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Biomechanical
Principles to be Investigated
Relationship between the center of mass and the base of support of the
body
Purpose
Depending upon the desired outcome(s) of a given sport, an athlete
may desire to move his/her body as quickly as possible in a given
direction (i.e., to be as unstable as possible without losing one’s
balance) or to maintain his/her body position while executing a specific
series of movement patterns (i.e., to be as stable as possible). The
location of the center of mass of the body with respect to its base of
support at any moment in time dictates where along the stability /
instability continuum an athlete resides. Why? This lab will
investigate the relationship between an individual’s center of mass and
his/her base of support for two sport-specific movement patterns with
opposite desired outcomes.
Rationale
Analysis of the location
of the center of mass of athletes with respect to their base of support
for a variety of sports reveals a shifting continuum within the extremes
of maximum stability and maximum instability.
Each sport and its associated movement patters dictate where along
this continuum optimal performance resides.
This lab will analyze two very different activities (i.e., the
weight-lifting squat exercise and the track sprint start) to demonstrate
the need for maximum stability vs. maximum instability in a desired
direction.
The incidence of low back pathologies (e.g. disc herniations, endplate
stress fractures, spondylolyses, and spondylolistheses) in football
players prior to collegiate participation is well documented (McCarroll et
al., 1986; Kreighbaum and Barthels, 1996).
The explosive nature of body movement patterns in this sport and
the need to move large opponents requires the development of both maximum
relative and absolute power. Free
weight lifting in the form of parallel squatting, deadlifting, lunges,
cleans, etc. performed correctly can develop lower extremity strength and
power. Correct body alignment
during execution of these types of free weight exercises involves keeping
the center of mass over the mid-point of the base of support of the body
(i.e., maximizing stability), which minimizes both compressile and
shearing forces along the lumbar spine.
Under normal parallel squatting conditions, Lander et al. (1990)
calculated the joint compression forces at the L5-S1 joint to equal 10,473
N (2304 lb) and the shearing forces to equal 3843 N (846 lb) for six
athletes squatting between 150-175 kg (330-385 lb).
If parallel squatting technique deviates from optimal form: (a)
trunk held in lordotic position, (b) shoulder girdles retracted, (c) toes
pointed slightly outward, and (d) body weight positioned over entire
surface area of the feet; the compression and shearing forces exerted on
the low back increase exponentially as does the potential for injury (Nordin
and Frankel, 1989). Coaches
must be taught that maintaining proper form and technique during these
types of free weight exercises must be emphasized over creating a
competitive atmosphere aimed solely at identification of the total amount
of weight lifted by position.
Contrary to maximizing stability during a squat exercise, the start position
of a track sprinter, should maximize instability in a forward direction.
In the sprint start position, the center of mass is located just
beyond the front edge of the base of support.
When the gun sounds to start the race, the runner’s primary
objective is to clear the starting blocks with explosive forward momentum.
As the hands lift off of the track, the center of mass is located
outside of its base of support creating a state of maximum instability.
The center of mass must be quickly re-established over its base of
support to prevent falling. This
starting position is utilized to achieve forward movement as quickly as
possible.
References
Cholewicki, J., McGill, S.M., & Norman, R.W. (1991).
Lumbar spine loads during the lifting of extremely heavy weights. Medicine
and Science in Sports and Exercise. 23
(10): 1179 – 1186.
Lander, J.E., Simonton, R.L., &
Giacobbe, J.K.F. (1990).
The effectiveness of weight-belts during the squat exercise. Medicine
and Science in Sports and Exercise.
22 (1): 117 – 126.
McCarrol, J.R., Miller, J.M.,
& Ritter, M.A. (1986). Lumbar spondylolysis and spondylolisthesis in college
football players. American
College of Sports Medicine.
14: 404 – 405.
Norkin, C.C., & Lavangie, P.K. (1992).
Joint Structure and Function (2nd ed.) Philadelphia: F. A.
Davis Company. |
