Muscle function- grading the strength of contraction

===Lecture Details===
Wayne Sturrock; Week 4 MED1022; Physiology
===Lecture Content===
Work = load x distance, muscle converts chemical energy (ATP) into electrical energy manifested as work. 20-25% is converted, the rest is dissipated as heat. The two most important features are the amount the muscle shortens and the velocity at which it shortens. The lighter the load the further and faster the muscle can shorten. Velocity = distance shortened/time. In an isotonic contraction, as a weight becomes heavier the speed at which the muscle shortens becomes slower and the maximum distance it can shorten becomes less. The relationship between load and velocity of shortening is the force- velocity relation. Fastest velocity occurs with zero load. Vmax is determined by myosin ATPase activity because this determines how quick crossbridges can cycle. Lowest velocity is 0 because muscle cannot shorten enough to move load (isometric contraction). Further than this is lengthening contraction (eccentric). 
Power is the rate at which work is done. Power = work/time = load x distance/time = force x velocity. Depends on how heavy the load is and how fast it is moving it. Maximum power is a balance between force and velocity, is when there is about 1/3 maximum force and 1/3 maximum velocity. 
Twitch contraction lasts a lot longer than the AP that set it off. Potential lasts about 2-5msec, contraction about 100msec. Summated contractions are multiple APs that build on each other, giving a higher overall force. Summation is a mechanical event associated with contraction. 
Tetanic contraction is response to repetitve stimulation, builds to tetanic contraction. At low frequencies tetanic contractions are unfused, at high frequencies they are fused. It can produce a maximum force the muscle is capable of generating. This is due to Ca usually being taken up quickly when released in one burst so it can't create a maximum contraction, with repeated APs it sticks around and can generate a full force. It is possible to do because the AP is so much shorter than the contraction itself. 
Each muscle fibre only recieves one nerve terminal. A motor unit is the group of muscle fibres innervated by one neuron. They vary widely in size. Small motor units are used for fine, delicate movements, large in coarse, powerful. Motor units can be recruited to allow for grading of contraction and allows the contractile work to be shared between fibres. Asynchronous activation allows for constant whole muscle force while allowing periods of rest for motor units. Smaller motor units usually have lower thresholds for excitation. 
Strength of contraction is graded by varying muscle length, varying frequency of excitation and varying number of motor units recruited.
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