Muscle loss through the years
Starting in your 30s, you can expect to lose as much as 1 to 2% of your muscle mass each year for the rest of your life. As the amount of muscle shrinks, you burn fewer calories and your ability to metabolize food diminishes, leaving you more vulnerable to weight gain, diabetes, and other negative health conditions.
Maximize metabolism in minutes
Interval training - alternating between high-intensity bursts of movement and a moderate pace - has been shown to amp up metabolism for up to 24 hours post workout. "You don't have to do a lot to see the benefits," says Wayne Westcott, PhD. "Aim for 15-25 minutes of interval training 3-4 days per week." If you're just getting started or have a lot of weight to lose, do walking or stationary cycling intervals, which are easier on the joints (or take to the water for extra safety and shock absorbency). If you want to challenge yourself, lace up your training shoes and jump rope or go for a run.
Source - Alex Hutchinson, Ph.D. - Author of Which Comes First, Cardio Or Weights