Muscle energy and fatigue

Glycolysis alone nets 2 ATP per glucose; turning pyruvate into lactate makes none but regenerates NAD+. Complete aerobic breakdown gives about 30–32.

• Correct: 2:
• About 30–32:
• 36–38:

ATP binding lets the myosin head detach from actin; splitting it then re-cocks the head. Without ATP, heads stay attached, as in rigor mortis.

• Correct: Makes the head release actin:
• Releases calcium from the SR:
• Pulls tropomyosin over actin:

Stored ATP and creatine phosphate are right beside the myosin heads, and creatine kinase remakes ATP in one step, so they supply most of the ATP in the first seconds of an all-out effort.

• Aerobic respiration of fatty acids: Aerobic respiration is the slowest system to deliver ATP; in the first seconds of a sprint it supplies only a small share.
• Anaerobic glycolysis of glucose from the blood: Glycolysis speeds up within seconds, but in the first 5 seconds creatine phosphate supplies more. And muscle glycolysis runs mainly on stored glycogen, not glucose from the blood.
• Correct: Stored ATP and creatine phosphate: Correct. Creatine phosphate dominates the first seconds.
• Lactate taken up from the blood: Lactate is a fuel for steadier work later. At the start of a sprint, muscle is making lactate, not using it.

Each creatine phosphate remakes one ATP, so the total available is 5 + 20 = 25 mmol. Time = 25 mmol ÷ 2.5 mmol per second = 10 seconds.

• 2 seconds: 2 seconds is how long the stored ATP alone would last (5 ÷ 2.5). Creatine phosphate adds much more.
• 8 seconds: 8 seconds counts only the creatine phosphate (20 ÷ 2.5) and leaves out the stored ATP.
• Correct: 10 seconds: Correct. (5 + 20) ÷ 2.5 = 10 seconds.
• 62.5 seconds: 62.5 comes from multiplying 25 by 2.5. Time is the amount divided by the rate.

A one-minute all-out run spends creatine phosphate early and relies heavily on fast glycolysis, which burns glycogen and produces lactate. Aerobic respiration is climbing too. Through it all, ATP itself falls only modestly, because it is remade almost as fast as it is used.

• Rate of glycolysis: up. Rising ADP and phosphate and falling creatine phosphate speed up the glycolytic enzymes, which supply ATP fast.
• Lactate production: up. Glycolysis makes pyruvate faster than the mitochondria can take it in, so pyruvate is turned into lactate, regenerating NAD+.
• Creatine phosphate: down. Creatine phosphate is spent in the first seconds to remake ATP and is not rebuilt until she rests.
• Muscle glycogen: down. Glycolysis runs mainly on glycogen stored in the fibers, and it uses glucose wastefully, 2 ATP each.
• ATP concentration in the fibers: down. It falls only modestly, to roughly three quarters of its resting level, far less than creatine phosphate, because the energy systems remake ATP almost as fast as it is spent.

The energy systems overlap. At 30 seconds, creatine phosphate is largely spent, glycolysis supplies the largest share and aerobic respiration a growing one. No system switches off; only their shares change.

• Glycolysis has replaced creatine phosphate, which has switched off: Creatine kinase keeps working as long as some creatine phosphate remains, and aerobic respiration runs from the start too.
• Aerobic respiration has taken over now that the start is over: Aerobic respiration is rising, but at 30 seconds of an all-out effort it supplies well under half of the ATP.
• Creatine phosphate is still supplying nearly all the ATP: Stored ATP and creatine phosphate last only about 10 seconds of all-out work.
• Correct: All three run; glycolysis supplies the most: Correct. All three run; their shares shift over time.

Because myoglobin binds oxygen more tightly, oxygen moves from the blood onto myoglobin, and myoglobin gives it up only when oxygen in the fiber drops low, as at the start of exercise or during a strong contraction. It is a small reserve and a shuttle toward the mitochondria.

• Correct: It pulls oxygen from the blood and keeps it until oxygen in the fiber falls very low: Correct. Tighter binding makes it a reserve released at low oxygen.
• It stops the mitochondria from ever running short of oxygen: The store is small, enough for seconds of hard work. Mitochondria can still run short of oxygen in sustained hard exercise.
• It makes the fiber less able to take up oxygen from the blood: Tighter binding does the opposite: it helps draw oxygen from the blood into the fiber.
• It lets the fiber make ATP without mitochondria: Myoglobin only holds oxygen. ATP made with oxygen is made in the mitochondria.

Blood lactate returns to resting levels within about an hour, so it cannot explain soreness two days later. Downhill walking is mostly eccentric work for the thigh muscles, which causes small injuries inside the fibers; the soreness comes from that damage and the repair that follows.

• Lactate has crystallized in the muscle fibers: Lactate dissolves and is cleared or burned within about an hour; it does not crystallize in muscle.
• His muscles have run out of creatine phosphate: Creatine phosphate is rebuilt within minutes of rest.
• Correct: Small fiber injuries from eccentric work: Correct. Delayed soreness comes from small muscle injuries, especially after eccentric work.
• Myoglobin has leaked out of his damaged muscle fibers: A small leak of muscle proteins can accompany the injury, but the soreness itself comes from the injury and repair, not from myoglobin.

Breaking down creatine phosphate and ATP releases inorganic phosphate. It weakens each cross-bridge's pull and reduces calcium release from the SR, and it is one of the best-supported causes of fatigue in hard, repeated contractions.

• Lactate poisoning the myosin heads: Lactate is a fuel, and adding it to rested muscle does not weaken it.
• ATP falling to zero: ATP stays near three quarters of its resting level even in severe fatigue.
• Correct: Inorganic phosphate building up: Correct. Phosphate buildup is a leading cause of peripheral fatigue.
• Myoglobin running out of iron: Myoglobin keeps its iron; it may give up its oxygen, but that does not destroy it.