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Background: 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.

Methods: 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.

Results: Muscle responses differed significantly depending on contraction type. Dynamic contractions induced immediate viscoelastic changes, with significant reductions in relaxation time after eccentric (p = 0.027), concentric (p = 0.026), and mixed contractions (p < 0.001), while no changes were observed after isometric contraction (p = 0.285). Stiffness remained stable across all contraction types (p > 0.05). Mixed contractions showed a biphasic response in decrement with a significant effect across series (p = 0.049), identifying decrement as the most sensitive indicator of early fatigue, whereas isometric contraction produced no significant modifications in any parameter.

Conclusions: 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.

Keywords: muscle contraction, muscle fatigue, resistance training, exercise, biomechanical phenomena

The study confirmed that the acute response of the biceps brachii to loading depends primarily on the type of contraction. Dynamic contractions, including eccentric, concentric, and mixed forms, produced immediate and clearly detectable changes in viscoelastic properties, particularly in relaxation, creep, and decrement. These findings indicate that these parameters are highly sensitive to early stages of fatigue. The mixed contraction revealed the most complex response pattern, showing significant changes across subsequent series as well as differences compared with other contraction types, which highlights the complexity of adaptive mechanisms occurring during alternating phases of movement. In contrast, the isometric contraction displayed biomechanical stability and did not show significant changes in any of the analyzed parameters. This confirms its distinct characteristics and its limited influence on short-term reorganization of muscle tissue. The stiffness parameter proved to be the least responsive to brief loading, suggesting that rapid fatigue-related changes occur mainly within viscoelastic rather than structural properties.

Overall, the results indicate that monitoring the complete biomechanical profile of the muscle, especially decrement as an inverse indicator of elasticity, may serve as a valuable tool for assessing fatigue, adaptation, and readiness for loading. The findings also confirm the usefulness of myotonometry for evaluating short-term muscular adaptations and emphasize the dominant role of contraction type in shaping the immediate mechanical response of muscle tissue.