PubMed · 6723616
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D W Repperger, D B Rogers, J W Frazier, K E Hudson. 1984. A task difficulty--G stress experiment.. https://doi.org/10.1080/00140138408963474
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The amplitude of open-loop pointing movements to step displacements in target position is influenced by the amplitude of simultaneously produced saccadic eye movements. The time course over which this occurs was addressed in the present study. Analysis of the pointing kinematics showed that saccade amplitude had its effect only during the initial acceleration of the hand. Moreover, the magnitude of the initial acceleration was correlated with the difference in the onset times of the eye and hand movements: the closer in time the saccadic and pointing responses were initiated the larger the initial hand acceleration. Taken together, these results demonstrate that saccades influence the kinematics of simultaneously produced limb movements but only over a limited time frame.
1. The effect of age and body size on the bouncing mechanism of running was studied in children aged 2-16 years. 2. The natural frequency of the bouncing system (fs) and the external work required to move the centre of mass of the body were measured using a force platform. 3. At all ages, during running below approximately 11 km h-1, the freely chosen step frequency (f) is about equal to fs (symmetric rebound), independent of speed, although it decreases with age from 4 Hz at 2 years to 2.5 Hz above 12 years. 4. The decrease of step frequency with age is associated with a decrease in the mass-specific vertical stiffness of the bouncing system (k/m) due to an increase of the body mass (m) with a constant stiffness (k). Above 12 years, k/m and f remain approximately constant due to a parallel increase in both k and m with age. 5. Above the critical speed of approximately 11 km h-1, independent of age, the rebound becomes asymmetric, i.e. f < fs. 6. The maximum running speed (Vf, max) increases with age while the step frequency at remains constant (approximately 4 Hz), independent of age. 7. At a given speed, the higher step frequency in preteens results in a mass-specific power against gravity less than that in adults. The external power required to move the centre of mass of the body is correspondingly reduced.
Complex movement execution theoretically involves numerous biomechanical degrees of freedom, leading to the concept of redundancy. The kinematics and kinetics of rapid straightening up movement from the squatting position were analysed with the optoelectronic ELITE system in 14 subjects. We found multiple acceleration and deceleration peaks for the hip, knee and ankle joints during the early extension phase of the movement. In order to test the temporal coordination between the angular acceleration of these joints, conjugate crosscorrelation functions (CCF) between each set of two variables were calculated. We found a bimodal distribution of the maximum CCF in positive and negative values suggesting the existence of two distinct strategies, the in-phase and the out-of-phase strategy for each pair of joints. The hip and knee coordination strategies (in- or out-of-phase) were well conserved in each subject for repetitive movements. Combination of joint pair strategies was more reproducible for the hip-knee/knee-ankle pair than for the other combinations, suggesting that the straightening up strategies are organised around the knee. We conclude that mastering of the redundancy problem can be realised by using coordination strategies characterised by opposed joint acceleration patterns.