THE VARIATION IN STRENGTH DECREMENT OF LOWER EXTREMITY MUSCLE GROUPS AND BIOMECHANICAL PLASTICITY IN OLDER ADULTS
Age-associated biomechanical plasticity (BP) has been established as the distal to proximal shift of joint mechanical output in old adults while walking. The cause of BP is still unknown, but changes in muscle strength of the lower extremities due to age are thought to be one of the underlying causes of BP. Old adults who had overall weaker lower extremities have been shown to have increased BP while walking on level and incline surfaces, however individual muscle groups have not yet been evaluated. We hypothesize that one causal factor of BP with age is that hip extensor muscles are more similar in strength in young and old adults than are ankle plantarflexor muscles, thus enabling old adults to walk with larger mechanical contributions from hip muscles as compensation for reduced contributions from ankle muscles. The purposes of the study were 1) compare muscle strengths of hip extensors and ankle plantarflexors between young and old adults, 2) verify BP in old adults by comparing hip and ankle joint torques and powers between age groups in level and incline walking & 3) examine the relationship between the relative strength in hip vs ankle muscles and the magnitude of BP in old adults during these gaits. 14 young (20yrs) and 22 old (76yrs) adults performed maximal isometric and isokinetic standing hip extension (20° of hip flexion) and seated ankle plantarflexion (15° of dorsiflexion). Age-based comparisons of muscle strength were made with 2X3 factor repeated measures ANOVAs, p<0.05. The same participants performed incline and level walking while ground reaction forces and 3D kinematics were obtained data. Walking joint torques and powers were calculated with inverse dynamics and were assessed using peak hip-to-ankle ratios with larger ratios indicating a larger shift of mechanical output to the hip. 2X2 factor repeated measures ANOVAs (p<0.05) for level and incline conditions were used to compare the torque and power ratios between age groups, with significant differences indicating BP. Pearson's correlations (p<0.05) were used to examine the relationship between walking power/torque ratios and the ratio of hip to ankle muscle strength in old adults. Old adults' hip extensor and ankle plantarflexor muscles were significantly weaker than young by 20% and 39%, (p<0.05). Old adults displayed a significant increase in hip/ankle ratios for both torque and power during level and incline conditions, indicating that the old adults exhibited BP (p<0.05). However, the correlations between ratios of hip and ankle isometric strength and hip/ankle peak torque and power were not significant for either level or incline walking. These findings suggest that there is a variation in strength decrement of individual lower extremities muscle groups due to age which may partially cause BP with age. However, we were unable to identify a relationship between the hip/ankle muscle strength ratio and BP, indicating the possibility of multiple factors contributing to BP.
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