ch8 muscle forces interacting with joints

What is wrong with the following explanation taken from a popular Therapeutic Exercise text?

"Other machines contain an elliptical cam that varies the resistance through the ROM. The cam is an attempt to account for changes caused by varying length-tension relationships, and the machine is called a variable resistance machine. Less resistance is provided at the beginning and the end of the ROM." NOTES: >said pulley moment arm doesnt change so resistant torque stays constant in range of motion TRUE >eliptical cam varies length-tension relationships/resistance; less resistance at beg and end is less for BICEPS machine but not for any other cam machine; on a cam you should see same number of motor units

What is wrong with the following example?

"With weight training resistance is provided in the form of free weights, machines, resistance bands, and body weights. All training is considered dynamic, or with movement. The types of resistance may be constant or variable. Constant resistance has a fixed weight or amount of resistance throughout the movement. Examples of dynamic constant resistance are free weights and barbells. A 10 LB DUMBBELL PROVIDES 10LBS OF RESISTANCE THROUGHOUT MOVEMENT. Dynamic, variable resistance machines work w cams, pulleys, and levers to vary the weight during the range of movement. Weight machines w grips or handles attached to pulleys that are attached to weight stacks are examples of dynamic, variable resistance machines." >wrong bc changes throughout movement (it supplies 10lbs of resistance FORCE but not 10lbs of resistant torque)

Measuring Muscle Torques

(see pic) down arrow is weight of forearm, up arrow is biceps brachii -Diagram of the upper and lower arm. The weight of the forearm (31.2N) is creating a torque, causing the arm to extend at the elbow. Because the line of force representing the weight of the forearm is perpendicular to the forearm, the physical distance from the axis of rotation ( elbow ) to that line of force is the perpendicular distance. This 90 degree angle created by the line of force interacting with the forearm represents a sine value of 1.0.....(0.2m X 1.0=.2m). We also know the muscle insertion site distance from the fulcrum, and we know the angle of pull of the muscle, so we can calculate perpendicular distance associated with the effort torque. Problem: The biceps brachii exerts a force on the radius thereby causing elbow flexion. We want to MATCH the effort torque to the resistant torque so the forearm does not move. We first calculate the resistant torque. Note that the resistant torque line of force is perpendicular to the forearm. So the physical distance = the perpendicular distance. -diagram indicates angle of pull=1.396 radians or 80 degrees; insertion site is at physical distance of 0.05 meters from fulcrum; resistant torque weight of forearm=31.2N; resistant torque being applied at 90 degree angle, so phys distance is perp distance(31.2N times 0.2N=6.24Newtonmeters)..gravity is pulling arm down w torque of 6.24NM...to hold arm in that position, need to create effort torque = to 6.24NM (effort torque=x number of newtons times the moment arm(sine of radians of pull times physical distance)...sine of 80 degrees is 0.9848..multiply times physical distance 0.05); so equation is: 6.24nm=xN* (0.9848times0.05) then 6.24Nm=xN*0.04924 then x=126.7N of effort force is needed to equal resistant torque Resistant torque: Forearm weight = 31.2N Forearm center of mass = .2m Tr = 31.2 · .2m = 6.24 Nm Calculating an effort torque (Te) that equals the Tr: Te = xN · moment arm (sine r X distance from fulcrum to muscle insertion) 6.24Nm = xN · (.9848 X .05) 6.24Nm = xN · .04924m x=126.7N *Sine Table: r = 1.396 or 80 degrees has a sine value of 0.985

Knee function after patellectomy. A 12- to 48-year follow-up.

