Muscle Roles in Movement

Muscle Roles

• Muscles rarely act alone; groups of muscles interact to produce desired movements.
• This interaction allows a single muscle to serve multiple roles.

Agonist

• These muscles are the prime movers.
• They perform the main action.
• Example: Iliopsoas in hip flexion

Secondary Movers

• These muscles assist the agonist.
• Example: Sartorius in hip flexion

Antagonist

• These muscles oppose the prime mover's action.

• They relax and lengthen to allow agonist movement

• Located on the opposite side of the joint from the movers.

• Example: Plantar flexors are antagonists to dorsiflexors during ankle dorsiflexion.

Synergists

• These muscles work together with agonists.
• They either contract or relax to modify agonist action.
• They prevent unwanted movements.
• There are different types of synergists:

Conjoint Synergists (Mutual Neutralizers)

• Two muscles work together to create a movement neither could do alone.
• Example: medial and lateral hamstrings during pure knee flexion

Neutralizers

• They neutralize undesired actions of prime or secondary movers.
• Example: Lateral rotators counteract undesired motion of adductors during hip adduction.

Stabilizers or Fixators

• These muscles surround proximal joints, contracting to allow distal joints to move smoothly.
• Their contractions are usually isometric.
• Example: Rectus abdominis muscles stabilize proximal attachments of psoas major during straight leg raises.

Support

• Muscles that hold body parts in position during an action.

• They act isometrically (same length) to hold body parts in place.

• Contract in agonist and antagonist simultaneously, usually in stressful situations.

• Example: Longitudinal trunk muscles stabilize the elbow during flexion against resistance.

Range of Muscle Extensibility and Contractility

• Length and arrangement of muscle fibers (e.g., longitudinal vs. pennate).
• Number of joints the muscle crosses.
• Resistance of antagonist muscles.
• Load opposing the muscle.

Physiological Cross-Section of a Muscle

• Area of section that cuts every muscle fiber perpendicularly.
• Determines the potential force of contraction.
• Absolute muscle strength: ~3-4 kg per cm² cross sectional area

Muscle Architecture and Contraction

Longitudinal Muscles

• Long fibers parallel to muscle's long axis.
• Create wide range of motion but with less force.
• Smaller physiological cross-sectional area.
• Example: Sartorius

Pennate Muscles

• Short fibers arranged obliquely to muscle's long axis.
• Create more force for a shorter range of motion.
• Larger physiological cross-sectional area.
• Example: Rectus femoris (bi-pennate)
• Pennation angle increases the force, but decreases contractile strength.

Differences Between One-Joint and Two-Joint Muscles

One-Joint Muscle

• Ideal for movement in a single joint.
• Sufficient shortening to move the segment without maximum angular displacement.

Two-Joint Muscles

• Ideal for performing movement patterns across multiple joints.
• Disadvantage of active or passive insufficiency

Muscle Insufficiency

Active Insufficiency

• Muscle cannot shorten further without losing tension.
• Example: Maximal hip flexion with knee extension.

Passive Insufficiency

• Muscle cannot stretch further without causing pain.
• Example: Forced hip flexion with maximal knee extension.

Tendon Action of Two-Joint Muscles (Tenodesis)

• Passive tension without active contraction.
• Muscle's elongation creates movement across two or more joints.
• Example: Wrist movement and finger flexion.

Case Study

• A 65-year-old woman has limited elbow flexion. What are the possible causes? (This needs additional context for a full answer).