[Three-dimensional analysis of variations of the forces associated with the climbing task in adolescents]

The efficient performance of voluntary movements requires a precise coordination between posture and movement. Anticipatory postural adjustments are believed to minimize the perturbation of balance triggered by the movement. This is particularly important in rock climbing where the unstable posture requires a fine balance control to preserve equilibrium. The aim of this study was to determine the spatial and temporal characteristics of the postural adjustments and movement in a climbing task for ten right-handed beginner children (5 boys and 5 girls) aged 10 to 15 years. A climbing ergometer was used for the experiment. The apparatus consists of four plates on which artificial climbing holds are secured. Using three-dimensional strain gauges we analyzed the three components of the forces applied at each hold. The signals were sampled at 100 Hz amplified and recorded on a personal computer. The climbing frame was adjusted to the anthropometry of each child. The task was to move one foot from one hold to another, and to actually climb onto the latter. Three levels of difficulty, corresponding to 1/3, 2/3, and 3/3 of shank length were tested. Subjects performed 5 trials with each foot at each level. The effect of three variables were analyzed: gender, laterality (right or left moving foot) and task difficulty. The force distribution was analyzed over a 400 ms period of steady state before the onset of movement. For all subjects and for all conditions, a greater force was applied to the right foot for the vertical forces and on the left foot for the lateral forces. In other words the subjects were standing over their right foot instead of exactly spreading their weight between the two legs. The onset of movement was always preceeded by a reproducible sequence of anticipatory postural adjustments involving the three supporting segments. This sequence was the same for the three components and started with the controlateral hand followed by the homolateral hand and then the controlateral foot. On the hand holds, the anticipation increased with the difficulty of the task, whereas movement time remained constant across all experimental conditions. The effect of task difficulty on the latencies of the anticipatory postural adjustments was more pronounced for the female subjects. The analysis of the load variations occurring during the movement along the three axes showed that the weight was transferred mainly to the controlateral foot but also to the hands. However the homolateral hand was never very involved. On the vertical component, there was a main effect of laterality. For a left foot movement, the force variation on the right controlateral side was more important than for a right foot deplacement. In addition there was a main effect of gender showing that the boys "pulled" more on their hands than the girls. This effect was stronger for a right foot movement. Compared with adults and age-matched boys, the girls seemed to use a more lateral, an expert-like, strategy.