-83 knees (69 patients) which had had patellectomy for anterior knee pain (52), patellofemoral osteoarthritis (25) or comminuted fractures (6). The patients were questioned about their symptoms and the function of the operated knee 14 to 50 years after operation. In the group with anterior knee pain, 76% achieved good results and were satisfied with the operation. Only 54% of the osteoarthritis group had satisfactory relief of pain and most had progressive deterioration of function. 16 patients who had had unilateral patellectomy were assessed by dynamometry, ultrasound and radiography. The average quadriceps muscle power was 60% of that on the normal side although two patients had stronger muscles in their operated than in their unoperated legs

Assessment of Stresses in the Cervical Spine Caused by Posture and Position of the Head

-A model of the cervical spine was created with realistic values in Cosmosworks, a finite element assessment package. Calculations were made and then forces were extracted in newtons and then converted into pounds. We made the calculations using neck + head, which gave an average weight of 60 newtons (6kg or 13.2 pounds). The center of mass was located 16cm above C7 or 15cm from the top of the skull. Results: The weight seen by the spine dramatically increases when flexing the head forward at varying degrees. An adult head weighs 10 to 12 pounds in the neutral position. As the head tilts forward the forces seen by the neck surges to 27 pounds at 15 degrees, 40 pounds at 30 degrees, 49 pounds at 45 degrees and 60 pounds at 60 degrees -Neither an explanation of finite element package nor the distance from the fulcum to the center of mass was provided. -Using the definition of torque, the values given in the article, and an assumption that the center of mass was located 7.9 inches from the fulcrum, the resistant torque created by a13.2 lb head and neck positioned 7.9 inches from the fulcrum and tilted at a 15 degree angle: T = 13.2 lbs (.2588 X 7.9) = 26.9 lb/'

Examples of lever systems within the body: 1st class:

-Elbow extension: (tricep working with forearm) We are looking at pic of woman doing dumbell workout with arms over and behind head.. the triceps with the ulna serving as the lever. The triceps contraction causes elbow extension. Example: Pushups. The triceps contracts causing the arm to strengthen at the elbow. -Contraction of the gastrocnemius muscle (like on tip toe) is also an example of a 1st class lever. But this example can be confusing. Make sure to note where the center of gravity is located. In order to be balanced the CG must be located in the area of the foot contacting the ground (or within the base of support) -note: can change resistance force (by changing weight of dumbell); you lose mechanical advantage anytime force isnt perpend to lever (anytime it isnt physical distance??) note: line of force must pass through center of support to be balanced; on tip toe resistant force line is passing through area in foot associated w the toes; one fulcrum at ankle and one at metatarsal phalange joint; effort is gastrocnemius muscle (pulls on calcaneal bone)...goes effort, fulcrum, resistance (same with dumb bell example elbow effort, fulcrum at middle of elbow, resistance at weight of forearm and weight of hands

Not all forces are applied perpendicular to the lever

-In this diagram, both the resistance and the force are applied perpendicular to the lever. So the physical distance from the fulcrum to the point of force application is also the perpendicular distance. Note that the force lines interact with the lever to create right angles (perpendicular). -second pic(draw line from where says force to triangle): In the example the force line and the resistance line do not interact in a perpendicular fashion. So the physical distance is no longer the perpendicular distance; The perpendicular distance from the fulcrum to the line of force is the shortest distance from the fulcrum to the line of force. (no longer 2 meters in length(the physical distance)..it is shorter...phys distance will be longer..perp distance will be shorter) We can estimate the perpendicular distance. Simply extend the force line, place your pencil on the fulcrum, and draw a line that intersects with the force line, creating a right angle. Note this is the shortest distance from the fulcrum to the line of force. Also note that this perpendicular distance is always shorter than the physical distance.

Effects of arm length on performing an iron cross.

-Note that the levers (arms) are fixed at the site of the rings. Resistant moment arm for Gymnast 2 is larger(longer arm??). So, effort torque must be increased. -A similar situation where a gymnast's CG is located away from the rings. The farther away from the rings, the greater the resistant torque. -Modifications can be made to decrease the amount of effort torque. Here we see a gymnast using the elastic recoil in rubber bands to decrease the resistant force, thereby reducing resistant torque. -notes: bending elbow moves base of support closer; force is reduced by creating an upward force

Why do we have a patella?

