Muscle Mechanics and Motor Units: Physiology and Electromyography

MOTOR UNIT

Single somatic motor neuron and all the skeletal muscle fibers it controls.

Motor Point

Specific region on muscle where a motor neuron enters.

Minimum Electrical Resistance

Location of minimum electrical resistance → maximum muscle twitch response.

SMALL MOTOR UNITS

One neuron controlling

LARGE MOTOR UNITS

2,000 muscle fibers, responsible for powerful, large scale movement.

SPARK OF CONTRACTION

Signal arrives = electrical signal travels down motor neuron → neuromuscular junction (NMJ).

Neurotransmitter Release

@ NMJ, neuron releases ACh → crosses tiny gap and binds to receptors on muscle fiber.

SINGULAR MUSCLE TWITCH

Single, brief contraction measured to understand different phases of muscle response.

LATENT PERIOD

Muscle has been excited but hasn't contracted.

CONTRACTION PERIOD

Muscle is actively shortening, generating tension.

RELAXATION PERIOD

Muscle returns to original length as Ca pumped back.

SUMMATION

Nervous system adds contractions together to provide more powerful force.

SPATIAL SUMMATION

Summing in space, voltage; how we fine tune muscle force from a light twitch to a powerful grip.

THRESHOLD STIMULUS

The minimum stimulus required to elicit a response.

TEMPORAL SUMMATION

Summing in time, frequency; if stimuli are delivered fast enough, muscle can't relax in between.

TETANUS

Contractions fuse into a smooth, sustained state due to rapid stimuli.

ELECTROMYOGRAPHY

Used to detect, amplify, measure electrical activity of muscles.

MUSCLE FATIGUE

During sustained, maximum contraction (tetanus), muscle force will eventually decline.

Depletion of Energy Reserves

Causes of muscle fatigue include depletion of ATP.

Metabolic Byproducts

Build up of metabolic byproducts (lactic acid) contributes to muscle fatigue.

Reduced Blood Flow

Reduced blood flow or oxygen supply can lead to muscle fatigue.

Ion Concentration Changes

Changes in ion concentrations (potassium) affect electrical signals and contribute to fatigue.