Biomechanics of Resistance Exercise. Biomechanics Biomechanics is the study of body movement The body's mechanics are muscles, bones, tendons and ligaments.

Biomechanics of Resistance Exercise

Biomechanics • Biomechanics is the study of

body movement

• The body's mechanics are muscles, bones, tendons and ligaments

• Makes use of the laws of physics and engineering concepts

Force• Defined as any interaction, a push or pull,

between two objects that can cause an object to accelerate or decelerate

• Forces are characterized by magnitude (how much) and direction (which way are they moving)

• For example a push on the ground generated by knee extension and hip extension may cause the body to accelerate upward-jump

Characteristics of a Force• Point of Application:

–Specific point at which the force is applied to an object

–Insertion of the muscle

• Line of Action:–Direction of the force

–A straight line of infinite length

Types of Forces

• Gravity• Inertia• Friction• Tension• Compression

• Shear• Torsion• Torque• Muscle Force• Ground

Reaction Force

Gravity

• Downward force on an object

• Equal to the object’s mass multiplied by motion

• Mass is the amount of matter that constitutes an object and expressed in kilograms

• Mass is constant, regardless of where it is measured (earth or on the moon)

Gravity• The pound is a unit of force not mass

–The mass of a BB stays constant while its weight varies according to motion

• The kilogram on a weight refers to its mass

• It is incorrect to say that an object weighs a certain number of kilograms (weight relates to force not mass)

–The mass of the BB is 45kg

Center of Gravity• The CG (center of mass) of a body refers to its

balance point   • The CG is that point where all of the weight of the

body is concentrated  (CG over BOS)• Anterior to S2 when standing

Center of Gravity• Changes when body is in motion

– Crouching, kneeling, or a sitting position will lower the center of gravity and increase stability. 

– A wrestler and defensive lineman will increase stability by lowering their CG

Center of Gravity• Narrow base of support is unstable

• When lifting or carrying, keep objects COG close to yours

• Lifting objects far from COG may result in fatigue, sprain, strain, joint damage…

Center of Gravity and Exercise• Move limbs and weight away from COG to increase

resistance

• Keep limbs and weight close to COG to decrease resistance

Inertia• Resistance to movement• Related to mass of the object• The greater the mass of an object, the

greater its inertia and harder to move it• “Cheat lifting movements” • A baseball player can train with a heavy

bat that provides greater inertial resistance, which slows down the swing speed

Friction• A force acting parallel (sandwiched)

between two objects during motion–When two surfaces pressed

together rub against each other.–Belt or brake pad, slide board,

football sled• Fluid Resistance:

–Pushing an object through a liquid or gas.

–Swimming, cycling, skydiving, baseball, rowing, kayaking, golf

Tension• A stretching force pulling at both ends

–Pull of a contracting muscle tendon

Compression• A force that tends to flatten or squeeze an object

– Lifting a BB presses the ends of the bones together and is produced by muscles and gravity along the length of the bones

• Necessary for the development of bone growth• If a large compressive force is applied, and the load

surpasses the stress limits, a fracture or break will occur

Shear Force• Applied parallel to the surface of the object

– Across the surface of a bone• A good example of shear force is seen with a

simple scissors. • The two handles put force in different directions

on the pin that holds the two parts together.

Torsion• Twisting force

– Foot is planted and the body changes direction (ankle sprain)

Torque• The force which causes rotation

• Rotation occurs about a pivot point

• Line of action of force must act at a distance from the pivot point

Muscle Force• A muscle can only generate a pulling or

tensile force

–Elbow extension: the triceps brachii pulls on the olecranon process of the ulna

–Elbow flexion: biceps brachii pulls on the radial tuberosity of the radius

–Movement at any joint occurs by opposing pairs of muscles and gravity assists in the motion

Ground Reaction Force (GRF)• Force provided by the surface an individual is moving

on (i.e., sandy beach, gym floor, grass lawn, concrete sidewalk) and the reaction to the force the body exerts on the ground

• If the body is pushing down and forwards, the ground reaction force is up and backward; if the body pushes down and backward, the ground reaction force is up and forwards

Levers of the Musculoskeletal System• Involve bones, joints, and skeletal

muscles • Bring about most movements of the limbs

and the whole body• Body movements characteristic of sport

and exercise act primarily through levers of the skeleton in order to exert forces on the ground, object, and other people

• Muscles not acting through bony levers include those of the face, tongue, heart, arteries, and sphincters

Parts of a Lever• Lever: a rigid or

partially rigid structure that can rotate about a pivot point or fulcrum.