-The patella moves the tendon away from the knee's axis of rotation, thereby increasing the perpendicular distance of the effort torque. -A patellectomy, the removal of the patella, may result in as much as a 40% reduction in knee extension torque.

note

-changing one small element in motor program can take a long time to adjust (making bike wheel turn opp direction when steer took guy 8 months to relearn..you cant just unplug/get rid of and then start new motor program) -effort torque and resistant torque change throughout motion (not necessarily changing together) -force line is perpendicular only at 90 degrees in arm

exam notes

-dont need to know ch5 math/conversions but still need to know exam 1 stuff -B: R=100 times 25 E=2.5x = 20(15) >the third arm in part B is calculated for you? C: center of mass above wrist and resistance? going down so no torque -also for C: R=25 times 360 E=5x -for sticking point: most efficient in ARM CURL but not in all sticking points!! -long moment arm=most advantageous/most efficient

internal and external torques

-effort torque=assists in desired outcome -resistant torque=hinders the desired outcome

notes

-why if 90degrees biceps brachii is the most efficient torque..how come this is usually a sticking point? bc only considering effort torque...when look at resistant torque that explains it (change in resistant torque is much greater?dramatically increases (decreases before/after 90 degrees) bc 3rd class lever -****at 90 to 100 degrees is the resistant torque and the effort torque at the most advantageous place they can be at? YES (both at maximal possible perp distance at 90 degrees) -recruit most motor units/(most effort torque??) at sticking point (every mm has dif sticking pt)

force vs effort notes

-end output=force (bc muscles do not attach at fulcrum.. so talking about torque) -torque is effort; effort torque=what helps you accomplish what you are trying to do and can be internal(muscle) or external(gravity..like pushing on something) -resistance could be internal or external resistance -theres always a force involved with effort (making an effort to move means there are forces to help you) -force is a part of effort (muscle force is a part of effort/torque) -in movement: ask what are the effort torques and what are resistant torques -he can ask: resistant torque = blah, and sine of angle of pull= blah what muscle force will you need?; or given physical distance and sine, what is perp distance; he wont give table of sine, he will just give us the sine; at 90 degrees the sine is 1, so 1 times the physical distance is 1 -patella doesnt help effort force it helps effort torque -in sit up if put weight behind head or chest you move center of mass/perp distance..changing both parts of torque; if put arms behind head you are just changing perp distance (only one part of torque) -when feet are secured you tend to hyperextend back(bad) and not working ab muscles as well -lifting: changing both resistant torques(of upper torso and box).. decreases (and effort torque decreases) with proper lifting -using a levered door knob/jar top changes what? resistant torque: NO (resistant torque is spring inside), effort torque: YES (bc increases perp distance) -if took knife and tried to loosed by banging on jar then you are changing resistant torque -arm math pic: in illustration A: what are the two resistant forces: 20N and 120N >what is the moment arm for the effort torque: 5cm >C: what is the resistant torque created by the weight of the forearm? no torque, so 0 (there is a resistant torque created by A.. the one by B is an effort torque) >whats the resistant torque(whats pulling arm down in A..ther wight of the forearm is pulling arm down): 20 times 15..plus weight part is 120 times 36 (add both together=all the torque pulling arm down, which equals 5x and you solve for x).. resistance: 120times36 + 20times15=5x (divide each side by 5 to find the effort torque aka x) >if changed hand position from 5cm to 4.5cm, is it/do u need? more or less muscle force/effort? MORE bc reduced perp distance so need to make more force to produce same result...longer perp distance=have more force so dont need to produce as much to match?????? -cam changes resistant torque moment arm!!!!! if muscle torque system decreases then cam decrease..if muscle becomes more efficient then cam will make the resistance more efficient...firing same number of motor units through whole range of motion -polar moment of inertia is radius of bone (center of cylinder to outside) with exercise bone density increases so polar mom of inertia increases -noth resistance and effort decrease if reduce perp distance... line down is resistance -stabilizing=compresion, joint dislocation=?; stick pt not compre or dislocating? -cam: same number of motor untis regardless of where u are TRUE(not true in free weight) >stick pt=ef and resistant are FARTHEST from eachother! they are not matched

torque

-on axis=no torque -slightly away from fulcrum=some torque -far away/close to end on lever=max torque -if coming from side heading towards fulcrum=no torque(see pic after elbow pic)

