• Ca2+;
  • calcium;
  • contraction;
  • damage;
  • development;
  • eccentric;
  • exercise;
  • fatigue;
  • force;
  • inflammation;
  • injury;
  • motor;
  • mouse;
  • myoblast;
  • myotube;
  • necrosis;
  • nerve;
  • neuron;
  • primary fibers;
  • proteinase;
  • regeneration;
  • secondary fibers;
  • synapse;
  • thrombin;
  • trypsin


Introduction: Protease-activated receptors (PARs) may play a role in skeletal muscle development. We compared the contractile properties of slow-twitch soleus muscles and fast-twitch extensor digitorum longus (EDL) muscles from PAR-1 null and littermate control mice. Methods: Contractile function was measured using a force transducer system. Fiber type proportions were determined using immunohistochemistry. Results: Soleus muscles from PAR-1 null mice exhibited longer contraction times, a leftward shift in the force–stimulation frequency relationship, and decreased fatiguability compared with controls. PAR-1 null soleus muscles also had increased type 1 and decreased type IIb/x fiber numbers compared with controls. In PAR-1 null EDL muscles, no differences were found, except for a slower rate of fatigue compared with controls. Conclusions: The absence of PAR-1 results in a slower skeletal muscle contractile phenotype, likely due to an increase in type I and a decrease in type IIb/x fiber numbers. Muscle Nerve 50: 991–998, 2014