Response to the Reviewer’s major concern.

We would like to thank the reviewer for the time, consideration of our manuscript, and helpful comments for its improvement. In this letter, we would like to respond to the important concerns raised by the reviewers, including the important issue ofthe sEMG interelectrode distance.

We understand that the reviewer believes that the relationship between sEMG (mean power frequency and root-mean-square) and underlying motor unit activity would have been improved if the interelectrode distance was smaller. The logic is that the frequency response of the surface electrodes would be closer to that of the needle electrode, and would be able to detect individual motor unit spikes. While the logic is correct, the approach is problematic for two reasons.

First, bipolar electrodes actually function is a comb filter. As the interelectrode distance decreases, the band-pass for the first lobe actually reduces signal power associated with the sEMG interference pattern. That is, the high-pass component of the first lobe becomes progressively higher. The effect is illustrated quite nicely in Figure 2.16 (page 49) in Basmajian and DeLuca (1985). However, motor unit firing rates have their largest impact upon frequency content of the sEMG interference pattern below 40 Hz, which would be most affected by shorter interelectrode distance (Lago and Jones, 1977; 1981).

Second, a smaller interelectrode distance is even more problematic in terms of biasing the results towards the most superficial muscle fibers away from where the needle is situated, within the muscle. It is true that Moritani and Muro (1986) used a smaller (1 cm) interelectrode distance as the reviewer suggests. However, Moritani and Muro (1987) also acknowledge that the smaller interelectrode distance may have biased the results. Detection volume is proportional to interelectrode distance (Lynn et al., 1978). Thus, smaller interelectrode distances record from the most superficial fibers, which in the biceps brachii are predominantly type II fibers. In a later study using the same surface and indwelling EMG methodology, Moritani et al. (1992) increased the interelectrode distance for the surface electrodes to 2 cm as we did. The specific purpose was to increase the detection volume to include the deeper muscle fibers where the recording needle is situated. The surface recordings would then better reflect the electrical activity of the same muscle fibers as detected by the needle, which would also include type I fibers.

The reviewer does raise a point of interest that should be included in the discussion and we will gladly include a discussion on how interelectrode distance may influence our results.

Refs

  1. Basmajian JV, DeLuca CJ. (1985). Muscles alive: Their functions revealed by electromyography. Baltimore, MD: Williams & Wilkins.
  2. Lindström LH, Magnusson RI. Interpretation of myoelectric power spectra: A model and its application. Proceedings of the IEEE 1977; 65: 653-62.
  3. Lynn PA, Bettles ND, Hughes AD, Johnson SW. (1978). Influences of electrode geometry on bipolar recordings of the surface electromyogram. Medical and Biological Engineering and Computing, 16, 651-660.
  4. Moritani, T, Muro, M. Motor unit activity and surface electromyogram power spectrum during increasing force of contraction. Eur J Appl Physiol 1987; 56: 260-265.
  5. Moritani, T, Sherman, WM, Shibata, M, Matsumoto, T, Shinohara, M. Oxygen availability and motor unit activity in humans. Eur J Appl Physiol 1992; 64: 552-556.

The point-by-point response

Reviewer 1:

Abstract: you conclusion could have been different if different EMG recording techniques were used.
The authors agree that interelectrode distance affects the frequency content of the sEMG signal. The following paragraph has been added to the discussion:
Page 15, line 38: “It is the case that the interelectrode distance impacts the frequency content of the surface EMG signal, as the bipolar recording electrodes act as a comb filter (Basmajian and De Luca, 1985). In this investigation, a 2 cm interelectrode distance was used. While a smaller interelectrode distance (1 cm, for example) may have altered the relationship between motor unit firing rate and MPF, the spatial selectivity of such an electrode configuration would bias the results toward more superficial motor units (Moritani and Muro, 1987). Therefore, the relationship between indwelling and surface EMG parameters would be a comparison of superficial muscle fibers from the surface recording, and deeper fibers from the indwelling signal, which is not the purpose of the study. The 2 cm interelectrode distance employed in this and other investigations (Moritani et al., 1992) was chosen to increase the detection volume, thereby better reflecting the activity of motor units detected by the indwelling electrode.”
Your figures 2 and 7 are not impressive although there were statistically significant correlations.
The authors agree that although relationships demonstrated in Figures 2 and 7 were statistically significant, they are weak correlations and are not impressive. This point has is highlighted in the Results and Discussion sections.
Page 9, line 46: “There was a weak positive relationship between recruitment
force and peak motor unit firing rate (r2=0.08)…”
Page 10, line 34: “…when the individual trials from all subjects were used instead of the means across subjects (Figure 7), the strength of the relationship between surface EMG RMS and motor unit firing rate was much lower (r2=0.19).”
Page 11, line 21: “When all trials were examined collectively, or for independent subjects, the relationship between motor unit firing rates and both RMS amplitude and MPF were very weak.”