Proper lifting technique:

-proper: both the upper torso and the box create resistant torques by bending at the knees and pulling the box closer to the feet; the resistant torque perp distances are decreased -improper: legs extended and leaning over to pick up

Researchers have measured the actual moment arms for various muscles at various angles and hand positions/through ranges of motion (moment arms don't change...we all have different moment arms though and changing hand position can increase or decrease moment arm associated with the muscles involved)

-see moment arms/perp distance(cm) associated with biceps brachii and triceps brachii muscles -note that when elbow is extended, the biceps moment arm is largest with the hand in supinated position(palm to front), but when elbow flexed, is largest w the hand in a neutral position -for biceps: perp distance was longer in elbow extended supinated and elbow flexed neutral (longer=more advantageous...for biceps start supinated and go to neural to be efficient from bottom up in weight lift..if want more motor units to fire, do opp) -for triceps: perp distance was longer in elbow extended neutral and elbow flexed neutral -the biceps moment arm is most advantageous in the elbow flexed neutral position (least during elbow extended neutral) -Note that the triceps brachii has a most advantageous position with elbow extended rather than flexed.(least during elbow flexed sup) Compare these findings to the sticking point within the range of a push-up. note: for tricep in push up, get stuck when get down low at flexion...tricep is least efficient in elbow flexed; bicep is least efficient elbow extended(shorter perp dist), bicep sticking point is at 90-100degrees -you can change perp distance by arranging body parts; perp distances change through the range of motion

CAM SYSTEMS

-see picture? -Pulley: Distances from fulcrum to outer edge are equal. -Cam: distances from fulcrum to outer edges are not equal. So a cam system can alter the amount of leverage available to the resistant force. A larger distance will create a larger resistant torque. -Isoinertial machines use changing cam profiles to create mechanical advantages or disadvantages associated with resistant torque. The cam profile is designed to approximate the change in anatomical mechanical advantages or disadvantages through the range of motion. note: specifically designed to control the moment arm thats associated with resistant torque (ex: if moment arm increases 25% then resistant torque will only increase 25% (same with decrease..decreases same)); cam machines rotate back and forth (distance goes from ex: 2 to 3 to 2); designed to give the weight a perpendicular distance which changes through rotation); same number of motor units through whole range of motion(in free weights it changes); cam causes you to fire a large number of motor units throughout the exercise (theres no easy points); same level of difficulty in range of motion; never know how much weight you are lifting; enhances moment arm/resistant torque so you dont need as much weight bc get same effect (still feels heavy/does same) bc put a pin in weights and move them; each persons ratio change is same/very similar (perp distances/moment arm will vary)

sticking point notes

-sticking pts=difference btw effort and resistant torques is largest -at 90 to 100 degrees the largest for resistant and effort torque? yes (both most efficient at that point but you get stuck -eff and resist torques change through range of motion and at some pt have a sticking point -sticking point is where muscle is fatigued

see picture on first pg of ch8: 1st, 2nd, and 3rd class levers

1st class: force(arrow up)...fulcrum... resistance(arrow down) >see saw design...fulcrum doesnt have to be in the exact middle..just has to be inbetween them 2nd class: fulcrum..resistance down....force up >effort has advantage 3rd class: fulcrum...force up...resistance down >fulcrum on one side and effort/force is closer to fulcrum >poor at generating torque; better at generating speed even tho not that great at speed(ex: arm brachii..small contraction>>a lot of movement at distal end) -The muscle attachment sites are located at a point distant from the axis of rotation. Thus, the force generated by the muscle contraction is considered to be an off axis force, or a torque. -gravity would be resistance ex, effort from biceps is force example note: triangle=fulcrum or pivot point or axis of rotation

