The influence of small fibre muscle mechanoreceptors on the cardiac vagus in humans

We have previously shown that activation of muscle receptors by passive stretch (PS) increases heart rate (HR) with little change in blood pressure (BP). We proposed that PS selectively inhibits cardiac vagal activity. We attempted to test this by performing PS during experimental alterations in vagal tone. Large decreases in vagal tone were induced using either glycopyrrolate or mild rhythmic exercise. Milder alterations in vagal tone were achieved by altering carotid baroreceptor input: neck pressure (NP) or neck suction (NS). PS of the triceps surae was tested in 14 healthy human volunteers. BP, ECG and respiration were recorded. PS alone caused a significant decrease ( P < 0.05) in R–R interval (962 ± 76 ms at baseline compared to 846 ± 151 ms with PS), and showed a reduction in HR variability, which was not significant. The decrease in R–R interval with PS was significantly less ( P < 0.05, n = 3) following administration of glycopyrrolate (−8.1 ± 4.5 ms) compared to PS alone (−54 ± 11 ms), and also with PS during handgrip (+10 ± 10 ms) compared with PS alone (−74 ± 15 ms) ( P < 0.05, n = 5). Milder reductions in vagal activity (NP) resulted in a small but insignificant further decrease in R–R interval in response to PS (−107 ± 17 ms compared to PS alone −96 ± 13 ms, n = 5). Mild increases in vagal activity (NS) during PS resulted in smaller decreases in R–R interval (−39 ± 5.5 ms) compared to PS alone (−86 ± 17 ms) ( P < 0.05, n = 8). BP was not significantly changed by stretch in any tests. The results indicate that amongst muscle receptors there is a specific group activated by stretch that selectively inhibit cardiac vagal tone to produce tachycardia.     

Comparison of blood pressure and heart rate responses to isometric exercise and passive muscle stretch in humans

The responses of mean arterial blood pressure (BP_a) and heart rate (f _c) to isometric contraction and passive stretch were compared in seven healthy male subjects at identical external forces. They were investigated in the sitting position with the hip and knee joint flexed to 90°. Each subject performed two tests, separated by a day, in which the stimuli were applied in random order. After 5 min of rest they performed either 10-min static plantar flexion of one calf (200 N) or 10 min of passive calf muscle stretch at the same load. After 5-min rest, the second stimulus was applied for a further 10 min followed by 5-min rest. The second test was identical except for the sequence of the stimuli. The BP_a was measured by a noninvasive and continuous method. Contraction of the vastus lateralis, gastrocnemius lateralis, and soleus muscles were determined by the myo-electric activity (electromyogram, EMG) by means of surface electrodes. The EMG activity of the vastus lateralis muscle remained at resting values throughout the experiments. Increases in EMG activity could only be detected for the triceps surae muscles during isometric contraction. During the initial 2 min of stimulation the BP_a and (f _c), responses to active contraction and passive stretch were comparable. Thereafter, both parameters showed significantly higher values during contraction. It was concluded that mechanical stress may have contributed to the early response of BP_a during both passive stretch and voluntary contraction but that chemical stimuli were needed to maintain the peripheral cardiovascular drive.     

