Muscle contraction generates discrete sound bursts.

Isolated frog sartorius muscles were stimulated to shorten under lightly loaded conditions. A piezoelectric transducer was placed alongside the muscle to record sounds generated during contraction. Shortening was accompanied by the generation of a series of discrete sound bursts. The bursts were found to be moderately repeatable among successive contractions; 44% repeated from contraction to contraction. The duration of each sound burst was on the order of 400 mus, and the temperature dependence of the interval between successive bursts had a Q10 of approximately 2. Sound intensity was variable: average acoustic power ranged from 0.05-0.4 mW/g, or approximately 1% of the heat generated during contraction. The generation of discrete bursts of sound during contraction, rather than continuous sound, implies that contractile behavior may be discontinuous.     

Generalized motor programs for rapid bimanual tasks: a two-level multiplicative-rate model

A model is proposed for the timing of rapid bimanual movements. It combines (a) the notion of a generalized motor program (GMP) with invariant relative timing, (b) the two-level concept of timing control with a central level of control and a peripheral level where the observations are made, and (c) the hypothesis that a single GMP simultaneously controls both limbs. Our method is based on the analysis of temporal intervals measured among landmarks taken from the bimanual kinematic traces. We show that sets of tetrad ratios — each composed of two pairs of covariances among four temporal intervals in the actions — should be equal to 1.0 if the hypothesis is correct. In addition, we show that these tetrad ratios should deviate systematically from 1.0 under certain, biologically realizable violations of the model. Data from human subjects show that the results generally conform to the basic model. Simulations are used to illustrate other violations of the model and to explore characteristics of the sampling distribution of the tetrad ratios under the model.     

Age-associated differences in motor output variability and coordination during the simultaneous dorsiflexion of both feet

Older adults are more variable than young adults on tasks that demand the simultaneous control of more than one effector, and the difference between age groups may be related to their different capacity of coordinating the force output of the involved effectors. The goal of this study was to determine whether age-associated differences in motor output variability during tasks involving the simultaneous dorsiflexion of two feet can be partially explained by differences in coordination and possibly attenuated by physical training. Ten young and 22 old adults (10 trained and 12 untrained old adults) volunteered to participate in the study. Trained older adults had experience in a high-intensity mixed modality training (MMT) regime for a minimum of 1?year. Volunteers performed successive trials of a constant force task and a goal-directed task, with and without visual feedback. Within- and between-trial variability were calculated and coordination was quantified using the uncontrolled manifold (UCM) approach (i.e., co-variation of the force outputs of both feet were used to quantify a motor synergy index). Older adults exhibited greater variability and lower synergy (p?p?相似文献   

Time to task failure and motor cortical activity depend on the type of feedback in visuomotor tasks

The present study aimed to elucidate whether the type of feedback influences the performance and the motor cortical activity when executing identical visuomotor tasks. For this purpose, time to task failure was measured during position- and force-controlled muscular contractions. Subjects received either visual feedback about the force produced by pressing a force transducer or about the actual position between thumb and index finger. Participants were instructed to either match the force level of 30% MVC or the finger position corresponding to the thumb and index finger angle at this contraction intensity. Subjects demonstrated a shorter time to task failure when they were provided with feedback about their joint position (11.5 ± 6.2 min) instead of force feedback (19.2 ± 12.8 min; P = 0.01). To test differences in motor cortical activity between position- and force-controlled contractions, subthreshold transcranial magnetic stimulation (subTMS) was applied while executing submaximal (20% MVC) contractions. SubTMS resulted in a suppression of the first dorsal interosseus muscle (FDI) EMG in both tasks. However, the mean suppression for the position-controlled task was significantly greater (18.6 ± 9.4% vs. 13.3 ± 7.5%; P = 0.025) and lasted longer (13.9 ± 7.5 ms vs. 9.3 ± 4.3 ms; P = 0.024) compared to the force-controlled task. The FDI background EMG obtained without stimulation was comparable in all conditions. The present results demonstrate that the presentation of different feedback modalities influences the time to task failure as well as the cortical activity. As only the feedback was altered but not the mechanics of the task, the present results add to the body of evidence that suggests that the central nervous system processes force and position information in different ways.     

