The Centers for Disease Control and Prevention offer these exercise recommendations for adults age 18 to 64:
• 2½ hours of moderate-intensity aerobic activity every week and muscle-strengthening activities that work all major muscle groups at least two days a week.
• 1 hour and 15 minutes of vigorous-intensity aerobic activity every week and muscle-strengthening activities that work all major muscle groups at least two days a week.
• An equivalent mix of moderate- and vigorous-intensity aerobic activity and muscle-strengthening activities that work all major muscle groups at least two days a week.
Every day, our bodies are burning calories.
We burn calories when we exercise, whether it's lifting weights or running or playing a game of basketball. But we also burn calories performing daily tasks: cleaning the house, gardening or walking the dog — just not as many calories.
A 150-pound person who goes mountain biking for 30 minutes can burn 290 calories, according to WebMD's fitness calorie calculator, the Fit-o-Meter. During the same amount of time, that person burns 341 calories by jumping rope or 511 calories by running up stairs.
But if that person opts for 30 minutes of yoga, he or she will only burn 85 calories. Sitting on the couch playing guitar for 30 minutes will burn 102 calories.
So why do some activities burn more calories?
For the most part, the answer lies in a few factors: muscle mass, intensity and gender, said Bridget Raach, a Clark College instructor in the fitness trainer program and Crossfit coach.
"The intensity of the activity is going to be a big predictor of how much calorie burn will be," she said.
Intensity, however, is relative to the person and their fitness level on any given day, Raach said.
Take Lance Armstrong, for example. Armstrong may ride his bicycle up a hill at a fast pace; his intensity level could be a 10 out of 10. Someone else may ride up that same hill at a much slower pace, in a lower gear and still reach the same intensity level, Raach said.
External weights can also make an activity more intense. For example, jumping jacks will burn calories. But doing jumping jacks while wearing a weight vest will burn more calories, she said.
"You're doing more than just moving," Raach said.
In addition to intensity, muscle mass plays a significant role in how many calories will be burned during activity.
"For the most part, muscle is what burns calories," Raach said. "The less muscle you have, the less calories you burn."
And when it comes to muscle mass, women are generally at a disadvantage, she said.
Women can build muscle mass, but not as much as men. Testosterone enables men to produce more muscle mass than women. That added muscle mass makes men more able to burn calories than women, Raach said.
"Life is not fair," she said.
But muscle mass can also relate to the number of muscles the body is using during an activity. Activity that incorporates more of the body's muscles — lunges as opposed to bicep curls, for example — burn more calories, Raach said.
In the end, an activity that may burn 400 calories for one person, may only burn 100 calories for another.
"It's not one size fits all," Raach said. "And yet, we're all human. We all need to move our bodies."