Effect of single-limb inertial loading on bilateral reaching: Interlimb interactions

This study employed the paradigm of asymmetric limb loading during bilatera l arm reaching to examine the motor system's ability to independently organ ize the discrete movement of both upper limbs to equidistant targets when o ne of the limbs is loaded under specific timing constraints. The loading pr ocedure involved attaching two different Velcro strapped weights to the rig ht wrist, thus increasing the right arm's mass by 25% (1 kg) and 50% (2 kg) . Movements were captured by a high-speed digital camera (240 Hz), while el ectromyographic (EMG) activity of selected elbow and shoulder muscles of bo th limbs was recorded (1,000 Hz) simultaneously. The results revealed that the mechanisms used by the system to compensate for unilateral limb loading were as follows: First, addition of an inertial load resulted in an increa sed movement time and concomitant decrease in peak velocity of both the upp er arm and forearm of only the loaded limb and was scaled to the added weig ht. Second, for the EMG parameters, adjustments to the inertial load were p rimarily characterized by an increase in burst duration of all muscles, wit h load-specific changes in activity and onset time: the elbow antagonist (b iceps) demonstrated a decrease in activity with the 50% load, and the elbow agonist (triceps) had an earlier onset with the 25% load. Concomitant adju stments on the unloaded limb consisted primarily of an increase in burst du ration of the shoulder and elbow agonists (pectoralis and triceps), an earl ier triceps onset solely with the 25% load, and a decrease in activity of t he biceps solely with the 50% load. Third, with the exception of biceps act ivity, the amplitude of EMG activity was invariant across changes in load f or both the loaded and unloaded limb. This lack of modulation in activity m ay have been related to the inability of performers to meet the time constr aint of simultaneous bilateral limb arrival to the end targets. This inabil ity can be the result of an active strategy selection process to safeguard the actions against interference or alternatively it could simply be a cons equence of the biomechanical properties of the system in relation to task c onstraints. These issues are discussed in the light of the present findings and those of previous studies.