Introduction We examined correlations among echo intensity and muscle thickness versus measures of athleticism and isometric strength in healthy children. Methods B-mode ultrasonography was used to examine the vastus lateralis (VL) and rectus femoris (RF) in 28 boys (mean age = 12 years). Tests of athleticism included jump height and peak velocity, sprint speed, and agility. Peak torque and the rate of torque development (RTD) at 100 and 200 ms from torque onset were assessed. Results Several significant correlations existed after controlling for age and mass. RF thickness showed the strongest correlations with the dependent variables. Both RTD time intervals, but not peak torque, correlated with VL thickness. Peak torque was greatly influenced by age and mass. Echo intensity and muscle thickness correlated highly with agility performance. Conclusions VL and RF echo intensity and muscle thickness may help predict athleticism, and to a lesser extent, RTD, in boys. Muscle Nerve 55: 685–692, 2017