Core-Hip
Flexor Interaction
The "core"
has become one of the hottest topics in the fitness and
sports medicine world over the past decade. It is often an
overused term that actually causes more confusion than clarity in a lot
of situations. The term is usually intended to describe the area
in the center of the body which encompasses the abdominal muscles, low
back muscles, and many times the muscles of the hip, including the
glutes, ab/adductors, and hip
flexors. 
In this conversation, we will use the term "core"
to describe the
muscles of the abdomin and low back which are primarily responsible for
stabilizing the pelvis. These are the Transverse Abdominis and
Multifidus muscles. They are very deep within the abdominal
cavity and are very difficult for many of today's athletes to
engage.
When an athlete isn't able to engage the core
to stabilize the pelvis,
there are a great number of issues that can arise. These issues
are both performance degregation and injury. One major downfall
that often occurs when the Transverse Abdominis and Multifidus aren't
working properly is the that hip flexors are forced
to work overtime to
help stabilize the pelvis. While they are intended to work
synergistically to flex the trunk and femur, the hip flexors are not
intended to stabilize the pelvis. When they become
synergistically dominant, either injury or performance degregation will
occur.
A prime example of when this occurs is during an active straight leg
raise. On the surface, the test gives us an idea as to how
flexible the athlete is in their hamstrings.
This is true to a
certain extent because if an athlete has an extreme hamstring
mobility
issue, it will show up in this movement. However, the hamstrings
are the only muscles that can be evaluated with this test.

If an athlete is tight in the hip flexor in the leg
which is on the
floor, they will also be positive for an active straight leg
raise. This is because the hip flexor will put the
pelvis in an
anteriorly rotated position, putting the hamstring
on prestretch, and
making the hamstrings
appear to be less flexible than they really
are.
The third muscle group which can be evaluated during this
movement is the core
(which brings us back to the current topic).
If the core
is not firing properly to stabilize the pelvis, the hip
flexors will turn on in an effort to make up for the core's area of
weakness. When the hip
flexors activate, the pelvis rotates, the
hamstrings
appear "tight" and we have a positive test.

This same hip flexor
dominance will show up in a number of other
athletic positions as well. Somehow, the hip flexors have the
ability to be dominant, weak, and innactive, all in the same
athlete.
The key to preventing such an issue is training the inner unit of the
core
to be very active and stable. Proper core
training will
ensure that the athlete's body understands when the hip flexor is
supposed to work and when it is the core's
turn to turn on and
stabilize.
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