Download SHRSCC 4 Musculoskeletal Function in the Sprint and

Specialist Certification Program
The Sprint, Hurdle, & Relay
Events
Musculoskeletal Function in the
Sprint and Hurdle Events
Musculoskeletal Architecture
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Muscular Architecture. All muscles in the body are constructed differently. These differences
produce differences in their abilities to produce force and in their speeds of contraction, even
though the actual contractile mechanisms are similar.
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Sarcomere Arrangement. A sarcomere is capable of contracting to approximately one
half of its resting length. If the predominant arrangement of the sarcomeres is end to
end (in series), the excursion (distance covered during contraction) of the muscle is long
and much distance is covered in a short time. Conversely, if the predominant
arrangement of sarcomeres is side by side (in parallel), the excursion is diminished, but
force production is enhanced. Long, slim muscles (primarily series arrangements) are
typically fast contracting. Short, thick muscles (with primarily parallel arrangements) are
typically slow contracting, but produce great force.
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Fiber Arrangements. The fiber arrangement in a muscle dictates its characteristics as
well. Parallel arrangements of the fiber put the fibers parallel to the line of pull of the
muscle. All fibers act along the same line. However, often we see fibers placed in a
slanted pattern called a pennate arrangement. In this setup, the effective excursion of
each fiber is reduced, slowing contraction. However, pennate arrangements allow much
fiber to be packed into a small area, enhancing the force producing capabilities of the
muscle.
Lever Systems
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Third Class Levers. Much of the musculoskeletal system operates as third class levers.
The fulcrums of these levers are located at the joint, the force is applied a short distance
from the joint, and the resistance is found even further from the joint. The anchoring of
the fulcrum of this system is crucial to efficiency of the lever, and has direct implications
for the training of absolute static strength.
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Angles of Pull. As the third class lever moves through its range of motion, angles of pull
of the muscle against the bone change, and unique forces are created at the joint.
Initially, much of the force is transmitted into the joint, producing a compression force.
In the middle of the range of movement we find the most effective locomotive forces.
Finally, at the end of the movement, we see a dislocation force occurring.
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Single and Double Jointed Muscles.
arrangements about a joint.
In locomotive joints, we see two types of muscles
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Single Jointed Muscles. Single jointed muscles cross one joint, and may act upon that
joint only.
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Double Jointed Muscles. Double jointed muscles cross two joints and may act upon
either, depending upon the stabilization patterns produced elsewhere. Double jointed
arrangements are very efficient, effectively acting as a pulley system in the way they
transmit force.
Spinal Loading and Postural Integrity
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Spinal Loading. Spinal loading refers to the process of force transmission and reception by the
spinal column.
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Postural Integrity. Postural integrity refers to proper functional state of the core of the body
during performance. Postural integrity is critical and prerequisite to proper function of the
limbs. It is important in three realms.
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Postural Stability. The core of the body must be adequately stabilized to provide a solid
base from which force may be applied. This stabilization is important when the body
must be subjected to impact. Failure to do so compromises displacement of the body, as
displacement producing forces become absorbed as individual angular movements
within the body.
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Postural Alignment. The core of the body must be aligned correctly in order to position
the limbs for efficient operation. We are most concerned with alignment in two realms,
the alignment of the head with respect to the spine, and the alignment of the pelvis
with respect to the spine.
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The Head. In nearly all athletic endeavors, we desire a neutral alignment of the
head. This insures muscle relaxation, stability of the body, and proper vestibular
function.
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The Pelvis. In nearly all athletic endeavors we desire a neutral alignment of the
pelvis. This places the pelvis in best position to move freely, the legs in best
position to apply force, and the body in the best position to avoid injury.
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The Spine. Alignment of the spine itself is another factor that must be
considered. It must be positioned in a way to best accept loading. Most spinal
misalignments are pathological, (lordosis, scoliosis) but can be corrected to
some degree with well designed training.
Uniformity of Movement. This stabilized and aligned postural unit (head, spine, and
pelvis) must move in some predictable fashion so that forces can best be transmitted to
the ground, and forces applied to the postural unit can be best accepted. Erratic
movements or radical changes in the path of movement make force application difficult.
Elastic Energy
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Elastic Energy Production. Muscles are capable of contracting with more force when they
operate elastically, using the stretch shortening cycle. Postural muscles are capable of elastic
operation, like all other skeletal muscle. This elastic response may occur in linear or rotational
fashion.
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The Spinal Engine and Pelvic Origination. Human locomotion originates with small movements
in the lumbar spine. These are amplified by the pelvis, and amplified to a greater degree by the
legs. Thus, freedom of movement of the spine and pelvis are crucial to running economy.
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Oscillations of the Pelvis. The pelvis during running oscillates in the frontal and transverse
planes, allowing elastic energy to be developed in the postural musculature. This aids running
economy by increasing stride length at any given frequency of movement.
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Postural Compromise and Elastic Energy. Postural problems decrease the freedom of the pelvis
to move or produce asymmetrical oscillations. This decreases elastic energy production,
destroying efficiency. These problems are commonly associated with excessive instability.
Summations and Transmissions of Force
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Summations of Force
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Usage of All Joints. In athletics we are usually concerned with generating great forces.
All of the joints in the body are capable of producing some force. Therefore, it follows
that we want to use all available joints when force production is a priority.
