Relaxation electromechanical delay of the quadriceps during selected movement velocities.

The purpose of this study was to quantify the time between the cessation of EMG activity and cessation of torque production, or relaxation electromechanical delay, (R-EMD) of the quadriceps at three angular movement velocities. A Biodex dynamometer passively moved the right knee of 25 males through a given range at three velocities (10, 60 and 120 degrees per second). Subjects were instructed to actively extend their knees to a visual target, then to immediately relax. Biodex torque and position data, as well as surface EMG from the right Vastus Medialis (VM), Rectus Femoris (RF) and Vastus Lateralis (VL) were sampled simultaneously. EMG cessation was determined when activity fell below a threshold based on the muscle's resting EMG. Torque cessation was determined when the slope of the relaxation curve decreased to 10% of the initial value. R-EMD time for each quadriceps head at each velocity was determined by calculating the difference between these two times. To examine reliability, subjects were retested four days later. Approximately two thirds of the subjects were unable to consistently perform the motor task of abrupt relaxation at some or all of the tested velocities. This variability was ascribed to motor control issues. Average R-EMD times for all subjects, muscle segments and velocities ranged from 249 +/- 68 ms to 276 +/- 51 ms during the first test session, and 239 +/- 46 ms to 300 +/- 59 ms during the second session. These data are important in identification of physiologically meaningful cessation of muscle contraction, and may be beneficial in research studies focusing on the areas of motor control and motor learning, computerized movement analysis, and prediction models for the determination of muscular force from the EMG signal.