FORCE VELOCITY RELATIONSHIP ::
Many a times a muscle has to work under load eg lifting of the weight and moving upwards by limb if the load is applied to muscle after it has started contracting it is known as after load contraction eg lifting of weight if the load is applied before the muscle starts contracting or when muscle is relaxing (isotonically) it is free load eg moving up of the load by hand the work done under free loaded condition is greater as compared to that done when muscle is afterloaded the velocity of contraction decreases with increase in force
in free loaded muscle the muscle gets stretched prior to contraction the length of the muscle thus increases like a rubber band higher the initial length more powerful is the muscle contraction the work efficiency thus increases with the increase in free load in after load the load is applied after the muscle starts contracting so the initial length remains constant in this condition the efficiency of the muscle work reduces with increase in load
smooth and cardiac muscles ::
A lthough the basic physiology of contraction of smooth and cardiac muscles is similar to that skeletal muscles there are marked differences in physiological characterisitics some properties of cardiac muscles are discussed in chapter 10 smooth muscles are discussed in this section
smooth muscles have a wide range of function they control the movement of material through most hollow organs for example they propel material in the gastro intestinal tract they restrict flow in blood vessels and bronchi and they expel material from the bladder and vas deferens smooth muscles also control pilo erection and influence the output of sensory information into receptors as in case of iris of the eye that controls the amount of light reaching the retina
at molecular level also pronounced differences are seen between smooth muscles and skeletal muscles for instance smooth muscle cells possess few mitochondria the sarcoplasmic reticulum is poorly developed and located close to the sarcolema there is no postjunctional thickening and the myofilaments of action and myosin are irregularly arranged
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