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Information from the central and peripheral nervous systems is continuously integrated to produce a stable gait pattern. However, stride duration fluctuates in a complex manner in healthy subjects, exhibiting long-range autocorrelations that can span over hundreds of consecutive strides. The present study was conducted to explore the mechanisms controlling the long-term fluctuation dynamics of gait. In the first part of the study, stride duration variability was evaluated on a treadmill during forward (FW) and backward walking (BW). Despite the modification of the biomechanical constraints imposed on the locomotor system, the characteristics of the long-range autocorrelations remained unchanged in both modes of locomotion (FW: H = 0.79 ± 0.04 and α = 0.58 ± 0.13; BW: H = 0.79 ± 0.11 and α = 0.53 ± 0.25). In the second part of the study, stride duration variability was assessed while the subjects were performing a dual-task paradigm that combined gait and mental calculation. The long-term variability of stride duration was similar during usual walking (H = 0.80 ± 0.06 and α = 0.57 ± 0.13) and in dual-tasking (H = 0.77 ± 0.06 and α = 0.52 ± 0.16), whereas walking altered the performance of the cognitive task. Hence, the biomechanical and cognitive interferences imposed in the present study were not sufficient to induce a modification of the long-range autocorrelations highlighted in walking variability. These observations underline the robustness of the long-range autocorrelations.

Long-range dependency has been found in most rhythmic motor signals. The origin of this property is unknown and largely debated. There is a controversy on the influence of voluntary control induced by requiring a pre-determined pace such as asking subjects to step to a metronome. We studied the cycle duration variability of 15 men pedaling on an ergometer at free pace and at an imposed pace (60 rpm). Revolution time was determined based on accelerometer signals (sample frequency 512 Hz). Revolution time variability was assessed by coefficient of variation (CV). The presence of long-range autocorrelations was based on scaling properties of the series variability (Hurst exponent) and the shape of the power spectral density (α exponent). Mean revolution time was significantly lower at freely chosen cadence, while values of CV were similar between both sessions. Long-range autocorrelations were highlighted in all series of cycling patterns. However, Hurst and α exponents were significantly lower at imposed cadence. This study demonstrates the presence of long-range autocorrelations during cycling and that voluntary intent can modulate the interdependency between consecutive cycles. Therefore, cycling may constitute a powerful paradigm to investigate the influence of central control mechanisms on the long-range interdependency characterizing rhythmic motor tasks.

A group of regulatory neurons in the spinal cord has been found to reduce sensory feedback to muscles in mice. Removal of these neurons leads to repetitive limb oscillations during reaching. See Article p.43 Humans and other animals execute limb movements with a seemingly effortless precision that relies on sensory feedback and its refinement by inhibitory microcircuits. A new study identifies presynaptic inhibition in the spinal cord, a regulatory filter mediated by Gad2-expressing GABAergic interneurons that form connections with the terminals of sensory afferents, as part of a hardwired gain control system crucial for the smooth execution of movement. Thomas Jessell and colleagues demonstrate that activation of Gad2-expressing neurons inhibits neurotransmitter release from sensory afferents. Selective ablation of these neurons in mice causes pronounced oscillations during goal-directed forelimb reaching movements, a behaviour captured by a model of sensory feedback at high gain.