Force potentiation during eccentric contractions in rat skeletal muscle

Abstract

AIM: Post-activation potentiation refers to an acute enhancement of contractile properties following muscle activity. Previously, the effects of prior muscle activation on eccentric force at tetanic activation frequencies have only been sparsely reported. This paper aimed to study acute activity induced effects on eccentric force of slow and fast twitch muscles and characterize them in relation to post-activation potentiation. METHODS: We elicited eccentric contractions in isolated rat extensor digitorum longus and soleus muscles by actively lengthening muscles at a constant velocity. We assessed contractile properties by measuring force over shortly interspaced, identical eccentric and isometric contractions. We then analysed stretch force, isometric peak force, rate of force development and relaxation times. Finally, we compared the time courses for the development and cessation of changes in stretch force to known features of post-activation potentiation. RESULTS: In extensor digitorum longus muscles stretch force consistently increased in a contraction-to-contraction manner by up to 49% (95% CI: 35-64) while isometric peak force simultaneously showed minor declines (8%, 95% CI: 5-10%). The development and cessation of eccentric force potentiation coincided with development of twitch potentiation and increases in rate of force development. In soleus muscles we found no consistent eccentric potentiation. Characterization of the increase in eccentric force revealed that force only increased in the very beginning of an active stretch. CONCLUSION: Eccentric force at tetanic activation frequencies potentiates substantially in extensor digitorum longus muscles over consecutive contractions with a time course coinciding with post-activation potentiation. Such eccentric potentiation may be important in sport performance.