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PubMed1990

Neural mechanisms underlying the clasp-knife reflex in the cat. I. Characteristics of the reflex.

1. The goal of this study was to characterize the clasp-knife reflex by the use of stretch and isometric contraction of ankle extensor and flexor muscles in decerebrated cats with bilateral dorsal hemisections of their spinal cords at segment T12. 2. Stretch of an extensor muscle evoked inhibition in both homonymous and synergistic extensor muscles. The similarities between homonymous and synergistic inhibition suggest that similar neural mechanisms were responsible. 3. Homonymous and synergistic clasp-knife inhibition showed several characteristic features: 1) inhibition was evoked only by large stretches that produced significant muscle force. Short stretches that did not produce large forces evoked only excitation; 2) the magnitude of clasp-knife inhibition increased with increasing initial motor output, as reflected in the level of rectified EMG; 3) the time course of reflex inhibition evoked by ramp-and-hold stretch was characterized by segmentation of EMG during ramp stretch, dynamic overshoot of inhibition at the end-of-ramp stretch, and slow but usually complete decay of inhibition during maintained stretch; 4) inhibition persisted beyond the termination of stretch, and 5) inhibition showed adaptation to repeated stretch. 4. Isometric contraction of the soleus or medial gastrocnemius, produced by electrical stimulation of the muscle nerve, also evoked powerful synergistic-reflex inhibition via similar mechanisms as stretch-evoked, clasp-knife inhibition. Stretch evoked a greater degree of inhibition than did contraction, indicating that receptors responsive to both stretch and contraction contribute to clasp-knife inhibition. 5. The reflex effects produced by stretching the soleus or medial gastrocnemius were not confined to the homonymous and close synergistic muscles. Extensor muscles were inhibited and flexor muscles were excited throughout the hindlimb, which paralleled the pattern of a flexion-withdrawal reflex evoked by cutaneous stimulation. 6. Stretch of a flexor muscle, the tibialis anterior, evoked the same spatial pattern and time course of reflex action as stretch of an extensor muscle--inhibition of extensor muscles and excitation of flexor muscles throughout the hindlimb, including homonymous excitation of the tibialis anterior. 7. We conclude that neither Golgi tendon organs nor secondary spindle afferents are likely to contribute significantly to clasp-knife inhibition because their responses to stretch and isometric contraction differ from the reflex actions evoked by stretch and contraction.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗

PubMed1996

Breathing when chest wall muscle are tonically contracted for isometric, non-respiratory tasks.

We have previously shown that regional chest wall impedance increases when the chest wall muscles are tonically contracted to perform isometric, non-respiratory tasks. To test how this affects breathing, we measured respiratory frequency, tidal volume, end-tidal PCO2, electromyographic activity (EMG) at four points on the chest wall surface, and regional displacements across six planes of the chest wall during maintenance of three different postures that necessitated strong tonic respiratory muscle contraction. These postures included a static push-up, a bilateral leg-lift and a partial sit-up. The subjects (n = 8) were able to maintain the postures for 1.5-2.5 min, and strong tonic EMG activity was observed in each posture at all points measured. The rate and depth of breathing and pattern of regional chest wall displacements were variable within the group of subjects and among the three postures. However, minute ventilation increased and end-tidal PCO2 decreased in each subject during each posture (P < 0.05). In six of the eight subjects, transdiaphragmatic pressure (Pdi) was measured during 1 min of the same exercises. The ratio of the breathing fluctuation in Pdi to tidal volume was at least twice as high compared with rest, except for two subjects during the leg-lifts. We conclude that strong tonic contraction of the chest wall muscles impedes, but does not limit, breathing, and that there is no single breathing strategy used during such conditions.

Adult↗

PubMed1975

Energy liberation and chemical change in frog skeletal muscle during single isometric tetanic contractions.

