426-44b biomechanics of weight lifting

8/13/2019 426-44B Biomechanics of Weight Lifting

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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.

426-44b biomechanics of weight lifting

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