ELECTROMECHANICAL DELAY IN ISOMETRIC MUSCLE CONTRACTIONS EVOKED BY VOLUNTARY, REFLEX AND ELECTRICAL-STIMULATION

Electromechanical delay (EMD) in isometric contractions of knee extens ors evoked by voluntary, tendon reflex (TR) and electrical stimulation (ES) was investigated in 21 healthy young subjects. The subject perfo rmed voluntary knee extensions with maximum effort (maximal voluntary contraction, MVC), and at 30%; 60% and 80% MVC. Patellar tendon reflex es were evoked with the reflex hammer being dropped from 60 degrees, 7 5 degrees and 90 degrees positions. In the percutaneous ES evoked cont ractions, single switches were triggered with pulses of duration 1.0 m s and of intensities 90, 120 and 150 V. Electromyograms of the vastus lateralis and rectus femoris muscles were recorded using surface elect rodes. The isometric knee extension force was recorded using a load ce ll force transducer connected to the subject's lower leg. The major fi nding of this study was that EMD of the involuntary contractions [e.g. mean 22.1 (SEM 1.3) ms in TR 90 degrees; mean 17.2 (SEM 7 0.62) ms in ES 150 V] was significantly shorter than that of the voluntary contra ctions [e.g. mean 38.7 (SEM 1.18) ms in MVC, P < 0.05]. The relationsh ips between EMD, muscle contractile properties and muscle fibre conduc tion velocity were also investigated. Further study is needed to expla in fully the EMD differences found between the voluntary and involunta ry contractions.