BACKGROUND: Indices of muscle quality vary in the literature with some emphasizing skeletal muscle composition, and others the ratio between muscle function and mass or size. The study of muscle quality is important as it is increasingly clear that factors other than muscle size are critically important for physical function. The purpose of this study was to determine relationships between different measures of muscle quality and maximal walking velocity (MWV) in young and older males. METHODS: Thirty healthy, young (n = 15, age = 20.7 ± 2.2 yrs) and older (n = 15, age = 71.6 ± 3.9 yrs) males underwent dual energy x-ray absorptiometry scans and ultrasound imaging to examine leg lean mass and cross-sectional area of the quadriceps, respectively. Echo intensity, an indirect indicator of skeletal muscle tissue composition, was also derived from grey-scale analysis of ultrasound images. Participants performed isometric and concentric isokinetic (60 deg/sec and 180 deg/sec) testing of the quadriceps. Peak torque was defined as the highest 500 ms and 25 ms rolling average for isometric and isokinetic contractions, respectively. Normalized strength was calculated via two methods for each velocity: 1) dividing peak torque by cross-sectional area, and 2) dividing peak torque by leg lean mass. Contractile acceleration was calculated from the velocity-time curve during unloaded knee extensions. For MWV, participants were instructed to walk as “fast as safely possible” along a 4 m course. Partial correlations, controlling for age and BMI, were conducted to examine relationships between MWV and muscle-related variables. RESULTS: MWV was positively correlated with contractile acceleration (r = 0.414; p = 0.029), but no other measures (p > 0.05). CONCLUSION: A greater capacity of the quadriceps to increase velocity rapidly was associated with a faster MWV. Contractile acceleration is indicative of muscle quality aspects (i.e., rate of activation, cross-bridge rate), but is not typically used as a standalone muscle quality metric. Muscle strength relative to size or mass, regardless of testing velocity, was not related to physical function in our smaller sample of males. It is likely the muscle group tested, and limitation of a single physical function measure were influential in these outcomes.

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