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Fatigue-induced reductions of torque and shortening velocity are muscle dependent.

INTRODUCTION
In addition to torque (force) loss during dynamic fatiguing contractions, an important aspect of fatigue is the reduction in shortening velocity. Relatively few reports have studied changes in shortening velocity in response to fatigue, and inconsistencies in the results might depend on the muscle tested. To better understand fatigue-related changes in shortening velocity in different muscles, we compared the fatigue response of two muscles in humans with different physiological properties using the same criterion for task failure.


METHODS
During separate sessions for the triceps brachii and soleus, 10 young males performed repetitive shortening contractions with a moderate load (50% of maximum voluntary isometric contraction (MVC) torque) until they achieved a 50% reduction in loaded shortening velocity at task failure.


RESULTS
In the unfatigued condition, these muscles expressed different physiological features with similar power output but approximately 1.5 times slower shortening velocity, approximately 1.4 times slower normalized evoked 50-Hz maximum rate of relaxation, and approximately 47% lower postactivation potentiation in the soleus compared with the triceps brachii. During the fatiguing task, significantly less total work was performed in the soleus ( approximately 1986 J) compared with the triceps brachii ( approximately 3940 J), suggesting greater endurance in the triceps brachii. Despite similar relative reductions in shortening velocity in both muscles at task failure, the soleus had a smaller reduction in MVC torque than the triceps brachii at task failure. Furthermore, in both muscles, there was a greater reduction at task failure and a faster restoration of shortening velocity compared with MVC torque.


CONCLUSION
Our findings suggest that fatigue-related mechanisms that reduce torque are not the same as those that reduce shortening velocity.