorylation of the regulatory light chains (R-LC) of myosin. It was our purpose in the present study to determine if posttetanic potentiation and corresponding R-LC phosphorylation were also attenuated in disuse-atrophied muscles. Two weeks after a spinal hemisection (T12), twitch and tetanic contractile characteristics were measured in situ in control, sham-treated and atrophied (hemisected) muscles. Posttetanic potentiation 20 s after a 2 s tetanic contraction (200 Hz) was depressed in atrophied muscles (128.7 +/- 2.6%; mean +/- SEM) when compared to sham-treated (149.9 +/- 2.4%) and control (142.9 +/- 2. 7%) muscles. Atrophied muscles demonstrated a significant increase in R-LC phosphorylation from rest (0.05 +/- 0.04 moles of phosphate/mole of R-LC) to posttetanic conditions (0.21 +/- 0.03 moles of phosphate/mole of R-LC), and less phosphorylation than control and sham-treated muscles (0.43 +/- 0.06 and 0.49 +/- 0.03 moles of phosphate/mole of R-LC, respectively) after tetanic stimulation. The preservation of the potentiation-phosphorylation relationship in atrophied muscles is consistent with the hypothesis that R-LC phosphorylation may be the principal mechanism for twitch potentiation.

Contractile properties of rat gastrocnemius muscle during staircase, fatigue and recovery

Slowing of relaxation is one of the anticipated changes in the contraction of fatigued skeletal muscle. However, interpretation of the mechanism(s) contributing to slowed relaxation may be affected by the measurement technique employed. In this study, relaxation was measured in three ways: (i) traditional half-relaxation time; (ii) peak rate of relaxation; and (iii) late relaxation time, measured from 50 to 25% of peak developed tension. When rat gastrocnemius muscle was stimulated indirectly in situ at 10 Hz, developed tension increased in 10 s to 185%, then decreased to 39% after 1 min with little additional change over the next 4 min. After 10 s of inactivity, developed tension was 60% of the initial value, but did not recover further over the next 20 min. The half-relaxation time transiently decreased at the start of stimulation, then by 20 s was considerably prolonged. Within 10 s of recovery, half-relaxation time returned to prestimulation values but became prolonged again by 10 min of recovery. The peak rate of relaxation was proportional to the developed tension at all times except 2.5-10 s of 10 Hz stimulation, at which time acceleration of relaxation was evident, and 15-20 s of the 10 Hz stimulation when it was relatively decreased. The late relaxation time increased during the repetitive stimulation, returned to control level early in recovery, then increased again, by 5 min of recovery. The diverse responses indicated by these indices of relaxation potentially discriminate different mechanisms which contribute to slowing of relaxation in fatigue, a point which would be missed if a single method of measurement of relaxation was employed.

Myosin light chain phosphorylation during staircase in fatigued skeletal muscle

It has been reported that the peak of the staircase or the enhanced tension response during low frequency stimulation is delayed in fatigued fast muscle. Our purpose was to determine if the rate and extent of regulatory myosin light chain (P-LC) phosphorylation, a molecular mechanism associated with the positive staircase, are also altered by fatigue. The staircase contractile response, muscle metabolites and phosphate incorporation by the P-LC were assessed at 0, 5, 10 or 20 s of 10-Hz stimulation, in either non-fatigued (control) or fatigued (10 Hz for 5 min, followed by 20 min of recovery) rat gastrocnemius muscle in situ. The concentration of adenosine triphosphate (ATP) in fatigued muscles, 21 +/- 0.9 mmol.kg-1 (dry weight) was significantly lower (P < 0.05) than in the control muscles, 26.1 +/- 1.5 mmol.kg-1. In both groups, ATP content was significantly lower after 20 s of 10 Hz stimulation. The P-LC phosphate content (in mol phosphate.mol-1 P-LC) was 0.10, 0.38, 0.60 and 0.72 after 0, 5, 10 or 20 s of 10 Hz stimulation in control muscles, but only 0.03, 0.08, 0.11 and 0.19 at these times in fatigued muscles. Although the absolute magnitude of tension potentiation was attenuated in proportion to the depressed twitch amplitude, these surprisingly low levels of phosphorylation were associated with 0, 48, 79 and 86% potentiation of the developed tension at these times in contrast with 0, 71, 87 and 49% potentiation in control muscles. These data demonstrate that while the rate and extent of phosphate incorporation is depressed in fatigued muscle, tension potentiation is still evident.(ABSTRACT TRUNCATED AT 250 WORDS)

