Muscle Contraction and Movement: Sliding Filament Mechanism and Coordination

Skeletal muscle contraction follows the sliding filament model:

1. Action potential → Ca²⁺ release from sarcoplasmic reticulum.
2. Ca²⁺ binds troponin C → tropomyosin shifts → exposes myosin‑binding sites on actin.
3. Myosin heads bind actin, undergo power stroke (ADP/Pi release), pulling thin filaments toward center.
4. ATP binds myosin → cross‑bridge detachment; ATP hydrolysis re‑cocks myosin head.
5. Cycle repeats while Ca²⁺ and ATP are available.

Force Gradation

• Motor unit recruitment (size principle).
• Firing rate (frequency coding).
• Muscle length (optimal at L₀).
• Velocity (force‑velocity relationship).

Synergistic Coordination

Agonists, antagonists, synergists, and fixators work together. Example: walking—quadriceps extend knee while hamstrings flex; co‑activation stabilizes joint.