We appreciate the interest by Dr. Lanza (Lanza 2018) in our recent work exploring the age-related neuromuscular effects on the human hamstrings, and for providing us the opportunity to clarify our methodology for assessing voluntary activation (VA).
As noted in our paper (Kirk et al. 2018), there are limitations when using pad stimulation to fully activate motor units, and in our experimental setup we were unable to reliably excite the sciatic nerve, located deep in the lower gluteal region. The stimulation current was increased to a level that activated as much of the hamstrings muscle group as possible to elicit maximal twitch torque with the muscle at rest. We found that during the incremental increase of current the twitch torque from the hamstrings muscles would decrease in some participants, due to the spread of current that activated muscles from the anterior or medial thigh compartments. In some participants this required repositioning of the pad stimulation electrodes; furthermore, extraneous muscle activation was assessed by manual palpation with visual inspection of the anterior and medial thigh muscles. Although not stated in our paper (Kirk et al. 2018), the ratio of the resting twitch torque (before the MVC) to the MVC torque was 13.7% (SD 4.9) in the old and 15.6% (SD 7.8) in the young, and these were not statistically different between age groups (one-tail unpaired t-test, P = 0.27). Furthermore, all stimulation procedures were performed by the same investigator to ensure consistency.
With regard to the calculation of VA using the interpolated twitch technique (Todd et al. 2004), the torque ratio of the postcontraction (potentiated) resting twitch to the MVC was 17.7% (SD 5.4) in the old and 17.4% (SD 6.7) in the young, and these were not statistically different (one-tail unpaired t-test, P = 0.46). Furthermore, these values are within the acceptable range (13–32%) (Folland and Williams 2007).
The absolute VA values although important, but as suggested by Dr. Lanza being reliable and consistent with the application of the technique between groups is critical in this study. We trust the readership now better appreciates those aspects despite some limitations acknowledged with the pad stimulation technique.
DISCLOSURES
No conflicts of interest, financial or otherwise, are declared by the authors.
AUTHOR CONTRIBUTIONS
E.A.K., K.J.G., and C.L.R. conception and design of research; E.A.K., K.J.G., and C.L.R. performed experiments; E.A.K., K.J.G., and C.L.R. analyzed data; E.A.K., K.J.G., and C.L.R. interpreted results of experiments; E.A.K., K.J.G., and C.L.R. prepared figures; E.A.K., K.J.G., and C.L.R. drafted manuscript; E.A.K., K.J.G., and C.L.R. edited and revised manuscript; E.A.K., K.J.G., and C.L.R. approved final version of manuscript.
REFERENCES
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