The length-tension relationship is a fundamental property of muscle. In its classic form, which is used in muscle models incorporated into studies of motor control, the length-tension relationship is measured during maximal activation via tetanic electrical stimulation in whole muscles or during high intracellular calcium levels in single muscle fibers. In this study, we measured the length-tension relationship of the cat soleus muscle during different levels of natural activation consisting of recruitment and rate modulation of motor units generated by the crossed extension reflex. The ipsilateral dorsal roots were cut to eliminate sensory feedback from the soleus. Length-tension was measured by large shortening steps that transiently allowed force to drop to zero. Force then recovered to a new steady value as the shorter length was maintained for several seconds. The effects of various levels of crossed extension activation on length-tension were compared with direct electrical stimulation of the muscle at 5, 10, 20, and 100 Hz. At all levels of crossed extension, the slope of the length-tension function was much steeper than the slope for tetanic stimulation at 100 Hz. Most slopes for crossed extension fell between the slopes seen with electrical stimulation at 10 and 20 Hz. There was a modest overall tendency for slope to decrease with the level of crossed extension activation. Because much of the normal movement repertoire requires submaximal activation, muscle models based on the tetanic length tension relationship will greatly underestimate the contribution of this relationship to force modulation at different muscle lengths.
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