A brief contraction has complex effects on summation of twitch pairs in human adductor pollicis

Abstract

During electrically evoked contractions of skeletal muscle, the interplay between twitch duration and the time between electrical stimuli (inter-pulse interval, IPI) determines how effectively twitch forces summate into high force contractions. A brief muscle contraction can impair summation by abbreviating twitch duration, though it is not clear if these impairments occur at all physiologically relevant IPI. This study was designed to test how a brief contraction affects summation of nominally isometric twitch pairs with IPIs lasting 10-5000 ms. Left adductor pollicis muscles of human participants (n = 9) were electrically activated using stimulus pairs applied both before (Pre) and after (Post) a 10 Hz, 1.0 s contraction. Force-time records were mathematically separated into Pulse 1 (single twitch) and Pulse 2 (summated twitch) components. The ratio of Pulse 2 peak force to Pulse 1 peak force was used as our measure of summation effectiveness.Consistent with the observed decline of Pulse 1 duration at Post relative to Pre (4.7 ± 0.6%; P < 0.001; duration was defined as the time from stimulation to the time required for active force to decline by 50%), summation effectiveness was higher at Pre than at Post at IPIs of 100-333 ms. Summation effectiveness was not different between Pre and Post at IPIs of 50-83 ms or 500-5000 ms. Intriguingly, summation effectiveness was higher at Post than at Pre at IPIs of 10-25 ms. In summary, a brief contraction has complex effects on the relationship between inter-pulse interval and summation effectiveness. Future experiments are needed to reveal the mechanisms behind this novel observation. K E Y W O R D Sforce regulation, muscle relaxation, potentiation, skeletal muscle, unfused tetanus 1 When multiple electrical stimuli are applied to a muscle at a sufficiently high frequency, twitches merge into higher force contractions, a process referred to as summation. Summation is influenced by the time between successive stimuli (i.e. the inter-pulse interval, IPI), twitch force and twitch duration. Twitch summation is a non-linear process. In a pair of twitches, the force attributable to the second twitch is typically higher than the force attributable to the first twitch if the second twitch is initiated before the muscle has completely relaxed from the first twitch (

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