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 ± 2 y.o., BMI 24.1 ± 2.8 kg/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 12 weeks 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 12 weeks, 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.