Contractile properties of the human triceps surae following prolonged exercise and beta-blockade

Sixteen healthy males volunteered to perform both an incremental maximal and prolonged submaximal treadmill test with beta-blockade (2 X 80 mg oral propranolol per day) or matched placebo in a blind crossover design. Prior to and following the prolonged exercise, electrical stimulation of the triceps surae was performed to examine contractile properties. During the maximal test, the heart rate (HR) was reduced at all times by beta-blockade. The time to exhaustion in this test was significantly reduced by beta-blockade (P less than 0.03), while the maximal oxygen uptake (VO2 max) was not significantly lower (P = 0.06). In response to prolonged treadmill walking at 60% of VO2 max, the HR was reduced but VO2, respiratory quotient and ventilation were not affected by beta-blockade relative to placebo. Plasma concentrations of free fatty acids increased during exercise in the placebo but not beta-blocked treatment (P less than 0.0001). Plasma noradrenalin and adrenalin increased with exercise; the increase in adrenalin with beta-blockade was greater than that with placebo (P less than 0.0001). The RPE obtained at intervals during the prolonged exercise were greater for beta-blockades than placebo. Eight of 16 subjects were unable to complete full 90 min with beta-blockade; but all 16 completed the test with placebo. The electrically evoked twitches in the triceps surae muscle group after exercise did not differ in peak torque or one-half relaxation time compared to pre-exercise. The time to peak twitch torque was significantly shorter after exercise. No differences in twitch were observed due to beta-blockade. The tetanic responses at 10, 20, 50 and 100 Hz were not affected by either exercise or the beta-blockade. In conclusion, an increased subjective estimate of fatigue (RPE) was observed during prolonged exercise with beta-blockade. This subjective fatigue did not relate to altered peripheral muscle force production during electrical stimulation. The results suggest either a central rather than peripheral origin of fatigue, or fatigue in a muscle group not examined by stimulation of the triceps surae.     

Mechanics of human triceps surae muscle in walking, running and jumping

Length changes of the muscle-tendon complex (MTC) during activity are in part the result of length changes of the active muscle fibres, the contractile component (CC), and also in part the result of stretch of elastic structures [series-elastic component (SEC)]. We used a force platform and kinematic measurements to determine force and length of the human calf muscle during walking, running and squat jumping. The force-length relation of the SEC was determined in dynamometer experiments on the same four subjects. Length of the CC was calculated as total muscle-tendon length minus the force dependent length of the SEC. The measured relations between force and length or velocity were compared with the individually determined force-length and force-velocity relations of the CC. In walking or running the negative work performed in the eccentric phase was completely stored as elastic energy. This elastic energy was released in the concentric phase, at speeds well exceeding the maximum shortening speed predicted by the Hill force-velocity relation. Speed of the CC, in contrast, was positive and low, well within the range predicted by the measured force-velocity properties and compatible with a favourable muscular efficiency. These effects were also present in purely concentric contractions, like the squatted jump. Contractile component length usually started at the far end of the force-length relation. Inter-individual differences in series-elastic stiffness were reflected in the force and length recordings during natural activity.     

Contractile properties of the human triceps surae muscle during simulated weightlessness

The effect of a 120-day period of bed rest on the mechanical properties of human triceps surae muscle was studied in a group of male volunteers (n = 6, mean age 38 years). The results shows that the contractile properties of skeletal muscle in response to disuse change considerably. Time to isometric peak tension of the triceps surae muscle increased from 120 (SEM 3.0) ms to 136 (SEM 2.9) ms (P < 0.01), half relaxation time from 92 (SEM 2.1) ms to 100 (SEM 1.6) ms (P < 0.05) and total contraction time from 440 (SEM 9.9) ms to 540 (SEM 18.7) ms (P < 0.001). Isometric twitch force (F _t) decreased by a mean of 36.7% (P < 0.05), maximal voluntary contraction (MVC) and maximal force (F _max) by a mean of 45.5% and 33.7%, respectively (P < 0.05-0.01). The valueF _max:F _t ratio increased by 3.6% (nonsignificant). The difference betweenF _max and MVC, expressed as a percentage ofF _max and referred to as force deficiency, has also been calculated. Force deficit increased by a mean of 60% (P < 0.001) after bed rest. Force-velocity properties of the triceps surae muscle calculated according to an absolute scale of voluntary and electrically evoked contraction development decreased considerably. The calculations of the same properties on a relative scale did not differ substantially from the initial physiological state. The results would suggest that muscle disuse is associated with both atrophy and a reduction in contractility in the development ofF _max and decreased central (motor) drive. The change in the triceps surae muscle contractile velocity properties may indicate changes in the kinetically active state in the muscles.     

