Muscle thickness correlates to muscle cross-sectional area in the assessment of strength training-induced hypertrophy

Muscle thickness (MT) measured by ultrasound has been used to estimate cross-sectional area (measured by CT and MRI) at a single time point. We tested whether MT could be used as a valid marker of MRI determined muscle anatomical cross-sectional area (ACSA) and volume changes following resistance training (RT). Nine healthy, young, male volunteers (24 +/- 2y.o., BMI 24.1 +/- 2.8kg/m(2)) had vastus lateralis (VL) muscle volume (VOL) and ACSA(mid) (at 50% of femur length, FL) assessed by MRI, and VL MT measured by ultrasound at 50% FL. Measurements were taken at baseline and after 12weeks of isokinetic RT. Differences between baseline and post-training were assessed by Student's paired t test. The relationships between MRI and ultrasound measurements were tested by Pearson's correlation. After RT, MT increased by 7.5 +/- 6.1% (P<.001), ACSA(mid) by 5.2 +/- 5% (P<.001), and VOL by 5.0 +/- 6.9% (P<.05) (values: means +/- SD). Positive correlations were found, at baseline and 12weeks, between MT and ACSA(mid) (r=.82, P<.001 and r=.73, P<.001, respectively), and between MT and VOL (r=.76, P<.001 and r=.73, P<.001, respectively). The % change in MT with training was correlated with % change in ACSA(mid) (r=.69, P<.01), but not % change in VOL (r=.33, P>.05). These data support evidence that MT is a reliable index of muscle ACSA(mid) and VOL at a single time point. MT changes following RT are associated with parallel changes in muscle ACSA(mid) but not with the changes in VOL, highlighting the impact of RT on regional hypertrophy.