Talk Test

“In order to enhance endurance performance, some training has to occur around the lactate threshold intensity level. When subjects in the study had difficulty talking, they were very close to that lactate threshold intensity. Because of this, athletes could gauge their intensity based on ability to talk comfortably.” 

“If you are beginning an exercise program and can still talk while you’re exercising, you’re doing OK,” Quinn says. “But if you really want to improve, you’ve got to push a little bit harder.”

(excerpt from http://www.unh.edu/news/cj_nr/2011/sep/bp13exercise.cfm)