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Biomechanics of ResistanceTraining
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Muscle Skeletal System
Bones and Muscles Joints Ligaments Tendons
Origins, insertions, and Actions
Agonist and AntagonistSynergist - assists directly in a movement(counteractions)
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Physics of the Muscular System
Lever Fulcrum - Pivot pointMoment arm - Perpendicular distance from theline of action to the fulcrumTorque - degree to which a force rotates anobject around a fulcrum
Muscle force - Force generated from the activityResistive force - Force generated by a sourceoutside of the body
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Musculoskeletal System
Skeleton Muscles function by pulling against bones that rotate
about joints and transmit force through the skin to the
environment. The skeleton can be divided into the axial skeletonand the appendicular skeleton.
Skeletal Musculature
A system of muscles enables the skeleton to move. Origin = proximal (toward the center of the body)
attachment Insertion = distal (away from the center of the body)
attach-ment
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Mechanical Advantage
mechanical advantage: The ratio of themoment arm through which an applied forceacts to that through which a resistive force
acts. A mechanical advantage greater than1.0 allows the applied (muscle) force to beless than the resistive force to produce anequal amount of torque. A mechanicaladvantage of less than 1.0 is a disadvantagein the common sense of the term.
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Torque
The degree to which a force tends torotate and object about a specified axis of rotation (fulcrum)
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Levers
First-class lever Second-class lever
Third-class lever
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First Class Lever
Muscle and resistive force act opposite thefulcrum
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Second Class Lever
muscle and resistive force act on the sameside as the fulcrum, with the muscle forceacting through a moment arm longer thanthat through which the resistive force acts
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Third Class Lever
muscle and resistive force act on the sameside as the fulcrum, with the muscle forceacting through a moment arm shorter thanthat through which the resistive force acts
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Variations in Tendon Position
Insertion further from axis of rotationshould be stronger(B) However a decrease speed associated with
movement
Insertion closer to axis of rotation will beweaker(A) However an increase is speed is associated
with movement
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Tendon Insertion and Joint Angle
Figure 4.9 (next slide) The slide shows changes in joint angle with
equal increments of muscle shortening whenthe tendon is inserted (a) closer to and (b) farther from the joint center.
Configuration (b) has a larger moment arm
and thus greater torque for a given muscleforce, but less rotation per unit of musclecontraction and thus slower movement speed.
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Figure 4.9
Reprinted, by permission, from Gowitzke and Milner, 1988.
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The Patella and Mechanical Advantage
Figure 4.6 (next slide) (a) The patella increases the mechanical
advantage of the quadriceps muscle group bymaintaining the quadriceps tendons distancefrom the knees axis of rotation.
(b) Absence of the patella allows the tendon
to fall closer to the knees center of rotation,shortening the moment arm through which themuscle force acts and thereby reducing themuscles mechanical advantage.
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Figure 4.6
Reprinted, by permission, from Gowitzke and Milner, 1988.
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Moment Arm and Mechanical Advantage
Figure 4.7 (next slide) During elbow flexion with the biceps muscle,
the perpendicular distance from the joint axisof rotation to the tendons line of action variesthroughout the range of joint motion.
When the moment arm (M) is shorter, there is
less mechanical advantage.
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Figure 4.7
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Moment Arm
Figure 4.8 (next slide) As a weight is lifted, the moment arm (M)
through which the weight acts, and thus theresistive torque, changes with the horizontaldistance from the weight to the elbow.
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Figure 4.8
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Key Point
Most of the skeletal muscles operate at aconsiderable mechanical disadvantage.Thus, during sports and other physicalactivities, forces in the muscles and ten-dons are much higher than those exertedby the hands or feet on external objects or
the ground.
