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Biomechanics of Weight Lifting
Basic Factors1. Relative TorqueMuscle vs. Resistance
2. Exercise Variations to Target Specific
Muscles
3. Anatomical Considerations
a) Accommodating Joint Movement
b) Protecting Tissue Structure
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Relative Torque
1.Muscle Torque (TM)The torque or
tendency to rotate at a joint produced by
muscle action or muscle force (FM)
2.Resistance Torque (TR) - The torque or
tendency to rotate at a joint produced by
resistance or weight
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Muscle Torque
FM
TM
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Resistance Torque
Resistance
TR
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Relative Torques
The type of muscle contraction that occurs isdependent on the relationship between TMand TR.
FM
Resistance
TM
TR
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Relative Torques
When TM> TRConcentric Contraction (Themuscle shortens and the weight is lifted.)
When TM< TREccentric Contraction (The muscle
lengthens and the weight is lowered.)
When TM= TRStatic or Isometric Contraction
(The muscle length doesnt change and the weight
is held steady.)
Movement occurs as muscle tension is adjusted tochange muscle torque relative to resistance torque.
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Torque produced by muscle forceRelated to the force
produced by the muscle (FM) and the distance from the muscle
insertion to the center of rotation of the joint (DFA).
DFA
FMFRO
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Torque produced by resistanceRelated to the totalweight of the resistance (R) and the distance from the
resistance to the center of rotation of the joint (DRA).
DRA
R
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For most muscles in the body, DFAis less than DRA.Therefore muscle force must usually be much greater
than the resistance in order to lift a weight.
DFA
DRA
FM
R
FRO
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Strength vs. Speed in the
Musculo-Skeletal System
A
B
The musculo-skeletal lever
systems generally favor
speed over strength.
Although the mechanicaladvantage favors the
resistance arm, in the time
that the muscle insertion
moves a given distance
(red arrow), the resistance
moves a much greater
distance (blue arrow).
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Strength vs. Speed in the
Musculo-Skeletal System
A
B
In other words, the end of a
limb is moving at a greater
velocity than the attachmentsof the muscles that produce
that movement.
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Some joints are
structured so that
strength is favored
over speed (as in the
calf muscles when
rising onto the toes).
DRA
DFA
R
FM
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Exercise Variations
Changing the range of motion of an exercise
or the position of a limb segment can
change the muscles targeted by that exercise
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Exercise Variations
Example 1: There are three common ways of
performing curls. Each of these will emphasize a
different flexor of the elbow.
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Exercise Variations
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1) Forward Curls Emphasize the Biceps Brachii (A primary flexor
of the elbow when the forearm is supinated )
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2) Reverse Curls
Emphasize the Brachialis (A primary flexor for all
positions of the forearm)
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3) Hammer Curls Emphasize the Brachioradialis (A primary flexor
when the forearm is halfway between full
supination and full pronation)
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Exercise Variations
Example 2: Variations on the bench press can
emphasize different aspects of the Pectoralis
Major
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Exercise Variations
Clavicular Head
Sternal Head
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1) Inclined Bench Press
Emphasizes the clavicular head of the Pectoralis
Major
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2) Declined Bench Press
Emphasizes the sternal head of the Pectoralis
Major
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3) Flat Bench Press
Uses both heads of the Pectoralis Major
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Anatomical Considerations
In order to lift safely, its important to
accommodate normal joint movement and tissue
structure
Joint StabilityMay be required when joint movementcould produce injury.
Movement AccommodationMany exercises need to
allow for the natural movement of active joints.
Tissue ProtectionThe direction or amount of forceapplication in certain exercises may be critical to
prevent injury to specific tissues.
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Joint Stability
EXAMPLE: Anterior or posterior movement in
the spinal column may cause injury or nerve
impingement when performing squats and otherexercises involving a load placed on the back
when it is moving.
To reduce the chance of injury the spinal column
is rigidly arched (hyperextended) or set toincrease stability.
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Setting the Back The back needs to be set when doing squats.
One of the primary reasons is to stabilize the
spinal column.
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Movement Accommodation
When the knees go from an extended to a
flexed position there is a natural lateral
rotation of the lower leg once the kneeflexes more than 100to 150.
In order to accommodate this lateral
rotation when performing squats, the feetshould be turned out slightly and the knees
should be in line with the toes.
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Foot and Knee Position
The positioning of the feet and knees when
performing squats are important so that the normal
movements of the knee arent restricted.
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Movement Accommodation
When performing dips the width of hand
placement affects the positioning of the humerus
in the shoulder joint. If the hands are too wide, this usually leads to
excessive internal rotation
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Hand Placement
Placing the hands too wide when doing dips may
lead to excessive internal rotation at the shoulder.
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Tissue Protection
Cartilage is designed to handle compressive stress
but is most easily damaged by shear or tensile
stress.
Compression Tension
Shear
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Tissue Protection
When doing squats, if the lower leg leans forward
too far, this creates excessive shear stress across
the knee. Therefore, the lower leg should be asvertical as possible to reduce this effect.
Shear
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Lower Leg angle
Keeping the lower leg as vertical as possible
reduces shear stress at the knee, thereby reducing
the chance of injury to the meniscus cartilage.
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Tissue Protection
When doing leg extensions, the anterior cruciate ligamenthas full tension in it when the knee is within 100to 150of
full extension.
In the last part of the exercise, if it is taken to full
extension, there may be excessive tension in this ligament. Performing full range leg extensions with heavy weight
may stretch the anterior cruciate ligament over time.
This action will also tend to cause excessive wear on the
articular cartilage on the upper facet of the posteriorsurface of the patella
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Tissue Protection
AnteriorCruciate
Ligament
Upper facet of
posterior surface
of patella
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Tissue Protection
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Leg Extensions
Performing full leg extensions with heavy weight
may compromise the structural integrity of the
anterior cruciate ligament and damage the patella.
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