Nociceptive EMG suppression in triceps brachii muscle in humans

Objective: Inhibitory spinal reflexes in human hand muscles mediated by cutaneous afferents (=cutaneous silent periods, CSPs) serve to protect the hand from injury. Proximal muscles are included in a complex protective pattern implementing both excitatory and inhibitory reflexes. Methods: We investigated the functional organization of CSPs in the triceps brachii muscle (TB) – the prime elbow extensor – assessing different stimulus intensities and elbow positions. Thirteen healthy subjects underwent recurrent electrical index finger tip stimulation with different intensities while volitionally activating TB against resistance. The rectified and averaged electromyogram (EMG) was recorded from TB with surface electrodes while the elbow joint was held in full flexion, full extension, and at an angle of 90°. Results: Digit II stimulation resulted in distinct CSPs in TB. Increasing stimulus intensities caused a more profound EMG suppression. Different elbow positions had a significant influence on CSP onset and end latency, and duration. Conclusions: The present findings are consistent with a functional organization of protective reflexes in the proximal human upper limb across several metameric segments. Spinal inhibitory neurons serving to rapidly suppress basic muscle synergies may compete with neural circuitry involved in postural control.