Recent data obtained from Rana temporaria sartorius muscles during an isometric tetanus indicate that the time-course of phosphocreatine (PC) splitting cannot account for the total energy (heat + work) liberation (Gilbert et al. 1971. J. Physiol. (Lond.) 218:)63). As this conclusion is important to an understanding of the chemical energetics of contraction, similar experments were performed on unpoisoned, oxygenated Rana pipiens sartorius muscles. The muscles were tetanized (isometrically) at 0 degrees C for 0.6, 1, or 5 s; metabolism was rapidly arrested by freezing the muscles with a specially designed hammer apparatus, and the frozen muscles were chemically analyzed. Comparable myothermal measurments were made on frogs from the same batch. Results of these experiments indicate: (a) The energy liberation parallels the PC and ATP breakdown with a proportionality constant of 10.7 kcal/mol; (b) comparably designed experiments with sartorius muscles of R. temporaria revealed that the ratio of energy liberation to PC splitting was significantly greater than that observed in R. pipiens sartorius muscles; (c) there is no systematic difference between experiments in which metabolism was arrested by the hammer apparatus and others using a conventional immersion technique.

Adenosine Triphosphate↗

PubMed2002

The effects of voluntary contraction effort on quadriceps femoris electromyogram median frequency in humans: a muscle and sex comparison.

The purpose of this study was to examine the effect of voluntary contraction efforts on the median frequency (f(med)) of the electromyogram (EMG) recorded from the quadriceps femoris muscle in healthy men and women. A group of 30 healthy volunteers (15 men, 15 women) were assessed for EMG activity of the vastus medialis (VM), vastus lateralis (VL), and rectus femoris (RF) muscles during isometric contractions with the knee at 60 degrees flexion. Subjects performed a series of 5 s maximal voluntary isometric contractions that anchored the perceptual range with a "10" on a 10-point scale. Sub-maximal isometric contractions were then separately performed at the following perceived effort levels on the 10-point scale: 1, 2, 3, 4, 5, 6, 7, 8 and 9, in a random order. Subjects were instructed to maintain the contraction at each perceived level of effort for 5 s. The f(med) of the three muscles was assessed using a power spectrum analysis performed over 11 consecutive, 512 ms, epochs overlapping each other by half their length during the middle 3 s of each contraction. The f(med) for each of the 11 epochs was then determined for each muscle, followed by calculation of the means and normalized coefficients of variation [(standard deviation/mean)x100%] for each contraction. The results demonstrated that the mean f(med) of VL was significantly greater than those of the other two muscles, and that f(med) of RF was significantly greater than that of VM. The VL muscle demonstrated a significant increase in mean f(med) across the contraction efforts, compared to the VM and RF muscles that displayed a significant decrease. The men displayed significantly higher f(med) values for the VM muscle than did the women, as well as showing a significantly greater increase across the contraction efforts for the VL muscle. The variability of f(med) was shown to be significantly higher for the VM muscle, compared to the VL and RF muscles. The findings of this study suggest that the f(med) statistic is most sensitive to contraction intensity efforts for the VL muscle, and that men display significantly higher values for the VL and VM muscles, compared to women.

Adult↗

PubMed1985

Ventilatory response to carbon dioxide and to isometric muscle contraction after administration of placebo, propranolol, mepindolol and salbutamol.

Healthy males (23-29 years) volunteered as subjects. We studied the ventilatory response to carbon dioxide and to isometric exercise (50% of maximum voluntary contraction) after administration of propranolol (20 mg), mepindolol (5 mg), salbutamol (20 mg) and placebo (single dose orally). The increase of pulmonary ventilation (VE) activated by central (CO2) and reflex stimulus (hand-grip) did not differ statistically between the four drugs. Analysis of VE in terms of inspiratory drive (Vt/Ti) and timing ratio (Ti/Ttot) showed that during CO2 stimulation mepindolol and salbutamol increased VE predominantly by the increase of Vt/Ti, presumably through the direct stimulation on the respiratory center. During rebreathing, Ti/Ttot increased significantly after administration of placebo and propranolol, so VE increased by a rise of Vt/Ti and by an increase of an effective 'timing component' (Ti/Ttot). Propranolol does not modify the ventilatory response to CO2 and hand-grip when VE is analyzed in terms of Vt/Ti and Ti/Ttot.

Adult↗

PubMed1991

Force response to rapid length change during contraction and rigor in skinned smooth muscle of guinea-pig taenia coli.

