Physical Demand but Not Dexterity Is Associated with Motor Flexibility during Rapid Reaching in Healthy Young Adults

Abstract

Healthy humans are able to place light and heavy objects in small and large target locations with remarkable accuracy. Here we examine how dexterity demand and physical demand affect flexibility in joint coordination and end-effector kinematics when healthy young adults perform an upper extremity reaching task. We manipulated dexterity demand by changing target size and physical demand by increasing external resistance to reaching. Uncontrolled manifold analysis was used to decompose variability in joint coordination patterns into variability stabilizing the end-effector and variability de-stabilizing the end-effector during reaching. Our results demonstrate a proportional increase in stabilizing and de-stabilizing variability without a change in the ratio of the two variability components as physical demands increase. We interpret this finding in the context of previous studies showing that sensorimotor noise increases with increasing physical demands. We propose that the larger de-stabilizing variability as a function of physical demand originated from larger sensorimotor noise in the neuromuscular system. The larger stabilizing variability with larger physical demands is a strategy employed by the neuromuscular system to counter the de-stabilizing variability so that performance stability is maintained. Our findings have practical implications for improving the effectiveness of movement therapy in a wide range of patient groups, maintaining upper extremity function in old adults, and for maximizing athletic performance.

Fig 1. Experimental set-up.

Fig 2. Time normalized joint position data.

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 ID 4 and 0 kg condition and the right panel of the ID 4 and 2 kg condition.

Fig 3. Log transformed V_Ratio (V_RatioT) averaged across ID and physical demand conditions for all phases of the reaching movement and at the end-point of reaching.

Vertical bars denote standard error of the mean.

Fig 4. GEV and NGEV averaged across ID and movement phases for the physical demand conditions.

Vertical bars denote standard error of the mean. Note that the statistical analysis was performed on the log transformed GEV (GEV_T) and NGEV (NGEV_T).

Fig 5. Log transformed V_Ratio (V_RatioT) averaged across ID and movement phases for the physical demand conditions.

Vertical bars denote standard error of the mean.

Fig 6. Log transformed V_Ratio (V_RatioT) and V_RatioPerm (V_RatioPermT) averaged across ID and physical demand conditions for all phases of the reaching movement and at the end-point of reaching.

Vertical bars denote standard error of the mean.

Fig 7. Log transformed V_Ratio (V_RatioT) and V_RatioPerm (V_RatioPermT) averaged across ID and movement phases for all physical demand conditions.

Vertical bars denote standard error of the mean.

Fig 8. Effects of dexterity demand on end-effector accuracy expressed as standard deviation error in mm.

* p =. 05. Vertical bars denote standard error of the mean.

Fig 9. Effects of physical demand on end-effector accuracy expressed as standard deviation error in mm.

* p =. 05. Vertical bars denote standard error of the mean.