Basics in Exercise Physiology

Patricia A. Deuster

Selasi Attipoe

The opinions and assertions expressed herein are those of the authors and should not be construed as reflecting those of the Uniformed Services University of the Health Sciences or the Department of Defense.

SKELETAL MUSCLE PHYSIOLOGY

Skeletal Muscle Fibers

Basic Units

The basic units of skeletal muscle include sarcomeres, myofilaments, myofibrils (basic units of contraction), muscle fibers (cells), fascicles (bundles of about 150 muscle fibers each), and a muscle (Fig. 5.1). Sarcomeres, areas from Z line to Z line and functional contraction units of myofibrils, contain two types of myofilaments: thick (myosin) and thin (actin) myofilaments, which are repeated throughout muscle myofibrils.
Other structures within sarcomeres are I bands, A bands, Hzones (absence of actin), and M bands (sarcomere’s center). Transverse tubules (T tubules; see Excitation-Contraction Coupling), found at the A-I junction of sarcomeres, and the sarcoplasmic reticulum (SR) are the primary regulators of calcium influx into muscle units. The sarcolemma is the muscle cell membrane.
Satellite cells are muscle “stem” cells. They are activated to a full cell cycle from the quiescent state in response to heavy resistance training and cell damage. Activation of satellite cells is necessary for normal growth and regeneration of tissue damage. Insulin growth factor-1 (IGF-1) is a primary regulator of satellite cells (27,28).

Muscle Proteins

Key muscle proteins include actin, myosin, troponin, and tropomyosin. Actin and myosin constitute myofilaments within myofibrils; myosin is the site of adenosine triphosphate (ATP) binding. Troponin complexes on actin molecules are the calcium binding sites for initiating contraction. Tropomyosin, found in thin filaments of muscle fibers, inhibits contraction until modified by troponin.

Muscular Contraction

Motor Neurons

Alpha and gamma motor neurons initiate and regulate muscle contraction. Alpha motor neurons initiate contractions in contractile (extrafusal) fibers, whereas gamma neurons innervate muscle spindle (intrafusal) fibers of muscle.

Motor Unit

Motor units (basic functional unit of movement) consist of alpha motor neurons, synaptic junctions (motor endplate), and the muscle fibers they innervate.
One motor neuron can innervate 10 to several thousand muscle fibers. Release of acetylcholine by neurons initiates contraction, and degradation of acetylcholine by cholinesterase terminates action potentials.

Excitation-Contraction Coupling

Muscle contraction is triggered by an electrical impulse involving acetylcholine release, arrival of the impulse at the sarcolemma (after crossing the synaptic junction), and subsequent entry of calcium into myofibrils. Electrical impulses travel quickly into the interior of muscle cells, down the T tubules with release of calcium from the SR to myofibrils.
Once calcium stores are released from the SR, a nonstoppable contraction is initiated: Calcium entry activates crossbridge linkages and contraction of sarcomeres.
Sliding-Filament Theory: This theory states that muscle contraction occurs when two major myofilaments (actin and myosin) slide past one another through a series of crossbridge linkages. Myosin crossbridge linkages combine, detach, and recombine in an oscillatory pattern such that at any point in time about 50% of myosin heads are attached to actin binding sites. No myofilament actually changes length.
All or None Law: A motor neuron initiating an action potential will contract all muscle fibers innervated by that motor neuron simultaneously. Muscle fibers achieve gradation of contraction strength by recruiting fewer or more motor neurons to initiate contraction and/or by changing the frequency of action potentials to sustain contractions.

Sensory Receptors

Muscle Spindles: These specialized intrafusal muscle fibers are located between and among extrafusal fibers deep within the interior of muscles. These proprioceptors sense and relay the length or velocity of muscle movement (e.g., patellar reflex).
Golgi Tendon Organs: These sensory organs detect differences in tension and muscle length. Although quiescent during
periods of slow to moderate muscle activity, they increase discharge when muscle load is excessive; they protect against injury due to overloading.

Figure 5.1: Basic anatomy and structure of skeletal muscle. Reprinted with permission from http://www.medicdirect.com and http://www.medicdirect.co.uk.

Skeletal Muscle Fiber Types

Skeletal muscle fiber types are characterized by differences in morphology, histochemistry, enzyme activity, surface characteristics, and functional capacity (17,18,19,23,24,27) (Table 5.1).