• Fulcrum (pivot point): axis of rotation or the point about which the lever pivots

• The axis is the point of rotation about which the lever moves

Levers• Bones of the body act

as levers which create a mechanical advantage of speed or strength

• The fulcrum is formed by the joint

• Effort is any force applied to the lever

• Load or resistance is a force that resists the motion of the lever

Muscle Force • Effort Arm/Force Arm• The force exerted by a muscle at either of its

ends when its electrochemically stimulated to shorten

Moment Arm (Lever Arm or Force Arm)• Moment arm of

the muscle force• Moment arm of

the resistive force

• Distance between the point where the force acts and the point of rotation

Application• Levers =

humerus, radius, and ulna

• Effort= contraction of biceps

• Load= weight of the arm, gravity…

• Fulcrum or axis of rotation = humeroulnar joint

Mechanical Advantage (MA)• Mechanical

Advantage: created by a machine that enables people to do work while using less force

• Used to allow a small effort to move a large load.

• It is calculated by dividing the load by the effort

Mechanical Advantage (MA)• A normal human can't lift a 1,000-pound vehicle

off the ground, so you need the mechanical advantage of a jack.

• If the weight of the vehicle (the load) is 1,000 pounds and the weight of the jack handle (the effort) is seven pounds, then the mechanical advantage provided by the jack is 1,000/7, or "one thousand to seven."

Mechanical Advantage (MA)• A MA greater than

1.0 means that muscle force exerted is less than resistive force

• A MA less than 1.0 means that muscle force exerted is greater than the resistive force

• A MA less than 1.0 is a disadvantage

Musculoskeletal System• Most of the skeletal muscles operate at a

considerable mechanical disadvantage.• Variations in Tendon Insertion

–Tendon insertion farther from the joint center results in the ability to lift heavier weights.• This arrangement results in a loss of maximum speed.

• This arrangement reduces the muscle’s force capability during faster movements

Three Classes of Levers• Each providing

different levels of mechanical advantage

• The levers are referred to as class 1, class 2 and class 3.

• Differs by the force you apply (effort), load (opposing force), and fulcrum (pivot point)

First Class Lever• Fulcrum is placed between the load and the

effort

• The force you apply is on the opposite side of the fulcrum to the force you produce

First Class Lever• If the two arms of the lever are of equal

length, the effort must be equal to the load

• If the effort arm is longer than the load arm, as in a crowbar, the effort travels farther than the load and is less than the load

• The mechanical advantage is calculated by measuring the length of the lever on either side of the fulcrum.

First Class Lever

• Triceps Extension:

–Muscle force and resistive force act on opposite sides of the fulcrum

First Class Lever• The head is raised off

the chest

• As the head is raised, the facial portion of the skull is the resistance, the fulcrum is between the atlas and occipital bone, and the effort is the contraction of the muscles of the back

First Class Levers• Balanced Movements

– Fulcrum near the middle

– Seesaw

– Erector spinae extending the head

• Speed and Range of Motion– Fulcrum is near the effort

– Scissors

– Triceps extending the elbow

• Force motion – Fulcrum is near load

– Crow bar

Second Class Levers• Load between the effort and the fulcrum • A wheelbarrow is a second-class lever. The wheel’s

axle is the fulcrum, the handles take the effort, and the load is placed between them.

• The effort always travels a greater distance and is less than the load.

Second Class Lever• Gastrocnemius and

soleus in plantarflexing the foot to raise the body on the toes

• The ball of the foot is the fulcrum, gastrocnemius and soleus are the muscle force, and the body is the resistance

Second Class Levers

Third Class Levers• Effort placed between the load and the fulcrum

• Increase distance and speed not force

• A hammer acts as a third-class lever when it is used to drive in a nail: the fulcrum is the wrist, the effort is applied through the hand, and the load is the resistance of the wood.

Third Class Lever• DB Biceps Curl• The fulcrum is the elbow, the effort is applied by

the biceps muscle, and the load is the weight in the hand.

• Most movements of the body are produced by third class levers

Third Class Lever• Pitchers use their arms

as third-class levers. • During a pitch, the

pitcher’s forearm pivots at the elbow, which is the fulcrum.

• The muscles of his forearm supply the force, and the ball provides the resistance.

• The lever action of a pitch magnifies the speed a pitcher can give to the ball.

