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Articles
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Contributions of the ankle and knee muscles to sprint starting.
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Introduction
Starting is an important component of the sprint race, especially the 100 m dash, since any
errors made will affect a significant proportion of the race. The sprint start has been extensively
analyzed (e.g., Dickinson, 1934; Henry, 1952; Baumann, 1976; Gagnon, 1976; Desipres 1973) yet
there is little information available as to which muscle groups contribute to the motion, how much
and how does each contribute, and in what order. It was the purpose of this research to
investigate these aspects of sprint starting using link segment indirect dynamics.
Review and theory
Link-segment indirect dynamics is a method for computing net forces and moments of force at
the joints from body segment parameters and kinematics derived from filmed human motions. It is
assumed that the computed moments of force are primarily caused by muscular contractions and,
thus, inverse dynamics offers an excellent tool for evaluating muscle functioning. In actuality
moments of force may also be caused by ligaments, joint capsules, bones, and other structures.
The net moments of force represent the summed moments of force of all the structures that act
across a joint, whether or not they cross more than one joint.
Unfortunately, solving for actual muscle functions is usually impossible because in human
motion analyses most systems of equations are mathematically indeterminate. Thus, the concept
of the “single equivalent muscle” is used to simplify the real situation. This concept assumes that
there is a pair of “muscles” at each joint– one extensor and one flexor. It is also assumed that
pairs of muscles act in reciprocal inhibition, that is, only one of the pair can be active at any given
instant.
A single equivalent muscle can be said to contract concentrically when its action (i.e., moment
of force) assists the joint’s angular motion (i.e., directions are the same). Conversely, the muscle
can act eccentrically when its moment resists the joint’s motion. These characteristics may be
determined by computing the powers produced by the moments of force. These powers are
calculated by taking the product of moment of force times joint angular velocity. When the power
is positive then the moment is said to be concentric. When the power is negative the moment is an
eccentric one. Integration of the power over time yields the work done by the moment or the
energy dissipated (Elftman, 1939; Winter, 1979; Robertson & Winter, 1980).
Methodology
Two subjects who compete nationally in the sprints or sprint hurdles were filmed at 50 fps in a
laboratory while starting from a Kistler force platform. Each subject was filmed starting with only
one foot on the force plate. Three starts of each foot were filmed with the best trials of each
subject being selected for linked- segment analysis.
The subjects ran with their regular running attire and spiked footwear on a material that is used
for indoor running tracks. A cutout piece of the material was affixed to the force platform in such
a way as to be isolated from the rest of the indoor track. This was necessary to obtain accurate
horizontal forces. Previous tests have shown that as much as 50% of the horizontal force can be
lost if the indoor track extends in one piece across the force platform.
The subjects used special indoor starting blocks that were separate for each foot. During the
filming the athletes were allowed to prepare as is usual in a sprint race. The athletes were
instructed to sprint as fast as possible for the first seven metres. As incentive their times over this
distance were recorded and reported to them. The athletes, who regularly trained on the same
track surface, were allowed sufficient practice trials to accommodate to the laboratory setting.
The film data after processing as projected onto a table equipped with a Numonics digitizer
interfaced to microNova computer. Markers at the approximate joint centres of the shank and
foot segments were digitized to an accuracy of 0.3 mm. These data were subsequently transferred
along with the synchronized force recordings to a mainframe computer for processing by the
computer software, BIOMECH (Kinesiology Dept., Univ. of Waterloo). This package performed
the aforementioned inverse dynamics to yield the powers, work, moments of force, and joint
angular velocity histories of the ankles and knees.
Result and discussion
Angular kinematics. The two athletes exhibited similar kinematic patterns at both joints and for
both legs. Both athletes exhibited slowly increasing plantar flexion and knee extension peaking
just prior to toe-off. The peak velocities of these joints occurred almost simultaneously in contrast
with the notion that the sequence of joint movements should be knee first then ankle.
These movements were followed immediately by rapidly rising dorsiflexion and knee flexion.
The most dramatic pattern being the knee flexion which reached velocities exceeding 15 rad/s.
Ankle kinetics. The two athletes exhibited different patterns at the ankle. One athlete had a
single weak plantar flexor concentric contraction which produced only 12 J of work and a peak
moment of 70 N.m. The other athlete produced a much larger plantar flexor moment which
peaked at 125 N.m and yielded three work periods. An initial concentric period followed by an
eccentric period and then another concentric period. The final concentric period had a peak power
output of over 400 W compared with a peak of only 200 W by the other athlete.
The front leg of the two athletes begins with a much larger moment of force (100 N.m vs 0 for
the back leg) since this leg is in a better position to carry the body’s weight. As with the back leg
a plantar flexor moment dominates throughout the thrust period. The peak moment for both
athletes was nearly 200 N.m. The functions of this moment were similar for both athletes. Initially,
the moment is isometric, that is, its power output is nearly zero yet its moment of force is not
zero. After about 0.3 s the moment becomes eccentric allowing the ankle to dorsiflex slightly
before plantar flexing. This brief dorsiflexion may act as a counter-movement enabling a more
power plantar contraction. The final plantar flexor concentric contractions prior to toe-off reached
powers of between 500 and 700 W and produced between 50 and 100 J of work.
After toe-off the work and power output of the ankle muscles drops very low since the
muscles are no longer acting to propel the body but are only needed to dorsiflex or plantar flex
the foot.
Knee kinetics. The muscles of the back legs of the two athletes had quite different functions. In
one athlete the knee extensors predominated acting eccentrically to prevent excessive knee
flexion. In the other, the knee extensors acted concentrically causing extension and forward
motion of the upper body. Finally, prior to toe-off the knee flexors dominated with an eccentric
contraction preventing hyper- extension of the knee similar to the role that they play during the
push-off phase of running (Robertson, in press).
The knee muscles of the front leg of both athletes had concentric periods of contraction of the
knee extensors prior to toe-off as might be expected. These contractions had peak powers of 400
and 800 W and peak moments of, approximately, 100 N.m. The athlete that had the highest
power also had a preceding knee flexor concentric contraction. This contraction caused the knee
to flex and perhaps enabled a more forceful knee extensor contraction, however, the additional
time required could be disadvantageous.
Additional time spent in the blocks means the athlete will be behind at the start and unless the
athlete’s velocity is greater he/she will not be able to make up for the loss if other factors are
equal. At this stage of the research the author is uncertain as to whether more time spent creating
thrust and consequent velocity is more important than getting away earlier. Previous studies
comparing the “bunch” and “elongated” starts showed that even though the bunch start gets the
athlete out of the blocks earlier an elongated start gets the athlete out faster, however, over 5, 10,
and 50 yd. the elongated start produced faster times (Henry, 1952).
References
1. Baumann, W. Biomech. V-B, 194-199, 1976.
2. Desipres, M. Biomech III, 364-369, 1973.
3. Dickinson, A.D. Res. Quart. 5(supp): 12-19, 1934.
4. Elftman, H. Amer. J. Physiol. 125:339-356, 1939.
5. Gagnon, M. Biomech. V-B, 46-50,1976.
6. Henry, F.M. Res. Quart. 23: 301-318, 1952.
7. Robertson, D.G.E. Biomech. X, in press.
8. Robertson, D.G.E. & Winter, D.A. J. Biomech. 13:845-854, 1980.
9. Winter, D.A. Biomechanics of Human Motion, Wiley: Toronto, 1979.
Author
D. Gordon E. Robertson,
Kinanthropology Department
University of Ottawa |
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