This is a 10-week human study involving 24 younger (20-35 y) and 24 older (65-85 y) healthy individuals. All participants will undergo unilateral immobilization of a knee for 7-10 days, followed by 4 weeks of heavy resistance exercise training (HReT). Half of the participants (12 younger and 12 older) will also undergo 4 weeks HReT prior to the immobilization. Prehabilitative exercise may confer protective effects on subsequent immobilization, and the various underlying mechanisms involved

Inclusion Criteria:

• Age: Between 20-35 years old or between 65-85 years old.
• BMI: Between 18.5 and 35 kg/m2.
• Healthy and well, with normal dietary habits (not vegan).

Exclusion Criteria:

• More than one muscle biopsy previously taken from both vastus lateralis muscles.
• Smoking.
• Pregnancy.
• Past or present substance or alcohol abuse.
• Medical/surgical conditions that are assessed to affect protein synthesis or the participant's involvement in the trial.
• Knee pain that could affect the completion of the training intervention.
• High intake of supplements that increase the risk of bleeding during muscle biopsy (such as fish oil and ginger).
• Participation in regular strength training within the past year.
• Regular participation in structured sports.
• Current use of blood-thinning medication.
• Previous participation in trials involving oral intake of heavy water.
• Metal in the body preventing MRI scanning.
• For the older group, not having gone through menopause, and for the younger group, not having a regular menstrual cycle.