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Joint Characteristics. All of the body’s joints are unique with respect to structure, and
thus force generation capabilities. Some are slow, some fast. Some weak, some strong.
Generally speaking proximal joints are slow, but produce large forces. Distal joints are
weak, but produce force quickly.
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Firing Orders. In efficient movement, we use slower, stronger joints to overcome
inertia, then faster, weaker joints once movement has been initiated and inertia is
overcome. Thus, the most effective patterns of joint usage progress from proximal to
distal. We refer to a unique timing of the contributions of these joints as a firing order.
There is a unique firing order for any movement that yields the most force, involving all
joints in a certain sequence.
Transmissions of Force
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Force Transmission. Sometimes joints are not in involved in force production, but force
transmission. When proximal joints are acting, forces are being transmitted through the
distal joints. Thus stabilization of the distal joints early in movement is important to
performance. Failure to stabilize these joints results in dissipated force and poor force
transmission.
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Joint Positioning. The initial positioning of a joint is important to performance. The joint
must be positioned in some way that allows the joint to act in the intended direction,
effectively contributing to performance.
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Distal Segment Positioning. In most efficient force transmission schemes, the force
generated in a proximal joint is transmitted through a distal segment of a limb. In these
cases, the force should be applied along the long axis of that segment, in order to best
transmit force and prevent dislocating forces.
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Joint Communication. Joints effectively communicate with each other, through
networks of muscle and fascia, and in reflexive activity. Similar relationships, joint
angles, and degrees of flexion and extension exist in comparable joints of the arms and
legs at many points during performance.
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The Distal Positioning Phenomenon. Positioning of the distal joint in a limb is especially
important for this reason. The positioning of the distal joint of a limb dictates the
characteristics of the firing order of that limb. As these joints communicate, messages
are sent from the distal joint throughout the limb. These affect the timing and efficiency
of more proximal joints.
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Transfers of Angular Momentum. In many cases, the flexion seen in a moving limb is
not produced by volitional flexion, but results from a transfer of angular momentum to
the distal segment of the limb when the proximal segment changes direction. The most
common occurrence of this type is the knee flexion seen in the recovery phase of a
running stride. The knee flexors remain totally relaxed, yet the knee flexes as a result of
the transfer of momentum when the direction of the thigh changes from backward to
forward.
Stability
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Dynamic Stability. Human locomotion is composed of alternating periods of stability and
instability. At touchdown of any step, the center of mass of the body is over the base of support,
and the body is stable. Yet an instant later, the center of mass is projected well past the base of
support, producing instability until the next step grounds. We call this process of losing and
regaining stability dynamic stability. While we then do expect certain degrees of instability in the
locomotive process, excessive instability can produce harmful consequences.
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Tradeoffs. Each step in running yields a tradeoff. Grounding the step in front of the body’s
center of mass interrupts horizontal progress. Grounding the step behind the body’s center of
mass preserves momentum, yet introduces instability. For this reason, optimal locomotion
patterns locate the grounding of each step under the body’s center of mass, optimizing the
tradeoff between those two goals.
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The Stability Reflex. When the human body is subject to excessive instability, movement
patterns change involuntarily in order to improve stability levels. These changes are seldom
consistent with good technique and high levels of performances. Typically we observe three
movement strategies in cases when excessive instability is introduced into the movement
pattern.
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Grounding Strategies. When grounding strategies are used, the next step is hurried
and/or location of a step is altered in order to regain stability. This results in altered
center of mass/base of support relationships. Reaching steps are a common grounding
strategy. Another common grounding strategy is plantar flexion of the foot. Plantar
flexion is a convenient motor tool to hasten ground contact.
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Multilink Strategies. When multilink strategies are used, the body shifts its parts in
some plane to regain balance. This results in compromised force summations and
transmissions.
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Stiffening Strategies. When stiffening strategies are used, the body stiffens joints
and/or the postural unit in order to preserve stability. This results in diminished
amplitudes of movement and elastic energy production.
Reflexive, Cyclic and Anticipatory Concerns
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The Rationale for Cause and Effect Coaching
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Error Accumulation. At the speeds we see in competition, time available for error
correction is minimal. Also, at high speeds, more of the movement is reflexive, and less
of the movement is under cognitive control. For this reason, errors in execution usually
produce other errors. Thus, a philosophy of cause and effect coaching is useful.
Backtracking, or looking at points prior to a mistake in order to find the roots of that
mistake is a valuable coaching tool.
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Considerations for Cyclic Activities. Cyclic activities are activities that reproduce a
certain pattern of movement. In cyclic activities, the errors we see normally reproduce
themselves, appearing the same way, at the same point in each cycle.
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The Rationale for Foretracking. Sometimes, errors are not caused by previous errors, but in
anticipation of certain circumstances. Habitual faulty movements near the end of a skill often
interfere with proper execution at the start of the skill. These circumstances exist as expected
movements or perceptions, and affect prior patterns of movement. At other times, these
circumstances may take the form of environmental factors. In either case, for these reasons,
foretracking is often an effective coaching tool. An example might be a hurdler whose takeoff
mechanics are disrupted because of a takeoff that occurs too close to the hurdle. The takeoff
appears faulty, but the problem is actually the hurdler’s anticipated misplacement with respect
to the hurdle.
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