Coordination between posture and movement: interaction between postural and accuracy constraints

Abstract

We examined the interaction between the control of posture and an aiming movement. Balance control was varied by having subjects aim at a target from a seated or a standing position. The aiming difficulty was varied using a Fitts'-like paradigm (movement amplitude=30 cm; target widths=0.5, 1.0, 2.5 and 5 cm). For both postural conditions, all targets were within the reaching space in front of the subjects and kept at a fixed relative position with respect to the subjects' body. Hence, for a given target size, the aiming was differentiated only by the postural context (seated vs. upright standing). For both postural conditions, movement time (MT) followed the well-known Fitts' law, that is, it increased with a decreasing target size. For the smallest target width, however, the increased MT was greater when subjects were standing than when they were seated suggesting that the difficulty of the aiming task could not be determined solely by the target size. When standing, a coordination between the trunk and the arm was observed. Also, as the target size decreased, the center of pressure (CP) displacement increased without any increase in CP speed suggesting that the subjects were regulating their CP to provide a controlled referential to assist the hand movement. When seated, the CP kinematics was scaled with the hand movement kinematics. Increasing the index of difficulty led to a strong correlation between the hand speed and CP displacement and speed. The complex organization between posture and movement was revealed only by examining the specific interactions between speed-accuracy and postural constraints.

Influence of obesity on accurate and rapid arm movement performed from a standing posture

“…When normal BMI subjects aim at targets of various difficulties, MTs are faster when they perform the task from a seated than from a standing posture. 20 In a different study, Savelsbergh et al 21 observed that children were able to catch more balls in a seated compared to a standing condition. 21,22 These experiments clearly suggest that a stable or controlled platform allows to improve the speed-accuracy characteristics of upperlimb movements.…”

“…21,22 These experiments clearly suggest that a stable or controlled platform allows to improve the speed-accuracy characteristics of upperlimb movements. [20][21][22] An alternative explanation for the increased MT relates to the mechanical consequence of the greater inertial load of the arm and forearm of the obese subjects. According to this hypothesis, moving heavier segments would require more force, and this could lead to a longer MT.…”

“…For instance, when aiming or reaching for an object from an upright standing position, proper balance control offers a stable or controlled platform for the goal-directed movement. [20][21][22] When aiming for a target as fast as possible, the speed and accuracy of a movement are two constraints that are used to define the motor performance. The relationship between these two variables has been formalized as Fitts' law.…”

Childhood obesity affects postural control and aiming performance during an upper limb movement

“…When aiming for a target as fast as possible, the speed and accuracy of a movement are two constraints that are used to define the motor performance. The relationship between these two variables has been formalized as Fitts' law [1,15,16]. It states that movement time (MT) is a linear function of the index of difficulty (ID = Log2 [2A/W]), where A is the amplitude of the movement and W the width of the target (accuracy constraint).…”

“…1) was assessed using a Fitts-like paradigm [1,15,16]. When aiming for a target as fast as possible, the speed and accuracy of a movement are two constraints that are used to define the motor performance.…”