Unintentional force drifts in the lower extremities.

We explored the phenomenon of unintentional force drift seen in the absence of visual feedback during knee extension contractions in isometric conditions. Based on the importance of knee extensors for the anti-gravity function, we hypothesized that such force drifts would be slower and smaller compared to those reported for the upper extremities. We also explored possible effects of foot dominance and gender on the force drifts. Young healthy persons produced isometric knee extension contractions to different levels, ranging from 15 to 25% of maximal voluntary contraction force, with the help of visual feedback, and then, the visual feedback was turned off. Force change over the time interval without visual feedback was quantified. In the absence of visual feedback, force drifted to smaller magnitudes. The drift magnitude expressed in percent of the initial force magnitude was smaller for smaller initial force levels, ranging between 8 and 15% of the initial force for the initial force magnitude of 15% and 25% of maximal voluntary contraction force. The time exponent of the force drift was independent of the initial force magnitude and was, on average, 6.45 s. There were no significant effects of foot dominance or gender, although the male subjects tended to show stronger scaling of the drift magnitude with the initial force level compared to the female subjects. The results show that unintentional force drift is a common phenomenon across limbs and muscle groups. This conclusion fits the theory of control with spatial referent coordinates and the general tendency of all natural systems to drift to states with lower potential energy.

Partly and Fully Supervised Physical Exercise Effects on Cognitive Functions and Movement Proficiency of Adolescents.

This study explored the effects of partly supervised physical exercise program (PSPEP) intervention compared to fully supervised physical exercise program (FSPEP) on cognitive functions, movement proficiency and problematic internet use (PIU) in adolescents presenting combined unhealthy lifestyle behaviors. METHOD: Over a period of 16 weeks the PSPEP group (n = 14) engaged in strength, balance and flexibility exercises three times per week with one supervised session and two unsupervised. The FSPEP group (n = 13) practiced dance activities for 2 to 4 days per week including in training sessions the same exercises as for the PSPEP group. Prior and after the study adolescents completed the PIU scale, performed movement proficiency and cognitive function tests. RESULTS: The PSPEP had significant effect on improvement of stress tolerance (p < 0.01, g = 1.08), while the FSPEP had significant effect on contributing general intelligence (p < 0.05, g = 0.90), color-word interference of the reading speed or color recognition (p < 0.01, g = 1.33), short- and long-term memory (p < 0.01, g = 1.72) and stress tolerance (p < 0.05, g = 1.06). The PSPEP had significant effect on improvement of the bilateral coordination (p < 0.01, g = 1.08). None of the two programs had effect on PIU. CONCLUSIONS: Engagement in PSPEP selectively contributed bilateral coordination and cognitive skills related to reaction to multiple stimuli. The FSPEP had multiple significant effects in improvement of cognitive outcomes.