1. Mechanical transients in fibre bundles of skinned smooth muscle of guinea-pig taenia coli at 21-22 degrees C were investigated by recording tension responses to length changes of up to 9%, complete within 0.3 ms. 2. The length-force relationship, recorded continuously during rapid stretch of a Ca(2+)-activated contracted muscle, was linear up to at least 2.5 times the isometric force, corresponding to a stretch of about 1%. The slope of the relationship (stiffness) increased with the velocity of stretch. 3. During rapid release (about 120 muscle lengths s-1) the length-force relationship was linear down to about 50% of the initial isometric force, reached at about 80 microseconds after the beginning of the release. At lower force the length-force relationship was concave upwards. The linear portion extrapolated to zero force at about -0.008 muscle lengths. In large releases the length-force plot approached the force baseline under an acute angle, and negative force was transiently exerted. 4. When the muscle was stretched back to the initial length after a shortening step, force transiently rose above the isometric force, but decayed back within a few milliseconds. Stiffness at the time of restretch was compared with that in the initial shortening step by plotting force vs. length, and was found to be decreased to 63% within 0.3 ms of a step to zero force. Stiffness decreased further with time at zero force, and after 256 ms was about 29% of the isometric value. 5. In rigor, caused by the introduction of ATP-free solution during the plateau of isometric contraction, fibre tension decreased to about 30% of the active tension, whereas stiffness relative to force increased; 82% of the initial stiffness in rigor was detected in a restretch immediately after a shortening step, decreasing to 59% at 256 ms. When the fibre was activated at suboptimal [Ca2+] to cause the same force as in rigor, stiffness was lower than in rigor and decreased more after a release. 6. After completion of a release-stretch cycle, stiffness was rapidly restored to the same value as in isometric contraction. Test stretches at different points in time after completion of the cycle revealed that most of the stiffness had been restored within 1 ms of the restretch, occurring concomitantly with a decay in force.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1989

Comparative effects of verapamil and D-600 on the contractile responses produced by neurotransmitters and depolarizing agents in the rat isolated seminal vesicle.

1. The comparative effects of verapamil and D-600, on isometric contractions produced by noradrenaline, acetylcholine, phenylephrine, clonidine, carbachol and high potassium, were studied in rat isolated seminal vesicle. 2. The aim was to see if verapamil and D-600 differed, in their selectivity, of blocking calcium, channels, and hence inhibition of isometric contractions produced in this preparation. 3. Verapamil (1 microM) and D-600 (9.5 microM), selectively, reduced the isometric contractions, greater reductions occurred in the KCl-induced contractions (control EC50 19 +/- 1.5 mM, in verapamil 58 +/- 1.3 mM, and in D-600 37 +/- 2.0 mM, means +/- SE, n = 8, P less than 0.001, less than 0.01, respectively). 4. It was concluded that KCl-induced contractions were voltage-dependent, whereas noradrenaline operated via both voltage and receptor-mediated mechanisms. 5. The clinical relevance of these results was discussed, and it may help our current understanding of the pharmacological management of sexual dysfunction.

Acetylcholine↗

PubMed1990

Gradual increase in leg oxygen uptake during repeated submaximal contractions in humans.

We examine whether muscle oxygen consumption (VO2) increases gradually during repeated submaximal isometric contractions. Six subjects made two-legged isometric quadriceps contractions at 30% maximal voluntary contraction for 6 s with 4 s of rest between until exhaustion (58 +/- 8 min). Blood samples were taken from the femoral vein and artery, and blood velocity was recorded by ultrasound-Doppler technique in the femoral artery. Blood flow was calculated from velocity and artery diameter values. Leg VO2 increased sixfold within the 1st min of exercise. A further doubling of the VO2 was seen during the remainder of the exercise, reaching 307 +/- 22 ml/min at exhaustion. This latter increase was due to a 54% increase in blood flow and a 34% increase in oxygen extraction. After 20 min of recovery VO2 was still 75% higher than preexercise values. The results show a twofold increase in energy demand of the working muscle during repeated constant-force isometric contractions. The increased energy cost of contraction is probably localized at the cellular level, and it parallels fatigue determined as decreased force-generating capacity.

Adenosine Triphosphate↗

PubMed1985

Altered calcium handling in experimental pressure-overload hypertrophy in the ferret.