System identification of human triceps surae stretch reflex dynamics

The interpretation of stretch-evoked reflex responses is complicated by the fact that the pattern of response will depend upon both the underlying reflex mechanisms and the time course of the stretch used to evoke the response. The objective of the present study was to use engineering systems analysis techniques to identify the dynamics of the human triceps surae (TS) stretch reflex in terms of its impulse response by deconvolving the position input from the observed response. Five normal subjects were instructed to maintain a tonic contraction of (TS) while subjected to repeated, computer-generated, stochastic perturbations of ankle position. Position, torque and smoothed, rectified surface EMGs were recorded and ensemble averaged over 25 stimulus presentations. Linear impulse response functions describing the dynamic relation between ankle velocity and TS EMG were found to account for a significant amount of the observed EMG variance (mean 60%). However, the impulse responses were noisy and the predicted EMG was systematically smaller than the observed EMG during the dorsiflexing phases of displacement. These findings suggested that a direction dependent nonlinearity might be present. Consequently, impulse responses relating half-wave rectified velocity to TS EMG were computed and found to be less noisy and to account for significantly more variance (mean 74%) than the purely linear model. The undirectional, velocity-sensitive impulse response functions were dominated by a large peak at about 40 ms followed by a smaller period of reduced activity. This is consistent with its mediation by primary spindle afferents. Although the shape of the impulse response remained unchanged, its amplitude, which provides a measure of relative gain, varied systematically with the level of contraction and the displacement amplitude. Multiple regression analysis demonstrated that most of the variation in the impulse response amplitude could be attributed to proportional increases with level of contraction (measured by average EMG) and proportional decreases with displacement amplitude.     

Comparison of the histochemical and contractile properties of human triceps surae

The purpose of the study was to compare the contractile properties determined from an electrically stimulated twitch with histochemically determined fibre type parameters of the human triceps surae. Muscle samples were obtained from the medial head of the gastrocnemius of ten male athletes. Ages ranged from 20 to 29 years. Muscle samples from the belly of the medial gastrocnemius muscle were obtained using the needle biopsy technique. The samples were treated histochemically for myosin ATPase to classify the fibres as either slow twitch (ST) or fast twitch (FT) and to determine fibre areas. Surface electrical stimulation was used to determine muscle twitch parameters. The contractile variables of the muscle twitch were latency (L), time to peak force (TPF), peak force (PF), half-contraction time (1/2 CT) and half-relaxation time (1/2 RT). Backward elimination procedures for dependent variables were used to determine which contractile properties best represented the histochemical profile of the muscles. Prediction formulas were developed for FT and ST percentages (R²=0·98, p<0·001), relative area percentage (R²=0·87, p<0·001), and ST area (R²=0·85, p<0·01). It was concluded that the use of the electrotensiometer (ETM) protocol was a valid testing procedure when studying physiological relationships of histochemical properties in intact human skeletal muscle. Protected by patent no. 4 688 581.     

Identification of time-varying dynamics of the human triceps surae stretch reflex

We examined the time-varying dynamics of the human triceps surae stretch reflex before, during, and after a large stretch was imposed upon the ankle joint, during a constant voluntary contraction of 15% of maximum voluntary contraction. Stretch reflex dynamics were estimated by superimposing a small stochastic displacement on many such stretches and using an ensemble-based time-varying identification procedure to compute impulse response functions relating the perturbation to the evoked electromyogram (EMG) at each point throughout the task. We found that stretch reflex magnitude (relating joint velocity to EMG) varied directly with baseline EMG activity during steady-state conditions before and after the large imposed stretch. Following the large stretch and the reflex activity it evoked, both background EMG and stretch reflex magnitude declined for up to 100 ms; changes in the stretch reflex were substantially greater in magnitude and followed a different time course from the corresponding changes in background EMG, however, indicating that stretch reflex properties were modulated independently of motoneuron pool activation level. Based on timing and the invariance of stretch reflex dynamics across time, it is argued that this behavior is largely mediated via peripheral neural mechanisms. This peripheral modulation of the stretch reflex presumably supplements various descending influences to adjust reflex properties.     

