Biomedical subjects

A J Knoek van Soest

Publications and source records attributed to A J Knoek van Soest.

5 recordsLinked to original sources

PubMed2005

Scaling and jumping: gravity loses grip on small jumpers.

There are several ways to quantify jumping performance, a common definition being the height gained by the body's centre of mass (CM) in the airborne phase. Under this definition, jump height is determined by take-off velocity. According to the existing literature on jumping and scaling, take-off velocity, and hence jumping performance is independent of size because the energy that differently sized geometrically scaled jumpers can generate with their muscles is proportional to their mass. In this article it is shown, based on a simple energy balance, that it is incorrect to presume that jump height does not depend on size. Contrary to common belief, size as such has does have an effect on take-off velocity, putting small jumpers at a mechanical advantage, as is shown analytically. To quantify the effect of size on take-off velocity, a generic jumper model was scaled geometrically and evaluated numerically. While a 70-kg jumper took off at 2.65 m/s and raised its CM by 0.36 m after take-off, a perfectly geometrically similar jumper of 0.7 g reached a take-off velocity of 3.46 m/s and raised its CM by 0.61 m. The reason for the better performance of small jumpers is their higher efficacy in transforming the energy generated by the actuators into energy due to vertical velocity of the CM. Considering the ecological and evolutionary relevance of different definitions of jump height, size-dependent efficacy might explain why habitual jumping is especially prominent among small animals such as insects.

Acceleration↗

PubMed2004

Mass perturbation of a body segment: 2. Effects on interlimb coordination.

The shifts in relative phase that are observed when rhythmically coordinated limbs are submitted to asymmetric mass perturbations have typically been attributed to the induced eigenfrequency difference (delta omega) between limbs. Modeling the moving limbs as forced linear oscillators, however, reveals that asymmetric mass perturbations may induce a difference not only in eigenfrequency (i.e., delta omega not equal 0) but also in the covarying low-frequency control gains (i.e., delta k not equal 0). Because the inverse of the low-frequency control gain (k) reflects the level of muscular torque (input) required for a particular displacement from equilibrium (output), asymmetric mass perturbations may result in an imbalance in the muscular torques required for task performance (related to delta k not equal 0). Thus, it is possible that the effects attributed to delta omega were in fact mediated by delta k. In 2 experiments, the authors manipulated delta k and delta omega separately by applying mass perturbations to the lower legs of 9 participants. The relative phasing between the legs was not affected by delta k, but manipulation of delta omega (while delta k remained approximately 0) induced systematic relative phase shifts that were more pronounced for antiphase than for in-phase coordination. That indication that the coordination dynamics is indeed influenced by an imbalance in eigenfrequency is discussed vis-a-vis the question of how such a merely peripheral property may affect the underlying coordination process.

Extremities↗

PubMed2003

Optical acceleration cancellation: a viable interception strategy?

Interception of fly balls requires active locomotion toward the point where catching can take place; as a result, the visual information guiding interception is affected by the catcher's own movement. The only interception theory currently available for a catcher standing in the plane of motion of the ball is Optical Acceleration Cancellation (OAC); in this strategy, the pseudo-optical variable "optical acceleration" (OA), if nonzero, specifies how the catcher should adjust his current velocity. We formulate a precise implementation of OAC where the catcher strives to maintain OA zero at all times and analyze its implications in terms of the catcher's interception behavior for different ball trajectories under air-friction-free, low-friction, and friction-dominated conditions. We conclude that the point in the ball trajectory where first visual contact (FVC) takes place determines to a large extent the ensuing interception behavior of the catcher. Conventional trajectories (FVC slightly above eye level, ball coming toward the catcher) result in fast acceleration to a constant velocity and successful interception. Trajectories with FVC below eye level typically result in unsatisfactory behavior of the catcher, who runs away from rather than toward the point of interception. In addition, ball trajectories are identified for which the OA equals zero even though the catcher is not on an interception course.

Acceleration↗

PubMed2003

The merits of a parallel genetic algorithm in solving hard optimization problems.

A parallel genetic algorithm for optimization is outlined, and its performance on both mathematical and biomechanical optimization problems is compared to a sequential quadratic programming algorithm, a downhill simplex algorithm and a simulated annealing algorithm. When high-dimensional non-smooth or discontinuous problems with numerous local optima are considered, only the simulated annealing and the genetic algorithm, which are both characterized by a weak search heuristic, are successful in finding the optimal region in parameter space. The key advantage of the genetic algorithm is that it can easily be parallelized at negligible overhead.

Algorithms↗

PubMed2002

Spontaneous leg movements in infants with and without periventricular leukomalacia: effects of unilateral weighting.

The present study was designed to investigate the contribution of the corticospinal tracts in the regulation and coordination of interlimb couplings and the spatio-temporal organization of kicking movements in young infants. Both healthy infants and those with differing degrees of periventricular leukomalacia (PVL) were subjected to a unilateral weight manipulation at the (corrected) age of 26 weeks. Infants with PVL were grouped according to the amount of damage in the area in which the corticospinal tracts are located as shown by neonatal MRI and confirmed with MRI recordings at 18 months. The main question asked was whether unilateral weighting would reveal different adjustment in infants with and without PVL and whether these differences were related to the severity of the lesions, if present. The major finding was that no differences were evident between groups in adjusting to the weight manipulation with regard to the tightness of interlimb couplings. This finding corroborates the suggestion that corticospinal influences are not directly involved in the regulation of these parameters. Although the same conclusion could be drawn concerning the kinematic details of kicks on the basis of group data, individual analyses revealed that kinematics in a few infants with PVL were markedly affected by the weighting. Thus, combining group with individual analyses may have additional value in the clinical interpretation of the effects of PVL on the neural functions of young infants.

Brain↗