Electromechanical delay measured during a voluntary contraction should be interpreted with caution

Electromechanical delay (EMD) is the time lag between muscle activation and muscle force production. It reflects both electrochemical processes (i.e., synaptic transmission, propagation of the action potential, excitation–contraction coupling) and mechanical processes (i.e., force transmission along the active and passive parts of the series elastic component). Consequently, it has been often used (more than 100 articles in PubMed) to evaluate muscle function, such as during fatigue tasks, in response to training programs, after anterior cruciate ligament reconstruction, and in patients with neuropathy or myopathy. EMD can be measured during voluntary, reflex, or electrically induced contraction. It should be considered a valid tool for evaluation of muscle function only if it not influenced by methodological artifacts. In our opinion, EMD measurements performed during voluntary contraction fail to meet this criterion, mainly because the electromyographic (EMG) electrode location may greatly influence detection of onset of muscle activation. In fact, because the onset of force production is due to the first recruited muscle fibers, it is crucial to detect precisely the onset of activation from these fibers. However, the use of classical EMG recordings, in which one pair of electrodes is placed somewhere between the motor point and the tendon, does not allow this to occur. Assuming a muscle fiber conduction velocity of 4 m/s, the propagation of the action potential from the motor point to a pair of electrodes placed 2 cm distal to the motor point would be 5 ms (in a fusiform muscle), which constitutes a relatively high error for EMD values between 40 and 60 ms. Also, due to the spatial heterogeneity in muscle recruitment, one would expect a nonuniform onset of activation within the muscle. In other words, intraand intersubject variations in EMD could be due in part (or totally) to differences in electrode location. To illustrate this drawback, we detected the onset of muscle activation from surface EMG recorded with an electrode array (64 channels; EMG-USB; LISIN-Ottino Biolettronica, Italy) during an isometric elbow flexion performed as quickly as possible. The results showed high variability in the onset times depending on the electrode location (up to 20 ms; Fig. 1). Because the use of electrode arrays would seem appealing, it does not totally solve the drawback just highlighted. The reason for this is that it mainly provides information from motor unit potentials (MUPs) recorded from the surface, where high-threshold motor units are predominant, and it cannot be used in all the synergist muscles involved in the task (e.g., deep muscles). In conclusion, for a proper interpretation of changes in EMD (across subjects and across time), EMD should be measured during electrically induced contraction by taking into account the difference between the stimulation artifact (very easy to detect) and the onset of force FIGURE 1. Experimental setup (A) and results (B). (A) The subject was seated upright. The angle between the upper arm and the forearm was fixed at 90 . He performed isometric elbow flexions as quickly as possible. (B) Electromyographic signals were amplified ( 500), bandpass filtered (10–500 HZ), and digitized at 4096 HZ. The onset of EMG activity was automatically detected (threshold 1⁄4 2 standard deviations beyond mean of baseline activity) and visually checked. The differences from the mean value calculated over 50 EMG channels are depicted. Note that the most proximal and distal lines of channels are not depicted. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]