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.

The coordination of joint movements during sit-to-stand motion in old adults: the uncontrolled manifold analysis

“…Previous studies on STS using the UCM analysis have adopted the head position, hand position 6) , ground reaction force 11) , and linear and angular momentum of COM 10) as the performance variables. The STS is characterized by three essential components, namely, (a) generating sufficient joint torque needed to rise (progression), (b) ensuring stability by moving the COM from a large BOS to a narrow BOS defined solely by the feet (stability), and (c) the ability to modify movement strategies used to achieve these goals depending on environmental constraints ( adaptation ) 3 ) .…”

“…The first phase is required for simultaneous COM deceleration in the horizontal direction and COM acceleration in the vertical direction. Greve et al 11) reported that older adults employed greater motor flexibility, stabilizing both horizontal and vertical movement of COM during STS. However, Scholz et al 9 ) reported that joint movement showed no kinematic synergy stabilizing the vertical path of COM.…”

“…In a previous study, STS under challenging task constraints, such as the changes in the BOS under the feet, increased the good variance that does not affect the performance variable 9,10) . Greve et al 11) reported that older adults employed greater motor flexibility, stabilizing ground reaction forces, and the horizontal and vertical movement of COM during STS. In contrast, some studies reported that, due to the inability to exploit motor abundance, older adults used a less flexible multi-joint coordination pattern to achieve the task [12][13][14] .…”

Motor synergies research in physical therapy: advantages of the uncontrolled manifold approach

“…For example, during sit to stand, do joint motions organize synergistically to stabilize the center of mass, the center of pressure, or the position of the head? 23,24 . In other words, the method allows the investigation of which performance variable is being prioritized by the control mechanisms.…”

Comparing Different Methods to Create a Linear Model for Uncontrolled Manifold Analysis

“…The uncontrolled manifold (UCM) method is a well-established approach to assessing the coordination of multiple degrees of freedom (DoF) in synergies that stabilize performance in human actions. The method has been applied to a variety of actions, such as sit-to-stance, finger-force production, and goal-directed reaching (Black, Smith, Wu, & Ulrich, 2007;Domkin, Laczko, Djupsjöbacka, Jaric, & Latash, 2005;Greve, Zijlstra, Hortobágyi, & Bongers, 2013;Klous, linear model is created by entering the joint angles as independent variables and the index-finger position as the dependent variable. Their relative relationships are described in the regression equations (see Equation 1; a separate equation is used for each direction of the fingertip position).…”