Reflex changes in muscle spindle discharge during a voluntary contraction

1. This study was undertaken to determine whether low-threshold cutaneous and muscle afferents from mechanoreceptors in the foot reflexly affect fusimotor neurons innervating the plantar and dorsiflexors of the ankle during voluntary contractions. 2. Recordings were made from 29 identified muscle spindle afferents innervating triceps surae and the pretibial flexors. Trains of electrical stimuli (5 stimuli, 300 impulses per second) were delivered to the sural nerve at the ankle (intensity: 2-4 times sensory threshold) and to the posterior tibial nerve at the ankle (intensity: 1.5-3 times motor threshold for the small muscles of the foot). The stimuli were delivered while the subject maintained an isometric voluntary contraction of the receptor-bearing muscle, sufficient to accelerate the discharge of each spindle ending. This ensured that the fusimotor neurons directed to the ending were active and influencing the spindle discharge. The effects of these stimuli on muscle spindle discharge were assessed using raster displays, frequencygrams, poststimulus time histograms (PSTHs) and cumulative sums ("CUSUMs") of the PSTHs. Reflex effects onto alpha-motoneurons were determined from poststimulus changes in the averaged rectified electromyogram (EMG). Reflex effects of these stimuli onto single-motor units were assessed in separate experiments using PSTHs and CUSUMs. 3. Electrical stimulation of the sural or posterior tibial nerves at nonnoxious levels had no significant effect on the discharge of the 14 spindle endings in the pretibial flexor muscles. The electrical stimuli also produced no significant change in discharge of 11 of 15 spindle endings in triceps surae. With the remaining four endings in triceps surae, the overall change in discharge appeared to be an increase for two endings (at latencies of 60 and 68 ms) and a decrease for two endings (at latencies of 110 and 150 ms). The difference in the incidence of the responses of spindle endings in tibialis anterior and in triceps surae was significant (P less than 0.05, chi 2 test). 4. For both triceps surae and pretibial flexor muscles the electrical stimuli to sural or posterior tibial nerves had clear effects on the alpha-motoneuron pool, whether assessed using surface EMG or the discharge of single-motor units. Based on EMG recordings using intramuscular wire electrodes, the reflex effects differed for the gastrocnemii and soleus. 5. In this study, reflex changes in the discharge of human spindle endings were more difficult to demonstrate than comparable changes in the discharge of alpha-motoneurons.(ABSTRACT TRUNCATED AT 400 WORDS)     

Local metabolite accumulation augments passive muscle stretch-induced modulation of carotid–cardiac but not carotid–vasomotor baroreflex sensitivity in man

We examined the effects of muscle mechanoreflex stimulation by passive calf muscle stretch, at rest and during concurrent muscle metaboreflex activation, on carotid baroreflex (CBR) sensitivity. Twelve subjects either performed 1.5 min one-legged isometric plantarflexion at 50% maximal voluntary contraction with their right or left calf [two ischaemic exercise (IE) trials, IER and IEL] or rested for 1.5 min [two ischaemic control (IC) trials, ICR and ICL]. Following exercise, blood pressure elevation was partly maintained by local circulatory occlusion (CO). 3.5 min of CO was followed by 3 min of CO with passive stretch (STR-CO) of the right calf in all trials. Carotid baroreflex function was assessed using rapid pulses of neck pressure from +40 to −80 mmHg. In all IC trials, stretch did not alter maximal gain of carotid–cardiac (CBR–HR) and carotid–vasomotor (CBR–MAP) baroreflex function curves. The CBR–HR curve was reset without change in maximal gain during STR-CO in the IEL trial. However, during the IER trial maximal gain of the CBR–HR curve was smaller than in all other trials (−0.34 ± 0.04 beats min⁻¹ mmHg⁻¹ in IER versus −0.76 ± 0.20, −0.94 ± 0.14 and −0.66 ± 0.18 beats min⁻¹ mmHg⁻¹ in ICR, IEL and ICL, respectively), and significantly smaller than in IEL ( P < 0.05). The CBR–MAP curves were reset from CO values by STR-CO in the IEL and IER trials with no changes in maximal gain. These results suggest that metabolite sensitization of stretch-sensitive muscle mechanoreceptive afferents modulates baroreflex control of heart rate but not blood pressure.     

Muscle sympathetic nerve activity responses to dynamic passive muscle stretch in humans

It is suggested that mechanoreceptors in muscle play an important role in the exercise pressor reflex. However, it has not been verified whether isolated stimulation of the mechanoreceptors can induce responses in muscle sympathetic nerve activity (MSNA) in young healthy individuals. We tested the hypothesis that passive stretch of muscle can evoke an increase in MSNA in healthy individuals. In 12 young subjects, leg calf muscles were passively stretched, or actively contracted for 5 s followed by a 15–25 s (random length) relaxation period. Stretch and contraction were each repeated 25 times. MSNA, heart rate and blood pressure were analysed, and averaged according to the onset of the force on a beat-by-beat basis. At the 1st to the 3rd heart beat from the onset of stretch, MSNA (199 ± 30%, P < 0.05) as well as heart rate (102.5 ± 0.7%, P < 0.05) increased transiently but significantly from the prior stretch baseline (100%), followed (from 3rd to 7th beat from the onset of stretch) by a transient increase in mean blood pressure (101.9 ± 0.3%, P < 0.05) from the baseline. Similar response patterns were observed during active muscle contractions. The present data show that MSNA responses to isolated stimulation of mechanoreceptors are measurable. Because of baroreflex engagement, the magnitude of the response is small and transient, and the haemodynamic consequences using this protocol may be limited.     