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Strength & Power
Acceleration - change in velocity per unittimeStrength - maximal force that a musclecan produce at a specific velocityWork = Force x distance
Power = Work/time
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Find Power
Subject BW 130 kg
1 RM Clean 150 kg Distance traveled .65m Time to lockout .6s
1 RM Deadlift 300kg Distance traveled .4m Time to lockout 2.5s
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Biomechanical Factors inStrength
Neural Control Motor unit recruitment Rate motor units are fired (rate coding)
Muscle Cross Sectional Area
Arrangement of Muscle Fiber Pennate arrangement
Muscle lengthJoint Angle
Muscle Contraction VelocityJoint angular speedStrength to Mass ratioBody Size
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Human Strength and Power
Basic Definitions strength: The capacity to exert force at any
given speed. power: The mathematical product of force and
velocity at whatever speed.
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Human Strength and Power
Biomechanical Factors in Human Strength Neural Control
Muscle force is greater when: (a) more motor units areinvolved in a contraction, (b) the motor units are greater in size, or (c) the rate of firing is faster.
Muscle Cross-Sectional AreaThe force a muscle can exert is related to its cross-sectional area rather than to its volume.
Arrangement of Muscle FibersVariation exists in the arrangement and alignment of
sarcomeres in relation to the long axis of the muscle.
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Key Terms
pennate muscle: A muscle with fibersthat align obliquely with the tendon,creating a featherlike arrangement.angle of pennation: The angle betweenthe muscle fibers and an imaginary linebetween the muscles origin andinsertion;0 corresponds to no pennation.
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Muscle Fiber Arrangements
Figure 4.11 (next slide) Muscle fiber arrangements and an example of
each
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Figure 4.11
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Human Strength and Power
Biomechanical Factors in Human Strength Muscle Length
At resting length: actin and myosin filaments lie next to
each other; maximal number of potential cross-bridgesites are available; the muscle can generate the greatestforce.When stretched: a smaller proportion of the actin and
myosin filaments lie next to each other; fewer potentialcross-bridge sites are available; the muscle cannotgenerate as much force.When contracted: the actin filaments overlap; the
number of cross-bridge sites is reduced; there isdecreased force eneration ca abilit .
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Muscle Length and Actinand Myosin Interaction
Figure 4.12 (next slide) The slide shows the interaction between actin
and myosin filaments when the muscle is at
its resting length and when it is contracted or stretched.
Muscle force capability is greatest when themuscle is at its resting length because of increased opportunity for actin-myosin cross-bridges.
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Figure 4.12
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Human Strength and Power
Biomechanical Factors in Human Strength Joint Angle
Amount of torque depends on force versus muscle length,leverage, type of exercise, the body joint in question, themuscles used at that joint, and the speed of contraction.
Muscle Contraction Velocity
Nonlinear, but in general, the force capability of muscledeclines as the velocity of contraction increases.
Joint Angular VelocityThere are three types of muscle action.
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Human Strength and Power
Biomechanical Factors in Human Strength Strength-to-Mass Ratio
In sprinting and jumping, the ratio directly reflects
an athletes ability to accelerate his or her body.In sports involving weight classification, the ratiohelps determine when strength is highest relativeto that of other athletes in the weight class.
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Human Strength and Power
Biomechanical Factors in Human Strength Body Size
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 thandoes the larger athlete.
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Sources of Resistance
Gravity Weight is horizontally closer to the joint it
exerts less resistive force (torque)
Weight is horizontally further from the joint itexerts more resistive force
9.8 m/s
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Section Outline
Sources of Resistance to MuscleContraction Gravity
Applications to Resistance TrainingWeight-Stack Machines
Inertia
Friction Fluid Resistance Elasticity Negative Work and Power
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Sources of Resistanceto Muscle Contraction
Gravity Applications to Resistance Training
When the weight is horizontally closer to the joint,
it exerts less resistive torque.When the weight is horizontally farther from a joint,it exerts more resistive torque.
Weight-Stack MachinesGravity is the source of resistance, but machinesprovide increased control over the direction andpattern of resistance.
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Cam-Based Weight-StackMachines
Figure 4.14 (next slide) In cam-based weight-stack machines, the
moment arm (M) of the weight stack
(horizontal distance from the chain to the campivot point) varies during the exercisemovement.
