Abstract

Background & Aim: Reaching is a daily occurrence requiring precise control of the arm, body and limbs to be successfully achieved. Generally, the volitional arm movement is preceded by postural muscle activity in the lower limb and trunk which create movement of the body and center of mass (CoM) towards the target. We have previously shown that in young persons these preparatory postural adjustments (or pPAs) are tuned to the direction of reach, ensuring a smooth transition from initial to final reach postures. However, the extent to which sensorimotor declines with aging affect the ability to produce tuned, direction-specific postural activity is currently unknown. As such, this study characterized muscle activity produced in the lower limb during goal-directed reaching movements while standing. Our aim was to identify the spatial tuning of such muscles in preparation for movement onset and whether muscle activation patterns prioritized balance control (and CoM stability ) over goal-directed movement generation. Methods: Ten right-handed older participants (6 males, Age: 68(6) years; Height: 1.70(0.58) m; Mass: 71(6.75) kg) stood in the center of a 180-degree semi-circular array of 13 light targets spaced at 15 degree intervals. Participants were asked to reach and point to the illuminated target at a natural speed. Focal muscle activity from the anterior and posterior deltiod (AD and PD) was recorded alongside bilateral lower limb muscles using surface electromyography. These included: tibialis anterior (TA), lateral gastrocnemius (GL), peroneus longus (PL), tensor fascia latae (TFL), vastus lateralis (VL), biceps femoris (BF) and gluteus medius (GM). Movement onset and termination were determined when finger tangential velocity exceeded (or reduced to) 3% of its peak during reaching. Finger kinematics including peak velocity, acceleration and deceleration phases were calculated. Postural adjustments were quantified during the 250 ms prior to movement onset (i.e. pPA period). Results: Finger kinematics showed a clear directional influence with movements to left-sided, contralateral targets (especially beyond 135 degrees) resulting in significantly lower peak velocities (p<0.001) and longer acceleration phases (p<0.001). Distinct muscle tuning was also seen for the lower limb and could be grouped spatially. Generally, activity biased muscles on the opposite side to the target. For example, right-sided targets (0-30 degrees) were characterised by activity from PD, left PF, left GM and bilateral GL with left-sided targets (105-180 degrees) showing preference to the right TA, PL, GM and BF. Conclusion: Considering the changes to finger kinematics and the functional roles of the spatially organised lower limb muscles pPAs may not continue to produce the necessary dynamics for movement initiation in a healthy aging cohort. Rather, they may act in a more traditional role of CoM stability, especially when the arm is required to cross the midline of the body.