Elevated plasma creatine kinase (CK) activity is often used as an indicator of exercise-induced muscle damage. Our aim was to study effects of contraction type, sex and age on CK efflux from isolated skeletal muscles of mice. The soleus muscle (SOL) of adult (7.5-month old) female C57BL/6J mice was subjected to either 100 passive stretches, isometric contractions or eccentric contractions, and muscle CK efflux was assessed after two-hour incubation in vitro. SOL of young (3-month old) male and female mice was studied after 100 eccentric contractions. For adult females, muscle CK efflux was larger (p < 0.05) after eccentric contractions than after incubation without exercise (698 ± 344 vs. 268 ± 184 mU·h^–1, respectively), but smaller (p < 0.05) than for young females after the same type of exercise (1069 ± 341 mU·h^–1). Eccentric exercise-induced CK efflux was larger in muscles of young males compared to young females (2046 ± 317 vs 1069 ± 341 mU · h^–1, respectively, p < 0.001). Our results show that eccentric contractions induce a significant increase in muscle CK efflux immediately after exercise. Isolated muscle resistance to exercise-induced CK efflux depends on age and sex of mice.

• Muscle lengthening contractions induce the highest CK efflux compared with similar protocol of isometric contractions or passive stretches.

• Muscle CK efflux is applicable in studying changes of sarcolemma permeability/integrity, a proxy of muscle damage, in response to muscle contractile activity.

• Isolated muscle resistance to exercise-induced CK efflux is greater in female compared to male mice of young age and is further increased in adult female mice.