Acsm Abstract Zach | 2006

TASK-DEPENDENT MODULATION OF AFFERENT PATHWAYS BETWEEN ELBOW FLEXOR MUSCLES. ZA Riley, BK Barry, MA Pascoe, and RM Enoka. Dept. of Integrative Physiology, University of Colorado, Boulder, CO USA. The relative activation of the elbow flexor muscles varies with changes in forearm posture. Presumably, the inhibitory afferent projection from the branch of the radial nerve that innervates brachioradialis to the biceps brachii motor neuron pool aids in the control of the supination action of the biceps brachii. PURPOSE: To determine whether the amount of inhibition of the biceps brachii muscle is modulated when changing forearm postures from neutral to pronation or supination. METHODS: The pathway was investigated using the spike-triggered stimulation technique. Single motor units in both the long and short heads of biceps brachii were examined by recording from the same motor unit during trials in 3 forearm postures: neutral, 45° supination, and 45° pronation. Peripheral nerve stimulation was delivered to the brachioradialis branch of the radial nerve at an intensity of 0.9x motor threshold for 0.5 ms duration. Subjects produced small elbow flexion forces (3.3 ± 0.5 %MVC) and maintained an average discharge rate of 11.5 ± 1.0 pps for the active motor unit. Stimuli were triggered at a delay of 30 ms following motor unit discharge, every 2-3 seconds for an average of 82 ± 26 stimulations. Inhibition was quantified as the percent change from the pre- and post stimulus interspike intervals as well as from X² statistics from the post-stimulus time histograms. RESULTS: Ten motor units (5 long head, 5 short head) were discriminated for all 3 postures. Significant differences were observed in 12 / 28 histograms, and an overall prolongation of the interspike interval corresponding to a decrease in discharge rate of the biceps brachii motor unit when the radial nerve was stimulated (P < 0.001). Preliminary results indicate that the magnitude of this inhibition increased from supination (1.81 %, P = 0.082) to neutral (3.31 %, P = 0.118) to pronation (6.47 %, P = 0.004). CONCLUSION: Stimulation of the radial nerve branch that innervates the brachioradialis produced an inhibitory effect on the discharge of single motor units in biceps brachii in each of the forearm postures, with the greatest inhibition occurring when the forearm was pronated. Supported by NINDS R01 NS43275 to RME.