Figure 3. Force-pCa relationship (A) and Hill plot analysis (B) of a single pre-flight soleus fibre.

The fibre was activated in solutions with free Ca²⁺ concentrations ranging from pCa 6.8 to 4.7. The initial and final contraction and every 4th or 5th intervening contraction, was performed at a pCa of 4.5. In A, forces have been plotted relative to peak force at pCa 4.5, i.e. P_r= (force at sub-maximal activation)/(force at pCa 4.5). Hill plots were fitted to the data (B). Half-maximal activation, or pCa₅₀, was defined as the mean abscissal intercept of the Hill plots fitted to data above and below P_r= 0.5. The Ca²⁺ activation threshold, calculated from the plot of P_r < 0.5, was defined as the pCa where log(P_r/(1 - P_r)) equalled -2.5. For this particular fibre, activation threshold and half-maximal activation occurred at pCa values of 7.09 and 6.02, respectively. The slope of the force-calcium relationship was determined for forces greater (n₁) and less (n₂) than 50 % of the maximal Ca²⁺-activated force. Subsequent gel electrophoresis indicated that this fibre expressed type I MHC.