Force decline after low and high intensity contractions in persons with multiple sclerosis

Abstract

Objective:Force decline during strong contractions is dominated by changes in the periphery whereasduring weaker contraction changes in voluntary activation become more important. We compared forcedecline and contributing factors in persons with multiple sclerosis (PwMS) during low and high intensitycontractions.Methods:Index finger abduction force, force evoked by electrical stimulation of the ulnar nerve at rest(RTw), and during MVCs were investigated in 19 PwMS and 19 controls. Participants performed contrac-tions in sets of six contractions (7 s-on, 3 s-off) at 25% or 80% MVC. After each set, a 5 s-MVC was per-formed with superimposed nerve stimulation followed by RTw. Contractions were repeated until MVCdropped below 80% of initial MVC.Results:Low compared to high intensity contractions caused a greater decline in voluntary activation anda smaller decline in RTw. Compared to controls, PwMS accomplished equal sets of contractions butshowed a smaller decline in RTw. Female PwMS showed poorer voluntary activation. The number oflow intensity contractions was associated with sense of fatigue in PwMS.Conclusion:Although, no difference in fatigability was observed, the mechanism contributing to forcedecline differed between PwMS and controls during submaximal contractions.Significance:During weak contractions, fatigue and fatigability are associated in PwMS.