Influence of contraction force and speed on muscle fiber conduction velocity during dynamic voluntary exercise

Before using electromyographic (EMG) variables such as muscle fiber conduct ion velocity (MFCV) and the mean or median frequency (MDF) of an EMG power spectrum as indicators of muscular fatigue during dynamic exercises, it is necessary to determine the influence of a joint angle, contraction force an d contraction speed on the EMG variables. If these factors affect the EMG v ariables, their influence must be removed or compensated for before discuss ing fatigue. The vastus lateralis of eight normal healthy male adults was s tudied. EMG signals during non-fatiguing dynamic knee extension exercises w ere detected with a three-bar active surface electrode array. EMG variables were calculated from the detected signals and compared with the angle of t he knee joint, the extension torque and the extension speed. The extension torque was set at four levels with 10% intervals between 40 and 70% of the maximum voluntary contraction. The extension speed was set at five levels w ith 60 degrees /s intervals between 0 and 240 degrees /s. Because the joint angle unsystematically affected the MFCV, EMG variables at a given joint a ngle were extracted for comparison. The influence of the extension torque a nd speed on the extracted EMG variables was clarified with an ANOVA and a r egression analysis. The statistical analyses showed that MFCV increased wit h the extension torque but did not depend on the extension speed. In contra st, MDF was independent of the extension torque but was dependent on the ex tension speed. MDF thus showed a behavior different from that of MFCV. It b ecame clear that if MFCV is used as an indicator of muscular fatigue during dynamic exercises, it is at least necessary to extract MFCV at a predeterm ined joint angle and then remove the influence of extension torque on MFCV. (C) 2001 Elsevier Science Ltd. All rights reserved.