The purpose of this study was to evaluate the relationship between changes in the intracellular handling of free, ionized calcium and the functional alterations that occur in cardiac hypertrophy. We developed a model of right ventricular pressure-overload hypertrophy in the ferret by critical banding of the pulmonary artery; this method produced significant degrees of hypertrophy within 1-3 months of surgery. We removed papillary muscles, 1 mm or less in diameter, from ferrets with cardiac hypertrophy and their age- and weight-matched controls, and loaded them with the bioluminescent calcium indicator, aequorin, in order to record intracellular calcium transients. Compared to the controls, the hypertrophied muscles demonstrated a prolonged duration of isometric contraction but a marked decrease in peak isometric tension development. The increased duration of isometric contraction in the hypertrophied muscles correlated with a similar prolongation of the calcium transient; we interpret this to mean that the rate of sequestration and perhaps release of calcium by intracellular stores is decreased in hypertrophy. In contrast, the amplitudes of the calcium transients were similar in muscles from both groups of ferrets, indicating that the diminution in peak tension developed by the hypertrophied muscles was not due to decreased availability of activator calcium. We conclude that the prolonged time course of tension development, but not the diminished peak isometric tension response, may be related to changes in intracellular calcium handling.

Aequorin↗

PubMed1998

Heart rate and blood pressure responses to isometric exercise in young and older men.

The aim of this study was to examine the isometric endurance response and the heart rate and blood pressure responses to isometric exercise in two muscle groups in ten young (age 23-29 years) and seven older (age 54-59 years) physically active men with similar estimated forearm and thigh muscle masses. Isometric contractions were held until fatigue using the finger flexor muscles (handgrip) and with the quadriceps muscle (one-legged knee extension) at 20%, 40%, and 60% of the maximal voluntary contraction (MVC). Heart rate and arterial pressure were related to the the individual's contraction times. The isometric endurance response was longer with handgrip than with one-legged knee extension, but no significant difference was observed between the age groups. The isometric endurance response averaged 542 (SEM 57), 153 (SEM 14), and 59 (SEM 5) s for the handgrip, and 276 (SEM 35), 94 (SEM 10) and 48 (SEM 5) s for the knee extension at the three MVC levels, respectively. Heart rate and blood pressure became higher during one-legged knee extension than during handgrip, and with increasing level of contraction. The older subjects had a lower heart rate and a higher blood pressure response than their younger counterparts, and the differences were more apparent at a higher force level. The results would indicate that increasing age is associated with an altered heart rate and blood pressure response to isometric exercise although it does not affect isometric endurance.

Adult↗

PubMed1993

An X-ray diffraction study of frog skeletal muscle during shortening near the maximum velocity.

Using an imaging plate exchanger and a synchrotron X-ray source, X-ray diffraction patterns were recorded from frog skeletal muscles shortening near the maximum velocity from a sarcomere length of 2.6 to 2.1 microns. The major findings were as follows: (1) the intensity of the second-order myosin meridional reflection, which decreased to 18% of the resting value during isometric contraction, increased to 34% during shortening, showing that the perturbation in the axial arrangement of myosin heads recovered. (2) The intensity of the third-order myosin meridional reflection increased by 32% on isometric contraction and decreased to 51% of the resting intensity during rapid shortening, showing disorder in the axial rearrangement of the myosin heads. (3) The axial Bragg spacing of the third-order meridional reflection, which increased by 1.22% during isometric contraction, was still 0.47% larger than the resting value during shortening, indicating that the increase is not entirely due to extension of the filament by tension. (4) The intensity ratio of the (1,0) and (1,1) equatorial reflections increased a small extent (from 0.57 to 0.62) during shortening, while the 5.9 nm and 5.1 nm actin layer-lines showed large intensity decreases (126 to 101% and 190 to 124% of the resting intensity, respectively), suggesting that considerable numbers of myosin heads are in the vicinity of the thin filament but not tightly bound to actin molecules during rapid shortening. These results are consistent with a model of muscle contraction in which myosin heads attach to actin in two different manners with weak and strong affinities and show that, during shortening, the number of tightly attached heads decreases by half, while the total number of attached heads decreases only slightly.

Animals↗

PubMed1980

Effects of active shortening on tension development of rabbit papillary muscle.

