Immediately following a weight training exercise, the muscle may seem full and tight for 15 to 30 minutes, or 'pumped'. The muscular 'pump' is caused by trapped plasma within the muscle. During muscular contraction, the contractile elements exert a force inward upon themselves; the muscle diameter increases as it shortens. During intense muscular contraction, this force inward momentarily occludes the vasculature, backing up blood flow through that particular muscle group. A compensatory increase of blood pressure forces plasma from the congested capillaries into the interstitial spaces of the muscle cells.
Bodybuilders commonly perform pumping-up exercises before appearing on stage to make their muscles appear fuller. For most of us, there is no real benefit from achieving a pump (except for a boost of one's ego). However, the inability to achieve a pump is one symptom of overtraining since fluid volume in the muscle, and possibly blood volume, decrease when glycogen stores are low. Intense cycling or swimming can also bring about a pump, or temporary fullness in the working muscles.
The burning sensation as experienced during (1) certain weight training exercises, (2) high repetition training, or (3) particular advanced training techniques, is caused by an accumulation of acid in the fatiguing muscle.
Anaerobic glycolysis utilizes carbohydrates and produces water and acid, or free hydrogen ions. Lactate helps prevent muscle acidosis by transporting positive hydrogen ions out of the muscles. This acid does not clear sufficiently if blood flow is impeded. The vasculature within the muscle is temporarily occluded by the surrounding muscle when it is contracted intensely for a relatively prolonged duration. The excessive acid acts upon nerve receptors, producing the localized burning sensation. Excessive acid accumulation also impedes muscular contraction. Acid interacts with calcium, rendering it unavailable for muscular contraction. After several regular workouts, the muscle(s) become better adapted to deal with acid accumulation. See types of muscular endurance and muscle fatigue & blood supply graph.
Many basic or compound exercises (e.g. squat, bench press, etc.) have a dynamic, or bell shaped resistance curve or they shift the resistance through multiple muscle groups throughout the exercise's range of motion, both allowing the muscles to momentarily relax between repetitions. This period of momentary relaxation between repetitions allows greater opportunity for momentary blood flow permitting the clearing of acid accumulation.
In contrast, many auxiliary or isolated exercises (e.g., leg extensions, lever fly) tend to have relatively continuous resistance curves, particularly when performed on variable resistance machines requiring the muscle to sustain contraction through the majority of the exercise's range of motion. This effect can be augmented by the use of cams and variable resistance levers which attempt to match the user's strength curve by a preset resistance curve.
Alternately, partials or not locking out can be used to restrict the range of motion on either compound or isolated exercises. These techniques also evoke intercellular acidosis by preventing the muscle to relax at the top and/or bottom of the movement, keeping it continuously contracted throughout the set.
Some evidence suggests that the resulting burning sensation may promote muscle development by via sarcoplasmic hypertrophy. Interestingly, mechanically occluding the venous return while moving relatively light loads can elicit metabolic changes and promote sarcoplasmic hypertrophy (Loenneke 2012). It is thought that hypoxia and intracellular acidosis of the muscle cells stimulate certain metabolic changes that are responsible for this effect.
Although using particular exercises or training techniques which bring about the burning sensation may be an effective means for training for sarcoplasmic hypertrophy or a particular type of muscular endurance, it does not appear to be the most effective method for developing strength, power, or myofibrillar hypertrophy.
It can be argued that compound exercises that do not induce burning sensation as readily, appear to more effective than isolated exercises in development of strength and muscular size. For example, leg press (a basic and compound exercise) is superior in developing both strength and muscular size compared to leg extension and hip extension (both auxiliary and isolated exercises) where the burning sensation is far more prevalent. In fact, isolated exercises may not provide additional benefits in the way of strength or muscular size in untrained men. (Gentil 2013)
Judging the effectiveness of an exercise on the burning sensation can be fallacious. 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 (see lower abdomen myth). Furthermore, beginners often feel muscular fatigue in unusual places until they become accustomed to that exercise (eg: neck fatigue during crunches). This is similar to the 'weak link' analogy (i.e. "A chain is only as strong as its weakest link"). Only after these 'weaker links' become conditioned, can the 'chain' (target muscle and assisting muscles) be exercised more effectively. In addition, the burning sensation has nothing to do with fat burning as popularly believed (See spot reduction myth).
Gentil P1, Soares SR, Pereira MC, Cunha RR, Martorelli SS, Martorelli AS, Bottaro M (2013). Effect of adding single-joint exercises to a multi-joint exercise resistance-training program on strength and hypertrophy in untrained subjects. Appl Physiol Nutr Metab. 38(3):341-4.
Loenneke JP1, Wilson JM, Marín PJ, Zourdos MC, Bemben MG (2012). Low intensity blood flow restriction training: a meta-analysis. Eur J Appl Physiol. 2012 May;112(5):1849-59.