Acute adaptations in dynamic torque and torque control to practicing with known and unknown sequences of concentric and eccentric muscle contractions in healthy young adults

Background During human movement, the neuromuscular system continuously adapts to varying and differently predictable demands. Practicing with sequences of known and unknown types of muscle contractions could train force control of muscle. We compared the acute effects of practicing with eccentric and concentric quadriceps muscle contractions at different velocities with known and unknown sequences, on knee extension torque, torque steadiness, and joint position sense in healthy young adults. Methods Nineteen physically active young men practiced unknown and known muscle loading conditions on an isokinetic dynamometer, comprising eccentric and concentric contractions at different velocities in randomized and predefined sequences (16 contractions per condition). Joint position sense, maximal voluntary isometric contraction torque, and torque steadiness before and after practices, as well as mechanical work during the practice contractions were measured. Results Maximal voluntary isometric contraction torque and joint position sense remained unchanged in both practice conditions, while torque steadiness variable error obtained at 70% of maximal voluntary isometric contraction torque decreased after known muscle loading. During eccentric and concentric practice contractions, mechanical work increased from trial to trial during known muscle loading. This improvement was limited to slow velocity concentric contraction after unknown muscle loading. Conclusion The acute effects of using known and unknown muscle loadings were limited. Only known loadings improved torque steadiness, suggesting more efficient motor unit control. The trial-to-trial mechanical work adaptations observed mainly in known muscle loadings suggest rapid neuromuscular adjustments influenced by load predictability, rather than uncertainty.