Electromechanical delay components during skeletal muscle contraction and relaxation : novel physiological insights and possible application in Myotonic Dystrophies

Muscle contraction and relaxation are physiological events which involve several mechanisms that are electrochemical and mechanical in nature. The functional insights into these processes can provide useful information in both clinical and physiotherapeutic fields. During contraction, the time lag between the onset of muscle electrical activation and the onset of force production was traditionally defined as electromechanical delay (EMD). A delay between the cessation of muscle electrical activity and the beginning of force decay can be observed during the relaxation phase (R-EMD). By an electromyographic (EMG), mechanomyographic (MMG) and force (F) combined approach, both EMD and R-EMD can be partitioned into two components, containing one the electrochemical and the other one the mechanical processes underlying muscle contraction and relaxation. The aim of the study was to assess the effects of fatigue on the different electrochemical and mechanical components of EMD and R-EMD. Inter- and intra-operator reliability of the measurements were also evaluated. Twenty-five participants underwent two sets of tetanic stimulations of the gastrocnemius medialis muscle, with 10 min of rest in between. After a fatiguing protocol of 120 s, tetanic stimulations were replicated. The same protocol was repeated on a different day. EMG, MMG and F signals were recorded during contraction. EMD and its two components (between EMG and MMG onset, delta-t EMG-MMG, and between MMG and F onset, delta-t MMG-F), together with R-EMD with its two components (between EMG cessation and the beginning of force decay, R-delta-t EMG-F, and from the initial force decrease to the negative peak of MMG, R-delta-t F-MMG) were calculated. After fatigue: (i) EMD, delta-t EMG-MMG and delta-t MMG-F lengthened by 18%, 16% and 22%, respectively; (ii) R-EMD, R-delta-t EMG-F, and R-delta-t F-MMG lengthened by 11%, 41%, and 67%. Reliability was always from high to very high. Fatigue altered the electrochemical and mechanical processes during both muscle contraction and relaxation. This EMG, MMG and F combined approach provided reliable measurements of the different delay components and may represent a valid tool also to investigate the electrochemical and mechanical involvement in some neuromuscular disorders, including myotonic dystrophies, where muscle contraction and/or relaxation are compromised. The progression of the pathology and the effects of therapeutic interventions could also be monitored.