Peripheral fatigue in patients with myotonic dystrophy type 1: an EMG, MMG and force combined approach

Aim: Clinical manifestations of myotonic dystrophy type 1 (DM1) involve myotonia and progressive muscle weakness. Moreover, an early onset and persisting muscle fatigability are often reported. The mechanisms underpinning this phenomenon are not completely clear. The assessment of the electromechanical delay during muscle contraction (DelayTOT) can provide deeper insights on the relative contribution of the electrochemical and mechanical events to muscle fatigue at peripheral level. Methods: DelayTOT components were assessed by an electromyographic (EMG), mechanomyographic (MMG), and force (F) combined approach in thirteen patients with DM1 (age: 38 \ub1 15 yrs; body mass: 75 \ub1 14 kg; stature: 1.78 \ub1 0.07 m; mean \ub1 SD) and in thirteen age- and body-matched healthy controls (HC) from tibialis anterior muscle during electrically evoked isometric contractions. Measurements were taken before, at the end of a fatiguing protocol (50 % maximum voluntary contraction, 6 s on/4 s off, until exhaustion), and after 2, 5 and 10 min during recovery. During contraction, stimulation (Stim), EMG, MMG and F signals wererecorded. DelayTOT and its three components (between Stim and EMG: Dt Stim-EMG, synaptic component; between EMG and MMG: Dt EMG-MMG, electrochemical component; and between MMG and F: Dt MMG-F, mechanical component) were calculated. The peak force (pF) was also determined. Results: No differences were found betweenDM1andHCfor fatiguing protocol duration. After fatigue, the reduction in pF and delays components were significantly larger in DM1 than in HC. While in HC the pF and delays components recovered within 5 min, in DM1 the pF, DelayTOT, Dt Stim-EMG and Dt EMG-MMG did not recover. Conclusions: The present findings suggest that the higher muscle fatigability inDM1could be mainly related to the pathologic alterations at the synaptic and electrochemical level during muscle contraction