Temporal changes in motor variability during prolonged lifting/lowering and the influence of work experience

Existing research indicates that repetitive motions are strongly correlated with the development of work-related musculoskeletal disorders (WMSDs). Resulting from the redundant degrees-of-freedom in the human body, there are variations in motions that occur while performing a repetitive task. These variations are termed motor variability (MV), and may be beneficial for reducing WMSD risks. To better understand the potential role of MV in preventing injury risk, we evaluated the effects of fatigue on MV using data collected during a lab-based prolonged, repetitive lifting/lowering task. We also investigated whether experienced workers used different motor control strategies than novices to adapt to fatigue. MV of the whole-body center-of-mass (COM) and box trajectory were quantified using cycle-to-cycle standard deviation, sample entropy, and goal equivalent manifold (GEM) methods. In both groups, there were significantly increased variations of the COM with fatigue, and with a more substantial increase in a direction that did not affect task performance. Fatigue deteriorated the task goal and made it more difficult for participants to maintain their performance. Experienced workers also had higher MV than novices. Based on these results, we conclude that flexible motor control strategies are employed to reduce fatigue effects during a prolonged repetitive task.     

Bayesian inference underlies the contraction bias in delayed comparison tasks

Delayed comparison tasks are widely used in the study of working memory and perception in psychology and neuroscience. It has long been known, however, that decisions in these tasks are biased. When the two stimuli in a delayed comparison trial are small in magnitude, subjects tend to report that the first stimulus is larger than the second stimulus. In contrast, subjects tend to report that the second stimulus is larger than the first when the stimuli are relatively large. Here we study the computational principles underlying this bias, also known as the contraction bias. We propose that the contraction bias results from a Bayesian computation in which a noisy representation of a magnitude is combined with a-priori information about the distribution of magnitudes to optimize performance. We test our hypothesis on choice behavior in a visual delayed comparison experiment by studying the effect of (i) changing the prior distribution and (ii) changing the uncertainty in the memorized stimulus. We show that choice behavior in both manipulations is consistent with the performance of an observer who uses a Bayesian inference in order to improve performance. Moreover, our results suggest that the contraction bias arises during memory retrieval/decision making and not during memory encoding. These results support the notion that the contraction bias illusion can be understood as resulting from optimality considerations.     

The influence of contraction type,prior performance of a maximal voluntary contraction and measurement duration on fine-wire EMG amplitude

We aimed to investigate the impact of time on fine-wire (fw) electromyography (EMG) signal amplitude, and to determine whether any attenuation is confounded by task type. Twenty healthy participants were instrumented with fw and surface (s) EMG electrodes at the biceps brachii bilaterally. Participants held a weight statically with one arm and with the other arm either repeated the same task following a maximum voluntary contraction (MVC) or repeated dynamic elbow flexion/extension contractions. Each task was repeated for 30 s every five minutes over two hours. EMG amplitude was smoothed and normalized to time = 0. Stable median power frequency of the s-EMG ruled out the confounding influence of fatigue. Repeated-measures ANCOVAs determined the effect of electrode type and time (covariate) on EMG amplitude and the confounding impact of task type. During the isometric protocol, fw-EMG amplitude reduced over time (p = 0.002), while s-EMG amplitude (p = 0.895) and MPF (p > 0.05) did not change. Fw-EMG amplitude attenuated faster during the dynamic than the isometric protocol (p = 0.008) and there was evidence that the MVC preceding the isometric protocol impacted the rate of decline (p = 0.001). We conclude that systematic signal attenuation of fw-EMG occurs over time and is more pronounced during dynamic tasks.     

Aging and the time and frequency structure of force output variability.

