In older, normal-weight women (60-75 yrs, BMI=18-28 kg/m2), an acute bout of higher-intensity (60% of maximum effort) aerobic exercise, in comparison to an isocaloric bout of lower-intensity (45% of maximum effort) exercise (caloric expenditure of 3.5 kcal/kg body weight for both), will result in differential:

1. decline in non-exercise activity calorie expenditure in the subsequent 48 hours after exercise;
2. increase in resting metabolic rate 48 hours after exercise;
3. increase in serum free triiodo-L-thyronine (free T3) concentrations immediately following exercise;
4. decline from pre-exercise in plasma leptin and serum free T3 concentrations 48 hours after exercise.

Inclusion Criteria:

1. Women
2. Age 60-75 years
3. normal weight
4. Weight-stable (± 5%) within the previous 3 months
5. Non-smoking for last year
6. Able to provide own transportation to study visits
7. Not currently involved in any other research study
8. Willing and able to participate in all aspects of the trial including research testing
9. Willing to give informed consent to participate

Exclusion Criteria:

1. Self-reported significant cardiovascular disease
2. Other self-reported medical conditions such as diabetes, chronic or recurrent respiratory conditions, active cancer, musculoskeletal disease
3. Medications known to affect exercise performance or metabolism
4. Excess caffeine use (> 500mg/day)
5. Any self-reported contraindications to exercise
6. Clinical depression
7. Abnormal cognitive function -