Which body muscle can resist fatigue? a) Skeletal muscle b) Cardiac muscle c) Smooth muscle d) Both a & b
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The textbook source confirms the answer clearly. Here is the full explanation:
Answer: c) Smooth Muscle
Smooth muscle is uniquely resistant to fatigue among the three muscle types.
Comparison of the Three Muscle Types
| Property | Skeletal Muscle | Cardiac Muscle | Smooth Muscle |
|---|---|---|---|
| Contraction speed | Fast | Intermediate | Slow, sustained |
| Fatigue resistance | Low to moderate (fatigues with sustained activity) | Moderate (rhythmic, but requires constant blood supply) | High - resistant to fatigue |
| Control | Voluntary | Involuntary | Involuntary |
| Location | Limbs, trunk | Heart | Viscera, blood vessels |
Why smooth muscle resists fatigue
As stated directly in Gray's Anatomy for Students:
"Its [smooth muscle's] contractions are less powerful than those of skeletal muscle, and it is resistant to fatigue."
And from Medical Physiology:
"Smooth muscle, like cardiac muscle, contracts only briefly... but must sustain this rhythmic activity for a lifetime."
The key reasons smooth muscle resists fatigue are:
- Slow, energy-efficient contractions - uses the "latch mechanism" where myosin cross-bridges remain attached with minimal ATP consumption, allowing prolonged force maintenance at very low energy cost.
- Aerobic metabolism predominates - smooth muscle relies primarily on oxidative phosphorylation, not anaerobic glycolysis (which generates fatiguing lactate).
- Functional necessity - smooth muscle lines the walls of blood vessels, the GI tract, the uterus, and airways. These structures must contract tonically for hours to years without rest.
Why the other options are wrong
- Skeletal muscle (a) fatigues readily, especially with high-intensity, sustained effort. Fast-twitch (Type II) fibers fatigue quickly; even slow-twitch (Type I) fibers fatigue eventually.
- Cardiac muscle (b) has excellent fatigue resistance for rhythmic contractions, but it does not maintain tonic sustained contractions like smooth muscle, and it depends heavily on continuous oxygen delivery - any ischemia leads to rapid dysfunction (angina, infarction).
- Both a & b (d) is therefore incorrect.
The correct answer is c) Smooth muscle.
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