Duration and intensity of force development have been shown to be less during active muscle shortening than during isometric contraction. The purpose of this study was to compare force developed during controlled shortening with that predicted by the Frank-Starling relation. Paillary muscles from the right ventricles of rabbits were arranged for isometric tension recording, and isometric contractions were recorded at several lengths. The muscles were then permitted to shorten at velocities of 0.2--6 mm/s, shortening beginning 150--200 ms after the stimulus. Length-tension-time curves constructed from the isometric contractions were used to determine predicted shortening tension (Pp), which was compared with actual tension during shortening (Ps) at corresponding times and lengths. Ps was significantly less than Pp and the ratio Ps/Pp decreased with increasing velocity of shortening. The decrease in Ps/Pp was directly related to the duration of shortening (P less than 0.001), suggesting that the fall of shortening tension reflected both the Hill force-velocity relation and shortening deactivation.

Animals↗

PubMed1995

Redistribution of cell membrane probes following contraction-induced injury of mouse soleus muscle.

Our aim was to study how mouse skeletal muscle membranes are altered by eccentric and isometric contractions. A fluorescent dialkyl carbocyanine dye (DiOC18(3)) was used to label muscle membranes, and the membranes accessible to the dye were observed by confocal laser scanning microscopy. Experiments were done on normal mouse soleus muscles and soleus muscles injured by 20 eccentric or 20 isometric contractions. Longitudinal optical sections of control muscle fibers revealed DiOC18(3) staining of the plasmalemma and regularly spaced transverse bands corresponding in location to the T-tubular system. Transverse optical sections showed an extensive reticular network with the DiOC18(3) staining. Injured muscle fibers showed distinctively different staining patterns in both longitudinal and transverse optical sections. Longitudinal optical sections of the injured fibers revealed staining in a longitudinally-oriented pattern. No correlations were found between the abnormal DiOC18(3) staining and the reductions in maximal isometric tetanic force or release of lactate dehydrogenase (P > or = 0.32). Additionally, no difference in the extent of abnormal staining was found between muscles performing eccentric contractions and those performing the less damaging isometric contractions. However, many fibers in muscles injured by eccentric contractions showed swollen regions with marked loss of membrane integrity and an elevated free cytosolic calcium concentration as observed in Fluo-3 images. In conclusion, a loss of cell membrane integrity results from contractile activity, enabling DiOC18(3) staining of internal membranes. The resulting staining pattern is striking and fibers with damaged cell membranes are easily distinguished from uninjured ones.

Animals↗

PubMed1996

Detrusor contractility and compliance characteristics in adult male patients with obstructive and nonobstructive voiding dysfunction.

PURPOSE: To understand better the contractility and compliance characteristics of the detrusor in patients with varying degrees of outlet obstruction, we analyzed urodynamic studies in elderly men with obstructive and nonobstructive voiding dysfunction. MATERIALS AND METHODS: All patients were evaluated with video urodynamics, including cystometry, isometric tests, voiding profilometry and post-void residual measurement. Bladder compliance, detrusor contractility, detrusor reserve, detrusor instability and the severity of outlet obstruction were determined in each patient. Patients were stratified into 4 groups: urodynamically normal, detrusor instability, outlet obstruction and outlet obstruction with detrusor instability. RESULTS: A significant correlation was found between the maximum isometric contraction pressure and the severity of obstruction in 168 patients. Maximum isometric contraction pressure was significantly greater in patients with than without obstruction, independent of detrusor instability. Although compliance was not significantly different among the groups, the proportion of patients with poor compliance (less than 30 ml./cm. water) was lowest in the normal group. The detrusor reserve was significantly less in patients with chronic retention (post-void residual more than 200 ml.) than in those with lower post-void residuals. CONCLUSIONS: The increase in detrusor contractility with increasing outlet obstruction suggests a compensatory response to obstruction. Furthermore, a decrease in bladder compliance does not appear to be a consistent finding in patients with outlet obstruction, although the proportion of patients with poor compliance is higher in the group with obstruction and/or detrusor instability than in those with normal urodynamic findings. The decrease in detrusor reserve in patients with high post-void residual volumes suggests that the detrusor reserve reflects the degree of detrusor decompensation.

Adult↗

PubMed1990

Muscle fiber conduction velocity and contractile properties estimated from surface electrode arrays.

