Ground Reaction Force & Stress Injuries Sports Biomechanics High Performance Sports Runner Sprinter

1. GRF (Ground
Reaction Force) &
Stress Injuries
Alpesh Jadhav
B.Sc. Sports & Exercise Sciences

2. Stress Fractures
• A Tiny Crack in a Bone caused by repetitive stress or force, often
from overuse.
• Forces that can cause a stress fracture could include repeatedly
jumping up and down or running long distances.

3. Causes
• Overuse
•Bone density
•Form
•Nutrition
•Change in surface
•Improper Equipment

4. List of Some of The Common Sites For Stress
Fractures In Runners

5. What research Studies Say ?
• Several studies have associated a High-arched Foot, with a greater incidence of
stress fractures (Giladi et al. 1985)
• One study found more femoral and tibial stress injuries in High-arched Feet and
more metatarsal stress fractures in individuals with low arches (Simkin et al.
1989).
• Muscle activity can modify the stress distribution in the foot.
• Sharkey et al. (1995) hypothesized that a consequence of fatigue during repetitive
exercise might be an increase in the loading of the metatarsals, and thus be a
factor in the mechanism of stress fractures.

6. • Stress fractures result from Repetitive Loading of bone, at levels
higher than can be sustained without a gradual breakdown of the
involved tissues.
• Stresses in the bone result from the ground reaction forces applied
to the feet, the internal muscle forces caused by muscle contraction,
and stress effects resulting from the specific composition and
orientation of the bones and joints in the lower extremity.

7. Ground
Reaction
Force

8. CONTACT FORCES
• Forces resulting from a direct interaction of two objects.
• The following contact forces are considered paramount in human
movement:
Ground Reaction Force (GRF)
Joint Reaction Force
Friction
Fluid Resistance
Inertial Force
Muscle Force
Elastic Force

9. Ground Reaction Force
• The Reaction Force provided by the surface upon which one is
moving.
• All surfaces on which an individual interacts provide a reaction
force.
• The individual pushes against the ground with force, and the
ground pushes back against the individual with equal force in the
opposite direction (Newton’s law of action–reaction).

10. • These forces affect both parties—the ground and the individual—and do
not cancel out even though they are equal in magnitude but opposite in
direction.
• The GRF changes in magnitude, direction, and point of application during
the period that the individual is in contact with the surface.

11. • As with all forces, the GRF is a vector and can be resolved into its
components.
• For the purpose of analysis, it is commonly broken down into its
components.
• These components are orthogonal to each other along a three-
dimensional coordinate system.

12. The components are usually
labeled:
• Fz Vertical (up–down),
• Fy AnteroPosterior (forward–
backward), and
• Fx MedioLateral (side-to-side).

13. The GRF is the sum of the effects of all masses of
the segments * the acceleration due to gravity.
i.e. the sum of the product of the masses and
accelerations of each segment.
This sum reflects the center of mass of the individual.
Consequently, the GRF acts at the center of mass of
the total body.

14. Use of GRF in Athletes
∑F = ma
Dividing a force by the mass, the result would be acceleration.
Hence,
a = ∑F/m
This value reflects the acceleration of the center of mass.

15. RESEARCH AREAS OF INTEREST
• Many researchers have related the Vertical GRF component to the
function of the foot during landings.

16. Dissipation of Energy:
Another view about Stress Fracture
Injuries

17. • In many athletic events which involve landing, the body experiences Very
High Impact Forces.
• The Vertical Ground Reaction Force can reach values that exceed body
weight by 14 times (Tupa et al. 1980; DeVita & Skelly 1992; McNitt-Gray
1993; Simpson & Kanter 1997; Requejo et al. 1998), which may result in
injuries (Dufek & Bates 1991; Nigg 1985).

18. • Two types of injury may occur due to extreme loads:
1. Injuries of Passive Anatomical Tissue (ligaments, cartilage,
intervertebral discs, etc.)
2. Injuries of muscles.

19. • The mechanisms underlying both injury types are not yet precisely
understood.
• If it is proven that the amount of mechanical energy absorbed by the
passive tissues during landing impact is a major contributor to their damage,
then the ability of active muscles to dissipate mechanical energy may be
very useful in protecting passive anatomical structures.

20. Dissipation of Energy
• A muscle subjected to Periodic Stretching and Shortening causing
dissipation of energy of oscillations.
• The ability of the muscle to dissipate energy increases with an
increase in activation level (Gasser & Hill 1924) and with the
magnitude of length change.

21. • A muscle’s ability to dissipate mechanical energy of the body seems to have
important implications for such athletic activities as landing in gymnastics, where
muscles acting eccentrically have to dissipate energy of the body in a short
period of time.
• The ability of muscles to dissipate energy is also important for preventing joint
angles from reaching the limits of their range of motion by decelerating body
segments.

22. 1) Stretching Active Muscles may lead to an enhancement of developed
force, work and power during subsequent isometric and concentric
actions.
2) This enhancement does not require additional metabolic energy
expenditure and may increase economy and efficiency of subsequent
isometric and concentric actions.

23. The amount of mechanical energy passively dissipated can be estimated
during barefoot landing on a stiff force plate after a drop jump.
To make this estimation, the percentage of energy dissipated by muscles is
obtained as:
Total negative work of joint moments during landing
ISL = ------------------------------------------------------------------ * 100%
Reduction in total energy of the body during landing
Where, ISL is the Index of Softness of landing

24. • In maximally soft landings, the total negative work of joint moments and the
reduction in total energy of the body were equal within the accuracy of
measurements (Zatsiorsky & Prilutsky 1987; Prilutsky 1990).
• The index ISL represents the percentage of total energy of the body just before
landing, which is dissipated by the muscles.
• The rest of the body’s energy is dissipated by Passive Structures.

25. • In the maximum stiff landings that the subject could perform, up to 30% of
the energy was dissipated passively (Zatsiorsky & Prilutsky 1987).
• If landing is performed on the heels by keeping the legs straight, no joint
work will be done and all the energy of the body will be dissipated in the
passive anatomical structures.
• Needless to say it would be very harmful for the body.
• It appears that athletes are able to regulate muscle behavior during
landing in order to maximize either ‘spring’ or damping properties of
the muscles (Dyhre-Poulsen et al. 1991).

26. • In several joints of the swing leg and the upper extremities, negative power
is developed prior to their range of motion limit (Morrison 1970; Winter
& Robertson 1978; Tupa et al. 1980; Prilutsky 1990, 1991).
• Thus, the muscle’s ability for energy dissipation and damping of high-
impact forces appears to play an important role not only in attenuating and
dissipating impact shock waves, but also in protecting joints from
exceeding their range of motion.

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28. Thanks