Muscle Tension Notes

Muscle tension is the force generated when a skeletal muscle contracts.
Myograms are graphical recordings of the changes seen in muscle tension when a muscle is stimulated.
A twitch is a single contraction/relaxation cycle of a muscle that is caused by a single stimulus.
A single EPP (End-Plate Potential) was produced, leading to a single action potential sent through the muscle.

A muscle twitch can be divided into three distinct periods:

Latent Period: This is the time it takes for calcium to be released from the sarcoplasmic reticulum (SR), bind to troponin, and for myosin to start the cross-bridge cycle. During this phase, no tension is generated yet.
Contraction Period: This is when cross-bridge cycling is actively occurring, leading to an increase in muscle tension.
Relaxation Period: Calcium is returned to the SR, cross-bridges begin to release, and connectin returns the muscle to its resting length, causing a decrease in muscle tension.

Skeletal muscles are consciously (voluntarily) controlled by the primary motor cortex within the cerebrum.
Lower motor neurons extend from the anterior horn of the spinal cord to the muscles.
Skeletal muscle fibers are individually stimulated, and every muscle fiber must have a synapse with a motor neuron at the neuromuscular junction.
A motor unit consists of one motor neuron and all the muscle fibers it stimulates.
Small motor units have less than five muscle fibers, allowing for precise control (e.g., moving fingers for typing or moving eyes for focus).
Large motor units have several thousand muscle fibers, generating more force but with less precision.
The size of the motor unit determines the degree of control.

Once an EPP is generated within a single muscle fiber, the muscle fiber will contract completely (All-or-nothing principle).
To generate varying levels of tension within a single muscle, the number of muscle fibers stimulated within that muscle needs to differ.
More muscle fibers stimulated; more tension generated.
To stimulate more muscle fibers, more motor units need to be recruited. This process is called Motor Unit Recruitment.
To recruit more motor units, the strength of the stimulus needs to increase. An increased amount of neurotransmitter released from the upper motor neurons will stimulate more lower motor neurons (thus more motor units).
Maximum tension of a muscle will be generated only when all motor units within that muscle have been stimulated.

The frequency of stimulation will affect the amount of muscle tension generated by a single muscle fiber.
If a muscle fiber is stimulated but allowed to completely relax before being stimulated again, it will produce a single twitch each time, generating the same level of tension.
If a muscle fiber is stimulated before it can fully relax from the previous stimulus, the “contractile forces” will add to the previous contraction and have a “summed” effect.

Twitch: Occurs with low frequency stimulation (less than 10 stimuli per second), allowing full relaxation between stimuli.
Wave Summation (Temporal Summation): Occurs when a muscle fiber is stimulated before it can fully relax from a previous stimulus, leading to an increase in tension. The “contractile forces” summate.
Incomplete Tetany: Results from stimulating the muscle with a frequency of 20-50 stimuli per second, allowing for very little relaxation between stimuli. Tension increases but does not reach maximum due to some calcium returning to the SR between stimuli.
Tetany (Complete Tetany): Occurs when the muscle is stimulated at a high frequency, allowing for no relaxation between stimuli. All calcium is released from the SR, none is returned, and maximal tension is generated, leading to a sustained contraction.

The amount of tension generated by a single muscle fiber depends on how many cross-bridges are involved in the contraction at the same time.
This is dependent on how much calcium is bound to troponin.
More calcium allows for more cross-bridges to be formed, generating more tension.
When a muscle fiber is stimulated before it can fully relax, not all calcium within the sarcoplasm can be returned to the SR before more calcium is released from the terminal cisternae.
This causes an increased calcium concentration within the sarcoplasm, allowing for more myosin binding sites to be exposed and a greater number of cross-bridges to be formed.

Skeletal muscles never fully relax, even when they aren’t being used.
A few motor units are always contracted due to involuntary somatic nervous stimulation.
Motor units being stimulated change continuously, so motor units do not become fatigued.
This ‘resting’ tension (muscle tone) is not enough to cause movement but:
- Establishes a constant tension on the muscle's tendon to stabilize the position of the bones and joints.
- “Primes” the muscle for contraction so it can respond more readily when stimulated.
- Maintains ‘resting’ length of muscle.

The amount of tension generated is influenced by the amount of overlap of thick and thin filaments when the muscle fiber is stimulated.
At resting length, the muscle can generate the maximum amount of tension because there is an optimal overlapping of the filaments.
This allows for the greatest number of cross-bridges to form when the muscle is stimulated.
If the muscle is already contracted or overly stretched when stimulated, less tension can be generated.
Fewer cross-bridges are formed due to:
- Already using all the cross-bridges (contracted).
- Not enough overlapping of the filaments (overstretched).

Isometric contractions occur when muscle tension generated is insufficient to overcome the resistance, or muscle tension is generated just enough to hold the load but not move it.
Skeletal muscle contracts, and muscle tension increases, but muscle length stays the same.
Examples:
- The muscle is maxed out and can’t generate any more tension (e.g., pushing on a wall or lifting a weight that is too heavy).
- The brain doesn’t want to move the load but hold it in place, so just enough tension is generated to hold the object but not move it (e.g., holding a baby in one position). Note: Tension will continue to increase, and over time, holding the baby will fatigue the arms.

Isotonic contractions occur when muscle tension generated is sufficient to overcome the resistance and moves the load.
Tension generated by the muscle remains constant as the length of the muscle changes.
Movement occurs at the joint.
Two types:
- Concentric Contraction: Occurs while the muscle shortens (e.g., lifting a baby).
- Eccentric Contraction: Occurs while the muscle lengthens (e.g., putting a sleeping baby into a crib). Still need tension to hold the load, but less tension is needed than in concentric contraction.