Central fatigue during a long-lasting submaximal contraction of the triceps surae

Our purpose was to study central fatigue and its dependence on peripheral reflex inhibition during a sustained submaximal contraction of the triceps surae. In 11 healthy subjects, superimposed twitches, surface electromyograms (EMG) from the medial head of the gastrocnemius (MG) and soleus (SOL) muscles, maximal compound motor action potentials (M_max), tracking error and tremor were recorded during sustained fatiguing contractions at a torque level corresponding to 30% of maximal voluntary contraction (MVC). When the endurance limit (401±91 s) of the voluntary contraction (VC-I) was reached, the triceps surae could be electrically stimulated to the same torque level for an additional 1 min in 10 of the 11 subjects. These subjects were then able to continue the contraction voluntarily (voluntary contraction II, VC-II) for another 85±48 s. At the endurance limit of VC-I, the superimposed twitch was larger than during the unfatigued MVC, while there was no significant difference between the twitch at the endurance limit of VC-II and MVC. The EMG amplitude of both MG and SOL at the endurance limit of VC-I was significantly less than that during the MVC. While the EMG amplitude of MG increased further during VC-II, SOL EMG remained unchanged, neither muscle reaching their unfatigued MVC values. This difference was diminished for SOL by taking into account its decrease in M_max found during VC-II, and relative EMG levels approached their MVC values. These results clearly indicate that a higher voluntary muscle activation was achievable after 1 min of electrical muscle stimulation, which continued metabolic stress and contractile fatigue processes but allowed for supraspinal, muscle spindle and/or motoneuronal recovery. It is concluded that peripheral reflex inhibition of -motoneurons via small-diameter muscle afferents is of minor significance for the development of the central fatigue that was found to occur during the first voluntary contraction.     

Comparison of the histochemical and contractile properties of human triceps surae

Summary Contractile and histochemical properties of the triceps surae were compared in 16 males and 4 females aged 20 to 49 years. Surface electrical stimulation was used to determine twitch, tetanic and fatigue parameters. From these tests, twitch tension (Pt), time to peak tension (TPT), half relaxation time 1/2 RT), tetanic tensions at 10, 20 and 50 Hz and an index of fatigue (FI) were calculated. A maximal voluntary contraction (MVC) was also performed. Muscle samples from the belly of the lateral gastrocnemius were obtained using the needle biopsy technique. The samples were treated histochemically for myosin ATPase and NADH-tetrazolium reductase in order to classify the fibres as either Type I, slow twitch (ST) or Type II, fast twitch (FT) and to determine fibre areas. Correlations were performed between the grouped male and female contractile and histochemical variables. The results demonstrated significant positive relationships between percentage of ST fibres (%ST) and TPT (r=0.49), and %ST and the ratio of tetanic forces at 10 Hz to 50 Hz (Po10/Po50) (r=0.55). No significant relationships were obtained for Pt, 1/2 RT, MVC or FI with any histochemical parameter. The results suggest that fibre type distribution determined using myosin ATPase is related to electrically stimulated isometric contractile speeds and not to voluntary force generation (MVC) or electrically induced fatigue.     

Control of the triceps surae during the postural sway of quiet standing

AIM: The present study investigated how the triceps surae are controlled at the spinal level during the naturally occurring postural sway of quiet standing. METHODS: Subjects stood on a force platform as electrical stimuli were applied to the posterior tibial nerve when the center of pressure (COP) was either 1.6 standard deviations anterior (COP(ant)) or posterior (COP(post)) to the mean baseline COP signal. Peak-to-peak amplitudes of the H-reflex and M-wave from the soleus (SOL) and medial gastrocnemius (MG) muscles were recorded to assess the efficacy of the Ia pathway. RESULTS: A significant increase in the H(max) : M(max) ratio for both the SOL (12 +/- 6%) and MG (23 +/- 6%) was observed during the COP(ant) as compared to the COP(post) condition. The source of the modulation between COP conditions cannot be determined from this study. However, the observed changes in the synaptic efficacy of the Ia pathway are unlikely to be simply a result of an altered level of background electromyographic activity in the triceps surae. This was indicated by the lack of differences observed in the H(max) : M(max) ratio when subjects stood without postural sway (via the use of a tilt table) at two levels of background activity. CONCLUSIONS: It is suggested that the phase-dependent modulation of the triceps surae H-reflexes during the postural sway of quiet standing functions to maintain upright stance and may explain the results from previous studies, which, until now, had not taken the influence of postural sway on the H-reflex into consideration.     

