It is well known that the force developed by skeletal muscles increases with temperature. In spite of the work done on this subject, the mechanism of force potentiation is still debated. Most of the published papers suggest that force enhancement is due to the increase of the individual crossbridge force. However, reports on skinned fibers and single molecule experiments suggest that crossbridge force is temperature independent. The effects of temperature on crossbridge properties in intact frog fibers were investigated here by applying fast stretches at various tension levels (P) on the tetanus rise at 5°C and 14°C to induce crossbridges detachment. Crossbridge number was measured from the force (critical force, Pc) needed to detach the crossbridge ensemble and the average crossbridge strain was calculated from the sarcomere elongation needed to reach Pc (critical length, Lc). Our results show that Pc increased linearly with the force developed at both temperatures, but the Pc/P ratio was considerably smaller at 14°C. This means that the development of a given isometric tension requires less crossbridges at 14°C than at 5°C and, consequently, the individual crossbridge develops a greater average force at high temperature. This mechanism accounts for all the tetanic force enhancement. The critical length Lc was independent of the tension developed at both temperatures but decreased significantly at high temperature suggesting that crossbridges at 14°C are more strained. The increased crossbridge strain accounts for the greater average force developed.