FORCE FROM CAT SOLEUS MUSCLE DURING IMPOSED LOCOMOTOR-LIKE MOVEMENTS - EXPERIMENTAL-DATA VERSUS HILL-TYPE MODEL PREDICTIONS

Muscle is usually studied under nonphysiological conditions, such as t etanic stimulation or isovelocity movements, conditions selected to is olate specific properties or mechanisms in muscle. The purpose of this study was to measure the function of cat soleus muscle during physiol ogical conditions, specifically a simulation of a single speed of slow walking, to determine whether the resulting force could be accurately represented by a Hill-type model. Because Hill-type models do not inc lude history-dependent muscle properties or interactions among propert ies,the magnitudes of errors in predicted forces were expected to reve al whether these phenomena play important roles in the physiological c onditions of this locomotor pattern. The natural locomotor length patt ern during slow walking, and the action potential train for a low-thre shold motor unit during slow walking, were obtained from the literatur e. The whole soleus muscle was synchronously stimulated with the locom otor pulse train while a muscle puller imposed the locomotor movement. The experimental results were similar to force measured via buckle tr ansducer in freely walking animals. A Hill-type model was used to simu late the locomotor force. In a separate set of experiments. the parame ters needed for a Hill-type model (force-velocity, length-tension, and stiffness of the series elastic element) were measured from the same muscle. Activation was determined by inverse computation of an isometr ic contraction with the use of the same locomotor stimulus pattern. Du ring the stimulus train, the Hill-type model fit the locomotor data fa irly well, with errors <10% of maximal tetanic tension. A substantial error occurred during the relaxation phase. The model overestimated fo rce by similar to 30% of maximal tetanic tension. A nonlinear series e lastic element had little influence on the force predicted by a Hill m odel, yet dramatically altered the predicted muscle fiber lengths. Fur ther experiments and modeling were performed to determine the source o f errors in the Hill-type model. Isovelocity ramps were constructed to pass through a. selected point in the locomotor movement with the sam e velocity and muscle length. The muscle was stimulated with the same locomotor pulse train. The largest errors again occurred during the re laxation phase following completion of the stimulus. Stretch during st imulation caused the Hill model to underestimate the relaxation force. Shortening movements during stimulation caused the Hill model to over estimate the relaxation force. These errors may be attributed to the e ffects of movement on crossbridge persistence, and/or the changing aff inity of troponin for calcium between bound and unbound crossbridges, neither of which is well represented in a Hill model. Other sources of error are discussed. The model presented represents the limit of accu racy of a basic Hill-type model applied to cat soleus. The model had e very advantage: the parameters were measured from the same muscle for which the locomotion was simulated and errors that could arise in the estimation of activation dynamics were avoided by inverse calculation. The accuracy might be improved by compensating for the apparent effec ts of velocity and length on activation. Further studies are required to determine to what degree these conclusions can be generalized to ot her movements and muscles.