Not all is lost: old adults retain flexibility in motor behaviour during sit-to-stand

Abstract

Sit-to-stand is a fundamental activity of daily living, which becomes increasingly difficult with advancing age. Due to severe loss of leg strength old adults are required to change the way they rise from a chair and maintain stability. Here we examine whether old compared to young adults differently prioritize task-important performance variables and whether there are age-related differences in the use of available motor flexibility. We applied the uncontrolled manifold analysis to decompose trial-to-trial variability in joint kinematics into variability that stabilizes and destabilizes task-important performance variables. Comparing the amount of variability stabilizing and destabilizing task-important variables enabled us to identify the variable of primary importance for the task. We measured maximal isometric voluntary force of three muscle groups in the right leg. Independent of age and muscle strength, old and young adults similarly prioritized stability of the ground reaction force vector during sit-to-stand. Old compared to young adults employed greater motor flexibility, stabilizing ground reaction forces during sit-to-sand. We concluded that freeing those degrees of freedom that stabilize task-important variables is a strategy used by the aging neuromuscular system to compensate for strength deficits.

Figure 1. Time normalized Joint position data in young and old participants.

The black line gives the mean, the dashed green line gives the standard deviation and the red line gives the standard error of the mean of the time normalized joint position data in degrees of the foot, ankle, knee, hip, trunk, neck, shoulder and elbow with the horizontal surface of the earth in young and old participants. Joint angles were calculated as depicted in figure 2 of Scholz et al 1999 . The vertical dotted black line indicates time where lift-off occurred.

Figure 2. V_RATIO for old and young participants.

V_RATIO of all performance variables for young and old participants in the anterior-posterior and vertical dimension of the CoM, head, GRF and linear momentum and for the angular momentum. V_RATIO of the CoM, head and GRF is given for lift-off (31–60%) and the extension phase (61–100%) of the chair rise. V_RATIO of the linear and angular momentum is given for lift-off, extension phase and the phases where greatest momentum change (0–50%; 30–80% and 10–80%) occurred. AP: anterior-posterior. V: vertical. LM: linear momentum. AM: angular momentum. Error bars represent standard error of the mean.

Figure 3. V_UCM and V_ORT for GRF in old and young participants.

Variance per degree of freedom for old and young participants in the vertical and anterior-posterior dimension of the GRF during lift-off (31–60%) and extension phase (61–100%) of the sit-to-stand task. DOF: degrees of freedom. V: vertical. AP: anterior-posterior. Error bars represent standard error of the mean.

Figure 4. Standard deviation of joint position data in old and young participants.

Across trial standard deviations for old and young adults of the foot, ankle, knee, hip, trunk, neck, shoulder and elbow joint in degrees during lift-off (31–60%) and the extension phase (61–100%) of the sit-to-stand task. Error bars represent standard error of the mean. Asterisk indicates significant difference between age groups (* = p<.05, ** = p<.01).