Myoelectric changes in the triceps surae muscles under sustained contractions

Summary Synergistic behaviour of triceps surae muscles (medial gastrocnemius-MG, lateral gastrocnemius-LG, soleus-SOL) during sustained submaximal plantarflexions was investigated in this study. Six male subjects were asked to sustain an isometric plantar flexor effort to exhaustion at two different knee angles. Exhaustion was defined as the point when they could no longer maintain the required tension. The loads sustained at 0 and 120 degrees of knee flexion represented 50% and 36% of their maximum voluntary contraction (MVC) respectively. MVC was measured at 0 degree knee flexion. During the contractions, electromyograms (EMG) from the surface of the triceps surae muscles were recorded. Changes in the synergistic behaviour of the triceps surae were assessed via partial correlations of the average EMG (AEMG) between three muscle combinations; MG/LG, MG/SOL, LG/SOL, and correlation between SOL/MG+LG and MG/SOL+LG. The latter combinations were based on either common fibre type or innervation properties. Two types of synergisms were identified: trade-off and coactivation. Trade-off and coactivation synergies were defined by significant (p<0.05) positive and negative correlations respectively. Coactivation synergism was found to occur predominantly under conditions of high load or reduced length of the triceps surae, and increased with the duration of the contraction. Trade-off synergism was evident when the muscles were at their optimum length and the loads sustained were submaximum. Complete shutdown of one muscle activity was ruled out. It is postulated that, in the absence of voluntary strategies on the part of the subjects, changes in the syznergistic behaviour of the triceps surae muscles, manifested through trade-off and coactivation, is dependent on the load placed on the muscle and the muscle effectiveness as characterized by the force/length curve.     

Renal sympathetic and circulatory responses to activation of the exercise pressor reflex in rats

We investigated the role played by the exercise pressor reflex in sympathetic regulation of the renal circulation in rats. In mid-collicular decerebrate rats, mean arterial pressure (MAP), heart rate (HR), left renal cortical blood flow (RCBF) and left renal sympathetic nerve activity (RSNA) were recorded before and during 30 s of static contraction of the left triceps surae muscles evoked by electrical stimulation of the tibial nerve, which activates both metabo- and mechanosensitive muscle afferents, and during 30 s of passive stretch of the left Achilles tendon, which selectively activates mechanosensitive muscle afferents. Static contraction (n = 17, +344 +/- 34 g developed tension) significantly (P < 0.05) increased MAP (+14 +/- 3 mmHg), HR (+6 +/- 1 beats min(-1)) and RSNA (n = 11, +19 +/- 5%) and significantly decreased renal cortical vascular conductance (RCVC, n = 11, -11 +/- 2%). Passive stretch (n = 20, +378 +/- 11 g) also significantly increased MAP (+11 +/- 2 mmHg), HR (+7 +/- 2 beats min(-1)) and RSNA (n = 15, +14 +/- 4%) and significantly decreased RCVC (n = 11, -12 +/- 3%). RCBF showed no significant changes during static contraction or passive stretch. Renal denervation abolished the decrease in RCVC during contraction (n = 12) or stretch (n = 13). These data indicate that both the exercise pressor reflex and its mechanically sensitive component, the muscle mechanoreflex, induced renal cortical vasoconstriction through sympathetic activation in rats.     

The influence of eccentric contractions and stretch on alpha motoneuron excitability in normal subjects and subjects with spasticity

