Low-intensity steady-state exercise performed at 60 to 70% of maximum heart rate increases mitochondrial density in human skeletal muscle.

Study: Effects of Exercise Training on Mitochondrial and Capillary Growth in Human Skeletal Muscle: A Systematic Review and Meta-Regression

The study found that low-intensity endurance exercise, like steady walking or cycling, boosts the number of energy-producing parts in muscles, which supports the idea that this type of workout improves muscle health.

Study: Markers of clinical and mitochondrial adaptation in response to moderate intensity continuous training: A systematic review and meta-analysis

The study looked at moderate, steady exercise like brisk walking or light cycling, and found it increased the number of energy-producing parts in muscles, which supports the idea that this type of exercise boosts muscle health.

Study: Passive heat therapy in sedentary humans increases skeletal muscle capillarization and eNOS content but not mitochondrial density or GLUT4 content.

The study looked at a type of light to moderate exercise that matches the kind described in the claim, and it found that this exercise increased the number of energy-producing parts in muscles, which supports the idea that this type of workout improves muscle health.