Isokinetic and isometric force measurements involving a dynamometer setup are widely used in training and in muscle assessment. For further understanding of the method, we investigated the activation of key functional muscles during isokinetic and isometric movements. During force measurements in an isokinetic Biodex System 3 ProTM, acoustic myography (AMG) was applied. Ten healthy subjects (5 men/5 women) in each decade from 20 to 69 years of age participated in the study. Measurements were carried out during extension and flexion of the ankle, knee and elbow joints. Muscle fibre use was measured by efficiency (E-score) and fibre recruitment (temporal (T-score) and spatial (S-score) summation). AMG measurements showed good reproducibility, and the recruitment pattern of muscle fibres did not change with gender or age. Overall, a significantly higher E-score (P < 0.05) was found at the lower angular velocities than at the higher ones, indicating a lower level of muscle efficiency at higher velocities. Muscles used for knee movement exhibited higher scores than muscles associated with the ankle and elbow joints, most likely related to the greater degree of force production at this joint compared to the ankle and elbow. The ability to activate and inactivate muscle fibres during periods of isokinetic activity becomes increasingly more difficult as the velocity increases. When assessing training effects in sports or rehabilitation, AMG in parallel with isokinetic measurements adds important additional information by giving a measure of possible improvements in efficiency and fibre use.