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Top: axial-plane MRI scans of the calf muscles of a young male aged 20 years and an older male aged 75 years matched for anthropometric characteristics and physical activity level. Bottom: sagittal-plane sonographs of the gastrocnemius medialis muscle in the same subjects. In the older individual (O) muscle fibre fascicle length and pennation angle are visibly smaller than in the younger adult (Y).