The present study examined the time and frequency structure of force output in adult humans to determine whether the changes in complexity with age are dependent on external task demands. Healthy young (20-24 yr), old (60-69 yr), and older-old (75-90 yr) humans produced isometric force contractions to constant and sine wave targets that also varied in force level. First, force variability on each force task increased with advancing age. Second, both time and frequency analysis showed that the structure of the force output in the old and older-old adults was less complex in the constant-force level task and more complex in the sine wave force task. Third, the alterations in force output with aging were primarily due to low-frequency bands <4 Hz. These results support the postulation that the observed increase or decrease in physiological complexity with aging is influenced by the relatively fast time scale of external task demands (Vaillancourt DE and Newell KM. Neurobiol Aging 23: 1-11, 2002).     

Longitudinal coordination of motor output during swimming in Xenopus embryos.

Little is known about the neural mechanisms that control the phenomenon of rostro-caudal delay. In Xenopus embryos there is a constant rostro-caudal delay of 2-5 ms mm-1 during fictive swimming. Rostro-caudal delay is not significantly correlated with cycle period. When NMDA is applied to the caudal spinal cord there is a decrease and in some cases a reversal in rostro-caudal delay. Conversely applying excitatory antagonists to the caudal spinal cord leads to an increase in delay. When caudal mid-cycle inhibition is reduced either pharmacologically using strychnine or surgically through hemisection of the spinal cord, there is an increase in rostro-caudal delay. Rostro-caudal delays are too small to be explainable on the basis of axonal conduction velocities and synaptic delays. This suggests that the central pattern generator of Xenopus behaves as a series of coupled oscillators and that the nature of the coupling, together with a longitudinal gradient in excitability associated with the oscillators, contributes to the observed rostro-caudal delay.     

Effects of phrenicotomy and exercise on hypoxia-induced changes in phrenic motor output.

To investigate models of plasticity in respiratory motor output, we determined the effects of chronic unilateral phrenicotomy and/or exercise on time-dependent responses to episodic hypoxia in the contralateral phrenic nerve. Anesthetized (urethane), ventilated, and vagotomized rats were presented with three, 5-min episodes of isocapnic hypoxia (11% O(2)), separated by 5 min of hyperoxia (50% O(2)). Integrated phrenic (and hypoglossal) nerve discharge were recorded before and during each hypoxic episode, for the first 5 min after the first hypoxic episode, and at 30 and 60 min after the final episode. Of 36 rats, one-half were sedentary while the other one-half had free access to a running wheel; each of these groups was split into three subgroups: 1) unoperated, 2) chronic left phrenicotomy (27-37 days), and 3) sham operated. Neither unilateral phrenicotomy nor running wheel activity influenced the short-term hypoxic phrenic response (during hypoxia) or long-term facilitation (posthypoxia). Posthypoxia frequency decline was exaggerated in phrenicotomized-sedentary rats relative to unoperated-sedentary rats (change in burst frequency = -23+/-4 vs. -11 +/-5 bursts/min, respectively; 5 min posthypoxia; P<0.05), an effect that was eliminated by spontaneous exercise. The results indicate that neither voluntary running nor unilateral phrenicotomy has major effects on time-dependent hypoxic phrenic responses, with the exception of an unexpected effect of phrenicotomy on posthypoxia frequency decline in sedentary rats.     

Effect of fatigue on maximal power output at different contraction velocities in humans.

The effect of fatigue as a result of a standard submaximal dynamic exercise on maximal short-term power output generated at different contraction velocities was studied in humans. Six subjects performed 25-s maximal efforts on an isokinetic cycle ergometer at five different pedaling rates (60, 75, 90, 105, and 120 rpm). Measurements of maximal power output were made under control conditions [after 6 min of cycling at 30% maximal O2 uptake (VO2max)] and after fatiguing exercise that consisted of 6 min of cycling at 90% VO2max with a pedaling rate of 90 rpm. Compared with control values, maximal peak power measured after fatiguing exercise was significantly reduced by 23 +/- 19, 28 +/- 11, and 25 +/- 11% at pedaling rates of 90, 105, and 120 rpm, respectively. Reductions in maximum peak power of 11 +/- 8 and 14 +/- 8% at 60 and 75 rpm, respectively, were not significant. The rate of decline in peak power during the 25-s control measurement was least at 60 rpm (5.1 +/- 2.3 W/s) and greatest at 120 rpm (26.3 +/- 13.9 W/s). After fatiguing exercise, the rate of decline in peak power at pedaling rates of 105 and 120 rpm decreased significantly from 21.5 +/- 9.0 and 26.3 +/- 13.9 W/s to 10.0 +/- 7.3 and 13.3 +/- 6.9 W/s, respectively. These experiments indicate that fatigue induced by submaximal dynamic exercise results in a velocity-dependent effect on muscle power. It is suggested that the reduced maximal power at the higher velocities was due to a selective effect of fatigue on the faster fatigue-sensitive fibers of the active muscle mass.     

