Muscle Energy Systems and Fiber Types: A Comprehensive Guide

All muscle contraction depends on ATP

ATP supply depends on availability of oxygen and organic

energy sources (for example, glucose and fatty acids); two main pathways of ATP synthesis: Anaerobic fermentation, Aerobic respiration/metabolism

Anaerobic fermentation

Enables cells to produce ATP in the absence of oxygen; yields little ATP and lactate (lactic acid - which needs to be disposed of by the liver); glycolysis (breaks down 1 glucose molecule to produce 2 ATP molecules by an enzyme); occurs in the sarcoplasm without the need for oxygen to be present; it is the primary energy source for short term maximum activity (30-40 seconds)

Aerobic respiration/metabolism

Uses the mitochondria to produce far more ATP than anaerobic fermentation; does not generate lactate; requires a continual supply of oxygen for the mitochondria; involves the Citric Acid Cycle and Electron

Transport Chain, both are located in the mitochondria; is the primary energy source at rest and for long term activity (activity after 40 seconds); uses both glucose and fatty acids to generate ATP; produces 30 molecules of ATP in the mitochondria

Immediate energy

For short, intense exercise (Ex: 100 m dash); oxygen is briefly supplied by myoglobin inside the muscle cell,

but this oxygen reserve is rapidly depleted; muscles meet most ATP demand by borrowing phosphate groups (Pi) from other molecules and transferring them to ADP to produce more ATP

Creatine kinase

Enzyme that obtains Pi from a phosphate-storage molecule creatine phosphate (CP) and gives it to ADP; catalyzes the reaction: ADP + Pi → ATP

Phosphagen system

The combination of ATP and CP which provides nearly all energy for short bursts of activity (~6 secs of sprinting)

Short-term energy

As the phosphagen system is exhausted, muscles shift to

anaerobic fermentation; muscles obtain glucose from blood and their own stored glycogen; in the absence of oxygen, glycolysis can generate a net gain of 2 ATP for every glucose molecule consumed; converts glucose to lactate (lactic acid) which then enters the blood stream

Anaerobic threshold (lactate threshold)

Point at which lactate becomes detectable in the blood

Glycogen-lactate system

The pathway from glycogen to lactate; produces enough ATP for 30-40 s of maximum activity

Long-term energy

After about 40 s, the respiratory and cardiovascular

systems start to deliver oxygen fast enough for aerobic

respiration to meet most of muscle's ATP demand; aerobic respiration produces more ATP per glucose than glycolysis does (another 30 ATP per glucose molecule); efficient means of meeting the ATP demands of prolonged exercise; after 3-4 min, the rate of oxygen consumption levels off to a steady state where aerobic ATP production keeps pace with demand; for 30 mins, energy comes equally from glucose and fatty acids; beyond 30 min, depletion of glucose causes fatty acids to become the more significant fuel

Muscle fatigue

Progressive weakness from prolonged use of muscles; fatigue in high-intensity (short duration) exercise results from: Potassium accumulation, ADP and Pi accumulation,; fatigue in low-intensity (long duration) exercise results from: Fuel depletion, Electrolyte loss, Central fatigue

Potassium accumulation

In the T tubules reduces muscle cell excitability by interfering with Ca2+ release from the SR

ADP and Pi accumulation

Slows cross-bridge movements, inhibit Ca2+ release and decreases force production in myofibrils

Fuel depletion

Glycogen and glucose levels decline

Electrolyte loss

Through sweat; decreases muscle excitability

Central fatigue

Ammonia released by active muscles inhibits motor neurons and cause less motor signals from the brain to the muscle cells

Excess postexercise oxygen consumption (EPOC), or oxygen debt

Elevated rate of oxygen consumption (excessive heavy/deep breathing) following exercise; EPOC can be six times the basal O2 consumption and can last an hour

Purpose: aerobically replenish ATP (some of which helps regenerate CP stores); replace oxygen reserves on myoglobin; provide oxygen to liver that is busy disposing of lactate, the liver converts lactate back to glucose when there is O2 present, the glucose then enters the blood stream and is delivered to muscle; provide oxygen to many cells that have elevated metabolic rates after

exercise

Skeletal muscle fibers types

Classified into two major physiological classes: Slow-twitch, slow oxidative (SO), red fibers, or type I fibers; Fast-twitch, fast glycolytic (FG), white fibers, or type II fibers

Note: most human muscles are a mixture of fast/slow fibers (their ratio in a muscle is genetically determined) and the muscle appears pink. Athletic training can "convert" some fast fibers to intermediate fibers which are more resistant to muscle fatigue; every muscle contains a mix of fiber types, but one type

predominates depending on muscle function; fiber type within a muscle differs across individuals; some individuals seem genetically predisposed to be sprinters, while others more suited for endurance

Slow-twitch, slow oxidative (SO), red fibers, or type I fibers

Well adapted for endurance, slow to contract; resist fatigue by oxidative (aerobic) ATP production; important for muscles that maintain posture (e.g., erector spinae of

the back, soleus of calf); grouped in small motor units controlled by small, easily excited motor neurons allowing for precise movements; thin cells with abundant mitochondria, capillaries, myoglobin (deep red color) and contain a form of myosin with slow ATPase, and a SR that releases calcium slowly

Fast-twitch, fast glycolytic (FG), white fibers, or type II fibers

Fibers well adapted for quick responses; utilize glycolysis (anaerobic fermentation) for energy, doesn't need O2; fibers are thick and strong; low myoglobin, low blood supply gives them pale color; abundant in quick (eye and hand muscles) and powerful muscles (gastrocnemius of calf and biceps brachii); grouped in large motor units controlled by larger, less excitable neurons allowing for powerful movements, but fatigues quickly; contain a form of myosin with fast ATPase and a large SR that releases calcium quickly

Intermediate, or fast oxidative (FO) fiber type

Known mainly in other mammals but relatively rare in humans except in some endurance-trained athletes