In situ investigation of the sarcomere force-length relationship in intact muscle using second harmonic generation microscopy

The sarcomere force-length (FL) relationship can be derived theoretically using thick and thin filament lengths. Here, we determined the mean sarcomere FL curve using directly- measured sarcomere lengths (SL) and muscle forces in an intact muscle and compared it to the theoretical FL curve. Tibialis anterior muscles of six mice were used. Muscle forces were measured for maximal contractions using an instrumented micro-manipulator and in situ SL were quantified by second harmonic generation microscopy. We found that the experimental FL curve was shifted to the left of the theoretical curve. Therefore, the practice of using mean SL to predict muscle forces in vivo may lead to erroneous predictions of the functional properties of skeletal muscles.