Muscle Tone

Muscle Electrodes
- Hardware used for detecting changes in muscle tension.
- Key measurements include voltage and frequency.
- Myographs are used to record muscle tension.

Engineers design artificial limbs and powered exoskeletons that mimic natural muscle function.
- These devices allow for intuitive control and help in preventing user fatigue.

Lab-grown muscle tissue aims to treat diseases and injuries, necessitating:
- Comprehensive understanding of exercise, tension, and growth factors.
- These factors trigger hypertrophy (muscle growth) and hyperplasia (increase in number of muscle fibers).

Neural robotic therapy devices are designed based on the understanding of isometric versus isotonic contractions to develop rehabilitation protocols for stroke and injury recovery.

Brain-computer interfaces translate motor unit recruitment patterns from neural signals.
Applications include control of external devices like wheelchairs and computer cursors.

Muscle tension: The force generated when a muscle is stimulated to contract.
- Lab experiments measure tension and display results using a myogram.

A muscle twitch is a brief contraction resulting from a single stimulus.
- Threshold: The minimum voltage required to trigger a twitch.

Latent period: Time after the stimulus before contraction begins, during which there is no change in tension.
Contraction period: Time when tension is increasing, starting as power strokes pull thin filaments.
Relaxation period: Time when tension is decreasing to baseline, initiated by the release of cross-bridges.

A motor unit consists of a single motor neuron and all the muscle fibers it innervates.
- All muscle fibers in a motor unit contract together when the neuron fires, generating movement force.
- Muscles contain a motor pool of multiple motor units, with muscle activity regulated by recruiting different units based on demand.

When stimulating voltage increases, more motor units are recruited to contract, known as recruitment or multiple motor unit summation.
- This explains the varying degrees of muscle force; fewer motor units are recruited to lift small objects (like a pencil) compared to larger ones (like a suitcase).
- Above a certain voltage, all motor units are recruited, causing maximum contraction regardless of further voltage increases.
- Recruitment occurs in order of size: smaller motor units are recruited first, followed by larger ones.

Muscle tension increases with increasing voltage, reaching maximum contractions.
- Voltage increments measured in mV.
Excited motor units:
- Small motor units recruited first.
- Medium motor units recruited next.
- Large motor units recruited last.

Increasing stimulus frequency while keeping voltage constant leads to wave summation:
- Contractile forces are added because relaxation is incomplete before the next stimulus arrives.
- This results in higher overall tensions.
- Ideal stimulus frequency for wave summation is about 20-50 stimuli per second.
If frequency increases further beyond this range, myogram results show incomplete tetany where twitches partially fuse.
Further increasing the frequency (e.g., 40-50 stimuli per second) leads to tetany, characterized by a smooth tension trace with no relaxation phase.
- Note: High-frequency stimuli can cause fatigue, leading to decreased muscle tension.

Muscle tone is defined as the resting tension in a muscle generated by involuntary nervous stimulation.
- Some motor units are randomly stimulated to ensure continuous adjustment, preventing fatigue.
- Muscle tone does not generate sufficient tension for movement and decreases during deep sleep.

Isometric contraction: Tension is insufficient to overcome resistance; muscle length remains the same (e.g., holding a weight still).
Isotonic contraction: Muscle tension overcomes resistance, resulting in movement;
- Tone stays constant while the muscle length changes. This includes:
- Concentric contraction: Muscle shortens while contracting (e.g., biceps brachii when lifting a load).
- Eccentric contraction: Muscle lengthens while contracting (e.g., biceps brachii when lowering a load).

During isometric contraction:
- Muscle tension does not produce movement despite tension generation.
During isotonic contraction:
- Muscle tension results in movement, with muscle shortening (concentric) or lengthening (eccentric).

In contractions, the muscle length correlates with muscle tension:
- Isometric phase: Muscle length remains unchanged while tension is generated.
- Isotonic phase (Concentric): Muscle shortens while contracting.
- Isotonic phase (Eccentric): Muscle lengthens while contracting.

Muscle fatigue refers to the reduced ability to produce muscle tension, primarily due to decreases in glycogen stores.
- Other contributing factors to fatigue include:
- Excitation at neuromuscular junction: Weakened excitation leading to insufficient neurotransmitter entry into the synaptic knob.
- Decreased synaptic vesicles reducing the number of available signals for contraction.
- Excitation-contraction coupling alterations in ion concentrations impairing action potential conduction and force production from the sarcoplasmic reticulum.
- Crossbridge cycling: Excessive Pi (inorganic phosphate) slows the release of ADP from the myosin head, making less Ca²⁺ available.

Sustained exercise induces notable changes in muscle:
- Endurance exercise enhances ATP production (resulting in an increase in the number of mitochondria).
- Resistance exercise leads to muscle hypertrophy, increasing both size due to enhanced syntheses of contractile proteins and greater glycogen reserves.
- Limited hyperplasia occurs (increase in fiber number).
Effects of inactivity include atrophy, defined as a decrease in muscle size due to disuse (e.g., someone in a cast).
- Atrophy is initially reversible, but may become permanent with extreme disuse.

Aging leads to a gradual loss of muscle mass which typically begins in the mid-30s.
- Consequences of this include decreased size, power, and endurance of skeletal muscles.
- There is a loss in the number and diameter of muscle fibers (reduction in myofibrils).
- Age-related changes include decreased oxygen storage capacity, diminished circulatory supply, and reduced recovery capacity after injury.
- A decrease in satellite cells, leading to fibrosis where muscle mass is often replaced by dense connective tissue along with reduced flexibility.