Young, healthy muscle (left column) appears pink and red. In contrast, the old muscle is marked by scarring and inflammation, as evidenced by the yellow and blue areas. This difference between old and young tissue occurs both in the muscle's normal state and after two weeks of immobilization in a cast. Exercise after cast removal did not significantly improve old muscle regeneration; scarring and inflammation persisted, or worsened in many cases. Credit: Morgan E. Carlson and Irina M. Conboy, UC Berkeley