Momentum Analysis of Sitback Failures in Sit-to-Stand Trials

We characterize successful sit-to-stand maneuvers and compare these to sitback and step failures, using whole body (WB) and upper body (HAT) linear and angular momentum parameters, Controls consisted of 11 normal elders. Six failed sit-to-stand maneuvers in which sitbacks occurred were extracted fi-om archived tests of patients, primarily patients with vestibular pathology, Whole body (WJ3) momentum and upper body or HAT momentum parameters were calculated. Sitback failures were: associated with lower HAT AP linear momentum at the time of maximum ground reaction force (GRF); i.e., just after liftoff. The sitback failures were also associated with lower knee torques at the time of maximum GRF and lower values of the maximum GRF. Thus, sitback failures appear to be due lack of adequate momentum and torque generation. These findings improve our understanding of the biomechanics of sit-tostand and the mechanism of lloss of balance during this demanding activity. -n Rising fi-om a chair is II common but demanding activity of daily living requiring coordination, balance, and adequate mobility and strength. Our studies and those of Pai and Rogers have focused on the significant role of upper body momentum (Pai and Rogers 1990; Pai and Rogers 1991; Riley, et al. 1991; Schenkman and Riley 1991; Pai, et al. 1994). We have observed numerous transient loss of balance incidents during sit-to-&“. We define two failure modes, “sitback” and “step” failures. “Sitback” failures, analyzed herein, occur when the subjects rise off the chair and then sit back down without reaching the standing position. We hypothesize that sitback failures will be preceded by inadequate momentum generation, thus confirming the role of momentrun control in sit-to-stand and dynamic balance control in gent:rnl. Methods Control subjects consisted of 11 elders selected at random from Medicare lists in the Boston area. The failed sit-to-stand maneuvers were extracted from our files in which various patient groups are represented; most were patients with vestibular dysfunction. All subjects provided informed consent in accordance with hlospital approved procedures. Six sitback failures in 6 subjects were analyzed. The selected trials were attempts to rise from a chair height of 100% knee height (kh) or greater at the subject’s self-selected pace with eyes open. Kinematic and kinetic data were acquired with a four camera SELSPOT IIYTRACK@ (Antonsson and Mann, 1991) data acquisition system sampling at 150hz. The kinematic and kinetic data and subject specific anthropometric data were incorporated into our 1 1-segment whole body model (Riley, et al. 1990; Hutchinson, et al. 1994) to estimate the body segment dynamics. Subjects rose from an armless, backless chaii with arms folded and held next to the trunk. The feet were set one on each force plate, parallel and lOcm apart in 18” ankle dorsiflexion. The subjects were directed not to move their feet during the sit-to-stand maneuver and to stand as still as possible once fully erect. Chair height was 100% of the subject’s knee height (kh) or greater, up to 120% kh, and the subjects rose at their chosen speed. A sit-to-stand trial was classified as a sitback failure if the buttocks contacted the chair again after initial liftoff. Failures were determined by visual observation at the time of trial and were confirmed by detailed analysis of the kinematic and force plate data. Whole body linear momentum and angular momentum about the body CG were evaluated.. Momentum parameters were also calculated for the upper body or HAT consisting of the head, trunk and pelvic segments and both arm segments. HAT angular momentum was evaluated about a point midway between the left and right hip joints. Both linear and angular momenta were normalized to subject mass (kg). Ttests with a 95% confidence level were used to evaluate the significance of inter group differences. Results