Nothing makes my hair hurt quite like listening to some guy who's heavier on biceps than brains sharing his "wisdom" about working out. Last week I wrote about the myth of "the pump" and its role in gym lore. A related issue is "the burn".
The burning sensation during certain resistance training is due to the accumulation of acid
in the fatiguing muscle. Anaerobic glycolysis uses carbohydrates
and produces water and acid, or free hydrogen ions. When blood flow is limited (as described in last week's post) the acid accumulates. All that acid creates a burning sensation but is not indicative of fat burning, muscle growth, or the effectiveness of an exercise, though it does impede the muscle's ability to contract further.
One other factor that is well explained by our friends at exrx.net is the fact that "it can also be misleading to judge an exercise's effectiveness
on a particular muscle group based upon local muscular
fatigue. For example, a common misconception is that leg raises
exercise the lower abdomen since most exercisers feel a burning
sensation in this area." In fact, the muscles involved in producing the movement (hip flexion) are not your abdominals. Your abdominal muscles are contracting isometrically in order to stabilize you, but they're not causing the movement, and as such – even though they're hard, and even though you "feel the burn" – leg lifts are not good for much other than getting you better at doing more leg lifts.