Comparison of acute responses to isotonic or isokinetic eccentric muscle action: differential outcomes in skeletal muscle damage and implications for rehabilitation.

Both isotonic and isokinetic eccentric muscle contractions are commonly used in muscle research laboratories to induce muscle damage, yet, the muscle damage outcomes between these 2 modes of eccentric contraction have not been compared. The purpose of this study was to compare modes of contraction for differences in muscle damage. 16 men were placed in the isotonic (IT: 110% of maximal isometric torque) or the isokinetic (IK: 120°/s) group, with each group performing 200 eccentric muscle actions of the knee extensors. Isometric peak torque, perceived soreness and CK activity were measured immediately pre and post exercise, and 48-h post exercise. Mean total work (1700 J) and peak torque per set (265 Nm) decreased over the 200 repetitions (p<0.01), and was not different between groups. Damage markers changed 48-h post exercise (p<0.05): peak isometric torque (-13%), creatine kinase activity (+200%) and self-perceived muscular soreness (+4 unit change). Significant group×time interactions (p<0.01) indicated that peak isometric torque was 22% lower, and creatine kinase and self-perceived muscular soreness were 330% and 3 unit difference higher in the IT as compared to the IK groups, 48-h post exercise. When equating for total work, skeletal muscle damage markers are higher during IT vs. IK modes. This reflects differences inherent in contraction type and suggests that this should be taken into account during physical rehabilitation.