Table 5.1 Characteristics of Major Skeletal Muscle Fiber Types*

Fiber Characteristics	Slow Twitch	Fast Twitch
	Type I	Type IIa	Type IIx
Other Terminology	Slow Oxidative (SO)	Fast Oxidative Glycolytic (FOG)	Fast Glycolytic (FG)
Aerobic Capacity	High	Med/High	Low
Glycolytic Capacity	Low	High	High
Myoglobin Content	High	Med	Low
Color	Red	Red	Pink/White
Fatigue Resistance	High	Med	Low
Glycogen Content	Low	Med	High
Triglyceride Content	High	Med	Low
Time to Peak Tension	Slow	Med	High
Myosin ATPase Activity	Low	Med	Med
Myosin Heavy Chain (MHC)	MHCIβ	MHCIIa	MHCIIx
Tension Cost*	Low	Med	Med
ATP/ADP	Low	Med	Med
ADP, adenosine diphosphate; ATP, adenosine triphosphate; Med, medium.
This chart represents current nomenclature for human skeletal muscle fiber types. Type IIb human muscle fibers are currently referred to as Type IIx because of the MHCIIx isoform found in human muscle. Type IIb muscle fibers are found in rodents and other species, with MHCIIb (19,21,22,23).
*Tension cost = ATPase activity to isometric tension ratio.

Human muscle fiber types include Type I, IIa, and IIx, due to the expression of different myosin heavy chain (MHC) forms (18,21,22,24).
Distribution of Type I (slow twitch) and Type II (fast twitch) fibers within normal populations depends on many factors
and shows extraordinary adaptive potential in response to innervation/neuronal activity, hormones, neural signaling, training, functional demands, and aging (19).
Muscle fiber types appear to change in response to these effectors in a sequential manner from either slow to fast or fast to slow (6,8,19).

BASIC DEFINITIONS IN EXERCISE PHYSIOLOGY

Pulmonary Ventilation

Minute ventilation ([V with dot above]E) is the total volume of air moved into and out of the lungs each minute; it is a function of tidal volume (V_T) and respiratory rate (f_B).
At rest, [V with dot above]E is between 5 and 7 L · min⁻¹, whereas during exercise it increases to between 60 and 180 L · min⁻¹, depending on the health of the person (Fig. 5.2). V_T increases by expanding both inspiratory and expiratory volumes; these “extra” volumes are called inspiratory and expiratory reserve volumes (IRV and ERV), respectively. The increase in V_T allows [V with dot above]E to increase 5- to 10-fold during exercise. An increase in f_B further augments [V with dot above]E (1,2,26).

Maximal Voluntary Ventilation (MVV)

MVV is the volume of air exchanged during repeated maximal respirations in a specified time (10-15 seconds). It is expressed as L · min⁻¹ and represents a measure of maximum breathing capacity for comparing to maximal [V with dot above]E during exercise ([V with dot above]E_max).

Breathing Reserve (BR)

BR is the difference between MVV and [V with dot above]E_max and is sometimes expressed as [V with dot above]E_max/MVV; it is the additional ventilation available during maximal exercise. If a person achieves maximal work capacity prior to attaining MVV, the person has a normal BR, whereas if MVV = [V with dot above]E, the person may have compromised pulmonary function. A typical BR is 11 L · min⁻¹, with normal [V with dot above]E_max/MVV ranging between 60% and 75%.

Figure 5.2: Tidal volume and breathing rate (left panel) and oxygen uptake and minute ventilation (right panel) in response to progressive maximal treadmill exercise. Dotted line through [V with dot above]E in right panel indicates departure from linearity.

Oxygen Uptake ([V with dot above]O₂)

Oxygen uptake ([V with dot above]O₂), based on the Fick equation, can be expressed as: [V with dot above]O₂ = CO × (a − vO₂) = HR × SV × (a – vO₂), where CO is cardiac output, a – vO₂ is arteriovenous O₂ content difference, SV is stroke volume, and HR is heart rate. For an average 70-kg adult, CO is about 5 L · min⁻¹ at rest and can increase to 20 to 30 L · min⁻¹ during strenuous exercise.
[V with dot above]O₂, determined during exercise by measuring respiratory gases, is related to the fractional percent of O₂ in inspired and expired air and [V with dot above]E. Inspired air contains 20.93% O₂ and expired air around 17.0%.
[V with dot above]O₂ is usually expressed in absolute units (L · min⁻¹) or relative to body weight (mL · kg⁻¹ · min⁻¹). Resting [V with dot above]O₂ ranges from 0.25 to 0.4 L · min⁻¹, and maximal exercise [V with dot above]O₂ values can exceed 5.0 L · min⁻¹ (15-70 mL · kg⁻¹ · min⁻¹) (Fig. 5.2). Higher relative values indicate higher aerobic fitness.