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ישן 18-02-07, 22:42   #2
igorek
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תאריך הצטרפות: Apr 2006
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Many people adopt belts in training for one of three reasons:

• They have observed others wearing them and have assumed that it will be a good idea for them to do so.
• Their backs are becoming sore and they believe that a back belt will help.
• They want to lift a few more pounds.

None of these reasons are consistent with the objective of good health. It would appear that if one must lift a few more pounds, wear a belt. If one wants to groove motor patterns to train for other athletic tasks that demand a stable torso, it is probably better not to wear one. Instead individuals are encouraged to train the core musculature and to perfect lifting technique.

Natures Belt – A brief review of the anatomy

The abdominal wall formed by the lateral obliques (external and internal) and transverse abdominis creates hoop forces around the abdomen (see figure 1). These hoop forces are the reason why the rectus abdominis has a “beaded” appearance—the lateral tendons transmit the lateral forces from the oblique muscles forming a continuous hoop around the abdomen. Further the intermuscular tendons and rectus fascia prevent the fibers of rectus from being ripped apart laterally from these hoop stresses. Posteriorly, the obliques connect with the lumbodorsal fascia (LDF) completing the hoop, or belt. The resulting “hoop stresses” and stiffness assist with spine stability. Training of this mechanism has been extensively discussed in McGill (2004).

Scientific Studies

The scientific studies can be divided into clinical trials and those that examined biomechanical, psychophysical, and physiological changes from belt wearing.
Clinical Trials: Clinical trials have been conducted to examine the effectiveness of belts to reduce injury in workers who perform lifting tasks. These types of studies are difficult to conduct. For example, the Hawthorne effect is a concern, as it is difficult to present a true double-blind paradigm to participants since those who receive belts certainly know so. In addition, logistical constraints on duration, diversity in occupations, and sample size are problematic. However, the data reported in the better-executed clinical trials cannot support the practice of universal prescription of belts to all workers involved in industrial lifting to reduce the risk of low-back injury (for example, Reddell and colleagues, 1992, Mitchell and colleagues, 1994, Wassell and colleagues, 2000). Weak evidence suggests that those people already injured might benefit from belts with a reduced risk of injury recurrence. However, evidence does not appear to support uninjured workers wearing belts to reduce the risk of injury; in fact, the risk of injury seems to increase during the period following a trial of belt wearing. Finally, some evidence suggests that the cost of a back injury may be higher in workers who wear belts than in workers who do not.

Biomechanical Studies: Researchers who have studied the biomechanical issues of belt wearing have focused on spinal forces, intra-abdominal pressure (IAP), load, and range of motion. The most informative studies are reviewed in this section.

Two studies in particular (Harman et al.,1989, and Lander et al., 1992) suggested that wearing a back belt can increase the margin of safety during repetitive lifting. Both of these papers reported ground reaction force and increased intra-abdominal pressure while subjects repeatedly lifted barbells. It was only assumed that intra-abdominal pressure is a good indicator of spinal forces. However, several studies have questioned the hypothesized link between elevated intra-abdominal pressure and reduction in low back load. For example, using an analytical model and data collected from three subjects lifting various magnitudes of loads, McGill and Norman (1987) noted that a buildup of intra-abdominal pressure required additional activation of the musculature in the abdominal wall. This activity resulted in a net increase in low back compressive load and not a net reduction of load, as researchers had previously thought. In addition, Nachemson and colleagues (1986) published some experimental results that directly measured intradiscal pressure during the performance of Valsalva maneuvers, documenting that an increase in intra-abdominal pressure increased, not decreased, the low back compressive load. Therefore, the conclusion that an increase in intra-abdominal pressure due to belt wearing reduces compressive load on the spine seems erroneous. In fact, such an increase may have no effect or may even increase the load on the spine.

Several studies have put to rest the belief that IAP affects low back extensor activity. Our own study (McGill and colleagues, 1990) suggested that there was no change in activation levels of the low back extensors nor in any of the abdominal muscles (rectus abdominis or obliques).

Both Reyna and colleagues (1995) and Ciriello and Snook (1995) found belts provided no enhancement of function in terms of alleviating the loading of back extensor muscles or fatigue, but these trials were not conducted over a very long period of time.

Both Lantz and Schultz (1986) and McGill and colleagues (1994) observed the range of lumbar motions in subjects wearing low back orthoses and belts, respectively. Generally, the stiffness of the torso was significantly increased about the lateral bend and axial twist axes when belt wearing but not when subjects were rotated into full flexion.

A data set presented by Granata and colleagues (1997) supports the notion that some belt styles are better in stiffening the torso in the manner described previously—namely, the taller elastic belts that span the pelvis to the rib cage. Furthermore, these authors also documented that a rigid orthopedic belt generally increased the lifting moment, while the elastic belt generally reduced spinal load. Even in well controlled studies, belts appear to modulate lifting mechanics in some positive ways in some people and in negative ways in others.

Studies of Belts, Heart Rate, and Blood Pressure: Hunter and colleagues (1989) monitored the blood pressure and heart rate of five males and one female performing dead lifts and one-arm bench presses and riding bicycles while wearing and not wearing a 10-cm (4-in.) weight belt. During the lifting exercise, blood pressure (up to 15 mmHg) and heart rate were both significantly higher in subjects wearing belts. Given the relationship between elevated systolic blood pressure and an increased risk of stroke, Hunter and colleagues (1989) concluded that individuals who may have cardiovascular system compromise are probably at greater risk when undertaking exercise while wearing back belts than when not wearing them. Our own work (Rafacz and McGill, 1996) supports these conclusions. Anecdotally, belts and associated pressures may possibly be linked with higher incidents of varicose veins in the testicles, hemorrhoids, and hernias. As of this writing, there has been no scientific and systematic investigation of the validity of these suggestions. It may be prudent to simply state concern and test these ideas in the future.

Psychophysical Studies: Some scientists and coaches have expressed concern that wearing belts fosters an increased sense of security that may, or may, not be warranted. Studies based on the psychophysical paradigm allow subjects to select weights that they can lift repeatedly using their own subjective perceptions of physical exertion. For example McCoy and colleagues (1988) found that subjects were willing to lift by approximately 19% when wearing a belt. This evidence may lend some support to the theory that belts give people a false sense of security.

A Look to the future

Training for high performance usually requires high speed movement. Also critical for ultimate performance is timed and coordinated stiffness and joint stability. Consider the golf swing that epitomizes the contrast between controlled relaxed motion through the backswing and violent whole body stiffening at the instant of ball contact only to be followed by more relaxed follow through motion. The same contrast can be generalized for generating punching power and for the impressive “hit”in football, for example. The best athletes are able to generate motion but know when to stiffen with extremely rapid muscle activation together with rapid muscle relaxation. Techniques for training this skill involve selective plyometric exercise which is generally encumbered by wearing a belt. Thus, different considerations apply for speed training and belt usage is considered inappropriate.
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