HETERONYMOUS REFLEXES IN THE PRIMARY AGONIST ARE ENHANCED WHEN SUPPORTING AN INERTIAL LOAD.

Abstract

Purpose/Hypothesis: Previous studies indicate that stretch reflexes in the human upper limb are influenced by the stability of the load supported during an isometric contraction. Our purpose was to assess whether changes in the sensitivity of spinal motor neurons to afferent feedback from accessory muscles contribute to reflex modulation when supporting an inertial load compared with exerting an equivalent force with the limb restrained. Number of Subjects: 13 healthy adults (aged 18–47 years, 9 men) participated in the study. Materials/Methods: Heteronymous reflexes were evoked in the first dorsal interosseus (agonist) and second palmar interosseus (antagonist) muscles by stimulating the median nerve at the wrist (1 ms pulse width; 3–5 s inter-stimulus interval) as subjects performed two motor tasks with the index finger. One task required subjects to exert a constant isometric force by pushing up against a rigid restraint, whereas the other task required subjects to maintain a constant angle at the metacarpophalangeal joint while supporting an inertial load suspended from the finger. Net torque (20% of maximum), the position of the index finger (0 degrees abduction), and the intensity of peripheral nerve stimulation were the same for both tasks. Twenty-four reflex responses were recorded in each muscle using surface or intramuscular electrodes, and the averaged EMG records were compared using a 2-factor ANOVA for repeated measures. Results: Heteronymous reflex responses were observed in 10/13 subjects (77%) for the first dorsal interosseus, but only 2/11 subjects (18%) in the second palmar interosseus. Despite similar tonic EMG of the agonist and antagonist muscles across tasks (P = 0.27), peak amplitudes of the short-latency (SL 28.6 ± 5.6 ms) and long-latency (LL 52.2 ± 3.5 ms) reflexes in the first dorsal interosseus were greater when subjects supported the inertial load (SL = 0.45 ± 0.19 vs. 0.57 ± 0.28 mV; LL = 0.32 ± 0.11 vs. 0.41 ± 0.19 mV; main effect of task P = 0.02). Conclusions: Agonist motor neurons exhibit heightened sensitivity to afferent feedback from median-innervated accessory muscles when controlling the position of an inertial load. Clinical Relevance: Findings indicate that the type of load used in strength and endurance training programs can influence the response of the primary agonist to concurrent activity of accessory musculature.