Physiology of Muscle Contraction

Properties:

	Contractility (obvious)
	Irritability - responds to stimulus
	Conductivity - stimuli is sent throughout the muscle fiber 
			(throughout the muscle in cardiac tissue)

Stimulation:
	Chemical - normal
	Electrical - physician
	Mechanical - hard blow (charley horse)

Studying Muscle Contracture

	Plotting - Myography
		Differences between myography and normal contracture
			1. All nerve fibers are enervated at once, as opposed to a gradual recruitment.
			2. A single stimulation is sent, as opposed to a volley 
				(slow volley is  5 to 6 per second, fast = 80-90).

	Approximations:  as measured in a frog gastrocnemius
		Latency period (0.01 seconds)
			Time after the stimulus is applied, and before the muscle begins to contract

		Contractile Phase (0.04 seconds)

		Relaxation Phase (0.05 seconds)

Summation of Muscle Contractions

	A single stimuli results in a single contraction (twitch)
		- This stimuli must be above a threshold level for it to cause a contraction
	If another stimuli is administered before a muscle Relaxes completely,
		contractile forces increase (summation)
	This can increase to a point of constant contraction (Tetanus)

Effect of Temperature

	Cooling a muscle increases contraction time (21% - 82%)
	Heating a muscle decreases contraction time (12%) and relaxation time (22%)
	Exercise induced heating is preferred to manual heating (ambient)
	Training results in increased muscle relaxation after exercise.

All or None Law
	If a stimuli causes a contraction, that contraction is maximal for the current conditions 
		(Nutrition, temperature, etc.)
	This only applies to a single motor unit
	This does not mean that a maximal contraction has occurred, as summation causes that.


Gradation of Response
	Two phases:   Recruitment and Rate Coding	

	Recruitment
		Fibers recruited from smaller motor units to larger motor units
			Allows the forces generate to be increased gradually
			Is accomplished by thresholds for recruitment / activation
				Smaller motor units have lower thresholds, are recruited more quickly
				Oxidative (low force) fibers also have lower thresholds
				This is called the "Size Principle" (should be size/type principle)

		Torn muscles around a single joint (hamstrings)
			Reciprocal Inhibition (agonist causes antagonist to relax)
				the force of the contraction

	Rate Coding
		Refers to the number of impulses sent
		Recall that more muscle force can be generated by increasing either:
			The number of muscle fibers recruited, or
			The rate that the impulses are sent (summation)
		Muscles with large amounts of muscle fibers, such as the deltoid, 
			predominantly recruit more fibers to increase the strength of a contraction
		Muscles with smaller numbers of motor units rely on increasing the stimuli to increase

		* Stressful or hypnotic states have seen people perform superhuman feats
		Is believed to be related to Rate Coding, or
			increasing the stimuli beyond normal levels in large muscles.

Muscle Fatigue

	With repeated stimuli, both contraction and relaxation begin to wane
		(Less of each occurs)
	The effect on the relaxation phase is larger
	When no relaxation is possible, this is contracture (often called muscle cramping)
	Where does fatigue occur ?
		Older research suggested the myoneural junction
			Nervous tissue is highly unfatigueable, and 
			Researchers have exhausted a muscle voluntarily and then stimulated it
				This has resulted in additional contraction
				This suggests that the muscle is not the primary source of fatigue
		Newer investigations have found that the action potential is undiminished in the muscle
			This suggests that the myoneural junction is not at fault
			Theory is that coupling between action potential and contraction
		This supports the idea of an acidic environment (lactate) inhibiting muscle contraction

	In the untrained performer, the large gains in muscular strength (20-50%) from weight training
		programs during the first few weeks are nearly all neural

Types of Contractions

	Contraction refers to muscle tension, not necessarily just muscle shortening
		Isometric
		Isotonic
		Isokinetic
		Concentric vs. Eccentric (positive vs. Negative work, w=fd)


Mechanical Factors

	Angle of Pull
		90 degrees to bone is optimal
		Forces transmitted are all used for movement, not stabilization
	Length of muscle (Length - Tension Relationship)
		Resting length is optimal
		More muscle mass is available for cross bridges to occur
	Speed of Contraction (Force - Velocity Curve for concentric / eccentric)
		Concentric: slower contractions result in more force
		Eccentric: faster contractions result in more force

Prestretch

	Muscles put on stretch before voluntary contraction
		perform more total work
		have a greater force output