Application of eccentric contractions and muscle stretch are clinically effective in reducing spasticity and increasing ROM (7). This may be explained by a change in the excitability of motoneurons supplying the spastic muscle. Excitability of motoneurons can be indirectly assessed using the H-reflex. Experiments were performed on 20 normal subjects and 17 subjects with spasticity resulting from neurological disorder. Subjects were seated in a secure position and the ankle joint was moved from 30 degrees plantarflexion to 20 degrees dorsiflexion at a velocity of 30 degrees/sec. Sixty eccentric contractions of the triceps surae muscle were performed using a Kin-Com dynamometer (Chattanooga Corp, Tennessee). Two protocols were used: (1) eccentric contractions only, and (2) eccentric contractions with a 5s stretch of the relaxed triceps surae after each contraction. Two sets of 10 H-reflexes were collected from the soleus muscle before (trial 1 & 2) and after (trial 3 & 4) eccentric and eccentric + stretch protocols. The mean peak to peak H-reflex amplitude was calculated for each trial and compared using ANOVA. Eccentric contractions resulted in a significant and maintained increase in the H-reflex in neurological compared to normal subjects (P < 0.05). Eccentric contractions in subjects with spasticity resulted in an increase in motoneuron excitability which may assist in corticospinal activation of motoneurons during voluntary movement. The eccentric + stretch protocol, resulted in a decrease in the mean amplitude of H-reflexes in neurological subjects, however, this was not significant. The application of a stretch following eccentric contractions decreased motoneuron excitability and may thus be beneficial to decrease spasticity whilst strengthening muscle.     

Reflex responses in plasma catecholamines caused by static contraction of skeletal muscle.

To examine a hypothesis of whether static muscle contraction produces a release of catecholamines from the adrenal medulla via reflex stimulation of preganglionic adrenal sympathetic nerve activity induced by receptors in the contracting muscle, we compared the reflex responses in a concentration of epinephrine (Ep) and norepinephrine (NEp) in arterial plasma during static contraction and during a mechanical stretch of the hindlimb triceps surae muscle in anesthetized cats. Static contraction was evoked by electrically stimulating the peripheral ends of the cut L(7) and S(1) ventral roots at 20 or 40 Hz. Mean arterial pressure (MAP) and heart rate (HR) increased 23 +/- 3.1 mmHg and 19 +/- 4.3 beats/min during static contraction. Ep in arterial plasma increased 0.18 +/- 0.072 ng/ml over the control of 0.14 +/- 0.051 ng/ml within 1 min from the onset of static contraction, and NEp increased 0.47 +/- 0.087 ng/ml over the control of 0.71 +/- 0.108 ng/ml. Following a neuromuscular blockade, although the same ventral root stimulation failed to produce the cardiovascular and plasma catecholamine responses, the mechanical stretch of the muscle increased MAP, HR, and plasma Ep, but not plasma NEp. With bilateral adrenalectomy, the baseline Ep became negligible (0.012 +/- 0.001 ng/ml) and the baseline NEp was lowered to 0.52 +/- 0.109 ng/ml. Neither static contraction nor mechanical stretch produced significant responses in plasma Ep and NEp following the adrenalectomy. These results suggest that static muscle contraction augments preganglionic adrenal sympathetic nerve activity, which in turn secretes epinephrine from the adrenal medulla into plasma. A muscle mechanoreflex from the contracting muscle may play a role in stimulation of the adrenal sympathetic nerve activity.     

In vivo muscle mechanics during locomotion depend on movement amplitude and contraction intensity

The effects of movement amplitude and contraction intensity on triceps surae and quadriceps femoris muscle function were studied during repetitive hopping. In vivo forces from Achilles and patellar tendons were recorded with the optic fibre technique from eight volunteers. The performances were filmed (200 Hz) to determine changes in muscle-tendon unit length and velocity. When hopping with a small amplitude (23° knee flexion during the ground contact phase), the Achilles tendon was primarily loaded whereas patellar tendon forces were greater in large-amplitude hopping (56° knee flexion). In spite of the different magnitudes of stretch in the quadriceps femoris muscle, the stretching velocity and activity patterns of the quadriceps muscle were similar in both conditions. Simultaneously performed electromyographic (EMG) recordings revealed that preferential preactivation of the gastrocnemius muscle was evident in both jumping conditions. The triceps surae muscle was strongly active in the eccentric phase of small-amplitude hopping. Results from hopping with small knee-joint displacement suggest that there may be a particular frequency and jumping height at which the elastic bouncing is best utilized and at the same time the concentric phase is most economical. Results also support earlier observations that the economy of the shortening phase must be compromised at some point in order to produce more power and improve the jumping height. Electronic Publication     

Changes in size of the stretch reflex of cat and man attributed to aftereffects in muscle spindles