Grabber Device:

A Grabber is useful for reaching objects that are hard to reach. However, the Grabber becomes an extension of the arm and therefore increases the resistant torque created by the weight of the Grabber and the weight of the object being retrieved. -Take note of the increase in resistant torque when the grabber is used in this manner. The axis of rotation is located at the shoulder. In the first figure, the CG of the arm plus the object in the hand combine to create the CG of the resistant forces. When the can is grabbed by the devise, the CG of the arm plus the grabber plus the can is located farther away from the axis of rotation. This arrangement creates a greater Tr. (first pic=holding can, second pic=grabbing can w grabber) -note: good for reaching but when get down...makes can a lot heavier/harder; dont want to use for heavy items..pull it as quickly as possible to axis of rotation

Sticking Points during a range of motion.

All free weight and body resistance exercises have a "sticking point" where the relationship between the effort torques and the resistant torques are most disadvantageous. When curling a weight, notice that both the effort and resistance moment arms are largest at about 90-100 degrees of flexion. HOWEVER, the increase in the resistant torque moment arm is quite a bit larger than the increase in the effort torque moment arm. Due to this disadvantage, the muscle force must be increased in order to keep the forearm moving. If the resistant torque is too large to overcome, the lifter gets stuck at that point. -Machines have been developed that take sticking points into account. By matching the resistant moment arm to the effort moment arm throughout the range of motion, sticking points can be eliminated.

cell phone neck

As the head tilts forward, the CM of the head creates a resistant torque. This resistant torque must be managed by contraction of the muscles responsible for keeping the head centered over its axis of rotation. It has been estimated that the forward head tilt can create up to 60lbs. of pressure in the cervical neck region.

Aching Back? Sitting Up Straight Could Be The Culprit

Back pain is the most common cause of work-related disability in the US, and a leading contributor to job-related absenteeism. A 135-degree body-thigh sitting posture was demonstrated to be the best biomechanical sitting position, as opposed to a 90-degree posture. The "slouch" position revealed a reduction in spinal disk height, signifying a high rate of wear and tear on the lowest two spinal levels. Across all measurements, the researchers concluded that the 135-degree position fared the best. As a result, Dr's advise patients to stave off future back problems by correcting their sitting posture and finding a chair that allows them to sit in an optimal position of 135 degrees note: slouching while sitting is #1 worst..sitting upright is 2nd worst

Counter Tops can also create a situation where the resistant torque is increased

By reducing the width of the counter top, the person is moved closer to the cupboard. Resistant torque created by the arm and any object in the hand will be reduced due to a decrease in the resistant torque moment arm.

Stabilizing(/compression) and Dislocating forces:

Cam machines attempt to provide maximum resistance throughout the range of motion. This can be problematic in that joint compression and dislocating forces vary throughout the range of motion, and are dependent on the amount of resistant torque that is present at these various angles. For ex: if resistant torque is large when the arm is extended at the elbow, the muscle torque necessary to overcome the large resistant torque may contribute to excessive compression forces within the joint. Note that when curling a dumbbell, the resistant torque is not large at elbow extension. Thus, effort torques needed to overcome the resistant torque created by the dumbbell are not large. Thus, the compression forces created by the effort torque are likewise not large NOTES: >dont want to have radius and ulna jamming into humerus; both stab/comp and disloc forces can be good >fewer motor units at comp/dislocating???; but not with cam(comp/disloc force increases?); can just do one set with cam machines; cam compresses joints so its bad for older ppl...we arent designed to handle this kind of system (debated)

Examples of lever systems within the body: 2nd class:

