Describe Various type of actions possible at the muscles. 5 marks
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Types of Actions Possible at Muscles
Muscle actions can be classified in two complementary ways: (A) by the functional role of the muscle and (B) by the type of contraction produced.
A. Functional Roles of Muscles
1. Prime Mover (Agonist)
The muscle primarily responsible for producing a specific movement. It generates the main force for the action.
- Example: Biceps brachii is the prime mover for elbow flexion.
2. Antagonist
A muscle that opposes the action of the prime mover. While the agonist contracts, the antagonist relaxes in a coordinated manner to allow smooth, controlled movement. Antagonists work in concert with agonists during the gait cycle to advance the limb through space.
- Example: Triceps brachii is the antagonist during elbow flexion.
3. Synergist
A muscle that assists the prime mover by adding force to the same movement or by eliminating unwanted movements that the prime mover would otherwise produce.
- Example: Brachialis assists biceps brachii in elbow flexion.
4. Fixator (Stabilizer)
A muscle that stabilizes the origin of the prime mover so it has a firm base from which to act. Fixators hold proximal segments steady while distal segments move.
- Example: Muscles of the rotator cuff stabilize the shoulder joint (glenohumeral joint) when arm movements are performed.
5. Neutralizer
A muscle that cancels out an unwanted secondary action of the prime mover, allowing only the desired movement to occur.
- Example: If a muscle both flexes and abducts a joint, a neutralizer prevents the abduction, allowing only flexion.
B. Types of Muscle Contraction
Based on the relationship between muscle tension and length change, contractions are classified as follows (Miller's Review of Orthopaedics):
| Type | Definition | Example |
|---|---|---|
| Isotonic Concentric | Muscle shortens during contraction; internal force > external force | Curling phase of a biceps curl (elbow into increasing flexion) |
| Isotonic Eccentric | Muscle lengthens during contraction; internal force < external force | Lowering phase of a biceps curl (elbow into extension) |
| Isometric | Muscle tension is generated but length remains unchanged | Pushing against a wall; most activity during normal gait |
| Isokinetic | Muscle contracts at constant velocity through full ROM | Exercises using a Cybex/dynamometer machine |
Most everyday muscle activity is eccentric — muscles activate while lengthening to control and decelerate movement, making eccentric contractions the most efficient for strengthening but also carrying the greatest risk of muscle injury. — Miller's Review of Orthopaedics, 9th Edition
Summary (5-mark key points)
- Agonist/Prime mover — produces the primary movement
- Antagonist — opposes the prime mover; ensures smooth joint motion
- Synergist — assists and refines the prime mover's action
- Fixator/Stabilizer — stabilizes the proximal origin of the acting muscle
- Types of contraction — concentric (shortening), eccentric (lengthening, most common), isometric (no length change), isokinetic (constant velocity)
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