CoursesPhysiologyMuscular systemSkeletal muscleSkeletal muscle force production

Skeletal muscle force production

Learning objectives

After completing this study unit, you will be able to:

Describe how muscle twitches contribute to tension generation.
Explain the relationship between muscle length and tension.
Compare and contrast isotonic and isometric contractions.

Introduction

Our central nervous system can finely tune the amount of force our muscles produce. The smallest force increment a muscle can produce is called a twitch, and it corresponds to the tension a muscle fiber can generate in a single contraction-relaxation cycle.

The tension generated by a muscle twitch depends on several factors, including:

Muscle fiber type: smaller muscle fibers generate lower tension for a longer time than larger fibers
Stimulation frequency: stimulation of a muscle fiber before it has fully relaxed results in overlapping muscle twitches that generate a higher peak tension than a single twitch
Length-tension relationship: excessively shortened or stretched sarcomeres before the start of the contraction generate lower tension

The central nervous system activates groups of fibers innervated by the same motor neuron, known as motor units. To increase muscle tension, the central nervous system increases the frequency of neural inputs to the active motor units (rate coding) and activates additional motor units (motor unit recruitment). Motor units are typically recruited according to the size principle, from smaller to larger. This strategy helps finely grade the amount of muscle force produced while limiting fatigue.

Muscle contraction can be classified into different types depending on how muscle length changes with contraction. During isometric contraction, gross muscle length does not change. During isotonic contractions, the muscle can shorten (concentric contraction) or lengthen (eccentric contraction).

Explore concepts

Wave summation and tetanus

Overlapping muscle twitches produce a higher peak tension due to wave summation. Depending on the stimulation frequency, wave summation can result in fused or unfused tetanus.

Length-tension relationship

Since muscle tension depends on the overlap between actin and myosin, sarcomere length prior to contraction affects muscle tension generation.

Motor unit: structure and function

A motor unit comprises a motor neuron and all the muscle fibers it innervates. The central nervous system modulates muscle force production by regulating the number of active motor units and their stimulation frequency.

Contraction types

Muscle contraction can be classified into different types depending on how muscle length changes during the contraction.

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Summary

Key points about muscle force production
Muscle twitch	Tension generated in a single contraction-relaxation cycle Duration primarily depends on muscle fiber type Peak tension primarily depends on muscle fiber type, stimulation frequency and sarcomere length prior to contraction.
Wave summation	Overlapping muscle twitches generate higher peak tension than individual twitches Can generate sustained contractions (tetanus): Unfused tetanus: submaximal tension, the muscle fiber partially relaxes between twitches Fused tetanus: maximal tension due to continuous crossbridge cycling
Length-tension relationship	Twitch tension depends on sarcomere length prior to contraction (determines actin and myosin head overlap, and therefore the number of crossbridges) Optimal length: maximal overlap zone, maximal peak tension Sarcomere <80% optimal length: lower tension (actin interferes with crossbridge binding) Sarcomere >120% optimal length: lower tension (fewer myosin heads can bind to actin
Motor unit	A motor neuron and all the muscle fibers it innervates To increase the tension produced: Recruitment (activation of additional motor units) Rate coding (higher stimulation frequency) Size principle: smaller motor units recruited before large ones; allows precise force control and limits fatigue
Muscle tone	Background level of contraction when the muscle is at rest Useful to stabilize joints and maintain posture High tone: hypertonia; low tone: hypotonia
Contraction types	Isometric: load = tension, constant gross muscle length Isotonic concentric: load < tension, muscle shortens Isotonic eccentric: load > tension, muscle lengthens