Muscle Physiology

Muscle physiology: Physiology is the study of the function of living systems, the mechanical, physical, and biochemistry of function and energy transfer. Muscle physiology is concerned with the types of muscle tissues (of which there are three). Here I discuss skeletal muscle specifically. A better understanding of muscle histology, physiology, and function will help us understand how to stretch more effectively.
Skeletal Muscle Morphology
Cross Bridges
Sliding Filament Theory
Energy Stores and Heat Production
Fibre Types
The Motor Unit
The Strength of Skeletal Muscle
Sources of Energy for Muscle Contraction
Factors that Influence Force Generation
Aging and Muscle Physiology

Skeletal Muscle Morphology

Skeletal muscle is made up of individual muscle fibers that are the building blocks of the muscular system.  Most skeletal muscle begins and ends with tendons and the muscle fibers are arranged in parallel between the tendinous ends so that force of contraction is additive. Each muscle fiber is a cylindrical, long multinucleated cell. There are no bridges between cells.
Muscle fibers are made up of fibrils and the fibrils are divisible into individual filaments. The filaments are made up of the contractile proteins.
Skeletal muscle contains the proteins myosin, actin, and tropomyosin.
The muscle fibrils are surrounded by structures made up of membranes that appear in electron photomicrographs as vesicles and tubules. These structures form the sarcotubular system.  The sarcoplasmic reticulum forms an irregular curtain around each of the fibrils between its contacts with the T system of transverse tubules that perforate the cell membrane of the muscle fibers.The function of the T system is to the rapid transmission of the action potential from the cell membrane to all the fibrils in the muscle. muscle morphology

Cross Bridges

The striations are called sarcomeres - the distance between the adjacent "Z lines".  The arrangement of thick and thin filaments that is responsible for the striations is diagrammed. The thick filaments which are about twice the diameter of the thin filaments are made up of myosin, the thin filaments are made up of actin, tropomyosin, and troponin.  The thick filaments are lined up to form the A bands, whereas the array of thin filaments forms the less dense I bands. The lighter H bands in the centre of the A bands are the regions where, when the muscle is relaxed, the thin filaments do not overlap the thick filaments.
  • A Band:   thick and overlapping thin filaments (myosin and actin/tropomyosin/troponin)
  • I Band:    thin filaments only (actin/troponin/tropomyosin)
  • Z Line:   anchors thin filaments
  • H Band: myosin
The attachment and detachment of the myosin  cross bridges to actin produces the shortening of the sarcomere and hence the shortening of the muscle fiber (muscle contraction).
muscle physiology

Sliding filament theory

Shortening of the sarcomere is the result of the formation and subsequent detachment of bonds between myosin and actin causing the filaments to slide past one another. sliding filament
  1. An action potential from a lower motor neuron reaches the motor end plate of the muscle fiber
  2. Acetylcholine is released by the neuron into the synaptic cleft
  3. Acetylcholine binds to receptors on the motor end plate resulting in depolarization of the cell membrane of the muscle fiber
  4. This action potential travels throughout the sarcotubular system causing calcium to flood into the muscle cell
  5. Calcium binds to the troponin protein causing movement of the tropomyosin to allow myosin to bond to actin molecules
  6. This cross bridge then moves causing the sarcomere to shorten resulting in muscle contraction.
  7. ATP binds to the myosin releasing the cross bridge
  8. As actin molecules are still exposed this binding and releasing will continue to occur causing further shortening of the sarcomere
  9. Relaxation occurs only when calcium is no longer being released into the cell faster than it is being pumped out of the cell

Energy Stores and heat production

Muscle contraction requires energy. The muscle fibers effectively convert chemical energy into mechanical energy. This of course is not 100% efficient and so releases heat. The immediate source of this energy is the phosphate derivatives in muscle produced from the metabolism of carbohydrates and lipids.

Thermodynamically. the energy supplied to a muscle must equal its energy output. The energy output appears in work done by the muscle contraction, the formation of phosphate bonds, and heat. The overall mechanical efficiency of skeletal muscle is up to 50 percent.  Consequently, heat production is significant.

The heat given off when at rest, is the external manifestation of the basal metabolic processes. Following contraction, heat production in excess of resting heat continues for as long as thirty minutes. This recovery heat is the heat created by the metabolic processes that restore the muscle to its precontraction state. If a muscle that has contracted isotonically is restored to its previous length, extra heat  is produced (relaxation heat). External work must be done on the muscle to return it to its previous length and relaxation heat is mainly a manifestation of this work.

Energy and work continues to take place at the cellular level long after physical work of the muscle has taken place.

Fibre Types

Fibre types vary based on isoenzyme activity of the myosin and the metabolism of the muscle.  Muscles containing many type 1 muscle fibers are called "red" muscles because they contain more myoglobin, and are darker than other muscles. These respond slowly and have a long latency, are adapted for long, slow posture maintaining contractions. A lot of the back supporting muscles  are red muscles. "White" muscles which contain mostly type IIB fibers achieve short twitch durations and are specialized for fine skilled movement. The muscles of the eyes and some hand muscles are fast muscles.
TypeI Type IIB TypeIIA
Myosin ATPase slow fast fast
sarcoplasmic reticulum activity moderate high high
Diameter moderate large small
Oxidative capacity high low high
The  fast/slow twitch fibre composition not only varies amongst muscles, but also amongst individuals. This variability in muscle physiology has been correlated with athletic performance. In mixed fibre muscles a short distance runner may have 75 % fast twitch, and a long distance runner may have as much as 75% slow twitch in the same muscle. Selective training for a preferential fibre type is beneficial for task specific sports.

