This study involves 24 younger (20-35 years) and 24 older (65-85 years) healthy adults. Participants will have one knee immobilized for 7-10 days, then do 4 weeks of heavy resistance exercise training (HReT). Half of them will also do 4 weeks of HReT before the knee is immobilized. Immobilization means keeping the knee still and not moving it. The study aims to see if exercising before immobilization can help protect the knee and understand how it works.

• Duration: 10 weeks total with 7-10 days of knee immobilization and 4 weeks of exercise.
• Participants: Must be between 20-35 or 65-85 years, healthy, and not have certain medical conditions.
• Considerations: Must be okay with having a muscle biopsy and not regularly exercise already.