1. This is a report of experiments carried out on the cat and on man, which demonstrate that conditioning of a muscle by contraction and movement can lead to changes in amplitude of stretch reflexes elicited in that muscle. 2. In triceps surae of the cat, the reflex response to a brief stretch was recorded after conditioning with a whole-muscle contraction followed by a pause at a length either 5 mm longer or shorter than the length at which the reflex was elicited. Following conditioning at the long length the reflex response was less than half as large as that following conditioning at the short length. 3. The changes in reflex amplitude could be correlated with an altered stretch responsiveness of muscle spindles in the soleus muscle. When the muscle had been held long during conditioning, a subsequent brief stretch applied at an intermediate length elicited fewer impulses in primary endings of spindles than after conditioning at a short length. 4. The same kind of experiment was then carried out on adult human subjects. When a tendon tap was applied to the Achilles tendon after a voluntary contraction and relaxation of triceps surae with the muscle at a long length, (foot dorsiflexed) the reflex was frequently less than half the size it had been after a contraction at a short length (foot plantarflexed). It was concluded that the same kind of spindle aftereffects as observed for cat soleus spindles were responsible for the changes in reflex amplitude. 5. It was found both in the cat and in human subjects that the changes in reflex amplitude after conditioning became progressively less as the test length was made longer. 6. The explanation put forward to account for these observations is that stable cross-bridges form between actin and myosin filaments of passive intrafusal (and extrafusal) fibers. When the muscle is shortened several seconds after a contraction at a long length, the intrafusal fibers, stiffened by the presence of cross-bridges, fall slack. Slack does not develop after a contraction at a short muscle length, as the fiber is stretched to the test length. Since any slack must first be taken up by the test stretch, there is a smaller afferent response and consequently a smaller reflex contraction in response to a tendon tap after conditioning at a long length.(ABSTRACT TRUNCATED AT 400 WORDS)     

Differences between motor unit firing rate,twitch characteristics and fibre type composition in an agonistic muscle group in man

Summary A comparison was carried out between the motor unit (MU) firing rate and the characteristics of the twitch and the fibre type composition of anconeus and triceps brachii. Fibre type composition (type I, type II) was determined in whole cross-sections of cadaver specimens. The proportion of type I fibre was 60%–67% in anconeus and 32–40% in the lateral head of triceps brachii. Reflecting these histochemical differences, the contraction time of anconeus and triceps was 92±9 ms and 68±9 ms respectively. It follows that anconeus can be classified as a slow muscle, as opposed to the lateral head of triceps. The relationship between MU firing rate and isometric force, tested at 90° elbow flexion, differed between the two muscles for force values below 30% of maximal voluntary contraction. No significant increase in MU firing rate was found in anconeus at forces above 5% of maximal voluntary contraction. It is concluded that even within a single agonistic muscle group acting at a single joint there is an adaptation of MU firing rate to the contractile characteristics of each muscle.     

The effects of acute passive stretch on muscle protein synthesis in humans.

We examined the effect of an isolated bout of maximal tolerated passive stretch on fractional muscle protein synthetic rate in human soleus muscle. Eight healthy males performed two separate trials with the same leg: one session of passive stretch and one of intermittent active isometric contraction at a force equivalent to that which occurred during the passive stretch trial. This force was approximately 40% of maximum voluntary contraction force and produced volitional fatigue in approximately 27 min. Intermittent passive stretch, for the same duration, elicited a 6.1 degrees increase in joint angle (P<.0005) with silent electromyography. Fractional protein synthetic rate from experimental and control soleus in each trial was assessed from biopsy samples over the period 10-22 hr postexercise by the incorporation rate of L-[1-13C] leucine into muscle. Protein synthesis was elevated in the soleus of the exercised leg following the active contraction trial by 49% (P<.05) but not following the passive stretch trial. Results indicate that a single bout of maximal passive stretch does not significantly elevate fractional muscle protein synthetic rate in humans and thus suggests that muscle stretch per se is not the stimulus for the muscle hypertrophy that occurs with resistance training.     

