Volume of ankle dorsiflexors and plantar flexors determined with stereological techniques

The validity of the methods used for determination of muscle mass has not been evaluated previously. We determined muscle mass by estimating muscle volume with assumption-free stereological techniques applied to magnetic resonance imaging (MRI) in 18 healthy untrained subjects (6 women, 12 men) aged 41 yr (29-64 yr; median, range). Muscle mass was also estimated by measuring leg circumference and cross-sectional muscle areas (CSA) from MRIs at three predetermined levels. Power [peak torque (PT)] of the ankle dorsiflexors and plantar flexors was estimated by using isokinetic dynamometry. Dorsiflexor volume (r(2) = 0.76, P < 5 x 10(-6)) and CSA (r(2) = 0.73, P < 5 x 10(-5)) were related to PT, whereas circumference was not (r(2) = 0.17, not significant). Correspondingly, a relationship to plantar PT was established for plantar flexor volume (r(2) = 0.69, P < 5 x 10-5) and CSA (r(2) = 0.46, P < 5 x 10(-3)) but not leg circumference (r(2) = 0.15, not significant). SDs of the residuals were smaller for the relationship between dorsiflexor PT and volume than between PT and CSA (0.42 vs. 0.45) for plantar flexors (1.5 vs. 2.0). By using the Cavalieri method, six MRI sections and 15 min of point counting are sufficient to obtain a valid estimate of the volume of the muscles of the lower leg.