A study of the relationship between muscle length, tension-time area and stiffness in cat areflexic soleus

The relationship between muscle length and both tension-time area and stiffness were studied in the isolated cat soleus muscle during tetanic isometric contraction at different stimulus rates. The results show that: 1. The area subtending the tension curve remains constant in a range of 8-10 mm of muscle length, for stimulation frequencies between 15 and 66 Hz. 2. The muscle stiffness, measured using different amplitude stretches, remains constant over changes in muscle length of 10 mm. 3. The stiffness is higher for smaller stretches than for larger ones. The data therefore show an approximately constant tension-time area and no significant changes in stiffness for variations in muscle length that exceed the physiological length variations during quiet standing. These results are discussed in the context of postural mechanisms.