Summary of Levers• Most body movements produced are

third class levers• We are more adapted to speed than

strength (short force arm/long weight arm)

• First class levers give the advantage of strength or speed depending on where the fulcrum is located

• Second class levers give the advantage of strength

Mnemonic• First Class Lever : E F L or L F E • Second Class Lever: F L E or E L F • Third Class Lever: F E L or L E F

• ETHEL (E F L) the FLEA (F L E) FELL (F E L)

Questions

1. Identify the 3 parts of a lever.

2. Draw and label a diagram of each of the 3 types of levers

Questions

3. In lifting an object, the biceps represents the _____________ force; the elbow represents the _____________;  the object lifted represents the ______________ force. This is an example of a ______________ class lever.

Identify the Classes of Levers

Questions5. In a first class lever, the

_________________ is between the effort and the load.

6. In an off-center first class lever (like a pliers), the load is larger than the effort, but is moved through a smaller_____________.

7. Identify two examples of first class levers

Questions

8. In a second class lever, the ________________ is between the fulcrum and the effort.

9.Identify two examples of second class levers.

Questions

10. In a third class lever, the ____________________ is between the fulcrum and the load.

11. Identify three examples of third class levers.

Questions

12. A man lifts a rock weighing 500 pounds by standing on the end of a lever. When the man later stood on a scale and weighed himself, he weighed in at 100 pounds.

How much mechanical advantage did the lever provide to the man in lifting the rock?

Questions13. A person wants to use a lever to lift a

large piece of machinery. The person uses a lever that was previously used to lift another piece of similar machinery and will be set up identically for this lift. The mechanical advantage in the last set up was 8. The machinery weighs 800 pounds. What force or weight must the person place on the lever to move the machinery?

Answers• MA = 500 pounds/100 pounds = 5• Conclusion: The man's body weight was

magnified 5 times by using the lever as a simple machine to lift the rock.

• Force with simple machine = (Force without simple machine)/(MA)Force with simple machine = 800/8 = 100 pounds

• Conclusion: The person would have to weigh or apply a force of 100 pounds to move the machinery.

Definitions• Strength:

– The capacity to exert force at any given speed

• Acceleration: – Increase in velocity or speed

• Deceleration: – Decrease in velocity or speed

• The force that a muscle exerts decreases as movement speed increases

• Power: – The time rate of doing work – force x distance/time = work/time

Work

• The effect of a force applied over a distance

• Work = Force x Distance

Work• If a work crew uses a pulley to hoist an

800-pound piano off the ground 20 feet into a second-story window, the amount of work done is 20 feet times 800 pounds, equaling 16,000 foot-pounds of work

• If it took the crew 40 seconds to hoist the piano, the average power would be 16,000 foot-pounds divided by 40 seconds, equaling 400 foot-pounds per second

Work and Power in SI Units• The worldwide standard of measures

is called System International (pg.35)

• Unit of Force is the newton (N)

• Power is Watts (W)

• Unit of Distance is the meter (m)

• Unit of Mass is the kilogram (kg)

Calculating Resistance Training Work• To accurately assess the work in a lifting

session measure the vertical distance the weight moves per repetition

• Work = Weight x Vertical Distance x Reps

• Consider the low and high points during the exercise movement

• Weight of the body should be considered• Compare a squat to a leg press

Biomechanical Factors in Human Strength

• Neural Control

• Muscle Cross-Sectional Area

• Arrangement of Muscle Fibers

• Muscle Length

• Joint Angle

• Muscle Contraction Velocity

• Strength-to-Mass Ratio

Neural Control• How many and which motor units are involved

in a muscular contraction (recruitment)

• During the first few weeks of resistance training, much of the strength improvement can be attributed to neural adaptations

• The brain learns how to produce more force from a given amount of muscle tissue

• Muscle force is greater when:

– More motor units are involved in a contraction, the motor units are greater in size, or the rate of firing is faster.

Muscle Cross-Sectional Area• Cross sectional area is proportional to tension

• Area is measured perpendicular to fiber length

Muscle Cross-Sectional Area

• Two people who have different heights but the same body fat % and weight will not have the same cross sectional area.

• The taller person will have less cross sectional area and less strength in proportion to body mass.

• Taller person will find it more difficult to perform pull-up’s, push-up’s and sprints.

• Strength of a muscle is not related to it’s volume

Arrangement of Muscle Fibers

Pennate Muscles• Pennate muscles (i.e., deltoid, rectus femoris,

tibialis posterior) have fibers that align obliquely with the tendon, creating a featherlike arrangement

• Pennate muscles have greater cross-sectional area (and greater tension development capability) than parallel muscles (extensor digitorum)

Muscle Length• At resting length: actin and myosin filaments lie

next to each other; maximal number of potential cross-bridge sites are available; the muscle can generate the greatest force.

