Muscle pre- and coactivity during downward stepping are associated with leg stiffness in aging

We have previously reported that elderly compared to young women executed d ownward stepping with substantially greater leg stiffness. Because antagoni st muscle coactivity increases joint stiffness we hypothesized that increas ed leg stiffness in aging is associated with increased muscle coactivity. W e also explored the possibility that the magnitude of the preparatory muscl e activity preceding impact also differed between young and old subjects. Y oung (n=11, 20.8 yr) and old (n=12, 69 yr) women performed downward steppin g from a platform set at 20% body height. The leg was modeled as a simple m ass-spring system. From video and ground reaction force data leg stiffness was computed as the ratio of force under the foot and the linear shortening of the limb. EMG activity of the vastus lateralis, biceps femoris, gastroc nemius lateralis, and tibialis anterior were recorded with a telemetric sys tem. Elders compared to young subjects had 64% greater leg stiffness during downward stepping. Muscle activity over a 200-ms period preceding touch do wn was 136% greater in elderly than in young subjects. Biceps femoris and t ibialis anterior coactivity during ground contact was 120% greater in the e lders. Muscle pre- and coactivity, respectively, accounted for about 50% of the variance in leg stiffness. In conclusion, elderly people elevate muscl e pre- and coactivity during downward stepping to stiffen the leg in compen sation for impaired neuromotor functions. (C) 2000 Elsevier Science Ltd. Al l rights reserved.