Acute Effects of Different Muscle Contraction Types on Biomechanical and Viscoelastic Properties of the Biceps Brachii Measured with Myotonometry

<b>Background:</b> Acute alterations in biomechanical and viscoelastic muscle properties provide important insight into early fatigue mechanisms; however, their dependence on specific muscle contraction types remains insufficiently understood. Therefore, the aim of this study was to quantitatively compare the acute effects of eccentric, concentric, isometric, and mixed contractions on the biomechanical and viscoelastic properties of the biceps brachii using myotonometry. <b>Methods:</b> Eighty healthy men aged 40 to 50 years were randomly assigned to four contraction conditions: eccentric, concentric, isometric or mixed concentric-eccentric. Each participant performed four sets of isolated biceps brachii exercise to volitional failure. Myotonometric measurements of tone, stiffness, decrement, relaxation and creep were collected before exercise and after each set. Changes within and between contraction types were analyzed. <b>Results:</b> Muscle responses differed significantly depending on contraction type. Dynamic contractions induced immediate viscoelastic changes, with significant reductions in relaxation time after eccentric (<i>p</i> = 0.027), concentric (<i>p</i> = 0.026), and mixed contractions (<i>p</i> < 0.001), while no changes were observed after isometric contraction (<i>p</i> = 0.285). Stiffness remained stable across all contraction types (<i>p</i> > 0.05). Mixed contractions showed a biphasic response in decrement with a significant effect across series (<i>p</i> = 0.049), identifying decrement as the most sensitive indicator of early fatigue, whereas isometric contraction produced no significant modifications in any parameter. <b>Conclusions:</b> Dynamic muscle work induces rapid and contraction-dependent shifts in viscoelastic properties, whereas stiffness appears resistant to short-term loading. Isometric contractions display minimal mechanical disturbance. Myotonometry proved effective in detecting early fatigue-related changes and decrement may serve as a key marker of short-term muscle adaptation.