The biphasic force-velocity relationship in whole rat skeletal muscle in situ

Edman has reported that the force-velocity relationship (FVR) departs from Hill's classic hyperbola near 0.80 of measured isometric force (J Physiol 404: 301-321, 1988). The purpose of this study was to investigate the biphasic nature of the FVR in the rested state and after some recovery from fatigue in the rat medial gastrocnemius muscle in situ. Force-velocity characteristics were determined before and during recovery from fatigue induced by intermittent stimulation at 170 Hz for 100 ms each second for 6 min. Force-velocity data were obtained for isotonic contractions with 100 ms of 200-Hz stimulation, including several measurements with loads above 0.80 of measured isometric force. The force-velocity data obtained in this study were fit well by a double-hyperbolic equation. A departure from Hill's classic hyperbola was found at 0.88+/-0.01 of measured isometric force, which is higher than the approximately 0.80 reported by Edman et al. for isolated frog fibers. After 45 min of recovery, maximum shortening velocity was 86+/-2% of prefatigue, but neither curvature nor predicted isometric force was significantly different from prefatigue. The location of the departure from Hill's classic hyperbola was not different after this recovery from the fatiguing contractions. Including an isometric point in the data set will not yield the same values for maximal velocity and the degree of curvature as would be obtained using the double hyperbola approach. Data up to 0.88 of measured isometric force can be used to fit data to the Hill equation.

Staircase, fatigue, and caffeine in skeletal muscle in situ

The specific locus of impairment in excitation-contraction coupling that is associated with skeletal muscle fatigue has not been identified. In the present study the phenomena of staircase and fatigue were studied in the rat gastrocnemius muscle in situ, and the effect of caffeine (50 mg kg-1) given prior to or during 5 minutes of stimulation was observed. A 10 Hz indirect stimulation resulted in a staircase response that proceeded for 10.4 +/- 1.6 (mean +/- SD) seconds, reaching a peak force value that was 70-75% higher than the initial contraction. After 5 minutes of stimulation and 20 minutes of rest, the staircase response was longer (17 +/- 3.1 seconds) and proceeded more slowly when the stimulation regimen was repeated. Caffeine accelerated the fatigue and reversed the effect of fatigue on the staircase response. Since caffeine enhances the release of Ca2+ from terminal cisternae, it is postulated that the accelerated fatigue in the presence of caffeine is indicative of a reduced availability of Ca2+ for release. This hypothesis would also explain the slower progression of staircase in the fatigued muscle.

Blood lactate accumulation decreases during the slow component of oxygen uptake without a decrease in muscular efficiency

Pulmonary oxygen uptake ([Formula: see text]) slowly increases during exercise above the anaerobic threshold, and this increase is called the slow component of [Formula: see text]. The mechanism of the increase in [Formula: see text] is assumed to be due to increasing energy cost associated with increasingly inefficient muscle contraction. We hypothesized that the increase in [Formula: see text] would be accompanied by a constant or increasing rate of accumulation of blood lactate, indicating sustained anaerobic metabolism while [Formula: see text] increased. Ten male subjects performed cycle ergometry for 3, 6, and 9 min at a power output representing 60% of the difference between lactate threshold and maximal [Formula: see text] while [Formula: see text] and blood lactate accumulation were measured. Blood lactate accumulation decreased over time, providing the energy equivalent of (mean ± SD) 1586 ± 265, 855 ± 287, and 431 ± 392 ml of [Formula: see text] during 0-3, 3-6, and 6-9 min of exercise, respectively. As duration progressed, [Formula: see text] supplied 86.3 ± 2.0, 93.6 ± 1.9, and 96.8 ± 2.9% of total energy from 0 to 3, 3 to 6, and 6 to 9 min, respectively, while anaerobic contribution decreased. There was no change in total energy cost after 3 min, except that required by ventilatory muscles for the progressive increase in ventilation. The slow component of [Formula: see text] is accompanied by decreasing anaerobic energy contribution beyond 3 min during heavy exercise.