When the cam is rotated in the directionshown from position 1 to position 2, themoment arm of the weights, and thus theresistive torque, increases.
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Figure 4.14
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Sources of Resistanceto Muscle Contraction
Inertia When a weight is held in a static position or when
it is moved at a constant velocity, it exerts constant
resistance only in the downward direction. However, upward or lateral acceleration of the
weight requires additional force.
Friction Friction is the resistive force encountered when one
attempts to move an object while it is pressedagainst another object.
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Sources of Resistanceto Muscle Contraction
Fluid Resistance Fluid resistance is the resistive force encountered by an
object moving through a fluid (liquid or gas), or by a fluidmoving past or around an object or through an orifice.
Elasticity The more an elastic component is stretched, the greater the
resistance.
Negative Work and Power Negative work refers to work performed on, rather than by, a
muscle. The rate at which the repetitions are performed determines
the power output.
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Joint Biomechanics: Concerns inResistance Training
BackBack InjuryIntra-Abdominal Pressure and Lifting Belts
Shoulders Knees
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Joint Biomechanics:Concerns in Resistance Training
Back Back Injury
The lower back is particularly vulnerable.Resistance training exercises should generally beperformed with the lower back in a moderately archedposition.
Intra-Abdominal Pressure and Lifting BeltsThe fluid ball aids in supporting the vertebral columnduring resistance training.Weightlifting belts are probably effective in improving
safety. Follow conservative recommendations.
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Fluid Ball
Figure 4.15 (next slide) The fluid ball resulting from contraction of
the deep abdominal muscles and the
diaphragm
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Figure 4.15
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Key Term
Valsalva maneuver: The glottis is closed,thus keeping air from escaping thelungs, and the muscles of the abdomenand rib cage contract, creating rigidcompartments of liquid in the lower torso and air in the upper torso.
h
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Joint Biomechanics:Concerns in Resistance Training
Shoulders The shoulder is prone to injury during weight training
because of its structure and the forces to which it issubjected.
Warm up with relatively light weights. Follow a program that exercises the shoulders in a
balanced way.
Exercise at a controlled speed.Knees The knee is prone to injury because of its location
between two long levers.
Minimize the use of wraps.
i i h i
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Joint Biomechanics:Concerns in Resistance Training
How Can Athletes Reduce the Risk of Resistance Training Injuries? Perform one or more warm-up sets with relatively
light weights, particularly for exercises that involveextensive use of the shoulder or knee.
Perform basic exercises through a full ROM.
Use relatively light weights when introducing newexercises or resuming training after a layoff of twoor more weeks.
Do not ignore pain in or around the joints.
(continued)
J i Bi h i
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Joint Biomechanics:Concerns in Resistance Training
How Can Athletes Reduce the Risk of Resistance Training Injuries? (continued ) Never attempt lifting maximal loads without proper
preparation, which includes technique instruction inthe exercise movement and practice with lighter weights.
Performing several variations of an exercise resultsin more complete muscle development and jointstability.
Take care when incorporating plyometric drills into
a training program.
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Movement Analysis and ExercisePrescription
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Major Body Movements
Figure 4.16 (next two slides) Planes of movement are relative to the body
in the anatomical position unless otherwise
stated. Common exercises that provide resistance to
the movements and related sport activities arelisted.
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Figure 4.16 (continued)
Reprinted, by permission, from Harman, Johnson, and Frykman, 1992.
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Key Point
Specificity is a major consideration whenone is designing an exercise program toimprove performance in a particular sport
activity. The sport movement must beanalyzed qualitatively or quantitatively todetermine the specific joint movementsthat contribute to the whole-bodymovement. Exercises that use similar jointmovements are then emphasized in theresistance training program.
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General Safety Tips
Warm upLift through a full range of motionUse light weight with new exercisesDo not ignore painDo not attempt maximal lifts without proper preparation
Post Work out icing can be effective Avoid bouncing