Electromyographic analysis of postural adjustments in two methods of balance testing.

The primary purpose of this study was to compare postural responses during two methods of balance testing: 1) a platform perturbation test (PP), with destabilizing forces of uniform magnitude given at the base of support, and 2) a postural stress test (PST), with destabilizing forces of increasing magnitude given at the waist. A secondary purpose was to examine the relationship between balance strategy scores and the patterns of muscle responses as determined by electromyographic analysis on the PST. Postural responses to backward sway of 17 healthy subjects, aged 60 to 79 years (means = 69.0, s = 5.3), were characterized bilaterally by EMG recording of the tibialis anterior, quadriceps femoris, gastrocnemius, and hamstring muscles. We examined three measures of bilateral postural responses: 1) latency to first muscle response (FR) among the four lower extremity muscle groups, 2) tibialis anterior muscle latency (TA), and 3) patterns of muscle responses. Our results demonstrated significantly shorter (p less than .001) TAs and FRs (in milliseconds) on the PST than on the PP (PST TA: = means = 80.3, s = 12.4; PP TA: = means = 142.6, s = 13.4; PST FR: means = 75.9, s = 10.4; PP FR: = means = 140.4, s = 13.6). The ankle strategy was the most frequently used pattern of muscle response during both the PP and the PST, but it occurred less frequently on the PST than on the PP (PP = 72.5%, PST = 50.9%) (p less than .05). No consistent relationship was observed between the balance strategy scores as determined by videotape analysis of the PST and the patterns of muscle responses as determined by EMG analysis.