Reports of the length dependence of fatigue are greatly exaggerated

Relative force depression associated with muscle fatigue is reported to be greater when assessed at short vs. long muscle lengths. This appears to be due to a rightward shift in the force-length relationship. This rightward shift may be caused by stretch of in-series structures, making sarcomere lengths shorter at any given muscle length. Submaximal force-length relationships (twitch, double pulse, 50 Hz) were evaluated before and after repetitive contractions (50 Hz, 300 ms, 1/s) in an in situ preparation of the rat medial gastrocnemius muscle. In some experiments, fascicle lengths were measured with sonomicrometry. Before repetitive stimulation, fascicle lengths were 11.3 ± 0.8, 12.8 ± 0.9, and 14.4 ± 1.2 mm at lengths corresponding to −3.6, 0, and 3.6 mm where 0 is a reference length that corresponds with maximal active force for double-pulse stimulation. After repetitive stimulation, there was no change in fascicle lengths; these lengths were 11.4 ± 0.8, 12.6 ± 0.9, and 14.2 ± 1.2 mm. The length dependence of fatigue was, therefore, not due to a stretch of in-series structures. Interestingly, the rightward shift that was evident when active force was calculated in the traditional way (subtraction of the passive force measured before contraction) was not seen when active force was calculated by subtracting the passive force that was associated with the fascicle length reached at the peak of the contraction. This calculation is based on the assumption that passive force decreases as the fascicles shorten during a fixed-end contraction. This alternative calculation revealed similar postfatigue absolute active force depression at all lengths. In relative terms, a length dependence of fatigue was still evident, but this was greatly diminished compared with that observed when active force was calculated with the traditional method.

Is a parallel elastic element responsible for the enhancement of steady-state muscle force following active stretch?

For over 50 years, it has been recognised that muscles from many different species of animals are able to generate a higher steady-state isometric force after active stretch than during a purely isometric contraction at the same length. This is known as `residual force enhancement' (rFE). The mechanism underlying this phenomenon remains controversial. One proposal is that an elastic element parallel to the cross-bridges becomes stiffer, or is engaged,when the muscle is activated and generates force when stretched. If this is indeed the sole mechanism, then rFE should be eliminated by subsequently shortening the muscle by a distance equal to or greater than the initial stretch. We tested this hypothesis using six intact single fibres from frog lumbrical muscle. The fibres were activated and stretched to generate rFE and then rapidly shortened by between 25% and 700% of the initial stretch distance. In contrast to previous reports, we found that rapid shortening induced a depression of subsequent isometric force. We used two methods to account for this force depression when calculating rFE, thereby obtaining upper and lower bounds for the true rFE. With both methods of calculation, rFE was significantly greater than zero when shortening distance was equal to stretch distance (P=0.0004 and P=0.03, respectively). Therefore, our hypothesis was not supported. We conclude that rFE is unlikely to be generated solely by a parallel elastic element.

Evaluation of the Monark Wingate ergometer by direct measurement of resistance and velocity

The purpose of this study was to assess the accuracy of the new basket-loaded Wingate ergometer introduced by Monark (Model 834E). Velocity was measured directly from the pedal switch while tension was measured with transducers on each end of the brake lacing. Moment of inertia of the flywheel was determined and accounted for in the calculation of power. Constant load tests (39.24 to 98.1 N), were done at pedaling speeds from 80 to 140 r x min(-1) (flywheel angular velocity = 30-50 rad x s(-1)). The load transmitted to the lacing at the front and back of the flywheel was 95.5 +/- 0.8% (mean +/- SEM) and 6.71 +/- 0.8%, respectively, of the load in the basket. Thus, the resultant tension (front minus back) was on average 88.8 +/- 0.57% of the applied load. The velocity recorded by the Monark Wingate Ergometer computer program (MWECP) was the same (100.4 +/- 1.56%) as that determined from the pedal switch directly. Five male mountain bikers performed a 30-s all-out test. Peak power calculated by MWECP (1181 +/- 55W) was always higher (p < .01) than that calculated from direct measures of tension and velocity (1102 +/- 66W), when not taking into account the moment of inertia. These experiments suggest that the basket-loaded Monark Wingate ergometer does not provide a correct calculation of power because of incomplete load transmission to the flywheel.

