Page 124 - Journal of Special Operations Medicine - Summer 2015

Injuries and Warm-up tasks and/or if they overexert or overuse the available
muscle mass or supportive structures, possibly leading
Active warm-up uses low-intensity activity to increase to acute or overuse injuries, respectively.
muscle temperature and allow a gradual transition
into more intense activity. There are two types of ac- Studies in BT generally have a sufficient number of re-
tive warm-up—general and specific. General warm-up cruits with both low and high BMI to show that either
uses activities like low-intensity running, cycling, jump- extreme will increase injury risk. Figure 3A shows the
ing, or lifting; specific warm-up uses activities similar results of a study that looked at this relationship among
to the exercises or sport that the individual is preparing male recruits in US Army BT. Recruits with either low
25
for. It has been hypothesized that there are many favor- or high BMI showed higher risk of injury than those
able physiologic effects from active warm-up. These in- with BMI in the two middle quartiles (a quartile is 25%
clude reductions in muscle viscosity that may increase of all recruits tested). The National Institutes of Health
the speed and force of muscle contractions; increase the considers individuals with “low” BMI to have a BMI
speed of nerve transmission, which could improve reac- value less than 18.5 kg/m . Data from several military
2
tion time; more rapid dissociation of oxygen from hemo- occupational specialties (MOS) (Figure 3B–D) indicate
globin, thus providing oxygen more rapidly to working that, generally, the higher the BMI, the higher the injury
muscles; and vasodilation, which increases blood flow risk. 26,27 The inability of these MOS studies to show that
in active tissues. 13,14 In experimental studies, more force lower BMI increases injury risk may be because very few
and greater muscle length were required to tear rabbit active duty Soldiers have a BMI less than 18.5 kg/m .
2
muscles that had been prewarmed. 15,16 No military stud- Future studies are needed to examine associations be-
ies have examined the effects of warm-up on injuries, tween low BMI and injury risk in active duty Soldiers.
but a number of studies on athletes have shown that
general and sports-specific warm-up routines appear to In summary, studies generally suggest that static stretch-
reduce the incidence of sports-related injuries. 17–20
ing will not reduce the incidence of injury but that
warm-up activities may be beneficial. Soldiers should
Injuries and Body Composition
maintain their BMI in the normal weight range, which
Studies that have looked at the relationship between the National Heart, Lung, and Blood Institute has de-
2 28
body fat and body composition have generally looked fined as 19–25 kg/m . Among Soldiers with a muscular
at body mass index (BMI) as a surrogate measure for build, BMI is likely to overestimate the amount of fat.
body fat. BMI is calculated as body weight divided by Soldiers need to develop and maintain a high level of
the square of height (kg/m ). physical fitness and optimal body composition not only
2
Generally, BMI shows a close relation- Figure 3 Associations between BMI and injury. (A) Recruits in US Army Basic
ship with body fat in both military Combat Training. (B) US Army Armor Soldiers and MP. (C) US Army Wheel
and civilian groups. 21–23 However, the Vehicle Mechanics. (D) US Army Band. BMI, body mass index; MP, military
relationship between BMI and injury police; sol-mo, Soldier-months.
is likely to be complex because indi-
viduals can have a high BMI either
because of higher body fat or because
of higher muscle and bone mass. If
high BMI reflects a larger percentage
of body fat relative to height, injury
risk might be increased because the
fat burden would (1) increase the in-
tensity of physical activity, leading to
24
more rapid fatigue; and (2) impose ad-
ditional repetitive stress on the muscu-
loskeletal system because of the greater
weight relative to height. In contrast to
high BMI, low BMI may reflect a pau-
city of either fat or muscle/bone mass,
or both. Low BMI may make Soldiers
more susceptible to injury if they lack
the muscle mass or strength in the sup-
portive structures (ligaments, bones)
required to perform certain physical

114 Journal of Special Operations Medicine Volume 15, Edition 2/Summer 2015