Protease activated receptors (PARs) are unique G-protein-coupled receptors that are activated by proteolytic cleavage. Four PAR isoforms (PAR-1 to 4) have been identified. PARs may play an important role in skeletal muscle development, as they are highly expressed in myoblasts and myotubes in tissue culture (Mackie et al., 2008), and show marked changes in expression in muscle fibres during embryonic development (Chevessier et al., 2001). In order to increase our understanding of the role of PAR-1 receptors in muscle development, we compared the contractile properties of slow and fast skeletal muscles from PAR-1 null (P-1N) and wild type (WT) mice. Experiments were conducted on 12 week old P-1N null (n=7) and WT littermate mice (n=5). This study was approved by the animal ethics committee of the University of Western Australia. Mice were anaesthetized by intraperitoneal injection of pentobarbitone (40mg/kg). The soleus (mainly slow twitch) and extensor digitorum longus (EDL) (fast twitch) muscles were surgically removed and connected to a force transducer system. The muscles were maintained in Kreb’s mammalian Ringer solution bubbled with Carbogen (95% O2 and 5% CO2). Twitch force characteristics, the force frequency relationship and maximum specific force (force normalised to muscle cross sectional area) were determined, and the rate of fatigue and post-fatigue recovery were assessed. Muscle fibre type proportions were determined using immunohistochemistry. The absence of PAR-1 receptors in soleus muscles resulted in a significantly greater mean peak twitch force (P-1N: 6.2 ± 0.231 N.cm-2, WT: 4.6 ± 0.392 N.cm-2, p<0.05) and mean twitch time to peak values (P-1N: 57.0 ± 3.0 ms, WT: 41.0 ± 1.0 ms, p<0.05) and produced a significant shift of the force-frequency curve to the left. In addition, the soleus muscles from the P-1N mice fatigued significantly more slowly (p<0.05) and exhibited a significantly faster post fatigue recovery (p<0.05) than muscles from WT mice. There was no difference in maximum specific force between soleus muscles from P-1N and WT mice. There was a 24% increase in the proportion of type 1fibres, and a 90% decline in the proportion of type IIb fibres in the soleus muscles from PAR-1 null mice compared to soleus controls. In P-1N EDL muscles, no significant differences were found, except for a slower rate of fatigue compared to controls. The results of this study indicate that the absence of PAR-1 receptors significantly alters the contractile properties of skeletal muscle, especially in the predominately slow twitch soleus. 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 fibre numbers. These results suggest that PAR-1 receptors may play a role in setting fibre-type proportions during skeletal muscle development.