Page 122 - Journal of Special Operations Medicine - Summer 2015

An Ongoing Series

The Importance of Physical Fitness for Injury Prevention:
Part 2

Joseph J. Knapik, ScD

ABSTRACT
This report examines associations between injuries and how the concept was developed, and described the as-
flexibility, stretching, warm-up, and body composition. sociation between injury and aerobic fitness and muscu-
Military studies show that either too much or too little lar endurance. Part 2 focuses on associations between
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flexibility increases injury risk. Static stretching prior injuries and flexibility, stretching, warm-up, and body
to exercise does not appear to reduce the overall injury composition.
incidence, although further research is needed on some
types of injuries. Static stretching also appears to reduce Injuries, Flexibility, and Stretching
strength and power (explosive strength). Warm-up (low
intensity activity prior to exercise or sports) appears to Flexibility is associated with injury, but the relationship
reduce injury risk. Body mass index (BMI; weight in kg/ is not what you might expect: either too much or too
height in m ) is a surrogate measure of body fat because little flexibility can increase injury risk. Let us look at
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it is highly related to laboratory measures of body fat. some data to better understand this. In one study, US
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However, Soldiers can also have a high BMI because of Army basic trainees were tested for hamstring flexibil-
higher muscle mass. If high BMI reflects a larger per- ity prior to basic combat training (BT), and their inju-
centage of body fat relative to height, injury risk might ries were tracked while they were in BT. Recruits were
be increased because the additional fat would increase separated into quintiles of flexibility, meaning that each
the intensity of physical activity, leading to more rapid group (quintile [Q]) represented 20% of all the recruits
fatigue and repetitive stress on the musculoskeletal sys- in the study. The lowest risk was among those in the
tem. Low BMI could reflect a paucity of fat or muscle/ middle quintile (Q3) and injury risk was highest among
bone, or both. Low BMI may make Soldiers more sus- those who had the most (Q5) or the least (Q1) flexibility
ceptible to injury if they lack the muscle mass or strength (Figure 1A). Another study looked at 423 Navy Sea,
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in the supportive structures (ligaments, bones) required Air, and Land (SEAL) candidates training at the Basic
to perform certain physical tasks, and if they overex- Underwater Demolition/SEAL School near Coronado,
ert or overuse the available muscle mass or supportive California. Prior to training, the flexibility of their an-
structures. Studies in basic combat training show that kles was tested and their injuries were recorded while
both high and low BMI increases injury risk. However, they were in SEAL school. The candidates were sepa-
studies among active duty Soldiers only show that injury rated into tertiles, meaning that each group (tertile [T])
risk increases as BMI increases, possibly because very represented 33% of all candidates. The candidates with
few active duty Soldiers have very low BMI (i.e., less the most (T1) and least (T3) flexibility had the higher
than 18 kg/m ). injury risk, while the group in the middle (T3) had the
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lowest injury risk (Figure 1B).
Keywords: body mass index; physical fitness; injury prevention
Flexibility can be improved by static stretching, and,
4,5
many years ago, stretching prior to physical activity
was recommended to reduce the likelihood of injury.
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Introduction
However, this idea had not been tested experimentally
This is the second of a two-part series discussing the at the time. A 1993 study conducted in Amsterdam,
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association between physical fitness and injuries in mili- The Netherlands, 421 male runners were placed into
tary operations. Part 1 defined physical fitness, described two groups—one group performed static stretching of

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