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Influence of Immobilization on Muscle

General Overview

• Immobilization is commonly used after injury or surgery, and it can occur with or without direct muscle injury.

• For instance, a fractured bone might require casting and immobilization, indirectly affecting associated muscles due to inactivity.

Impact on Muscle Protein Balance

• Normally, muscle protein synthesis and degradation are balanced.

• During immobilization, mechanical loads on muscles are significantly reduced, leading to increased protein degradation compared to synthesis.

• This imbalance results in muscle atrophy, characterized by a decrease in muscle size and strength.

• Animal studies indicate up to a 30% loss in muscle volume due to immobilization.

Effects of Positioning During Immobilization

• The position of the muscle during immobilization greatly influences atrophy outcomes.

• Greatest atrophy occurs when muscles are held in a shortened range, leading to a loss of sarcomeres in both series and parallel arrangements.

Clinical Implications of Atrophy

• Muscle atrophy negatively impacts muscle performance as muscle size diminishes.

• Altered force-length relationships:

  • Normal muscle function is represented by a specific force-length relationship (blue line).

  • When immobilized in a shortened position (red line), this relationship is disturbed, which can have substantial clinical implications post-immobilization.

Effects on Other Tissues

• Immobilization affects various tissues beyond just muscle, including:

  • Connective tissues

  • Tendons

• Tendons may experience changes in density and proportion, which can lead to increased stiffness.

• Long-term immobilization may also lead to fatty deposits in the muscle, contributing to reduced flexibility and increased stiffness, especially when muscles are held in shortened positions.

Summary

• The consequences of immobilization underscore the importance of movement and the balance of muscle protein dynamics for maintaining muscle health and function.