Tuesday, February 16, 2010


The so-called crunch or partial sit-up has all but eclipsed the old “full sit-ups,” and for good reason. The old style of sit-ups involved lifting the torso from a flat starting position to a seated position. In contrast, crunch sit-ups are so-named because they involve a partial range of motion, curling the torso to a position of maximum muscle contraction (about 1/3 the total motion of a full sit-up), which to some people feels like a “crunching” motion.

Old-style full sit-ups involved wasted motion, since past approximately the one-third part of the movement, the abdominals remained isometrically contracted, while hip flexors took over in the final 2/3 of the movement. This was problematic not only because of the wasted movement, but also because working the hip flexors in this manner tended to pull on the lower spine, thus irritating the lower back muscles. Crunch sit-ups obviated this shortcoming of full sit-ups while also providing more concentrated exercise for the abdominals.

Lately, even the efficiency of crunch sit-ups has been questioned. Criticisms in this regard involve the range-of-motion in typically executed crunch sit-ups. Starting the exercise with the back flat on the floor, or exercise board, does not allow a prestretch phase of the exercise. Prestretch is vital to inducing maximal muscular contraction during most exercises.

Several solutions exist to remedy this crunch sit-up deficiency. One method involves the use of a large ball, with which the exerciser lies on while doing crunch sit-ups. These “Swiss ball sit-ups,” allow a person to assume a correct biomechanical position favorable to allowing both prestretch and proper abdominal exercise motion. Such motion involves a curling of the torso rather than sit-ups with a stiff torso--which tends to shift the emphasis back to the hip flexor muscles.

But are these Swiss ball sit-ups safe and superior to normal crunch sit-ups? A recently published study compared Swiss ball crunches to standard crunch sit-ups. The study involved 8 men, average age, 23, who did four types of sit-ups:

1) Traditional crunches on a padded bench with feet flat on floor
2) Crunches over a Swiss ball with feet flat on floor
3) Crunches over a Swiss ball with feet resting on a bench the same height as the ball
4) Crunches done over a small, round moveable board (Wobble board), with feet flat on the floor.

To determine muscle activity during these exercises, all the study subjects wore electrodes on various points of their abdominals and side torso muscles, which send readings of electrical activity of the active muscles to an electromyograph (EMG) machine.

The results showed that maximal voluntary contraction (MVC) during standard crunch sit-ups was 21%, while external oblique muscle activity was 5%. During the Swiss ball version of crunches, MVC of the abdominals averaged 35%, with external oblique activation showing as 10% MVC. Exercising on the Swiss ball activated the obliques more than the other ab exercises because the obliques work to stabilize the torso, an effect heightened when doing crunches over the Swiss ball.

Thus, according to this study, doing crunches over a Swiss ball may more potently activate the abdominal muscles in comparison to standard crunch sit-ups.

Repeated aerobic sessions: better for fat-burning?

Is it better to do one long aerobic session, or break it up into 2 shorter sessions? A new study shows that if increased fat oxidation is your goal, dividing aerobic sessions may be the way to go. The study involved 7 untrained, drug-free men, who did 2 bouts of 60 minutes each of aerobics, separated by a one-hour rest. The intensity level of both sessions was 50% of maximal oxygen intake, considered low intensity. The goal of the study was to figure out if doing repeated bouts of aerobics modifies fat-oxidation during exercise.

The results showed a significantly higher fat-burning effect during the second exercise session. In fact, the measure of glycerol, a signal of fat release during exercise, was 2-3 times higher during the second session compared to the first. Apparently, the first session of aerobics acted as a “priming” session for the second session in regard to increased fat-burning.

Fat oxidation during exercise is determined by a balance between catacholamines, such as epinephrine and norepinephrine, and insulin. Catacholamines favor increased fat release from fat cells through stimulating beta-adrenergic cell receptors in the fat cells. Insulin inhibits fat release during exercise. In this study, plasma levels of norepinephrine remained the same during both exercise bouts, however levels of epinephrine were 2-3 times higher during the second exercise session, while insulin levels were significantly lower during session #2, a scenario that favors increased fat oxidation.

Interestingly, growth hormone, a substance associated with increased fat mobilization, was lower during the second session. The study authors attribute this to the higher level of circulating fatty acids in the blood during the second session. Higher levels of blood fats are a known inhibitor of growth hormone release. During the second session, higher levels of cortisol were also evident, and cortisol also blunts GH release. The higher levels of epinephrine evident during the second session are likely due to the lower blood glucose levels that were induced by the first session, thus explaining why the first session acted as fat-burning primer for the second session.

Although not examined in this study, I wonder if doing a high intensity weight-training session would have the same priming effect if followed by an aerobics session. There is no reason why it wouldn’t unless two conditions occurred: 1) a large carbohydrate intake was consumed shortly before the workout; 2) carbohydrates were consumed during the workout. Both conditions would serve to maintain blood glucose levels, which is great for energy purposes but an ineffective technique for inducing greater fat oxidation.

©,2013 Jerry Brainum. Any reprinting in any type of media, including electronic and foreign is expressly prohibited.

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