SN4
SHORT NOTE
5M
Differentiate between isotonic and isometric contraction.
Definitions
Isotonic contraction = the muscle shortens while tension remains constant.Isometric contraction = tension develops while the muscle length remains constant.
Comparison
| Feature | Isotonic | Isometric |
|---|---|---|
| Length | Changes (shortens) | Constant |
| Tension | Constant | Increases |
| External work done | Yes (work = load × distance) | No external work |
| Energy fate | Work + heat | All converted to heat |
| Latent period | Longer | Shorter |
| Oxygen consumption | Less | More for the same tension |
| Blood flow | Maintained (pump action) | Occluded → fatigues faster |
| Example | Walking, lifting a weight, flexing the elbow | Standing, holding a weight steady, pushing a wall |
Points of Importance
- Most natural movements are mixed — isometric first until tension equals the load, then isotonic
- In isometric contraction efficiency is zero in the physical sense, since no external work is done
- Isometric exercise raises blood pressure sharply (pressor response), as sustained contraction occludes the vessels
- Auxotonic contraction — both length and tension change, as in the heart
Concentric and Eccentric Contraction
| Type | Muscle | Example | Tension |
|---|---|---|---|
| Concentric | Shortens while contracting | Biceps lifting a weight up | Lower |
| Eccentric | Lengthens while contracting | Biceps lowering a weight slowly; quadriceps walking downhill | Highest |
- Eccentric contraction generates the greatest tension and is the chief cause of delayed onset muscle soreness
Applied Aspects
- Isometric exercise is avoided in hypertension and ischaemic heart disease because of the marked pressor response
- Isotonic (aerobic) exercise is preferred in cardiac rehabilitation — it improves cardiac output with less rise in blood pressure
- Isometric contraction of the quadriceps is used in physiotherapy when the knee joint must be immobilised
All questions in Nerve & Muscle Physiology
- Q1Define action potential. Describe its phases and ionic basis. Add a note on the refractory period. ★
- Q2Describe the structure of the neuromuscular junction and the steps of neuromuscular transmission. Add a note on the drugs acting on it. ★
- Q3Describe the sliding filament mechanism of skeletal muscle contraction. Explain excitation–contraction coupling and the role of calcium. ★
- Q4Classify nerve fibres. Describe the mechanism of impulse conduction and compare saltatory with continuous conduction. Add a note on nerve degeneration and regeneration. ★
- Q5Compare the structure and properties of skeletal, cardiac and smooth muscle. Describe the mechanism of smooth muscle contraction. ★
- SN1Write a short note on myasthenia gravis. ★
- SN2Write a short note on rigor mortis. ★
- SN3Define motor unit. Describe the innervation ratio and the size principle. ★
- SN4Differentiate between isotonic and isometric contraction.
- SN5Describe the changes following section of a peripheral nerve. Add a note on regeneration. ★
- SN6Describe the length–tension relationship of skeletal muscle and give its significance.
- SN7Write a short note on muscle fatigue. Add a note on oxygen debt.