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Abstract
It has been generally accepted that the steady-state isometric forces following active muscle shortening are depressed and following stretching are enhanced for in-vitro and in-situ muscle and muscle fibre preparations. However, these history-dependent properties are poorly understood for in-vivo human skeletal muscles during voluntary contractions. The primary goal of this dissertation was to determine the functional implications of history-dependent properties for in-vivo human voluntary movements, and furthermore, to gain insight into the possible mechanism(s) underlying history-dependent properties. Experiments were performed on in-situ cat soleus, using electrical stimulation, and on in-vivo human adductor pollicis, using maximal voluntary activation and electrical stimulation.
In cat soleus, force depression and force enhancement were not commutative and stretch-induced force enhancement was influenced by the preceding shortening in a dose-dependent manner. These results suggest that there is more than one mechanism required to explain force depression and force enhancement. In human adductor pollicis, it was demonstrated that history-dependent properties existed during maximal voluntary contractions, and that their characteristics were similar to those obtained during electrically elicited contractions. These results indicate that history-dependent properties are functionally relevant for human voluntary contraction.
Novel results found in the various experiments included the following: first, the steady-state force enhancement exceeded, on specific occasions, the maximum isometric force at optimal length; second, some of the force enhancement persisted after deactivation of the muscle, suggesting that part of the force enhancement was caused by a contribution from passive elements; third, force depression in the human adductor pollicis was associated with a proportional decrease in muscle stiffness, suggesting that perhaps force depression was caused by a decrease in the proportion of attached cross-bridges. It is concluded that history-dependent properties occur during maximal voluntary contractions and that force enhancement and force depression are caused by different phenomena.
