Muscle fiber types and exercise

Myoglobin is a red, iron-containing protein that binds oxygen in the cytosol of muscle fibers and releases it when oxygen in the fiber falls.

• Correct: Myoglobin:
• Creatine kinase:
• Troponin:

Anaerobic glycolysis nets 2 ATP per glucose, fast but wasteful. Aerobic respiration yields about 30–32 per glucose but needs oxygen and delivers ATP more slowly.

• Correct: 2 compared with about 30–32:
• About 30–32 compared with 2:
• The same number, but faster:

Stem cells are unspecialized cells that divide to renew themselves and produce daughter cells that differentiate into specialized cells.

• Correct: An unspecialized cell that can divide and give rise to specialized cells:
• A mature cell that has stopped dividing:
• A cell that has lost its nucleus:

Total = 90 + 70 + 40 = 200. Only slow oxidative fibers are slow-twitch: 90 ÷ 200 = 0.45, or 45%.

• 35%: 35% is 70 ÷ 200, the fast oxidative fibers, which are fast-twitch.
• Correct: 45%: Correct. 90 ÷ 200 = 45%.
• 55%: 55% is the fast-twitch share: (70 + 40) ÷ 200.
• 80%: 80% adds slow oxidative and fast oxidative fibers ((90 + 70) ÷ 200). Fast oxidative fibers are fast-twitch.

Oxidative fibers depend on oxygen arriving from capillaries around them. A small diameter keeps every mitochondrion close to the surface, so the diffusion distance for oxygen is short. A glycolytic fiber does not wait for oxygen, so it can be thick and hold many myofibrils.

• A thin fiber holds more glycogen per unit of its volume: Slow oxidative fibers actually have low glycogen stores; the glycogen-rich fibers are the thick, glycolytic ones.
• A thin fiber contracts faster than a thick one: Contraction speed depends on the myosin version and calcium handling, not on diameter. Slow oxidative fibers are thin and slow.
• Correct: Oxygen has less distance to diffuse: Correct. Short diffusion distance supports aerobic metabolism.
• A thin fiber needs no myoglobin: Slow oxidative fibers have the most myoglobin of all.

At marathon pace, small motor units of slow oxidative fibers do most of the work aerobically. A sprint recruits the large, fast glycolytic units too, which make ATP fast by glycolysis, produce lactate and tire quickly, so the burst cannot last.

• Recruitment of fast glycolytic fibers: up. By the size principle, the largest motor units, made of fast glycolytic fibers, join only when near-maximal force is needed.
• Rate of anaerobic glycolysis: up. Fast glycolytic fibers make their ATP mainly by glycolysis, and every working fiber's ATP demand rises steeply.
• Lactate production: up. Glycolysis makes pyruvate faster than the mitochondria can take it in, so pyruvate is converted to lactate.
• Share of ATP from aerobic respiration: down. Aerobic respiration keeps running, but glycolysis now supplies a much larger share of the far greater total.
• How long she can keep this pace: down. Fast glycolytic fibers fatigue within a minute or two as glycogen falls and phosphate builds up.

This is hypertrophy: existing fibers make more actin and myosin and add myofibrils, so each fiber's diameter grows. Muscle stem cells fuse with the fibers and add nuclei to support the extra protein.

• Existing fibers dividing into two: Mature skeletal muscle fibers never divide, and hyperplasia adds few if any fibers in adult humans.
• Correct: Existing fibers adding myofibrils: Correct. Hypertrophy of existing fibers.
• Fat building up between the fibers: Training builds muscle protein, not fat. Fat replacing fibers happens in diseases such as muscular dystrophy.
• Tendons lengthening to make room for more fibers: Tendons do not grow to hold new fibers; fiber number stays about the same.

In a cast, motor neurons still fire and fibers still get some activity and nerve contact. With the nerve cut, the fibers receive no action potentials at all. They atrophy severely, and over months to years, without reinnervation, they are replaced by fat and connective tissue.

• Milder atrophy than in the cast: Loss of the nerve causes more atrophy, not less; lack of strain is not protective here.
• About the same atrophy as in the cast: Denervated muscle loses much more than muscle that keeps its motor neurons.
• Hypertrophy as the fibers contract on their own: Skeletal muscle fibers contract only when their motor neuron fires; cut off, they cannot drive their own growth.
• Correct: Worse atrophy, ending in replacement by fat and scar: Correct. Denervation atrophy is severe and, without regrowth of the axons, permanent.

This is Duchenne muscular dystrophy. Without dystrophin, each contraction tears the sarcolemma, fibers die and leak creatine kinase, and repair gradually falls behind. Fat and connective tissue take the place of the lost fibers, so the calves look big while being weak.

• Hypertrophy of normal fibers compensating for weakness: Some fibers may enlarge early, but the bulk of the enlarged calf is not muscle; the boy is getting weaker, not stronger.
• Correct: Fibers replaced by fat and connective tissue: Correct. The large calves are fat and connective tissue, not muscle.
• Extra fibers made by hyperplasia: Hyperplasia adds few if any fibers in skeletal muscle, and the fibers here are dying.
• Swelling from lactate stored in the fibers: Lactate is not stored in fibers; it leaves and is used as fuel within about an hour.

Slow oxidative fibers belong to the smallest motor units, which are recruited first, so gentle walking uses mainly them. A maximal jump recruits all motor units, from small to large, so all three types contract.

• Walking: fast glycolytic. Jumping: slow oxidative: It is the reverse: low-force tasks use the small slow units, and a maximal effort adds the large fast ones.
• Walking and jumping: all three types equally: Recruitment follows force. A slow walk needs only a small part of the muscle's units.
• Correct: Walking: mainly slow oxidative. Jumping: all three types: Correct. Small slow units first; large fast units join for maximal force.
• Walking: none. Jumping: fast glycolytic: Walking uses the calf muscles. And even in a jump, the small slow units fire first and stay active as the larger ones join.