Muscle Mechanics and Energetics

1. The motor unit and the twitch

A motor unit is one motor neuron and all the muscle fibers it innervates; it is the smallest functional unit the nervous system can activate. Motor-unit recruitment is the process of activating more motor units to increase force, with small, fatigue-resistant units recruited first and large units last (the size principle). A single action potential causes a twitch: after a brief latent period (excitation-contraction coupling before tension rises), force climbs in the contraction phase and falls in the relaxation phase as calcium is resequestered.

2. Summation and tetanus

If a second stimulus arrives before the fiber fully relaxes, the twitches add together — summation — because calcium is released again before it has been fully pumped back. As stimulus frequency rises, the fiber produces a sustained, partially fluctuating contraction (incomplete tetanus) and finally a smooth, fused contraction (complete tetanus). Tetanus is normal physiological behavior (not the disease tetanus), and it underlies all graded, smooth voluntary force.

3. Length-tension and force-velocity relationships

The length-tension relationship states that force depends on sarcomere length because it depends on actin-myosin overlap. Force is maximal at the optimal sarcomere length, where overlap is greatest; at shorter lengths the filaments collide, and at longer lengths fewer cross-bridges can form, so force falls. The force-velocity relationship states that a muscle contracts with less force when it shortens faster (and produces the most force when shortening slowly or not at all). Contractions are isometric (tension changes, length constant) or isotonic (length changes against a constant load), the latter divided into concentric (muscle shortens) and eccentric (muscle lengthens while producing force, as when lowering a weight).

How it works

1. A motor neuron fires and produces a twitch in every fiber of its motor unit.
2. Higher force requires recruiting more motor units, smallest first.
3. Higher firing frequency causes summation, then incomplete tetanus, then complete tetanus.
4. Maximal force occurs at optimal sarcomere length and at slow shortening velocities.
5. ATP is regenerated by creatine phosphate (immediate), glycolysis (fast), and oxidative phosphorylation (sustained).
6. Fiber type selects the dominant pathway and sets fatigue resistance.

Common confusions

Memory aids

"Recruit small to large; length and speed set force; PCr-Gly-Ox is the fuel order." Motor units are recruited smallest-first, force peaks at optimal length and slow speed, and the three energy sources run in the order creatine Phosphate → Glycolysis → Oxidative phosphorylation.

Quick review

Topic Recap

Force is graded by motor-unit recruitment (small to large) and by summation of twitches into incomplete and complete tetanus. The twitch's latent, contraction, and relaxation phases reflect excitation-contraction coupling, cross-bridge cycling, and calcium resequestration. Force depends on sarcomere length (maximal at optimal sarcomere length) and on shortening velocity (the force-velocity relationship), with contractions classified as isometric or isotonic (concentric or eccentric). ATP is supplied by creatine phosphate, glycolysis, and oxidative phosphorylation, and slow-oxidative, fast-oxidative, and fast-glycolytic fibers differ in which pathway dominates and how quickly they fatigue.

Knowledge Check

1. What is a motor unit, and how does the nervous system increase force?
2. Name the three phases of a twitch and what causes each.
3. What is the difference between incomplete and complete tetanus?
4. Why does force fall when a muscle is stretched well beyond its optimal sarcomere length?
5. Match each energy source to its timing, and list the three fiber types.

Answers and Rationales

1. A motor unit is one motor neuron plus all the fibers it innervates. Force increases by recruiting more motor units (size principle) and by raising firing frequency. Rationale: recruitment and rate coding are the two ways the nervous system grades force.
2. Latent period (excitation-contraction coupling before tension), contraction phase (cross-bridge cycling while calcium is high), relaxation phase (SERCA calcium removal). Rationale: each phase maps to a distinct physiologic process.
3. Incomplete tetanus is a partially fused, fluctuating contraction at moderate frequency; complete tetanus is a smooth, fused contraction at high frequency. Rationale: fusion depends on whether the next stimulus arrives before calcium has been fully cleared.
4. Because beyond optimal length, actin-myosin overlap decreases, so fewer cross-bridges can form. Rationale: force tracks the number of cross-bridges, which the length-tension relationship describes.
5. Creatine phosphate (seconds, immediate), glycolysis (fast, anaerobic), oxidative phosphorylation (sustained, aerobic). Fiber types: slow-oxidative (Type I), fast-oxidative (Type IIa), fast-glycolytic (Type IIx). Rationale: timing and oxygen use define each pathway and fiber type.