The filled symbols show purely isometric reference contractions; the open symbols represent the steady-state isometric forces following active stretch of the muscle. Percentage values for residual force enhancement were estimated from the original figure published by Morgan et al. (2000) for all data on the ascending part of the force–length relationship. Out of the 11 comparisons that could be made, 10 show a clear force enhancement while in one instance the isometric reference force and the isometric force following active muscle stretching were the same. The average force enhancement estimated based on this figure was 5.7% for all 11 data points, with a peak value of 15.2% of force enhancement above the plateau force for the 3 pulse s⁻¹ condition at a normalized muscle length of about 6.5 mm. All measurements were made on an in situ preparation of cat soleus under physiological conditions.

Sarcomere lengths are non-uniform prior to stretch and after stretch, as has been shown previously by Rassier et al. (2003) and Telley et al. (2006). Furthermore, sarcomeres were stretched by different amounts during myofibril stretching. In this particular example, the shortest sarcomere prior to stretch became the longest after stretch. After stretch, the longest sarcomere (which is supposed to be the weakest and should be stretched beyond myofilament overlap) shortens towards the end of the contraction, while the shortest sarcomere (which is supposed to be the strongest and should be shortening) is stretched. This behaviour is inconsistent with the sarcomere length non-uniformity theory, and the expected rapid and uncontrollable stretching of the weakest sarcomere on the descending limb of the force–length relationship. Myofibril stress prior to stretching at an average sarcomere length of 2.7 μm is about 220 nN μm⁻², and following stretch, at an average sarcomere length of 3.1 μm, is in excess of 300 nN μm⁻², thereby showing residual force enhancement above the isometric force at the final length (e_f) and above the force at optimal length (e_o).