Kimberlee Jordan

Stability and the time-dependent structure of gait variability in walking and running

Participants were asked to walk and run continuously (5 min trials) at speeds associated with preferred gait transition speeds. During slow running the local dynamic stability of the head was decreased compared with fast walking, with the reverse being true for the local dynamic stability of the ankle. The standard deviation of relative phase of the knee and ankle also was greater during slow running than fast walking. These findings for stability were mirrored in the detrended fluctuation analysis of the peak to peak interval of the head and ankle. Taken collectively these results support the proposition that larger long range correlations in the stride interval are associated with decreases in measures of stability.

The Structure of Variability in Human Walking and Running is Speed-Dependent

The variability of gait in human walking and running is not random but shows self-similarity that is dependent on the speed of locomotion. The strength of the long-range correlations for each gait pattern follows a U-shaped function that is centered on the respective preferred speed of locomotion.

Heteronymous reflex responses in a hand muscle when maintaining constant finger force or position at different contraction intensities.

OBJECTIVE This study compared heteronymous reflex responses evoked in the first dorsal interosseous muscle by electrical and mechanical stimuli during force and position tasks performed at different target torques. METHODS Twenty-two healthy human participants contracted the first dorsal interosseus muscle either to produce a constant force against a rigid restraint (force task) or to maintain a constant position of the index finger (position task) against a constant load of 20, 40, and 60% of maximum. RESULTS The amplitude of the short-latency reflex evoked by electrical stimulation of the median nerve was significantly greater when maintaining finger position, whereas no difference was present for the long-latency responses. In contrast, the reflex responses (short- and long-latency) did not differ between tasks when elicited by tendon-taps. CONCLUSIONS Task difference in reflex responsiveness depended more on the type of stimulus applied than the reflex pathway and was consistent across three voluntary contraction forces. SIGNIFICANCE The results suggest that afferent input from homonymous and heteronymous pathways is modulated similarly at the spinal level during such tasks, and implies the significance of presynaptic inhibition during motor performance.

Long-range correlations in motor unit discharge times at low forces are modulated by visual gain and age

•  What is the central question of this study? Changes in visual gain do not influence the time‐dependent structure of motor unit discharge variability in young adults; however, the influence of visual gain on the regularity of motor unit discharge times in old adults is unknown. •  What is the main finding and its importance? We examined the association between visual information and the size and temporal structure of the variability in index finger force and motor unit discharge times when young and old adults performed isometric contractions with a hand muscle. We showed that the regularity with which motor units discharged action potentials decreased with increased gain of visual feedback, regardless of age.

Grip width and the organization of force output.

The authors investigated the structure of force production and variability as a function of grip configuration and width during precision grasping. Variability was studied in absolute (standard deviation) and relative (coefficient of variation) terms; in addition, the authors used approximate entropy to examine regularity. In Experiment 1, the participants (N = 14) used a 2-digit grasp (thumb, index), whereas in Experiment 2, the participants (N = 11) used a 3-digit grasp (thumb, index, middle). The level and regularity of force increased with grip width. The amount of variability was least at narrow grip widths for 2-digit grasping and greatest at narrow grip widths for 3-digit grasping. That pattern of findings is not necessitated by the mechanical equilibrium of grasping; thus, it also reflected adaptive neural reorganization of force output to task demands.