When you’re doing aerobic activity, such as walking or biking, exercise intensity correlates with how hard the activity feels to you. Exercise intensity is also shown in your breathing and heart rate, whether you're sweating, and how tired your muscles feel. The two basic ways to measure your exertion is by how you feel, and your heart rate. You can use either way of gauging exercise intensity. If you like technology, a heart rate monitor might be a useful device for you. If you feel you're in tune with your body and your level of exertion, you likely will do fine without a monitor. While engaged in moderate activity your breathing quickens, but you’re not out of breath. You develop a light sweat after about 10 minutes of activity and you can carry on a conversation but you can’t sing.
Everyone responds to a new exercise routine differently. When you first start exercising, the physical changes that you go through and indicators that you’re physically improving tend to be quite distinct. Some signs that it’s time to increase your intensity are if it feels too easy; you find yourself going through the motions without much effort. If your heart rate isn’t going up like it used to, or you can even carry a conversation through the whole workout. If you’re dreading your workout or can’t wait for it to be over, it might be time to switch it up.
For more information and original publication: http://www.mayoclinic.org/healthy-lifestyle/fitness/in-depth/exercise-intensity/art-20046887?pg=2 and http://www.discovergoodnutrition.com/2014/08/increase-workout-intensity/