The effect of tibialis anterior weakness on foot drop and toe clearance in patients with facioscapulohumeral dystrophy.

BACKGROUND: Facioscapulohumeral dystrophy is a genetic disease characterized by progressive muscle weakness leading to a complex combination of postural instability, foot drop during swing and compensatory strategies during gait that have been related to an increased risk of falling. The aim is to assess the effect of tibialis anterior muscle weakness on foot drop and minimum toe clearance of patients with facioscapulohumeral dystrophy during gait. METHODS: Eight patients allocated to a subgroup depending on the severity of tibialis anterior muscle weakness, assessed by manual muscle testing (i.e., severe and mild weakness), and eight matched control participants underwent gait analysis at self-selected walking speeds. FINDINGS: Walking speed, for all facioscapulohumeral dystrophy patients, and step length, for patients with severe weakness only, were significantly decreased compared to control participants. Minimum toe clearance was similar across all groups, but its variability was increased only for patients with severe weakness. A greater foot drop was systematically observed for patients with severe weakness during swing and only in late swing for patients with mild weakness. Individual strategies to compensate for foot drop remain unclear and may depend on other muscle impairment variability. INTERPRETATION: Although all patients were able to control the average height of their foot trajectory during swing, patients with severe tibialis anterior muscle weakness exhibited increased foot drop and minimum toe clearance variability. Manual muscle testing is a simple, cheap and effective method to assess tibialis anterior muscle weakness and seems promising to identify facioscapulohumeral dystrophy patients with an increased risk of tripping.

Motor control of landing in an unsteady environment.

BACKGROUND: When landing from a jump or a drop, muscles contract before touchdown to anticipate imminent collision with the ground, soften ground contact and allow to return to a stable standing position without stepping or rebounding. RESEARCH QUESTION: This study assesses the effect of the unsteadiness of the environment on the motor control of landing. The 'unsteady environment' was induced by asking participants to perform drop landings inside an aircraft that underwent trajectories parallel to Earth's surface. The participants also performed the same task in a 'steady environment' in our laboratory. METHODS: Ground reaction forces, lower limb joints' movements and the activity of lower limb muscles were recorded. The stability of the landing was assessed by the vertical and anterior-posterior stability indexes, center of pressure measures and by the coefficient of variation of kinetic and kinematic parameters. RESULTS: On one hand, participants slowdown their joint movements and reduce the knee joint excursion during landing, probably to avoid excessive movements that may induce imbalance. On the other hand, the stability of the landing is reduced while the variability of the movement is increased, illustrating a less stable and less consistent landing. In addition, whatever the environment, landing parameters associated with increased stiffness (i.e., increased impact forces and decreased joint range of motion) are correlated with decreased landing stability. SIGNIFICANCE: Overall, landings in the'unsteady environment' appear to be more cautious but less stable and less finely tuned. Since the stability of the landing is not directly influenced by the steadiness of the environment, this more cautious behavior could be, at least in part, related to the fear/apprehension induced by sudden acceleration variations of the frame of the aircraft.

Human motor control of landing from a drop in simulated microgravity.

Landing on the ground on one's feet implies that the energy gained during the fall be dissipated. The aim of this study is to assess human motor control of landing in different conditions of fall initiation, simulated gravity, and sensory neural input. Six participants performed drop landings using a trapdoor system and landings from self-initiated counter-movement jumps in microgravity conditions simulated in a weightlessness environment by different pull-down forces of 1-, 0.6-, 0.4-, and 0.2 g External forces applied to the body, orientation of the lower limb segments, and muscular activity of 6 lower limb muscles were recorded synchronously. Our results show that 1) subjects are able to land and stabilize in all experimental conditions; 2) prelanding muscular activity is always present, emphasizing the capacity of the central nervous system to approximate the instant of touchdown; 3) the kinetics and muscular activity are adjusted to the amount of energy gained during the fall; 4) the control of landing seems less finely controlled in drop landings as suggested by higher impact forces and loading rates, plus lower mechanical work done during landing for a given amount of energy to be dissipated. In conclusion, humans seem able to adapt the control of landing according to the amount of energy to be dissipated in an environment where sensory information is altered, even under conditions of non-self-initiated falls.

Motor control of landing from a countermovement jump in simulated microgravity.

Landing from a jump implies proper positioning of the lower limb segments and the generation of an adequate muscular force to cope with the imminent collision with the ground. This study assesses how a hypogravitational environment affects the control of landing after a countermovement jump (CMJ). Eight participants performed submaximal CMJs on Earth (1-g condition) and in a weightlessness environment with simulated gravity conditions generated by a pull-down force (1-, 0.6-, 0.4-, and 0.2-g0 conditions). External forces applied to the body, movements of the lower limb segments, and muscular activity of six lower limb muscles were recorded. 1) All subjects were able to jump and stabilize their landing in all experimental conditions, except one subject in 0.2-g0 condition. 2) The mechanical behavior of lower limb muscles switches during landing from a stiff spring to a compliant spring associated with a damper. This is true whatever the environment, on Earth as well as in environments where sensory inputs are altered. 3) The motor control of landing in simulated 1 g0 reveals an increased "safety margin" strategy, illustrated by increased stiffness and damping coefficient compared with landing on Earth. 4) The motor command is adjusted to the task constraints: muscular activity of lower limb extensors and flexors, stiffness and damping coefficient decrease according to the decreased gravity level. Our results show that even if in daily living gravity can be perceived as a constant factor, subjects can cope with altered sensory signals, taking advantage of the remaining information (visual and/or decreased proprioceptive inputs).