Change in length of relaxed muscle fascicles and tendons with knee and ankle movement in humans

Ultrasonography was used to measure changes in length of muscle fascicles in relaxed human tibialis anterior and gastrocnemius during passively imposed changes in joint angle. Changes in the length of muscle fascicles were compared to changes in the length of the whole muscle-tendon units calculated from joint angles and anthropometric data. Relaxed muscle fascicles underwent much smaller changes in length than their muscle-tendon units. On average, muscle fascicles in tibialis anterior [saw] 55 ± 13 % (mean ± s.d. ) of the total change in muscle-tendon length. This indicates nearly half of the total change in muscle-tendon length was taken up by stretch of tendon. In gastrocnemius, which has relatively long tendons, only 27 ± 9 % of the total change in muscle-tendon length was transmitted to muscle fascicles. Thus, the tendency for passive movement to be taken up by the tendon was greater for gastrocnemius than tibialis anterior ( P = 0.002). For these muscles, the relatively large changes in tendon length across much of the physiological range of muscle-tendon lengths could not wholly be explained by tendon slackness, changes in fibre pennation, or stretch or contraction history of the muscle. Our data confirm that when joints are moved passively, length changes [seen] by muscle fascicles can be much less than changes in the distance between muscle origin and insertion. This occurs because tendons undergo significant changes in length, even at very low forces.     

The time course and direction of lower limb vascular conductance changes during voluntary and electrically evoked isometric exercise of the contralateral calf muscle in man

This study aimed to clarify the direction and timing of the change of non-active lower limb vascular conductance at the onset of contralateral limb isometric exercise and to examine the mechanisms controlling this change. Fifteen human subjects performed 2 min of electrically evoked (Stim) or voluntary (Vol) ischaemic isometric calf plantar flexor exercise at 30 % maximum voluntary contraction (MVC). Blood pressure (BP) and heart rate were continuously recorded and blood flow in the non-active contralateral lower limb was recorded at 15 s intervals. In subsets of subjects the presence of inadvertent muscle contraction was monitored by calf muscle EMG and the effects of the sensation of electrical stimulation without muscle contraction (sham) were investigated. After 10-15 s conductance had increased significantly (   P < 0.05  ) in Vol and Stim by a mean of 15 and 12 %, respectively, whilst BP was unchanged. Following this initial increase conductance decreased progressively during Stim and Vol whilst blood pressure rose. No EMG activity was seen during either protocol. In the sham stimulation experiments where no contraction was evoked the conductance change at the onset of stimulation replicated that seen during Stim exercise. Increases in conductance were independent of central command and muscle force generation, were not activated in anticipation of exercise but could be activated secondarily to peripheral sensations associated with expected exercise. The explanation for our results might involve sympathetic withdrawal related to mental stress; however, a central pathway, which directly activates a vasodilator mechanism in passive calf muscle, remains a possibility.     

Significance of peripheral afferent input to the α-motoneurone pool for enhancement of tremor during an isometric fatiguing contraction

The objective of this study was to investigate the contribution of peripheral afferent input to the enhancement of isometric tremor during a sustained submaximal isometric contraction. It was hypothesised that during muscle fatigue, when excitatory drive is high, peripheral afferent input may augment oscillations in the stretch reflex arc and result in bursting motor-unit activity and increased tremor. Nine healthy subjects maintained isometric plantar flexions at 30% of their maximum voluntary contraction until the limit of endurance, under three test conditions. Two paradigms were used to reduce afferent input to the triceps surae α-motoneurone pool: (1) continued vibration of the Achilles tendon, and (2) ischaemic partial block of the tibial nerve. These were compared to a control experiment, in which there was no intervention. By recording H-reflexes from the gastrocnemius and soleus muscles, it was possible to assess the effectiveness of reducing the afferent input. When H-reflex suppression had stabilised, the fatiguing contraction was commenced and tremor was computed from the continuously recorded torque signal. Superimposed maximum twitches were elicited as indirect measures of excitatory drive. The increase in tremor root mean square throughout the fatiguing contraction was significantly less for both the vibration and ischaemic conditions. Furthermore, tremor mean power frequency decreased significantly with endurance time in the control experiment, while no significant change was seen in the other two experimental conditions. It is concluded that the enhancement of isometric tremor seen during a fatiguing submaximal isometric contraction is facilitated by peripheral afferent input to the α-motoneurone pool. Accepted: 9 February 2000     

The pressor response to voluntary and electrically evoked isometric contractions in man