Digastric muscle (muscle under jaw; when contracts it pulls mouth open); gravity pulls jaw down when you sleep but chewing muscles are taught(dont stretch) so doesnt let mouth completely gape open; have to forcibly open mouth if want to >.fulcrum at (tempromandibular)jaw joint, resistance is in masater muscle and temporalis muscle(when contract mouth closes),effort at diagastric muscle(farthest away from fulcrum)

Examples of lever systems within the body: 3rd class:

Elbow Flexion -biceps attachment to radius >fulcrum being elbow joint..short distance away is attachment for biceps muscle..resistance would be weight of forearm and any weight or object holding in hand

Let's see if we can apply our knowledge of torque to some specific rehabilitative exercises. Using figure A, calculate the two resistant torques and calculate the muscle force needed to match the resistant torques. (see pic)

Resistant torques in Figure A: 120N X 36cm + 20N X 15cm Effort torque in Figure A: 5cm X muscle force Resistant torque in Figure C: 360N X 25cm Effort torque in Figure C: 5cm X muscle force What about the weight of the forearm and hand? Notice that the line of force created by the weight of the forearm and hand passes directly through the axis of rotation. Therefore this force is not off axis and does not cause a torque. Now try to determine the equations for the effort and resistant torques in Figure B. note for B: 25*100=resistant torque; after equation get 2 effort torques somehow? note for C: res torque=360*25; no torque bc on axis force?; forearm not helping or hurting

Jar Opening:

Individuals often struggle when trying to remove a lid from a jar or bottle. >Using a leveraged jar opening devise allows the user to increase the effort torque perpendicular distance which increases the effort torque. >Using a rubber sheet placed directly over the jar top does not increase effort torque production. However, the rubber sheet increases the friction between the top and the application of the effort torque.

Isotonic Movement, Torque and Sticking Points:

Muscles attach to bones. These attachment sites are off-axis. This off-axis application of force creates torques. The biceps brachii attaches to the radius bone in the forearm. When the biceps brachii contracts, the resulting off-axis force creates an effort torque that causes flexion at the elbow (which serves as the axis of rotation). The physical distance from the proximal end of the radius to the attachment site is constant. If the biceps brachii attachment site is 5 cm from the proximal end of the radius, that 5 cm distance remains 5 cm throughout the movement. HOWEVER, the perpendicular distance from the axis of rotation to the point of effort force application does change throughout the range of motion. This change in perpendicular distance means that the effort torque will change throughout the range of motion. Likewise, the resistant torque changes throughout the range of motion.

A Prospective Study of Back Belts for Prevention of Back Pain and Injury

Neither frequent back belt use nor a belt-requirement store policy was significantly associated with back injury claim rates or self-reported back pain. Rate ratios comparing back injury claims of those who reported wearing back belts usually every day and once or twice a week vs those who reported wearing belts never or once or twice a month were 1.22 (95% confidence interval [CI], 0.87-1.70) and 0.95 (95% CI, 0.56-1.59), respectively. The respective odds ratios for low back pain incidence were 0.97 (95% CI, 0.83-1.13) and 0.92 (95% CI, 0.73-1.16). Conclusions: In the largest prospective cohort study of back belt use, adjusted for multiple individual risk factors, neither frequent back belt use nor a store policy that required belt use was associated with reduced incidence of back injury claims or low back pain." -note: prospective: follow over time (time consuming)..find ppl w back belt and w/o

PUSHUPS: Effects of body position on resistant torque.

Note that the muscle forces are being applied in the shoulder region and the resistant force (gravity) is also being applied in the shoulder region. The difference in resistance occurs due to the removal of the weight of the lower limbs, thus reducing resistant torque. When performing a push up the torso and lower body combine to serve as a lever fixed at the toes. The resistant torque is created by the weight of the body and is represented by a force line emanating from the center of mass. Given all else equal, the higher in the torso the CM is located, the greater the resistant torque. Push ups can be modified by relocating the axis of rotation from the toes to the knees. This relocation will shorten the perpendicular distance of the resistant torque, and will remove the weight of the lower legs and feet from the resistant torque equation, thereby reducing resistant torque. -resistant torque changes w changing leg position; weight of legs remains same regardless of knee angle but CM of legs is moved closer to axis of rotations hips

Effects of position of center of gravity on sit-up performance.

