RESUMO
This study examined the association between plantarflexion torque variability during quiet bipedal standing (QS) and during plantarflexion force- and position-matching tasks (FT and PT, respectively). In QS, participants stood still over a force plate, and the mean plantarflexion torque level exerted by each subject in QS (divided by 2 to give the torque due to a single leg) served as the target torque level for right leg FT and PT (performed with the participants seated with their right knee fully extended). During FT participants controlled the force level exerted by the foot against a rigid restraint, while during PT they controlled the angular position of the ankle when sustaining equivalent inertial loads. Standard deviation (SD) of plantarflexion torque was computed from torque signals acquired during periods with and without visual feedback. Significant correlations were found between plantarflexion torque variability in QS and FT (r = 0.8615, p < 0.0001 and r = 0.8838, p = 0.0003 for visual and no visual conditions, respectively) as well as between QS and PT (r = 0.8046, p = 0.003 and r = 0.7332, p = 0.0103 for visual and no visual conditions, respectively), regardless of vision availability. No significant differences were found between the correlations for Qs vs FT and QS vs PT (t(8) = 0.4778, p = 0.6455 and t(8) = 1.6819, p = 0.1310 for visual and no visual conditions, respectively), as assessed by "Hotelling-Williams" tests for equality among dependent correlations. The results indicate that simple measurements of plantarflexion torque fluctuations during FT and PT may be used to estimate balance ability. From a practical standpoint, it is suggested that rehabilitation protocols designed to regain/improve balance function may be based on the performance of FTs or PTs executed in a seated position.
Assuntos
Fenômenos Biomecânicos/fisiologia , Retroalimentação Sensorial/fisiologia , Pé/fisiologia , Equilíbrio Postural/fisiologia , Postura/fisiologia , Adulto , Humanos , TorqueRESUMO
The present study examined the association between plantar flexion torque variability during isolated isometric contractions and during quiet bipedal standing. For plantar flexion torque measurements in quiet stance (QS), subjects stood still over a force plate. The mean plantar flexion torque level exerted by each subject in QS (divided by 2 to give the torque due to a single leg) served as the target torque level for right leg force-matching tasks in extended knee (KE) and flexed knee (KF) conditions. Muscle activation levels (EMG amplitudes) of the triceps surae and mean, standard deviation and coefficient of variation of plantar flexion torque were computed from signals acquired during periods with and without visual feedback. No significant correlations were found between EMG amplitudes and torque variability, regardless of the condition and muscle being analyzed. A significant correlation was found between torque variability in QS and KE, whereas no significant correlation was found between torque variability in QS and KF, regardless of vision availability. Therefore, torque variability measured in a controlled extended knee plantar flexion contraction is a predictor of torque variability in the anterior-posterior direction when the subjects are in quiet standing. In other words, larger plantar flexion torque variability in KE (but not in KF) implies less stable balance. The mechanisms underlying the findings above are probably associated with the similar proprioceptive feedback from the triceps surae in QS and KE and poorer proprioceptive feedback from the triceps surae in KF due to the slackening of the gastrocnemii. An additional putative mechanism includes the different torque contributions of each component of the triceps surae in the two knee angles. From a clinical and research standpoint, it would be advantageous to be able to estimate changes in balance ability by means of simple measurements of torque variability in a force matching task.