Using an array of surface electrodes set 5 mm apart, we estimated the conduction velocities of muscle fibers during submaximal voluntary isometric contraction of human first dorsal interosseous muscle. The conduction velocity obtained by the averaging method ranged from 3.2 to 5.0 m/sec with a mean of 4.2 m/sec. Twitch tensions in the muscle detected during voluntary isometric contractions ranged from 0.31 to 5.97 g with a mean of 2.75 g based on an averaging method triggered by surface myoelectric signals. Threshold forces of the motor units varied from 120 to 930 g. The rise time of the force developed by isometric adduction ranged from 36.0 to 75.4 msec, with a mean of 55.4 msec. The conduction velocity of the muscle fiber showed a high correlation with the twitch (r = 0.71, n = 50; P less than 0.001) and threshold (r = 0.52, n = 50; P less than 0.001) forces, but a low one with rise time (r = -0.32, n = 50; P less than 0.05). The use of the averaging method with surface electrode arrays, especially for voluntary isometric contractions, shows that motor unit conduction velocity and contractile properties are functionally correlated.

Adult↗

PubMed2003

Passive stretches protect skeletal muscle of adult and old mice from lengthening contraction-induced injury.

We tested the hypothesis that a single bout of training with passive stretches or isometric contractions protects the extensor digitorum longus muscle in old mice from contraction-induced injury. Lengthening contractions produced similar decreases in force (approximately 70%-80%) and numbers of overtly injured fibers (approximately 15%-20%) in adult and old mice, but twofold greater inflammatory cell accumulation above untreated control values in old versus adult mice. For both age groups, prior training with passive stretches improved postinjury force almost twofold compared with untrained muscles and reduced injured fibers by one half. Training with passive stretches or isometric contractions reduced inflammatory cell accumulation following lengthening contractions by as much as two thirds in old mice, but not in adult mice. The data indicate that passive stretches provide some protection against contraction-induced injury in old mice, and that accumulation of inflammatory cells does not correlate strongly with force deficit and number of injured fibers.

Aging↗

PubMed1983

Acidotic depression of cyclic AMP accumulation and phosphorylase b to a transformation in skeletal muscle of man.

Intravenous infusion of adrenaline was performed in three healthy subjects on two occasions. In one case subjects performed a maximal isometric contraction before infusion. Biopsies were taken from the quadriceps femoris muscle before and after infusion for 0.5 and 2 min, and analysed for muscle pH, cyclic AMP, metabolites and activities of glycogen phosphorylase and synthetase. Isometric contraction resulted in a decrease of muscle pH to 6.60 (normal value at rest 7.0-7.1). By this experimental procedure the effect of adrenaline infusion could be studied on a muscle with normal pH and one with low pH. Cyclic AMP increased from 3 to about 9.5 mumol per kg dry weight after 0.5 min of adrenaline infusion. When isometric contraction preceded the infusion, cyclic AMP increased more slowly and was about 5.5 mumol per kg dry weight after the same time of infusion. Phosphorylase a constituted about 22% of total phosphorylase in resting muscle but increased rapidly to 80% after 0.5 min infusion. When exercise preceded infusion phosphorylase a decreased and was still lower after 2 min infusion. The results can be explained by inhibition of adenylcyclase and phosphorylase b kinase at low muscle pH.

Acid-Base Equilibrium↗

PubMed1982

Control scheme governing concurrently active human motor units during voluntary contractions.

1. The electrical activity of up to eight concurrently active motor units has been recorded from the human deltoid and first dorsal interosseous muscles. The resulting composite myoelectric signals have been decomposed into their constituent motor-unit action potential trains using a recently developed technique.2. A computer cross-correlation analysis has been performed on motor-unit firing rate and muscle-force output records obtained from both constant-force and triangular force-varying isometric contractions performed by normal subjects, and three groups of highly trained performers (long-distance swimmers, powerlifters and pianists).3. The temporal relationships between firing rate activity and force output have provided evidence that the deltoid of long-distance swimmers has a significantly higher percentage of slowly fatiguing fibres than that of normal subjects.4. Results showed that both muscles are incapable of producing a purely isotonic contraction under isometric conditions. Small, possibly compensatory force variations at 1-2 Hz result from a common drive to all active motoneurones in a single muscle pool.5. Rapid force reversals during triangular, force-varying isometric contractions appear to be accomplished through a size-related motor-unit control scheme. All firing rates decline prior to the force peak, but small motor units with slow-twitch responses tend to decrease their firing rates before large, fast-twitch motor units. This mechanism is not visually controlled, and does not depend on force rate in non-ballistic contractions.

Action Potentials↗