The Navy SEAL Physical Fitness Guide 177
Biomechanics of Load-Bearing
The duration of time your foot is on the ground during a normal gait does not
increase until pack loads are greater than 50% of your body weight. However, the length of
time your foot is in the air during the swing phase of your gait decreases with increasing
load-bearing weight. This response occurs to increase the length of time both feet are on
the ground for double support. As the load weight increases, your feet increase the ground
force downward, forward, rearward and in lateral directions. Shorter stride lengths will
increase stride frequency and help maintain normal walking patterns during moderate to
heavy load-bearing.
Shorter strides may minimize strain and possibly
reduce lower back and lower extremity injuries.
Load-Bearing and Walking Pace
Self-pacing during load-bearing results in a lower energy cost than a forced-pace.
However, not all missions allow self-pacing. When the pace is forced, a self-selected
decrease in walking pace will be observed as the load-bearing weight increases. The self-
selected walking pace or exercise intensity during load-bearing depends on many factors.
Some of the factors affecting the self-selected pace include:
" Load weight
" Aerobic fitness level (maximal aerobic capacity)
" Total distance walked
You will normally self-pace at 30% - 35%
of your maximal aerobic capacity when
carrying a moderate load of 50 pounds.
With lighter loads, self-pacing normally results in an exercise intensity of around
45% of your maximal aerobic capacity. However, an exercise intensity of 60% or greater
can be achieved when the load is light and the distance short. Remember: walking pace
and load weight carried during an infiltration or exfiltration contribute to the rate of
exhaustion. Figure 10-1 presents estimates of when exhaustion might occur at various
walking speeds and loads. High rates of energy expenditure (900 to 1000 calories per hour)
can be sustained for only 6 to 10 minutes. To be able to move quickly at this intensity, load
weight must be light.