Variant and invariant characteristics of the sit-to-stand task in healthy elderly adults

The purpose of this study was to identify kinetic, kinematic, and electromyographic (EMG) characteristics during sit-to-stand (STS) in healthy elderly subjects that were affected by changes in movement speed or initial starting position. Eight healthy elderly adults performed the STS movement at natural and fast speeds. Their ankles were placed in either 5 degrees or 18 degrees of dorsiflexion prior to the movement. A motion analysis system, single force plate, and EMG equipment were used to collect data on ankle, knee, and hip movement, vertical ground reaction forces, vertical and horizontal velocity of the head, and EMG activity from selected muscles. The data were further described according to three specific phases of the STS movement. Two-way analyses of variance for repeated measures were used to determine which of the kinematic, kinetic, and timing variables were affected by changing the speed of the movement or the initial ankle position. When asked to stand as fast as possible, muscle onsets and movement phases were shortened. However, when expressed as a percent of total movement time, normalized extensor muscle onsets were found to be earlier during fast movement of the task, whereas the other variables were not different. Changing the initial ankle position affected muscle onsets, duration of movement phases, and joint excursion. We conclude from these data that many of the characteristics of the sit-to-stand movement are speed invariant and therefore the STS movement is for the most part programmed.