Spot Reduction Myth

Contrary to what the infomercials suggest there is no such thing as spot reduction. Fat is lost throughout the body in a pattern dependent upon genetics, sex (hormones), and age. Overall body fat must be reduced to lose fat in any particular area. Although fat is lost or gained throughout the body it seems the first area to get fat, or the last area to become lean, is the midsection (in men and some women, especially after menopause) and hips and thighs (in women and few men). Sit-ups, crunches, leg-hip raises, leg raises, hip adduction, hip abduction, etc. will only exercise the muscles under the fat.

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Lower Abdominal Myth

It is widely believed the lower abs are exercised during the leg raise or other hip flexor exercises. However, it can be misleading to judge the mechanics of an exercise based on localized muscular fatigue. The primary muscle used in hip flexion is actually the Iliopsoas one of many hip flexors. The Iliopsoas, particularly the Psoas portion, happens to lie deep below the lower portion of the Rectus Abdominous. During the leg raise, the entire abdominal musculature isometrically contracts (contracts with no significant movement) to:

• Posture the spine and pelvis
  • Supports the weight of the lower body so the lumbar spine does not hyperextend excessively
  • Maintains optimal biomechanics of the Iliopsoas
    • Hips are kept from prematurely flexing if the lumbar spine and pelvis does not hyperextend excessively
    • Iliopsoas can contract more forcefully in a relatively slight stretched position
      • Bent knee (and hip) sit-ups actually place Iliopsoas at a mechanical disadvantage
• Counteracts Iliopsoas's pull on spine
  • Many people with weak abdominal muscles are not able to perform hip flexor exercises without acute lower back pain or discomfort

The combination of the local muscular fatigue, or a burning sensation from the isometrically contracted abdominal muscles, and from the working hip flexors produces fatigue in the pelvis area which we mistakenly interpret as the lower portion of the Rectus Abdominous being exercised. In movements where the Rectus Abdominous does Isotonically contract (contracts with movement), it flexes the spine by contracting the entire muscle from origin to insertion. The spine is not significantly flexed during the leg raise. Incidentally, both the spine and hip flexes during the full range op motion Sit Up and Leg Hip Raise. See Spot Reduction Myth above.

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High Repetitions Burn More Fat Myth

Performing lighter weight with more repetitions (15-20 reps, 20-30 reps, or 20-50 reps) does not burn more fat or tone (simultaneous decrease of fat and increase muscle) better than a heaver weight with moderate repetitions (8-12 reps). Weight training utilizes carbohydrates after the initial ATP and CP stores have been exhausted after the first few seconds of intense muscular contraction. Typically a set's duration is 20 to 30 seconds. For the average fit person, it requires 20 to 30 minutes of continuous aerobic activity with large muscle groups (e.g. Gluteus Maximus and Quadriceps) to burn even 50% fat; fat requires oxygen to burn. Performing a few extra repetitions on a weight training exercise is not significant enough to burn extra fat and may in effect burn less fat. If intensity is compromised, less fat may be burned when light weight is used with high repetitions. The burning sensation associated with high repetition training seems to be the primary deterrent for achieving higher intensities.
Higher volume weight training (i.e. 3 sets versus 1 set of each exercise) with short rest periods of approximately 1 minutes can stimulate a greater acute growth hormone release (Kraemer 1991, 1993; Mulligan 1996). Growth hormone is lipolytic in adults. It is hypothesized that maximal effort is necessary for optimizing exercise induced secretion of growth hormone. Growth hormone release is related to the magnitude of exertion (Pyka 1992) and is attenuated with greater lactic acidosis (Gordon 1994).
Intense weight training utilizing multiple large muscles with longer rest between sets may also accentuate body lipid deficit by increasing post training epinephrine. Intramuscular triacylgycerol is thought to be an important energy substrate following repeated 30 second maximal exercise with 4 minute recovery intervals (McCartney 1996, Tremblay 1994). Rest periods lasting approximately 4 minutes between maximal exercise exercise of very short duration is required for almost complete creatine phosphate recovery required for repeated maximal bouts (McCartney 1986). Insufficient recovery may compromise the intensity of the exercise and in turn, possibly decrease intramuscular triacylgycerol utilization following anaerobic exercise with significantly shorter rest periods.
For individuals attempting to achieve fat loss for aesthetics, the intensity of weight training can be a double edge sword. When beginning an exercise program, muscle mass increases may out pace fat losses, resulting in a small initial weight gain. Significant fat loss requires a certain intensity, duration, and frequency that novice exercisers may not be able to achieve until they develop greater tolerance to exercise. If an exercise and nutrition program is not adequate for significant fat loss, a lighter weight with higher repetitions may be recommended to minimize any bulking effects, although less fat may be utilized hours later. If an aerobic exercise and nutrition program is sufficient enough to lose fat, a moderate repetition range with a progressively heavier weight will accelerate fat loss with a toning effect. If a muscle group ever outpaces fat loss, the slight bulking effect is only temporary. For a toning effect, fat can be lost later when aerobic exercise can be significantly increased or the weight training exercise(s) for that particular muscle can be ceased altogether. The muscle will atrophy to a pre-exercise girth within months. Higher repetitions training may be later implemented and assessed.
It still may be recommended to perform high repetitions (e.g. 20-30) for abdominal and oblique training. It has been theorized muscular endurance may be more beneficial for lower back health than for muscular strength. Furthermore, moderate repetitions with a greater resistance can increase muscular girth under the subcutaneous fat, particularly in men, who have greater potential for muscular hypertrophy. Increasing the thickness around the waist with existing abdominal fat may further increase bulk, particularly in men who typically have greater intra-abdominal and subcutaneous fat in this area. The abdominal musculature is composed of relatively small muscle mass as compared to the glutes, quadriceps, hamstrings, chest, and upper back. Performing high reps with a lighter resistance should not compromise metabolism or muscle increases, as would performing high reps with light resistance on other, larger muscle groups. See Spot Reduction Myth above.
It is plausible that the high repetition myth was originated and later propagated by bodybuilders that used calorie restrictive diets to shed fat before a contest. Because of their weakened state from dieting, they were unable to use their usual heavier weights. When asked about their use of lighter weights, they explained they were "cutting up" for a contest. This is merely a theory, but it is easy to see how it may have been misunderstood that the lighter weight was used to reduce fat instead of actually being a result of their dietary regime.
Typically with weight training alone, the fat loss is equal to the muscle gain, give or take a few pounds. Certain dietary modification can have much greater impact on fat loss than with weight training alone. The ideal program for fat loss would include the combination of proper diet, weight training, and cardio exercise. Also see study summaries: Weight Training and Diet and Endurance and Weight Training.