CONTEXT-DEPENDENT REFLEX CONTROL - SOME INSIGHTS INTO THE ROLE OF BALANCE

Recent research suggests that the balance requirements of a task dicta te the reflexive response. However, these observations were inferred i ndirectly from either different tasks or different phases of the same task. This study directly tested the hypothesis of balance-dependent c ontrol during recovery from an unexpected trip. The subjects were trip ped in two different support conditions: unilimb support (provided by the stance limb) or trilimb support (provided by the stance limb and b oth arms placed on adjacent parallel bars). The subjects exhibited ant icipatory changes: they biased the body center of mass toward the supp ort limb in the mediolateral direction and elevated the swing limb hig her when there was a possibility of being tripped. The electromyograph ic (EMG) latencies were not influenced by the threat to equilibrium. T he magnitudes of the EMG reflexive response to the trip were clearly m odulated as a function of the threat to stability, not in a simple man ner, but rather in a complex manner, which optimized the recovery stra tegy. It is evident that the overriding concern, equilibrium control d uring locomotion, has a dominant influence on reflex modulation.