Note that the person with the higher center of gravity (F1) will have a greater resistant torque to overcome due to the increase in the resistant moment arm. -effort torque cant be changed (effort torque is by ab muscles and hip flexors) -putting hands behind/above head moves center of mass/gravity up and increases resistant torque

Lever System Machines: Flexion at the Hip.

Note that the weights are positioned at the end of the lever. So resistant torque will be increased due to the leverage gained by the position of the weights. Also notice that the increase in the resistant moment arm changes throughout the movement....in this case becoming larger as the subject flexes at the waist. -OPEN CHAIN exercise (KNOW)=where the feet or hands are NOT fixed in space, they are moving in space (distal end of this lever system mentioned is moving) note: closed chain is when hands or feet are fixed and are not moving, like a push up (body is moving but hands are not), a squat >research says open chain is not good to be doing esp with lots of resistance >shear=two objects moving in opp directions (shear effect w tibia/fib and femur in this open chain mentioned; thats why acl usually occur noncontact bc your femur wants to keep traveling forward and acl is what stops it from shifting forward, but put a twist in it and the acl gets caught); those with v shaped notch (compared to u shaped) are more likely to tear acl (u shaped have more wiggle room in condyles)

Diagnosing Back Pain

Part of the problem in using MRI to diagnose lower back pain is that there is not a good correlation between abnormal findings and symptoms. If you look at 100 middle-aged people who have never had back pain, two-thirds of them would have abnormalities on MRI (therefore its hard to tell what is causing the pain)

Leg Lifts:

Resistant torque changes with changing leg position. Note that the weight of the legs remains the same regardless of angle of knee flexion. However, the CM of the legs is moved closer to the axis of rotation (hips). This reduction in perpendicular distance reduces the resistant torque created by the weight of the legs. (the pic is of sitting pike position then bending knees sitting)

Torque formula

T=force times perpendicular distance -perpendicular distance/moment arm=the shortest distance(perpendicular distance) from the fulcrum/axis to the line of force (only when force is applied at a right angle is the perp distance the actual physical distance; anytime apply a force that is above or below 90degrees..you are loosing some torque and perp distance will be shorter than physical distance; to maximize torque, make it far from lever and perpendicular to lever) -torque causes angular motion

Torque example: (see pic on first pg of ch 8)

Teeter-totter (first class lever) with 6 weights attached at points along the board. Note that this depiction has the lines of force intersecting with the lever in a perpendicular fashion. Since this interaction creates a right angle (⊥), the physical distance from the fulcrum to the line of force is also the perpendicular distance. This is not always the case as we will see when we exam muscle forces interacting with the skeletal system. -In both cases, total weight on each side of the fulcrum = 60lbs. However, rearranging those weights changes the torque. >each weight is causing a torque. ex: 30lb weight times 10 ft from fulcrum(=300 torque units) PLUS 20lb times 20ft (=400)PLUS 10lb times 30ft (=300) >standard: ft-lbs; metric: N-meters >when change larger weight to farther away, you increase torque by moving more force away from fulcrum..causing rotation of teeter totter >when big bro moves up on see saw....little bro can use see saw with him (Whereas before was weighted down)..this is a change in torque when he scoots up

Rising from a seated position.