Muscle activation and the isokinetic torque-velocity relationship of the human triceps surae

Summary The influence of muscle activation and the time allowed for torque generation on the angle-specific torque-velocity relationship of the triceps surae was studied during plantar flexion using supramaximal electrical stimulation and a release technique on six male subjects [mean (SD) age 25 (4) years]. Torque-velocity data were obtained under different levels of constant muscle activation by varying the stimulus frequency and the time allowed for isometric torque generation prior to release and isokinetic shortening. To eliminate the effects of the frequency response on absolute torque the isokinetic data were normalized to the maximum isometric torque values at 0.44 rad. There were no significant differences in the normalized torques generated at any angular velocity using stimulus frequencies of 20, 50 or 80 Hz. When the muscle was stimulated at 50 Hz the torques obtained after a 400 ms and 1 s pre-release isometric contraction did not differ significantly. However, with no pre-release contraction significantly less torque was generated at all angular velocities beyond 1.05 rad · s^–1 when compared with either the 200, 400 ms or 1 s condition. With a 200 ms pre-release contraction significantly less torque was generated at angular velocities beyond 1.05 rad · s^–1 when compared with the 400 ms or 1 s conditions. It would seem that the major factor governing the shape of the torque-velocity curve at a constant level of muscle activation is the time allowed for torque generation.     

Response of the human triceps surae muscle to electrical stimulation during varying levels of blood flow restriction

The changes in muscle force associated with varying degrees of lower-limb ischaemia were investigated. Isometric torque production by the triceps surae muscle was measured during a 5-min continuous train of 2-Hz electrical stimulation in six healthy young adults under different thigh cuff occlusion pressures. The reproducibility of this protocol when performed under complete ischaemia (tested five times over a 2-week period) was assessed as having a coefficient of variation (CV) for fatigue (end/initial force) of [mean (SEM) 12 (1)%; n=5]. This compares favourably with that obtained for maximum voluntary contraction torque [CV 9 (1)%]. In six subjects, triceps surae muscle fatigue was assessed under thigh cuff pressures of 0, 6.7 kPa (50 mmHg, venous occlusion) and 28 kPa (210 mmHg, complete ischaemia), as well as two intermediate levels of occlusion that were established by cuff pressures of 13.4 (0.5) and 20.3 (1.1) kPa [103 (4) and 152 (8) mmHg, respectively]. These corresponded to ankle-brachial pressure indices of 1.3 and 0.8, respectively when subjects were seated, or 0.8 and 0.36 when supine. With undisturbed lower-leg circulation, force potentiated steadily over the 5 min of stimulation such that the final force was 135 (8)% of the initial value. With complete ischaemia, force fell to 47 (2)% of the initial value. Stimulation under thigh occlusion pressures of 6.7, 13.4 and 20.3 kPa elicited intermediate levels of reduction in force, graded according to the increasing restriction of perfusion. The results show that low-force twitch contractions, which themselves do not occlude blood flow, are extremely sensitive to impaired perfusion and may represent a viable alternative to established methods of muscle performance assessment in patients with blood flow insufficiency. Accepted: 5 November 1999     

Heterogeneous oxygenation in nonexercising triceps surae muscle during contralateral isometric exercise

To test whether changes in oxygenation of a resting skeletal muscle, evoked by a static contraction in a contralateral muscle, is uniform within a given skeletal muscle, we used near-infrared spectroscopy (NIRS). Seven subjects performed 2 min static knee extension exercise at 30% of maximal voluntary contraction. Changes in oxygenated hemoglobin (HbO₂) were monitored using multiple-channel NIRS (40 channels, 13 sources and 12 detectors) attached on the contralateral nonexercising triceps surae muscle. Changes in HbO₂ were expressed as a percentage of total labile signals. To characterize the distribution of changes in HbO₂, channels were compared between their positions on the triceps surae muscle, and represented as ‘proximal versus distal’ and ‘lateral versus medial’ portions. During static muscle contraction, the averaged changes in HbO₂ of all channels were correlated with those in calf blood flow (plethysmography; R ²=0.188, P<0.05) and with calf vascular conductance (R ²=0.146, P<0.05). HbO₂ did not differ significantly between the lateral and medial portions of the triceps surae muscle. In contrast, the decrease of HbO₂ in the proximal portion of the muscle was greater than that of the distal portion (P<0.05). These results indicate that the changes in oxygenation of a resting muscle, evoked by static contraction of the contralateral muscle, are heterogeneous.     

