Muscle Tension

Motor Unit

A somatic motor neuron and all the skeletal muscle fibers it stimulates for contraction and relaxation

Average Motor Unit Size

A single motor neuron typically controls about 150 muscle fibers.

Motor Unit Distribution

Muscle fibers of a motor unit are spread throughout a muscle rather than grouped together.

Small Motor Units

Contain few muscle fibers and allow precise, controlled movements.

Large Motor Units

Contain many muscle fibers and generate powerful movements.

Muscle Strength: Determinants

Depends on motor unit size and the number of motor units activated.

Motor Unit: Memory Trick

Small units = precision; large units = power.

Twitch Contraction

A brief contraction of all muscle fibers in a motor unit in response to a single action potential that’s triggered by a single action potential in a motor neuron, typically last 20–200 milliseconds (msec).

Myogram

A recording or graph of muscle contraction.

Latent Period

The brief delay between the stimulus and the beginning of muscle contraction, typically lasts 2 milliseconds, in which muscle action potential spreads across the sarcolemma and Ca²⁺ is released from the sarcoplasmic reticulum.

Contraction Period

The phase when muscle tension increases and reaches its peak, typically lasts 10–100 milliseconds, in which Ca²⁺ binds troponin, binding sites are exposed, and cross-bridges form.

Peak Tension

The maximum force produced during the contraction period.

Relaxation Period

The phase when muscle tension decreases and the muscle returns to resting state, typically 10–100 milliseconds., in which Ca²⁺ is pumped back into the SR, tropomyosin blocks binding sites, and cross-bridges detach.

Refractory Period

A period when a muscle fiber temporarily cannot respond to another stimulus from losing excitability immediately after being stimulated, typically 1 millisecond (skeletal) and 250 milliseconds (cardiac)

Second Stimulus During Refractory Period

The muscle responds to the first stimulus but not the second.

Twitch Contraction: Three Phases

Latent period → Contraction period → Relaxation period.

Motor Unit Recruitment

The process of activating additional motor units to increase muscle force, in which small motor units are recruited first, followed by larger motor units as force demands increase.

Muscle Tone

A continuous state of slight muscle contraction caused by alternating activation of small groups of motor units; keeps muscles firm but does not produce movement.

Muscle Tone: Function

Helps maintain posture, keeps muscles ready for action, supports organs, aids digestion, and helps maintain blood pressure.

Muscle Tone: Flaccid Muscle

A limp muscle that has lost muscle tone due to damage or loss of its motor nerve supply.

Isotonic Contraction

A contraction in which muscle tension remains relatively constant while the muscle changes length for body movements and moving objects.

Isotonic Contraction: Types

Concentric isotonic contractions and eccentric isotonic contractions.

Concentric Isotonic Contraction

A contraction in which the muscle shortens while generating enough tension to overcome resistance and produce movement. (e.g., lifting a book from a table using the biceps brachii.)

Eccentric Isotonic Contraction

A contraction in which the muscle lengthens while continuing to contract to make crossbridge resist the load and slow the muscle's lengthening (e.g., lowering a book back onto a table in a controlled manner)

Eccentric vs. Concentric Contractions

Concentric contractions shorten the muscle; eccentric contractions lengthen the muscle.

Eccentric Contractions and Muscle Soreness

Repeated eccentric contractions tend to cause more muscle damage and delayed-onset muscle soreness (DOMS) than concentric contractions.

Isometric Contraction

A contraction in which tension is generated but the muscle does not change length due to the tension produced is not great enough to overcome the resistance of the object being moved (e.g., holding a book still without moving it)