When seated, CG is an off axis force creating a resistant torque. A CG located away from the knee (axis of rotation) will require an increased effort torque. Sliding the feet backward will also decrease or eliminate the resistant torque. Leaning the upper torso forward and sliding the feet back moves the CG to the right of the axis of rotation, turning what was a resistant torque into an effort torque. -note: having 2 rails is better than 1. 1 is better than none

Example of a research study involving cams:

aim of this study was to determine the extent to which the resistance provided by variable-cam resistance training machines match joint torque capability. 8 variable-cam knee extension machines were assessed. The angle-torque relationship of the different training machines was highly variable, but consistently less curvilinear and significantly dif from knee extensor capabilities, with changes in torque of +2.5 to +22.2% (ascending limb) and +37.6 to -20.5% (descending limb). It is recommended that variable-cam resistance training machines be designed with the angle-torque relationship of the relevant joint and the inertial profile of the lifting exercise in mind

Similar effects of one- and three-set strength training on strength and muscle mass gains in upper and lower body in untrained subjects:

purpose of this study was to compare the effects of single- and multiple-set strength training on hypertrophy and strength gains in untrained men. The results demonstrate that 3-set strength training is superior to 1-set strength training with regard to strength and muscle mass gains in the leg muscles, while no difference exists between 1- and 3-set training in upper-body muscles in untrained men

formula for perpendicular distance

the sine of the angle(of pull) or radian multiplied by the physical distance from fulcrum to muscle insertion -aka sines of radians of pull times the physical distance -aka sines of radians/degrees associated with the force line interacting with the lever, multiplied by the physical distance from the fulcrum to the point where the force is applied to/muscle insertion -if applied at 90 degrees, the sine of 90degrees is one; if greater or less than 90, the sine will always be less than one and therefore the perp distance will always be shorter than the physical distance if the angle is less or greater than 90

Do Back Belts Prevent Injury? A Lack of Scientific Support

there has been a dramatic increase in # of workers who rely on back belts to prevent injury during lifting. Employers relying on back belts to prevent injury should be aware of the lack of scientific evidence supporting their use. -concluded that, because of limitations of the studies that have analyzed workplace use of back belts, the results cannot be used to either support or refute the effectiveness of back belts in injury reduction. The Institute does not recommend the use of back belts to prevent injuries among workers who have never been injured. Because the Institute's primary focus is on the prevention of injury, it did not address the use of back belts as medical treatment during rehabilitation from injury

The Universal Machine

was an early attempt to recognize that muscle forces vary throughout range of motion. This design of machine tried to take into account the efficiency of the triceps brachii as the elbow moved from flexion to extension. As previously noted, the triceps muscle is more efficient with elbow extension and less efficient with elbow flexion. The machine keeps the weights at a consistent distance from the fulcrum. So resistant torque does not change throughout the range of motion (note that the weights are fixed on 2 rods, causing the weights to simply move up and down throughout the range of motion). The subject uses a lever to move these weights. As the lever is pushed upward, the effort force is no longer being applied in a perpendicular fashion....in essence shorting the perpendicular distance from effort force application to the fulcrum. So as the elbow is extended, the triceps becomes more efficient. But, the line of force being applied to the external lever that the subject is using becomes shorter, canceling out the gains in efficiency that the triceps experiences as the elbow moves from flexion to extension. So, as the triceps becomes more efficient, the external lever moment arm becomes less efficient. The result is that the triceps has to work equally as hard throughout the entire range of motion. In a free weight situation (bench press), the triceps becomes more efficient as the elbow is extended. So the sticking point in the free weight exercise is at the bottom, where the triceps is least efficient. >changes effort torque not resistant torque >may only need 20lbs to feel like 100lbs

Lever doorknobs

works in the same manner as the jar opening devise. Effort torque is increased due to an increase in effort moment arm.

Rotational Effects of a Force: Moment (Torque)

•Muscles attach to bones. •Bones serve as levers rotating about joints. A lever is a rigid object fixed at a point. •Joints serve as axes of rotation. •Three classifications of lever systems found in the body: 1st,2nd, and 3rd class -notes: >force applied to bone >moment arm=angular displacement and acceleration of a lever or the perpendicular distance >moment and torque are same thing >when muscles contract they pull