Muscle force up to 50% of maximum does not affect cutaneous silent periods in thenar muscles

Objective The cutaneous silent period (CSP) is a spinal inhibitory reflex mediated by A-delta fibers. The amount of muscle contraction has been reported to influence CSPs, but this has not been confirmed in studies applying clinically feasible stimulus parameters. Methods Fourteen healthy subjects underwent comparison of CSPs while contracting the target muscle at different levels ranging from 10% to 50% of the maximum force, which was continuously monitored with a force transducer. Rectified surface electromyographic (EMG) recordings were obtained from abductor pollicis brevis muscle following ipsilateral recurrent noxious digit 2 and digit 5 stimulation. Results Group average CSP onset and end latency, CSP duration, and the magnitude of EMG suppression were not influenced by volitional muscle contraction ranging from 10% to 50% of the maximum. In contrast, background EMG levels increased significantly with the amount of muscle force. Conclusions CSPs prove to be robust nociceptive cutaneomuscular reflexes, which are – in contrast to previous reports – not significantly influenced by clinically easily applicable amounts of muscle force. Significance Non-dependence on the amount of volitional target muscle activation over a range from 10% to 50% of individual maximum force levels renders CSPs particularly suitable for clinical use in patients with peripheral and spinal disorders affecting the A-delta fiber system.