Revisiting the use of Hoffmann reflex in motor control research on humans

Research in movement science aims at unravelling mechanisms and designing methods for restoring and maximizing human functional capacity, and many techniques provide access to neural adjustments (acute changes) or long-term adaptations (chronic changes) underlying changes in movement capabilities. First described by Paul Hoffmann over a century ago, when an electrical stimulus is applied to a peripheral nerve, this causes action potentials in afferent axons, primarily the Ia afferents of the muscle spindles, which recruit homonymous motor neurons, thereby causing an electromyographic response known as the Hoffmann (H) reflex. This technique is a valuable tool in the study of the neuromuscular function in humans and has provided relevant information in the neural control of movement. The large use of the H reflex in motor control research on humans relies in part to its relative simplicity. However, such simplicity masks subtleties that require rigorous experimental protocols and careful data interpretation. After highlighting basic properties and methodological aspects that should be considered for the correct use of the H-reflex technique, this brief narrative review discusses the purpose of the H reflex and emphasizes its use as a tool to assess the effectiveness of Ia afferents in discharging motor neurones. The review also aims to reconsider the link between H-reflex modulation and Ia presynaptic inhibition, the use of the H-reflex technique in motor control studies, and the effects of ageing. These aspects are summarized as recommendations for the use of the H reflex in motor control research on humans.

Age-dependent adaptations to anticipated and non-anticipated perturbations after balance training in children

Postural control undergoes rapid changes during child development. However, the influence of balance training (BT) on the compensation of perturbations has not yet been investigated in children. For this purpose, young (6.7 ± 0.6 years) and old children (12.0 ± 0.4 years) were exposed to externally induced anticipated (direction known) and non-anticipated (direction unknown) perturbations on a free swinging platform before and after either child-oriented BT (INT; young: n = 12, old: n = 18) or regular physical education (CON; young: n = 9, old: n = 9). At baseline, old children exhibited less platform sway after perturbations than young children (p = .004; η²_p = 0.17). However, no differences were found between anticipated and non-anticipated perturbations. After training, INT reduced the platform sway path while CON remained stable (-11.1% vs. +2.7%; p < .001; η²_p = 0.26). Furthermore, the young INT group adapted statistically similarly in anticipated and non-anticipated situations (-7.9% vs. -12.6%; p = .556; r = 0.33), whereas the old INT group tended to improve more in anticipated perturbations (-15.1% vs. -8.2%; p = .052; r = 0.51). Thus, the maturity of the postural system seems to influence the extent of training adaptations in anticipated perturbations. Furthermore, this study provides evidence that BT can improve postural responses to external perturbations in children and may represent a useful intervention to prevent falls.

Medial Axis and Singularities

This paper is devoted to the study of the medial axes of sets definable in polynomially bounded o-minimal structures, i.e. the sets of points with more than one closest point with respect to the Euclidean distance. Our point of view is that of singularity theory. While trying to make the paper self-contained, we gather here also a large bunch of basic results. Our main interest, however, goes to the characterization of those singular points of a definable, closed set X ⊂ R n , which are reached by the medial axis.

The effects of predictability on inter-limb postural synchronization prior to bouts of postural instability

Anticipatory balance control optimizes balance reactions to postural perturbations. Predictive control is dependent on the ability of the central nervous system to modulate gain in accordance with specific task demands. Inter-limb synchronization is a sensitive measure of individual limb contributions to balance control and may reflect the coordination of gain modulation in preparation for instability. The purpose of the study was to determine whether gain modulation in advance of predictable bouts of instability was reflected in the extent of inter-limb synchronization. Two adjacent force plates were used to collect center of pressure (COP) data from 12 healthy young adults (27.5±3.4 years). Participants prepared for internal and external balance perturbations using a cueing paradigm with three auditory warning tones followed by an imperative tone. Perturbations were delivered in blocked and randomized conditions with two perturbation magnitudes (small and large). Inter-limb synchrony was calculated using the cross-correlation function of the COP excursions from the left and right foot for three seconds prior to perturbation onset in the anteroposterior (AP) and mediolateral (ML) direction. Inter-limb synchrony decreased in the AP and ML directions as perturbation magnitude became more unpredictable. The need to take a step or not knowing whether a step was required prior to postural instability reduced ML inter-limb synchrony. No differences were found between internal and external perturbations. Modulation of postural set was evident in the extent of inter-limb synchrony.

The role of the cerebral cortex in postural responses to externally induced perturbations

The ease with which we avoid falling down belies a highly sophisticated and distributed neural network for controlling reactions to maintain upright balance. Although historically these reactions were considered within the sub cortical domain, mounting evidence reveals a distributed network for postural control including a potentially important role for the cerebral cortex. Support for this cortical role comes from direct measurement associated with moments of induced instability as well as indirect links between cognitive task performance and balance recovery. The cerebral cortex appears to be directly involved in the control of rapid balance reactions but also setting the central nervous system in advance to optimize balance recovery reactions even when a future threat to stability is unexpected. In this review the growing body of evidence that now firmly supports a cortical role in the postural responses to externally induced perturbations is presented. Moreover, an updated framework is advanced to help understand how cortical contributions may influence our resistance to falls and on what timescale. The implications for future studies into the neural control of balance are discussed.