Electromyogram patterns during plantarflexions at various angular velocities and knee angles in human triceps surae muscles

To investigate the influence of the various knee angles and ankle angular velocities on synergistic muscle activities, the surface electromyograms (EMG) were recorded from the triceps surae muscles, i.e. lateral gastrocnemius (LG), medial gastrocnemius (MG) and soleus (SOL) muscles. Six healthy young men performed ankle plantarflexions at three ankle angular velocities of 6, 30 and 60°?·?s^?1 and three knee angles of 0, 30 and 60° (0° equalling full extension) under constant load (5% and 10% maximal voluntary contraction). At the fully-extended knee angle (0°), peak values of integrated EMG (peak iEMG) during ankle plantarflexions were significantly increased (P?P??1) the peak iEMG were significantly increased (P?P?相似文献   

Influence of stretch-shortening cycle on mechanical behaviour of triceps surae during hopping

Belli , A. & Bosco, A. 1992. Influence of stretch-shortening cycle on mechanical behaviour of triceps surae during hopping. Acta Physiol Scand 144 , 401408. Received 20 March 1 991 , accepted 3 December 1991. ISSN 0001–6772. Laboratories of Physiology, Universities of St Etienne and Lyon, France and Departments of Biology and Physical Activity, University of Jyväskylä, Finland. Six subjects performed a first series of vertical plantar flexions and a second series of vertical rebounds, both involving muscle triceps surae exclusively. Vertical displacements, vertical forces and ankle angles were recorded during the entire work period of 60 seconds per series. In addition, expired gases were collected during the test and recovery for determination of the energy expenditure. Triceps surae was mechanically modelled with a contractile component and with an elastic component. Mechanical behaviour and work of the different muscle components were determined in both series. The net muscular efficiency calculated from the work performed by the centre of gravity was 17.5±3.0% (mean ± SD) in plantar flexions and 29.9 ± 4.8% in vertical rebounds. The net muscle efficiency calculated from the work performed by the contractile component was 17.4 %% 2.9% in plantar flexions and 16.1 ± 1.47; in vertical rebounds. These results suggest that the muscular efficiency differences do not reflect muscle contractile component efficiency but essentially the storage and recoil of elastic energy. This is supported by the relationship (P < 0.01) found in vertical rebounds between the extra work and the elastic component work. A detailed observation of the mechanical behaviour of muscle mechanical components showed that the strategy to maximize the elastic work depends also on the force-velocity characteristics of the movement and that the eccentric-concentric work of the contractile component does not always correspond respectively to the ankle extension-flexion.     

Sag during unfused tetanic contractions in rat triceps surae motor units.

Contractile properties and conduction velocity were studied in 202 single motor units of intact rat triceps surae muscles activated by intra-axonal (or intra-myelin) current injection in L5 or L6 ventral root to assess the factors that determine the expression of sag (i.e., decline in force after initial increase during unfused tetanic stimulation). Sag was consistently detected in motor units with unpotentiated twitch contraction times <20 ms. However, the range of frequencies at which sag was expressed varied among motor units such that there was no single interstimulus interval (ISI), with or without adjusting for twitch contraction time, at which sag could be detected reliably. Further analysis indicated that using the absence of sag as a criterion for identifying slow-twitch motor units requires testing with tetani at several different ISIs. In motor units with sag, the shape of the force profile varied with tetanic frequency and contractile properties. Simple sag force profiles (single maximum reached late in the tetanus followed by monotonic decay) tended to occur at shorter ISIs and were observed more frequently in fatigue-resistant motor units with long half-relaxation times and small twitch amplitudes. Complex sag profiles reached an initial maximum early in the tetanus, tended to occur at longer ISIs, and were more common in fatigue-sensitive motor units with long half-relaxation times and large twitch amplitudes. The differences in frequency dependence and force maximum location suggested that these phenomena represented discrete entities. Successive stimuli elicited near-linear increments in force during tetani in motor units that never exhibited sag. In motor units with at least one tetanus displaying sag, tetanic stimulation elicited large initial force increments followed by rapidly decreasing force increments. That the latter force envelope pattern occurred in these units even in tetani without sag suggested that the factors responsible for sag were expressed in the absence of overt sag. The time-to-peak force (TTP) of the individual contractions during a tetanus decreased in tetani with sag. Differences in the pattern of TTP change during a tetanus were consistent with the differences in force maximum location between tetani exhibiting simple and complex sag. Tetani from motor units that never exhibited sag did not display a net decrease in TTP during successive contractions. These data were consistent with the initial force decrement of sag resulting from a transient reduction in the duration of the contractile state.