Context-dependent reflex control: some insights into the role of balance.

Recent research suggests that the balance requirements of a task dictate the reflexive response. However, these observations were inferred indirectly from either different tasks or different phases of the same task. This study directly tested the hypothesis of balance-dependent control during recovery from an unexpected trip. The subjects were tripped in two different support conditions: unilimb support (provided by the stance limb) or trilimb support (provided by the stance limb and both arms placed on adjacent parallel bars). The subjects exhibited anticipatory changes: they biased the body center of mass toward the support limb in the mediolateral direction and elevated the swing limb higher when there was a possibility of being tripped. The electromyographic (EMG) latencies were not influenced by the threat to equilibrium. The magnitudes of the EMG reflexive response to the trip were clearly modulated as a function of the threat to stability, not in a simple manner, but rather in a complex manner, which optimized the recovery strategy. It is evident that the overriding concern, equilibrium control during locomotion, has a dominant influence on reflex modulation.