Sustained Isometric Wrist Flexion and Extension Maximal Voluntary Contractions on Corticospinal Excitability to Forearm Muscles during Low-Intensity Hand-Gripping

Abstract

The wrist extensors demonstrate an earlier fatigue onset than the wrist flexors. However, it is currently unclear whether fatigue induces unique changes in muscle activity or corticospinal excitability between these muscle groups. The purpose of this study was to examine how sustained isometric wrist extension/flexion maximal voluntary contractions (MVCs) influence muscle activity and corticospinal excitability of the forearm. Corticospinal excitability to three wrist flexors and three wrist extensors were measured using motor evoked potentials (MEPs) elicited via transcranial magnetic stimulation. Responses were elicited while participants exerted 10% of their maximal handgrip force, before and after a sustained wrist flexion or extension MVC (performed on separate sessions). Post-fatigue measures were collected up to 10-min post-fatigue. Immediately post-fatigue, extensor muscle activity was significantly greater following the wrist flexion fatigue session, although corticospinal excitability (normalized to muscle activity) was greater on the wrist extension day. Responses were largely unchanged in the wrist flexors. However, for the flexor carpi ulnaris, normalized MEP amplitudes were significantly larger following wrist extension fatigue. These findings demonstrate that sustained isometric flexion/extension MVCs result in a complex reorganization of forearm muscle recruitment strategies during hand-gripping. Based on these findings, previously observed corticospinal behaviour following fatigue may not apply when the fatiguing task and measurement task are different.

Fatigue, induced via repetitive upper-limb motor tasks, influences trunk and shoulder kinematics during an upper limb reaching task in a virtual reality environment

“…Fatigue, defined as:” a symptom in which physical and cognitive function is limited by interactions between performance fatigability and perceived fatigability” [ 14 , 15 ], is a daily encountered condition that could be responsible for the kinematic changes observed in individuals with chronic RCRSP. It has been demonstrated that during upper limb work related tasks such as hammering [ 16 ], ratcheting [ 17 ] and simulated chain work [ 18 – 20 ], fatigue leads to kinematic adaptations, including decreased glenohumeral elevation, and increased trunk and scapular movements [ 14 , 15 , 21 ]. These observed kinematics changes following fatigue could be caused by different modulating factors [ 14 ] such as the reduction of proprioception acuity [ 15 ], muscle activation latency [ 22 ] and/or muscle force variability [ 23 , 24 ].…”

“…It has been demonstrated that during upper limb work related tasks such as hammering [ 16 ], ratcheting [ 17 ] and simulated chain work [ 18 – 20 ], fatigue leads to kinematic adaptations, including decreased glenohumeral elevation, and increased trunk and scapular movements [ 14 , 15 , 21 ]. These observed kinematics changes following fatigue could be caused by different modulating factors [ 14 ] such as the reduction of proprioception acuity [ 15 ], muscle activation latency [ 22 ] and/or muscle force variability [ 23 , 24 ]. As kinematics changes could increase mechanical load on the shoulder joint and lead to injuries, it is important to better understand how fatigue influences shoulder movements, to assess its potential harmful effects and improve preventive interventions.…”