Summary Blood pressure and heart rate changes during sustained isometric exercise were studied in 11 healthy male volunteers. The responses were measured during voluntary and involuntary contractions of the biceps brachii at 30% of maximal voluntary contraction (MVC), and the triceps surae at 30% and 50% MVC. Involuntary contractions were evoked by percutaneous electrical stimulation of the muscle.Measurements of the time to peak tension of maximal twitch showed the biceps brachii (67.0±7.9 ms) muscle to be rapidly contracting, and the triceps surae (118.0±10.5 ms) to be slow contracting. The systolic and diastolic blood pressures increased linearly throughout the contractions, and systolic blood pressure increased more rapidly than diastolic. There was no significant difference in response to stimulated or voluntary contractions, nor was there any significant difference between the responses to contractions of the calf or arm muscles at the same relative tension.In contrast the heart rate rose to a higher level (P<0.01) in the biceps brachii than the triceps surae at given % MVC, and during voluntary compared with the electrically evoked contractions in the two muscle groups.It was concluded that the arterial blood pressure response to isometric contractions, unlike heart rate, is primarily due to a reflex arising within the active muscles (cf. Hultman and Sjöholm 1982) which is associated with relative tension but independent of contraction time and muscle mass.     

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.     

Movement reduces the dynamic response of muscle spindle afferents and motoneuron synaptic potentials in rat

Among the mechanisms that may result in modulation of the stretch reflex by the recent history of muscle contraction is the history dependence observed under some conditions in the response properties of muscle spindles. The present study was designed to test one report that in successive trials of muscle stretch-release, spindle afferent firing during stretch, i.e., the dynamic response shows no history dependence beyond the initial burst of firing at stretch onset. Firing responses of spindle afferents were recorded during sets of three consecutive trials of triangular stretch-release applied to triceps surae muscles in barbiturate-anesthetized rats. All 69 spindle afferents fired more action potentials (spikes) during the dynamic response of the first trial, excluding the initial burst, than in the following two trials. The reduced dynamic response (RDR) was nearly complete after trial 1 and amounted to an average of approximately 12 fewer spikes (16 pps slower firing rate) in trial 3 than in trial 1. RDR was sensitive to the interval between stretch sets but independent of stretch velocity (4-32 mm/s). RDR was reflected in the synaptic potentials recorded intracellularly from 16 triceps surae alpha-motoneurons: depolarization during muscle stretch was appreciably reduced after trial 1. These findings demonstrate history dependence of spindle afferent responses that extends throughout the dynamic response in successive muscle stretches and that is synaptically transmitted to motoneurons with the probable effect, unless otherwise compensated, of modulating the stretch reflex.     

Acute effects of stretching on the neuromechanical properties of the triceps surae muscle complex

Previous research has shown that an acute bout of passive muscle stretching can diminish performance in certain movements where success is a function of maximal force and/or power output. Two possible mechanisms that might account for such findings are a change in active musculotendinous stiffness and a depression of muscle activation. To investigate the likelihood of these two mechanisms contributing to a post-stretch reduction in performance, we examined the acute effects of stretching on the active stiffness and muscle activation of the triceps surae muscle group during maximal single-joint jumps with movement restricted to the ankle joint. Ten males performed both static (SJ) and countermovement (CMJ) jumps before and after passively stretching the triceps surae. Electrical activity of the triceps surae during each jump was determined by integrating electromyographic recordings (IEMG) over the course of the movement. Triceps surae musculotendinous stiffness was calculated before and after stretching using a technique developed by Cavagna (1970). Following stretching, a significant decrease [mean (SD) 7.4 (1.9)%; P<0.05] in jump height for the CMJ occurred, but for the SJ, no significant (P>0.05) change in jump height was found. A small but significant decrease [2.8 (1.24)%; P<0.05] in stiffness was noted, but the magnitude of this change was probably not sufficient for it to have been a major factor underlying the decline in CMJ performance. Paradoxically, after stretching, the SJ exhibited a significant (P<0.05) decrease in IEMG, but the IEMG for the CMJ remained unchanged (P>0.05). It appears that an acute bout of stretching can impact negatively upon the performance of a single-joint CMJ, but it is unlikely that the mechanism responsible is a depression of muscle activation or a change in musculotendinous stiffness. Electronic Publication