RESPONSE

Abstract

Dear Editor-in-Chief: We appreciate Folland and William‘s comments regarding our recent paper (1). They suggest our method of calculating the central activation ratio (CAR) may have overestimated muscle activation. Multiple twitch interpolation (ITT) methodologies have been used to quantify activation during voluntary actions, each with its own advantages and disadvantages (2). They suggest, given our use of submaximal stimulation, the use of a different methodology to assess muscle activation-the ratio of the superimposed tetanus to passive tetanus. We agree that assessing muscle activation in this manner would also be appropriate. Applying this ratio to our data lowers average muscle activation from 92 ± 7 to 87 ± 13% (as indicated by Folland and Williams) and from 96 ± 4 to 89 ± 11% in the right and left thighs, respectively. This reduces the difference in muscle activation between ITT and the T2 MRI assessment employed in our study from 17 to 12% in the right thigh and from 22 to 15% in the left thigh. The difference in activation between methodologies, while remaining statistically different in the left thigh (P = 0.02), no longer reaches statistical significance in the right (P = 0.07). The 12% difference in muscle activation in the right thigh still represents a large effect (Cohen‘s D = 0.89) and suggests that the comparison was underpowered, particularly in light of the data from the left thigh. Indeed, when the data from the right and left thigh are pooled, doubling the sample size, the resultant 13% difference between the methods is significant (P = 0.002). We believe a revision of the data does not alter the conclusions of our recent study (1)-the ITT and T2 MRI methods yield markedly different values for muscle activation. It remains unclear whether the MRI method can detect differences in activation between clinical and healthy populations, as well as changes in voluntary activation that may occur with resistance training. However, given the lack of a gold-standard measurement of muscle activation, and the experimental and clinical importance of accurately assessing muscle activation, we hope the use of T2 MRI will be further explored in future studies. Tracee Kendall Jean Mayer USDA Human Nutrition Research Center on Aging Tufts University Boston, MA Christopher D. Black Christopher P. Elder Department of Kinesiology The University of Georgia Athens, GA DOI: 10.1249/mss.0b013e31804a8049