Age-related differences in trunk muscle reflexive behaviors

Reports of larger passive and similar intrinsic trunk stiffness in older vs. younger populations suggest a diminishing demand for reflexive contributions of trunk muscles to spinal stability with aging. It remains unclear, though, whether such diminishing demands result in deterioration of trunk muscle reflexive behaviors. A cross-sectional study was completed to assess age-related differences in the latency and likelihood of trunk muscle reflexive responses to sudden perturbations. Sixty healthy individuals, aged 20-70 years, were recruited to form five equal-sized and gender-balanced age groups. Using a displacement-control, sudden perturbation paradigm, the latency and likelihood of trunk muscle reflexive responses to sudden perturbations were estimated, and the influences of age, gender, and level of effort (20% versus 30% of maximum voluntary exertion-MVE) were evaluated. There were no consistent age-related differences found in any of the measures of trunk muscle reflexive behavior. However, the latency of muscle response to perturbation was generally higher among older individuals, and this difference was significant in the condition involving 30% MVE effort. With an increase in level of effort (from 20% to 30% of MVE), there was a ~7% increase in the latency of trunk muscle responses to anteriorly-directed perturbations as well as ~ 15% (21%) decrease (increase) in response likelihood during anteriorly (posteriorly) directed perturbations. Furthermore, the reflexive response likelihood of trunk muscles was 28% (58%) larger (smaller) in female vs. male participants during anteriorly (posteriorly) directed perturbations. Our results did not, in general, support the hypothesis of an age-related decay in reflexive trunk muscle behaviors. Larger reflexive responses were associated with lower trunk intrinsic stiffness among females and during a lower level of effort, suggesting a secondary role for reflexive responses in spinal stability. Such secondary compensatory responses appear, however, to be consistent over a wide age range.