Old adults preserve motor flexibility during rapid reaching

Abstract

Purpose: Our ability to flexibly coordinate the available degrees of freedom allows us to perform activities of daily living under various task constraints. Healthy old adults exhibit subclinical peripheral and central nervous system dysfunctions, possibly compromising the flexibility in inter-joint coordination during voluntary movements and the ability to adapt to varying task constraints.

Method: We examined how healthy old (75.4 ± 5.2 years, n = 14) compared with young adults (24.3 ± 2 years, n = 15) make use of the available motor flexibility to adapt to physical and dexterity constraints during a rapid goal-directed reaching task. We manipulated physical and dexterity demands by changing, respectively, external resistance and target size. Motor flexibility was quantified by an uncontrolled manifold (UCM) analysis.

Results: We found that healthy young and old adults employ similar motor flexibility as quantified by the ratio between goal equivalent and non-goal equivalent variability (V _Ratio) and were similarly able to adapt to increases in physical and dexterity demands during goal-directed rapid reaching (V _Ratio: p = .092; young: 0.548 ± 0.113; old: 0.264 ± 0.117). Age affected end-effector kinematics. Motor flexibility and end-effector kinematics did not correlate.

Conclusion: The data challenge the prevailing view that old age affects movement capabilities in general and provide specific evidence that healthy old adults preserve motor flexibility during a reaching task. Future studies applying UCM analysis should examine if experimental set-ups limit movement exploration, leaving possible age differences undetected.

Keywords: Ageing; Motor control; Motor flexibility; Reaching; Task demand; Uncontrolled manifold.

Fig. 1

Experimental set-up. Participants sat in an adjustable chair in front of a table so that the olecranon process with the elbow flexed at 90° was at tabletop height. The start posture was approximately 20° shoulder abduction, 90° elbow flexion and 90° pronation. To have a consistent start position, participants placed their right olecranon on an elbow support and the pointer tip in a pre-defined start position. The elbow support was positioned at the right side of the participants’ body at the same height with the table. The start position of the pointer tip was marked on the table with a dot of the size of the diameter of the pointer tip. During the start position the back of the pointer tool was placed against a wooden bar to release the load (Greve et al. 2015)

Fig. 2

Time normalized joint position data of the ID 4 and 0-kg condition for young and old participants. The blue line gives the mean, the dashed green line gives the mean of the within participant standard deviation and the red dashed line gives the standard error of the mean of the time normalized joint position data in degrees of the shoulder, elbow and wrist joint. The left panel gives the time normalized joint position data of the young and the right panel of the old participants. (Color figure online)

Fig. 3

GEV and NGEV (log transformed) averaged across age and dexterity constraints for both physical demand conditions. Vertical bars denote standard error of the mean

Fig. 4

Effective target width for young and old participants and both dexterity demand conditions averaged across physical constraints. Vertical bars denote standard error of the mean

Fig. 5

Effective target width for young and old participants and both physical demand conditions averaged across dexterity demand. Vertical bars denote standard error of the mean