Serotonergic modulation of respiratory motor output during tadpole development.

To test the hypothesis that serotonin (5-hydroxytryptamine; 5-HT)-receptor activation elicits age-dependent changes in respiratory motor output, we compared the effects of 5-HT bath application (5-HT concentration = 0.5-25 microM) onto in vitro brain stem preparations from pre- and postmetamorphic bullfrog tadpoles. Recording of motor output related to gill and lung ventilation showed that 5-HT elicits a dose-dependent depression of gill burst frequency in both groups. In contrast, the lung burst frequency response was stage dependent; an increase in lung burst frequency at low 5-HT concentration (< or =0.5 microM) was observed only in the postmetamorphic group. Higher 5-HT concentrations decreased lung burst frequency in all preparations. Gill burst frequency attenuation is mediated (at least in part) by 5-HT(1A)-receptor activation in an age-dependent fashion. We conclude that serotonergic modulation of respiratory motor output 1) changes during tadpole development and 2) is distinct for gill and lung ventilation.     

Phytochrome-controlled rapid contraction and recovery of contractile vacuoles in the motor cells of Mimosa pudica as an intracellular correlate of nyctinasty

Summary Using living thin sections (ca. 70–80 thick) of tertiary pulvini of Mimosa pudica, we have quantitatively determined that the bahavior of the contractile tannin vacuoles in the motor cells is under phytochrome control. Using material in which these vacuoles were in their most expanded state in white light, contraction was observable 3 min after the material was placed in continuous darkness. No contraction occurred if the cells were irradiated with 90 sec of far-red light; red light reversed this effect. Futhermore, the kinetics of change of the vacuolar conformation was closely paralled by that of the nyctinastic changes of the pinnule closure during the different treatments. When the section of pulvinus was irradiated with a microbeam of far red light in one part of the section, and the motor cell vacuoles in another area were monitored for contraction, they almost always responded.We therefore conclude that the contractile vacuole of the motor cell is an excellent cellular correlate of phytochrome-mediated nyctinasty in M. pudica, and discuss its possible causal role in regulating the phenomenon. It is further concluded that functional phytochrome is present in all parts of the pulvinus and that, upon absorption of the stimulus energy, an intercellular messenger is released which stimulates all the motor cells in the pulvinus.Abbreviations FR far-red - R red     

The myosin motor: muscle contraction and in vitro movement

The molecular mechanism of in vitro movement is assumed, by most investigators, to be identical to that of muscle contraction. We discuss this view, which raises various problems. We believe there are mechanisms for muscle contraction (in this case considerable forces are developed, with small displacements) and other mechanisms for in vitro movement (giving large displacements, without necessarily generating substantial forces). Hybrid models may explain muscle contraction. The traditional swinging-crossbridge model may explain in vitro movement. For muscle contraction, movement may result partly from the swinging-crossbridge mechanism and partly from other factors. Comparisons of different fibres at different moments of the Mg-ATPase cycle suggest that both the value of the isometric force in muscle and in vitro and that of the Mg-ATPase activity used in vitro need to be reconsidered. The recently reported dependence of the isometric active tension of smooth skinned fibres on temperature appears to be weaker than predicted by the swinging-crossbridge theory alone. This recent observation is compatible with the existence of other forces (electrostatic repulsions) decreasing with temperature as has been known for some years. From recent experimental data, we think the biochemistry of myosin and actomyosin should be reassessed, to try to find new details of the mechanisms of muscle contraction and in vitro motility.     