Absence of staircase following disuse in rat gastrocnemius muscle

Repetitive stimulation of mammalian fast-twitch skeletal muscles will normally result in a positive staircase response. This phenomenon was investigated in the rat gastrocnemius muscle following a 2-week period of tetrodotoxin-induced disuse. Muscle inactivity was imposed by superfusing tetrodotoxin in saline over the left sciatic nerve via an implanted osmotic pump. In situ isometric contractile responses to double pulse stimulation and repetitive stimulation at 10 Hz were determined the day after removal of the pump. Two weeks of disuse resulted in 40% muscle weight loss. A twitch contraction gave the same force when expressed per gram of wet muscle weight in control muscles, 317 +/- 24.6 (means +/- SE) g/g, as compared with tetrodotoxin-treated muscles, 328 +/- 24.2 g/g. Both contraction time and half-relaxation time were prolonged following treatment with tetrodotoxin. Repetitive stimulation at 10 Hz resulted in a positive staircase response in the control muscles, but not in muscles of the tetrodotoxin-treated rats. The observed changes in the time course of the twitch contraction with repetitive stimulation following tetrodotoxin-induced disuse are consistent with alterations in sarcoplasmic reticulum handling of calcium. It is not certain if there is a change following disuse in the mechanism normally associated with staircase or if this mechanism is merely opposed by an early fatigue.

Production of methionyl-minus ovine growth hormone in Escherichia coli and one-step purification

The overproduction of ovine growth hormone (oGH) in Escherichia coli is described, achieved in part by alteration of the codon usage for nine of the first 15 amino acids (aa) of the mature hormone. Recombinant oGH (re-oGH), representing 12% of the total cellular protein, was isolated from inclusion bodies by solubilisation using the cationic surfactant cetyltrimethylammonium chloride (CTAC). The hormone was refolded and subsequently purified to greater than 95% homogeneity in a single step using preparative reverse phase high performance liquid chromatography. The aa sequence analysis revealed that the N-terminus of the E. coli-derived polypeptide was identical to that of pituitary-derived oGH, and re-oGH displayed potent somatotropic activity in vivo.

Force-frequency and force-length properties in skeletal muscle following unilateral focal ischaemic insult in a rat model

AIM

Our purpose was to quantify skeletal muscle properties following unilateral focal ischaemic insult (stroke) in a rat model.

METHODS

Male rats were divided into two groups: stroke and 2 weeks recovery (n = 8) and control group (n = 7). Stroke was induced in the area of the motor neocortex containing hind limb corticospinal neurones. Contractile properties of the medial gastrocnemius muscle were measured in situ in both limbs. Force-length and force-frequency properties were measured before and 35 min after 5 min fatiguing stimulation.

RESULTS

Stroke resulted in bilateral tetanic fade during 200 Hz stimulation. When normalized to 100 Hz contractions, force at 200 Hz was 95.4 +/- 0.9% for the paretic muscles, 96.7 +/- 1.7% for non-paretic muscles and 102.2 +/- 1.0% for muscles of control rats (P = 0.006). Prior to fatiguing contractions, there was no difference in the length dependence of force. During repetitive contractions, active force fell significantly to 19 +/- 4 and 25 +/- 5% of initial force in paretic and non-paretic muscles of animals with a stroke respectively. In control animals active force fell to 37 +/- 5%. During repetitive contractions, fusion index increased in muscles of stroke animals to 1.0 +/- 0 but in control animals it was 0.95 +/- 0.02. There was selective force depression at short lengths for fatigued paretic muscle (significant difference at muscle lengths less than reference length -2 mm).

CONCLUSION

The tetanic fade at high stimulation frequencies indicates that there may be activation failure following focal ischaemic insult. The greater magnitude of fatigue and selective depression at short lengths following repetitive contractions should be investigated further.

O2 uptake and developed tension during and after fatigue, curare block, and ischemia

The purpose of this study was to investigate the effect of changes in developed tension on the ratio between O2 uptake and isometric developed tension in the in situ dog gastrocnemius-plantaris muscle group. O2 uptake and isometric developed tension of the muscle were measured during contractions at 1 twitch/s before and after fatigue with both single and twin impulses (6.5 V, 0.2-ms duration; the twins were separated by 10--20 ms). Twin impulses prior to fatigue raised developed tension to two times the tension developed with single impulses. Fatigue was obtained by stimulation at 10--20 impulses/s for 30--40 min. Twin impulses after fatigue returned developed tension to the level of single impulses before fatigue. O2 uptake and developed tension were also measured during the slower development of fatigue produced by continuous stimulation (3, 4, 5, and 6 impulses/s) as well as during "fatigue" induced by partial neuromuscular block with curare or ischemia. In all cases, there was no change in the O2 uptake per unit of tension developed, indicating a constant coupling between O2 uptake and developed tension.