A comparison of voluntary and involuntary measures of electromechanical delay.

Electromechanical delay (EMD) is a measurement used to assess the mechanical lag between muscle activation onset and force production. EMD measurements may be performed by voluntary or electrically evoked muscle activation. This study compared gastrocnemius EMD during voluntary and involuntary contractions and assessd the intrasession reliability of each set of measurements. Subjects were 15 volunteers (age 21 +/- 2 years, ht 171.8 +/- 10.0 cm, mass 76.1 +/- 13.4 kg). EMD measurements were recorded from the medial head of the gastrocnemius of each subject during voluntary and involuntary contractions. Order was counterbalanced between subjects. Subjects stood with the dominant leg on a force plate, the nondominant next to the force plate, and with their hands in contact with a bar in front of them. A supramaximal percutaneous stimulus was applied to the tibial nerve in the popliteal fossa for involuntary (electrically evoked) contractions. For voluntary contractions, subjects were instructed to rise up on the toes as quickly as possible. Four trials were collected for each condition with 30 s of rest between each. Repeated measures ANOVAs were performed for each condition to calculate an ICC (2,1). Means of the 4 trials for each condition were used to detect differences between groups. EMD was greater in the voluntary condition (22.8 +/- 8.2 ms) compared to the involuntary condition (9.7 +/- 3.1 ms; p < .001). Intrasession reliability for each condition was very strong (involuntary ICC (2,1) = .977; voluntary ICC (2,1) = .972). EMD measured during a single leg stance is much shorter when measured during an electrically evoked (voluntary) contraction. The difference in EMD between conditions is likely the result of differences in recruitment during the two types of contractions. Reliability within a measurement session was very strong for each of the conditions.