Effects of inspiratory flow on diaphragmatic motor output in normal subjects.

Increasing inspiratory flow (V) has been shown to shorten neural inspiratory time (TI(n)) in normal subjects breathing on a mechanical ventilator, but the effect of V on respiratory motor output before inspiratory termination has not previously been studied in humans. While breathing spontaneously on a mechanical ventilator, eight normal subjects were intermittently exposed to 200-ms-duration positive pressure pulses of different amplitudes at the onset of inspiration. Based on the increase in V above control breaths (DeltaV), trials were grouped into small, medium, and large groups (mean DeltaV: 0.51, 1.11, and 1.65 l/s, respectively). We measured TI(n), transdiaphragmatic pressure (Pdi), and electrical activity (electromyogram) of the diaphragm (EMGdi). Transient increases in V caused shortening of TI(n) from 1.34 to 1.10 (not significant), 1.55 to 1.11 (P < 0.005), and 1.58 to 1.17 s (P < 0. 005) in the small, medium, and large DeltaV groups, respectively. EMGdi measured at end TI(n) of the pulse breaths was 131 (P < 0.05), 142, and 155% (P < 0.05) of the EMGdi of the control breaths at an identical time point in the small, medium, and large trials, respectively. The latency of the excitation was 126 +/- 42 (SD) ms, consistent with a reflex effect. Increasing V had two countervailing effects on Pdi: 1) a depressant mechanical effect due primarily to the force-length (11.2 cmH(2)O/l) relation of the diaphragm, and 2) an increase in diaphragm activation. For the eight subjects, mean peak Pdi did not change significantly, but there was significant intersubject variability, reflecting variability in the strength of the excitation reflex. We conclude that increasing inspiratory V causes a graded facilitation of EMGdi, which serves to counteract the negative effect of the force-length relation on Pdi.     

Abdominal motor unit activity during respiratory and nonrespiratory tasks

Abdominalmuscles serve multiple roles, but the functional organization of theirmotoneurons remains unclear. To gain insight, we recorded single motorunit potentials from the internal oblique (IO) and transversusabdominis (TA) muscles of three standing subjects during quietbreathing, a leg lift, and an expiratory threshold load. Inspiratoryairflow, recorded from a pneumotachometer, provided tidal volumes andrespiratory cycle timing. Fine wires, implanted under ultrasonicimaging, detected single motor unit potentials that were visuallydistinguished by their spike morphology. From the number of spikes,firing profiles, times of occurrence in the respiratory cycle, andtheir onset, instantaneous, mean, and peak firing frequencies wededuced that 1) breathing patterns varied across tasks, 2) differentmotor units were recruited for each task with essentially no overlap,3) their firing displayed prominentexpiratory activity during each task, and4) the recruitment levels anddischarge patterns of IO and TA were different. We conclude that the IOand TA motor pools receive a strong central respiratory drive, yet eachpool receives its own distinct, task-dependent synapticinput.

     

Mechanisms underlying the regulation of motor unit contraction in the skeletal muscle.

The control of movements is made posible thanks to the activity of motor units in skeletal muscles. In the present paper the influence of frequency and pattern of motoneuronal firing on the tension of contraction and the tension-time area is presented and discussed. The most resistant to fatigue slow-twitch motor units are low susceptible to changes in a pattern of impulses and therefore they are well prepared to participate in long-lasting movements at low but rather stable levels of tension. Moreover, their contraction is very effective and it is performed at a low metabolic charge. Fast-twitch units have lower resistance to fatigue and they have higher tension but they have high susceptibility to a pattern of pulses and their tension can be effectively regulated by an increase or a decrease in the interpulse interval. Therefore, fast motor units are specialized to participate in the regulation of the movement force. The existence of different functional groups of motor units in skeletal muscles enables the performance of different motor tasks very effectively and at possibly low metabolic costs.