Roles or Functions of Muscles

1. Roles or Functions of Muscles • Agonists - Muscles which produce movement by concentric contraction • Primary Movers or Primary Agonists – Always recruited for a given movement • Secondary Movers, or Assistant Agonists – Recruited when there is additional resistance • Antgonists – Slow down or resist movement caused by agonists or an external force by means of eccentric contraction

2. Roles or Functions of Muscles • Stabilizers – Keep a limb or body segment stabilized (sometimes with an isometric contraction) • Ex. Rotator cuff muscles • Neutralizers – Prevent unwanted movement of one of the agonist muscles. • Ex. Elbow flexion - pronator teres recruited to counteract supination caused by biceps brachii

3. 2 Joint Muscles • a muscle and/or its tendons cross more than one joint advantage: (1) moving two joints simultaneously occurs in normal movement Running Walking Climbing stairs Throwing Kicking • (2) Co-contractions - protection of joints, bones

4. 2 Joint Muscles Advantages • (3) Multi-joint muscles combine to enhance movement. • Ex. hamstrings + quadriceps in power based activities such as sprinting and jumping (4) 2 joint muscle can be stretchedimmediatelybefore needed for joint movement ex. Gastrocnemius stretched by knee extension before needed for plantar flexion (walking, running, jumping)

5. 2 Joint Muscles • Cross two or more joints • May involve whole muscle group of part Biceps Femoris Long Head – Active in both knee flexion and hip extension (2 joint). Short Head – Only active in knee flexion.

6. 2 Joint Muscles (continued) The rectus femoris of the quadriceps group is active in both knee extension and hip flexion. Right Leg – Anterior View

7. 2 Joint Muscles (continued) The gastrocnemius is active in both knee flexion and plantar flexion. The hamstrings are active in both knee flexion and hip extension. Right Leg – Posterior View

8. 2 Joint Muscles • examples: • 1. rectus femoris • knee extension & hip flexion • 2. hamstrings (biceps femoris, semimembranosus, semitendinosus) • knee flexion & hip extension • 3. gastrocnemius • plantar flexion & knee flexion • 4. triceps brachii (long head) • elbow extension & shoulder (glenohumeral) extension • 5. biceps brachii • elbow flexion & shoulder (glenohumeral) flexion (short head)

9. 2 Joint Muscles • Muscles can produce or affect movement at any joint they cross • Examples: • The hamstring muscles cross the knee and hip and are therefore active in both hip extension and knee flexion. • The gastrocnemius is active both in knee flexion and plantar flexion. • The rectus femoris of the quadriceps is active in both knee extension and hip flexion

10. Disadvantages of 2 Joint Muscles • Active Insufficiency – muscle shortens • reduces # of cross-bridges • . Example: when the knee is in flexion (as in seated calf raises), the gastrocnemius produces little or no force in plantar flexion..

11. Disadvantages of 2 Joint Muscles • Passive Insufficiency – When 2 joint muscle stretched - high passive and active tension limiting range of motion • Example: when the knee is fully extended, tension in the gastrocnemius restricts the range of dorsiflexion at the ankle. When the knee is flexed, the gastrocnemius places less restriction on dorsiflexion.