Balancing with Vibration: A Prelude for “Drift and Act” Balance Control

Stick balancing at the fingertip is a powerful paradigm for the study of the control of human balance. Here we show that the mean stick balancing time is increased by about two-fold when a subject stands on a vibrating platform that produces vertical vibrations at the fingertip (0.001 m, 15–50 Hz). High speed motion capture measurements in three dimensions demonstrate that vibration does not shorten the neural latency for stick balancing or change the distribution of the changes in speed made by the fingertip during stick balancing, but does decrease the amplitude of the fluctuations in the relative positions of the fingertip and the tip of the stick in the horizontal plane, A(x,y). The findings are interpreted in terms of a time-delayed “drift and act” control mechanism in which controlling movements are made only when controlled variables exceed a threshold, i.e. the stick survival time measures the time to cross a threshold. The amplitude of the oscillations produced by this mechanism can be decreased by parametric excitation. It is shown that a plot of the logarithm of the vibration-induced increase in stick balancing skill, a measure of the mean first passage time, versus the standard deviation of the A(x,y) fluctuations, a measure of the distance to the threshold, is linear as expected for the times to cross a threshold in a stochastic dynamical system. These observations suggest that the balanced state represents a complex time–dependent state which is situated in a basin of attraction that is of the same order of size. The fact that vibration amplitude can benefit balance control raises the possibility of minimizing risk of falling through appropriate changes in the design of footwear and roughness of the walking surfaces

The Effect of Co-activation of Antagonist Muscles on Motor Cortex Excitability: A Transcranial Magnetic Stimulation Study

The effect of unilateral tonic muscle activity with and without co-activation of the antagonists on motor cortex excitability has been studied in seven right handed healthy volunteers. Contralateral motor evoked potentials (MEPs) were recorded from the first dorsal interosseous muscles of right hands in response to transcranial magnetic stimulation (TMS) during relax, isometric index finger abduction and antagonistic co-activation. The intracortical facilitation (ICF), short- and long-latency intracortical inhibition (SICI and LICI) were investigated by paired-pulse TMS. The unilateral tonic activation of the right hand facilitated MEPs in response to single-pulse TMS. The increase of MEP amplitudes was significantly greater during isometric index finger abduction compared to co-activation of antagonist muscles. During paired-pulse TMS with short interstimulus intervals, the SICI (interstimulus interval of 3 ms) was not influenced by the unilateral tonic activity while ICF (interstimulus interval of 13 ms) was suppressed. During paired-pulse TMS with longer interstimulus interval (100 ms) the LICI was not influenced during isometric index finger abduction while during antagonistic co-activation the LICI was significantly less pronounced. The decreased LICI is assumed to reflect mechanisms underlying the co-activation of antagonists

Changes in fluctuation of isometric force following eccentric and concentric exercise of the elbow flexors

The original publication can be found at www.springerlink.comThis study tested the hypothesis that eccentric exercise (ECC) would increase force fluctuation for several days following exercise; however, concentric exercise (CON) would not produce such an effect. Twelve men performed six sets of five reps of dumbbell exercise of the elbow flexors eccentrically with one arm and concentrically with the other, separated by 4-6 weeks, using a dumbbell set at 50% of maximal voluntary isometric contraction (MVC) measured at 90 degrees of elbow flexion. MVC, range of motion (ROM), upper arm circumference, plasma creatine kinase activity (CK), myoglobin concentration (Mb) and muscle soreness were assessed before, immediately after, 1 h and 1-5 days following both exercise bouts. Force fluctuations during 30, 50 and 80% MVC were quantified by coefficient of variation (CV) of the force data (sampling frequency: 100 Hz) for 4 s. Significantly (P < 0.01) larger changes in MVC, ROM, and upper arm circumference were evident following ECC compared to CON, and only ECC resulted in significant (P < 0.01) increases in CK and Mb, and development of muscle soreness. Significant (P < 0.01) differences existed between ECC and CON for changes in force fluctuations. CV increased significantly (P < 0.01) immediately and 1 h after ECC from baseline for 30, 50, and 80% MVC without a significant difference among the intensities, and no significant changes in CV were evident following CON. It was concluded that increases in force fluctuation were peculiar to ECC, but did not necessarily reflect muscle damage.Andrew P. Lavender and Kazunori Nosak