Fatigue effects on body balance

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Abstract

Body sway variables (sway area and sway path) were recorded by a dynamometric platform in 13 young subjects, standing quiet with feet together, with eyes open (EO) and eyes closed (EC), prior to and following two types of physical exercise (treadmill walking and cycle ergometer pedalling). Each exercise was performed under both fatiguing (above anaerobic threshold) and non-fatiguing conditions. Following fatiguing treadmill exercise, we observed a significant increase in body sway with respect to pre-exercise values. The increase was present under both visual conditions, affected both sway area and sway path and lasted until about 15 min from the end of the exercise. The Romberg quotient (the ratio of EC/EO of sway area, or sway path) significantly increased after the fatiguing exercise with respect to the non-fatiguing exercise. The mean position of the centre of foot pressure (CFP) was unchanged after the exercise. Fatigue induced an increase in the median frequency of oscillation of the centre of foot pressure, independent of the amplitude of sway. Non-fatiguing treadmill exercise induced no significant changes in sway or in its frequency content. Following fatiguing cycle ergometer exercise, a negligible increase or a decrease (under eyes closed condition) in body sway were observed. Non-fatiguing cycling exercise induced no significant changes or a decrease in sway. Control experiments showed that simple repetition of successive stance trials (without intercalated exercise) was able by itself to induce a decrease in sway. By taking this effect into account, both types of cycling exercises revealed a mild capacity to increase sway. We concluded that body sway increased after strenuous physical exercise, but was little affected by exercise performed below the estimated anaerobic threshold. The effects of fatigue on sway were short-lasting and of moderate extent, and therefore were not liable to seriously threaten body equilibrium.

Introduction

The mechanical characteristics of the muscles are not sufficient to compensate for the torques produced by external perturbations leading to stance destabilization. Therefore, balance is actively controlled by the central nervous system (CNS), which calls into action the various relevant postural muscles as and when needed (Nardone et al., 1990). During quiet stance, little activity on the part of the postural muscles is normally required (Schieppati et al., 1994), thanks to the ability of the CNS to anticipate minimal body destabilization. In turn, the CNS relies on proprioceptive, visual and vestibular input. Accordingly, when any of these inputs is defective, body sway increases and muscle activity increases concurrently, in order to maintain balance (see Dietz, 1992).

Apart from the role of the possible alterations in the sensory input, little information exists as to other causes that may affect the control of body balance under specific physiological conditions. In particular, it is not known how fatigue, which is typically an internal perturbation, may alter this function. It was therefore deemed worthwhile to address this issue, all the more so since fatigue is a phenomenon caused by a combination of processes which occur at different sites (Balestra et al., 1992; Sharpe and Miles, 1995; see Enoka and Stuart, 1992, and Gandevia et al., 1995), and is therefore likely to affect a complex task such as body balance in several ways.

With this aim, we compared common indicators of body sway (sway area (SA) and sway path (SP)) prior to and after two types of exercises (treadmill walking and cycle ergometer pedalling), each performed both below and above the threshold of fatigue. It was found that strenuous exercise does indeed affect body balance during maintenance of quiet upright stance, but the consequent increase in sway is short-lasting and of moderate extent. Preliminary results have been presented in abstract form (Schieppati et al., 1996).

Section snippets

Subjects

A total of 13 young healthy subjects were selected for testing (six males and seven females, aged 18–39 years). All gave their informed consent, although the specific aims of the experiments were not conveyed. The local ethics committee approved the use of the experimental procedure. Each subject attended two experimental sessions on different days (three of the subjects attended four sessions on separate days).

Balance assessment

The three components of the force acting on each of the four transducers of a

Fatiguing exercise

All subjects crossed the 60% value of the maximal heart rate in the first part of the exercise (6 min±1.0 SEM). Then the exercise lasted for 19.0±1.0 min (Fig. 1A). The final power output produced by the subjects at the end of fatiguing exercise was, on average, 122.8±13.8 W (Fig. 1B). The final heart rate attained was 93.0±3.1% of the maximum heart rate, i.e. well above the estimated AT (see Section 2) (Fig. 2A). All subjects perceived the exercise either as hard or very hard. On average, the

Discussion

Physical exercise produced various effects on body sway variables in young normal subjects during quiet upright stance. The effects depended on the type of exercise (treadmill walking as opposed to cycling), on the intensity of the work (above or below the estimated anaerobic threshold), and on the visual conditions. All effects appeared immediately after the exercise and had a short duration, vanishing by about 15 min after the end of the exercise.

Treadmill walking produced transient

Acknowledgements

The technical support of Dr Giuseppe Valmadre in preliminary experiments is gratefully acknowledged. Rosemary Alpress scrutinized the English. This study was supported in part by the grants Ricerca Corrente 1996/1997 and Ricerca Finalizzata 1993 from the Ministero della Sanità, Rome, Italy.

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