Do you feel sluggish and stiff when you start your workouts? Here's a simple way to get loose — and have more stamina.

A study published in the Journal of Sport and Health Science found that you can power up your muscles' performance with passive warmups (using heating pads or a hot shower) or active warmups (10 minutes on the stationary bike or mimicking less intensely your upcoming exercise by, for example, doing knee raises while walking or lifting light hand weights).

And for every 1.8 F your muscle temperature rises, you boost your power and speed by 3.5%.

The advantage of active warmups is that they alert your nervous system to upcoming movement patterns. The researchers recommend that you do them until you start to sweat lightly. Then you're ready for your workout.

But if you have trouble even doing the warmups, it's time to get tested for exercise intolerance. People with obesity, Type 2 diabetes, and inflammatory conditions are at risk. Your doctor may have you do a six-minute walking test to see how far you can go.

You can also undergo an exercise stress test that tracks your blood pressure, heart rate and rhythm, and oxygen level, or a cardiopulmonary exercise test that measures the depth and speed of your breaths.

If you're exercise intolerant, the Cleveland Clinic says eating more protein, getting enough vitamin D, losing weight, and treating conditions that interfere with working out are smart moves.