Axial reflexes are present in older subjects and may contribute to balance responses

We studied the response to axial taps (mini-perturbations) of a group of 13 healthy older subjects (mean age 63 ± 12 years, 7 females, 6 males), 12 of whom were also studied using larger applied (macro-) perturbations requiring active postural responses. The mini-perturbation consisted of a brief impulsive force produced by a mini-shaker applied to the trunk at the level of the shoulders and anteriorly at the upper sternum which was perceived as a tap. Acceleration, force platform, and EMG measurements were made. The average peak accelerations for the mini-perturbations were 108 mG (anterior) and - 78.9 mG (posterior). Responses overall were very similar to those previously reported for younger subjects: the perturbation evoked short latency responses in leg muscles, modulated by degree and direction of lean, and were largest for the muscle most relevant for the postural correction. The increases in the amplitude for the main agonist were greater than the increase in tonic activity. With both anterior and posterior lean, co-contraction responses were present. The size of the EMG response to the mini-perturbations correlated with the corresponding earliest EMG responses (0-100, 100-200 ms intervals) to the larger postural perturbations, timing which corresponds to balance responses. The balance responses evoked by the larger imposed postural perturbations may, therefore, receive a contribution through the reflex pathway mediating the axial tap responses, whose efferent limb appears to be the reticulospinal tract.