The Motor Unit

A motor unit is several muscle fibers innervated by a single motor neuron. The number of muscle fibers in a motor unit varies. In muscles of the hand  and eyes that require fine graded and precise movements, there are 3-6 muscle fibers per motor unit. Large muscles of the back in humans can contain more than 150  muscle fibers per motor neuron. motor unit

Each muscle fiber in a motor unit is of the same type; hence, motor units are divided into fast and slow units. Generally speaking, slow motor units are innervated by small, slowly conducting motor neurons and fast units by large rapidly conducting motor neurons. In large muscles the small slow units are first recruited during movements, are resistant to fatigue, and are the most used motor units. The fast motor units, which are more easily fatigued, are generally recruited with more forceful movements.

The Strength of Skeletal Muscle

Muscle physiology research has shown that mammalian skeletal muscle is capable of exerting 3-4 kg of tension per square centimeter of cross sectional area.

Sources of energy for muscle contraction

The high energy bond in the terminal phosphate  of ATP provides energy for muscle contraction.
  • Creatine phosphate - creatine phosphokinase allows creatine phosphate to convert ADP back to ATP. This reaction does not require oxygen and takes place quickly during short contractions. It is the primary source of energy to a muscle during contractions of 10 seconds or less.
  • Anaerobic glycolysis - During anaerobic glycolysis the muscle cell uses intracellular gycogen and glucose. Oxygen is not required and the energy lasts 30 to 60 seconds of the muscle fiber contraction. Lactic acid is the byproduct of this reaction.
  • Oxidative phosphorylation - Glucose and fat are oxidized to provide this energy. energy graph

Factors that Influence Force Generation

Human beings will forever be trying to outdo each other. Training  focuses on increasing speed, strength, and power in order to maximize athletic performance. Muscle physiology shows that the musculoskeletal system and neuromuscular system are complex and possess nonlinear intrinsic properties which vary depending on the environment, and the sport.

The mechanical output of a muscle depends on several intrinsic properties:
Force-length relationship
There is an optimal muscle length at which a maximum force can be generated. Force declines when the muscle is in a longer or shorter state.
Passive elasticity
Elasticity of the connective tissue component of the muscle (fascia, epimysium, perimysium, etc) also contributes to the overall force generated when a muscle is stretched beyond its optimal length.
Force-velocity relationship
There is a relationship between the force developed by a muscle and and the rate of change in a muscles length. As a muscle shortens more quickly the force that the muscle can generate declines exponentially. When a muscle is undergoing an eccentric contraction (lengthening and contracting)  the force generated increases as the speed of lengthening increases to a point.
Timing of muscle activation
There are delays between the time  the nervous system recruits a muscle fibre and the time the muscle fibre contracts. This is primarily due to the movement of calcium and cross bridge formation. Because muscles take time to "turn on"and "turn off"  there will always be a delay that can affect athletic performance.

Extrinsic properties that influence force are as follows:
  • stretch induced force enhancement
  • muscle temperature

Aging and Muscle Physiology

Research into muscle physiology shows that muscle loss with aging is primarily due to loss of muscle fibres, particularly fast twitch fibres (sarcipenia).  This leads to a reduction in the performance of the muscle. The rate of muscle loss occurs as follows:
Age 24 to 50        10% loss
Age 50 +              0.5 - 1.4% loss per year

This change will influence performance in terms of muscle power and muscle strength. According to muscle physiology studies muscle strength generally peaks  in both women and men in their 20's and this remains relatively unchanged until their 50's. At 60 muscle strength declines by approximately 15% or more each decade. Reduced muscle power is also probably due to changes in contraction velocity and neural activity.

Training has been shown to reduce the rate of loss of strength and power that comes with aging. Some athletes continue to compete into their 80's. With the loss of fast twitch fibres the older athletes tend to prefer endurance sports. In any marathon you will see a large number of older athletes in the 50-70 age range. This understanding of muscle physiology can help everyone train smarter.

Further Reading on Muscle Physiology
Deschenes MR. 2004. Effects of aging on muscle fibre type and size. Sports Med. 34:809–2

Frontera WR, Hughes VA, Fielding RA, Fiatarone MA, Evans WJ, Roubenoff R. 2000. Aging of skeletal muscle: a 12-yr longitudinal study. J. Appl. Physiol. 88:1321–26

Janssen I, Heymsfield SB, Wang ZM, Ross R. 2000. Skeletal muscle physiology mass and distribution in 468 men and women aged 18–88 years. J. Appl. Physiol. 89:81–88

Rantanen T, Masaki K, Foley D, Izmirlian G, White L, Guralnik JM. 1998. Muscle physiology, Grip strength changes over 27 years in Japanese-American men. J. Appl. Physiol. 85:2047–53

Vaillancourt DE, Larsson L, Newell KM. 2003. Effects of aging on force variability, single motor unit discharge patterns, and the structure of 10, 20, and 40 Hz EMG activity. Muscle physiology Neurobiol. Aging 24:25–35

Zajac FE. 1989. Muscle and tendon: properties, models, scaling, and application to biomechanics and motor control. Crit. Rev. Biomed. Eng. 17:359–411

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