102 research outputs found
Strategies for the integration of posture and movement during reaching in the cat
We have examined the relationship between the movement and the anticipatory postural adjustments (APAs) that precede that movement during a reaching task in the cat. We recorded ground reaction forces in all 3 planes from all 4 limbs as well as electromyographic (EMG) activity from limb and axial muscles. The reaching movement was always preceded by an APA that was characterized by a loading of the reaching forelimb and an unloading of the support forelimb. This loading of the reaching forelimb was preceded, and accompanied, by increased activity in shoulder and limb extensor muscles of the reaching limb; extensor muscle activity in the supporting limb was simultaneously decreased. An important finding from this study was that the onset of the APA and of the movement was temporally decoupled. Analyses of the onset of EMG activity showed that most of the muscles that we recorded could be classified as either related to the APA or related to the movement. These results support the idea of distributed, and perhaps independent, systems for the execution of the APA and of the prime movement. There was also postural activity in the supporting limb during the movement. Analysis of this activity, which is also anticipatory in nature, suggests that it was tightly linked to the movement. We suggest that this postural response is signaled as part of the command for movement. Some muscles, particularly the extensors of the reaching limb, received convergent input from the command signals for the APA and for the movement
Independent and convergent signals from the pontomedullary reticular formation contribute to the control of posture and movement during reaching in the cat
We have addressed the nature of the postural control signals contained within the discharge activity of neurons in the pontomedullary reticular formation, including reticulospinal neurons, during a reaching task in the cat. We recorded the activity of 142 neurons during ipsilateral reaching movements that required anticipatory postural adjustments (APAs) in the supporting limbs to maintain equilibrium. Discharge activity in 82/142 (58%) neurons was significantly increased before the onset of the reach. Most of these neurons discharged either in a phasic (22/82), tonic (10/82), or phasic/tonic (41/82) pattern. In each of these 3 groups, the onset of the discharge activity in some neurons was temporally related either to the go signal or to the onset of the movement. In many neurons, one component of the discharge sequence was better related to the go signal and another to the onset of the movement. Based on our previous behavioral study during the same task, we suggest that reticular neurons in which the discharge activity is better related to the go signal contribute to the initiation of the APAs that precede the movement. Neurons in which the discharge activity is better related to the movement signal might contribute to the initiation of the movement and to the production of the postural responses that accompany that movement. Together our results suggest the existence of neurons that signal posture and movement independently and others that encode a convergent signal that contributes to the control of both posture and movement
Descending signals from the pontomedullary reticular formation are bilateral, asymmetric, and gated during reaching movements in the cat
We examined the contribution of neurons within the pontomedullary reticular formation (PMRF) to the control of reaching movements in the cat. We recorded the activity of 127 reticular neurons, including 56 reticulospinal neurons, during movements of each forelimb; 67/127 of these neurons discharged prior to the onset of activity in the prime flexor muscles during the reach of the ipsilateral limb and form the focus of this report. Most neurons (63/67) showed similar patterns and levels of discharge activity during reaches of either limb, although activity was slightly greater during reach of the ipsilateral limb. In 26/67 cells, the initial change in discharge activity was time-locked to the GO signal during reaches of either limb; we have argued that this early discharge contributes to the anticipatory postural adjustments that precede movement. In 11/26 cells, the initial change in activity was reciprocal for reaches with the left and right limbs, although activity during the movement was nonreciprocal. Spike-triggered averaging produced postspike facilitation or depression (PSD) in 12/50 cells during reaches of the limb ipsilateral to the recording site and in 17/49 cells during reach of the contralateral limb. Some cells produced PSD in ipsilateral extensor muscles before the start of the reach and during reaches made with the contralateral, but not the ipsilateral limb; this suggests the signal must be differentially gated. Overall, the results suggest a strong bilateral, albeit asymmetric, contribution from the PMRF to the control of posture and movement during voluntary movement
Motor control of landing in an unsteady environment
Background: When landing from a jump or a drop, muscles contract before touchdown to anticipate imminent collision with the ground, soften ground contact and allow to return to a stable standing position without stepping or rebounding. Research question: This study assesses the eïŹect of the unsteadiness of the environment on the motor control of landing. The âunsteady environmentâ was induced by asking participants to perform drop landings inside an aircraft that underwent trajectories parallel to Earthâs surface. The participants also performed the same task in a âsteady environmentâ in our laboratory. Methods: Ground reaction forces, lower limb jointsâ movements and the activity of lower limb muscles were recorded. The stability of the landing was assessed by the vertical and anterior-posterior stability indexes, center of pressure measures and by the coeïŹcient of variation of kinetic and kinematic parameters. Results: On one hand, participants slowdown their joint movements and reduce the knee joint excursion during landing, probably to avoid excessive movements that may induce imbalance. On the other hand, the stability of the landing is reduced while the variability of the movement is increased, illustrating a less stable and less consistent landing. In addition, whatever the environment, landing parameters associated with increased stiïŹness (i.e., increased impact forces and decreased joint range of motion) are correlated with decreased landing stability. SigniïŹcance: Overall, landings in theâunsteady environmentâ appear to be more cautious but less stable and less ïŹnely tuned. Since the stability of the landing is not directly inïŹuenced by the steadiness of the environment, this more cautious behavior could be, at least in part, related to the fear/apprehension induced by sudden acceleration variations of the frame of the aircraft.
Calculation of the external work done during walking in very young children
During walking, muscles must perform positive work to replace the energy lost from the body at each step, even if the average speed is constant and the terrain level. Young children have immature and irregular walk, but little is known about the effect of this walking pattern on the muscular external work done. Our objective was to measure using force platforms and the method of Cavagna (J Appl Physiol 39:174-179, 1975) the amount of muscular external work done by 1-year-old-, 4-year-old children and adults during walking. We were interested to quantify the approximation made by measuring only the positive external work done and assuming it reflects the external work done. After having confirmed that young children were not able to walk at a constant average speed over a complete number of steps, we showed the effect of the selection of trials by measuring the external work done assuming the amount of positive work done is equal to the negative work done (supposing there is no acceleration or deceleration over a complete number of steps). We observed that even if young subjects were not able to walk at a constant lateral speed over a complete number of steps, the amount of work done to maintain the center of mass movements in the transversal plane is not more than 10% of the external positive work done. This observational study points out that the measurement of external work, a good summary indicator for the gait mechanics, may be interpreted precociously when the population studied walked irregularly
Effet sur la course de la sollicitation du rĂ©flexe dâĂ©tirement musculaire au niveau des muscles flĂ©chisseurs plantaires chez lâhomme : rĂ©sultats prĂ©liminaires
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