6 biomechanics

7/30/2019 6 Biomechanics

1/57

Biomechanics of ResistanceTraining


2/57

Muscle Skeletal System

Bones and Muscles Joints Ligaments Tendons

Origins, insertions, and Actions

Agonist and AntagonistSynergist - assists directly in a movement(counteractions)


3/57

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


4/57

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


5/57

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.


6/57


7/57

Torque

The degree to which a force tends torotate and object about a specified axis of rotation (fulcrum)


8/57

Levers

First-class lever Second-class lever

Third-class lever


9/57

First Class Lever

Muscle and resistive force act opposite thefulcrum


10/57

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


11/57

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


12/57

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


13/57

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.


14/57

Figure 4.9

Reprinted, by permission, from Gowitzke and Milner, 1988.


15/57

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.


16/57

Figure 4.6

Reprinted, by permission, from Gowitzke and Milner, 1988.


17/57

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.


18/57

Figure 4.7


19/57

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.


20/57

Figure 4.8


21/57

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.


22/57

Strength & Power

Acceleration - change in velocity per unittimeStrength - maximal force that a musclecan produce at a specific velocityWork = Force x distance

Power = Work/time


23/57

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


24/57


25/57

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


26/57

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.


27/57


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.


28/57

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.


29/57

Muscle Fiber Arrangements

Figure 4.11 (next slide) Muscle fiber arrangements and an example of

each


30/57

Figure 4.11


31/57


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 .


32/57

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.


33/57

Figure 4.12


34/57


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.


35/57


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.


36/57


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.


37/57

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


38/57

Section Outline

Sources of Resistance to MuscleContraction Gravity

Applications to Resistance TrainingWeight-Stack Machines

Inertia

Friction Fluid Resistance Elasticity Negative Work and Power


39/57

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.


40/57

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.


41/57

Figure 4.14


42/57


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.


43/57


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.


44/57

Joint Biomechanics: Concerns inResistance Training

BackBack InjuryIntra-Abdominal Pressure and Lifting Belts

Shoulders Knees


45/57

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.


46/57

Fluid Ball

Figure 4.15 (next slide) The fluid ball resulting from contraction of

the deep abdominal muscles and the

diaphragm


47/57

Figure 4.15


48/57

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


49/57


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


50/57


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


51/57


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.


52/57

Movement Analysis and ExercisePrescription


53/57

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.


54/57


55/57

Figure 4.16 (continued)

Reprinted, by permission, from Harman, Johnson, and Frykman, 1992.


56/57

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.


57/57

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

6 biomechanics

Documents