Aspects of Muscle Mechanics

Losses in Muscle Work During the Elongation and Contraction Cycle 

An isotonic type of contraction (reduction in length under constant stress) is exemplified by an isolated muscle excited to lift a weight hanging on one end - see Fig. 9.8. (The cross-sectional area experiencing loading is assumed to remain approximately constant.) By fixing both ends and applying an 

Pars pro toto the isometric contraction is discussed a bit more in detail (Fig. 9.9). By using a measuring device corresponding to Fig. 9.9a but not activating the muscle by an electric stimulus a certain lengthening force can be measured at a certain muscle length (prestress). From many such measurements force-length-curve of the not active muscle (curve A in Fig. 9.9c) can be derived. 

When the same measuring device is used and the force transducer is rather stiff, as a consequence of a supramaximal electric stimulus a (near) isometric contraction of the muscle under a certain prestress can be recorded. From many such measurements a force-length curve of the active muscle can be derived. It is known as the curve of the isometric maxima (curve B in Fig. 9.9c). 

Assuming a certain-prestress resulting in a muscle length L passive stress results in a passive force F. If the muscle is activated electrically in this position, it develops an active force F2 — F additionally, resulting in a total force F2. One can device from Fig. 9.9c, that the active force is a function of the prestress, that means the length of the muscle under the experimental conditions discussed. The muscle can develop its maximum active force when prestressed to its length in-situ (i. e. when incorporated in the living 

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