• When stretched: a smaller proportion of the actin and myosin filaments lie next to each other; fewer potential cross-bridge sites are available; the muscle cannot generate as much force.

• When contracted: the actin filaments overlap; the number of cross-bridge sites is reduced; there is decreased force generation capability

Length-Tension Relationships• The length at which a muscle can produce

the greatest force

Joint Angle and Muscle Contraction Velocity

• Joint Angle:

– Higher muscle forces results in higher torque, which means a greater ability to rotate the limb about a joint against resistive torque

• Muscle Contraction Velocity:

– Muscle force (tension) and speed of a contraction (rate) are inverse

– As the speed of a concentric muscle contraction increases, its ability to produce force decreases (overlapping of myosin and actin)

Muscle Actions

• Concentric: the muscle shortens because the contractile force is greater than the resistive force. The forces generated within the muscle and acting to shorten it are greater than the external forces acting at its tendons to stretch it

Muscle Actions• Eccentric: the muscle lengthens because the

contractile force is less than the resistive force. The forces generated within the muscle and acting to shorten it are less than the external forces acting at its tendons to stretch it.

• Isometric: the muscle length does not change because the contractile force is equal to the resistive force. The forces generated within the muscle and acting to shorten it are equal to the external forces acting at its tendons to stretch it.

Force-Velocity Relationship

Strength-to-Mass Ratio• Equals the force the person can exert

during movement divided by the mass of the body

• An individual weighing 150 pounds can leg press 200 pounds, the leg press strength to mass ratio is 200/150 = 1.33

• However, if the same strength gain is accompanied by a body weight increase of 170 pounds, the strength-to-mass ratio is decreased to 1.29

Strength-to-Mass Ratio• As body size increases, body mass increases

more rapidly than does muscle strength

• Given constant body proportions, the smaller athlete has a higher strength-to-mass ratio than does the larger athlete

• Bodybuilders vs. Gymnasts vs. Rock Climbers• Individuals must consider whether the

exercises they select will likely add body mass to a greater extent than increase in strength

• Greater strength-to-mass ratio means improved physical performance

Sources of Resistance to Muscle Contraction

• Gravity

• Inertia

• Friction

• Fluid Resistance

• Elasticity

• Pulleys

• Electronically Controlled Devices

Pulleys• A chain, belt or rope wrapped around a wheel • The mechanical advantage of a pulley system

is approximately equal to the amount of supporting ropes or strands.

• If you had a weight of 60 kg and you wanted to lift it using two pulleys that used two ropes looped between them, you would have a mechanical advantage (MA) of two

• Having two ropes that give you a mechanical advantage (MA) of two means your weight feels like one half of what it really is.

• When lifted with the help of the pulleys 60kg would only feel like 30 kg.

• Thus, the effort weight equals 30 kg

Pulleys• If a pulley setup has three supporting strands,

what would be the MA of the setup?• If the weight of an object being lifted is 100 kg

and the number of supporting ropes the pulley system has is four; what would be the systems MA?

• How much effort weight would you actually be lifting?

• The weight of an object is 30 kg, the mechanical advantage is three, how much effort weight would you need to raise the object?

• (Answers: 3, 4, 25 kg, 10 kg)

Elasticity and Electronically Controlled Devices

• Elasticity: –Springs or bands for resistance–Tension increases with stretch –Tension begins low and ends high

unlike human muscle groups in which there is a decline in force toward the end of the movement

• Electronically Controlled Devices:– Regulate degree of resistance (Life Circuit)

Application • When the weight is horizontally farther

than the joint, it exerts more torque• When the weight is horizontally closer

to the joint, it exerts less torque• During a BB curl the horizontal

distance from the BB to the elbow is greatest when the forearm is horizontal

• The lifter must exert the greatest muscle torque to lift the weight

Application• During a squat the combined center of

mass of the body and BB must be positioned above the foot so that the lifter does not fall

• Placing the bar in different positions causes the lifter to adjust body posture to keep center of mass over base of support to avoid falling

Application• BB positioned low on the upper back,

the trunk leans forward to keep the center of mass over the base of support which brings the bar farther from the hip and closer to the knees (hip extensors work harder)

• BB positioned high on the back or in front of the shoulders brings less trunk lean so the load is placed on the knees

Application• The Smith machine has the

advantage of allowing the feet to be positioned behind or in front of the center of mass of the body without causing a fall

• With feet farther forward, the trunk stays more upright which reduces the torque the back muscles generate making the quadriceps do more work

Application• Any exercise in which the whole

body is lifted involves work against gravity

• Doing an exercise faster increases acceleration forces; more work against gravity and inertia–Sprinting

–Power Clean