Chapter 4 Mechanical Work-Capacity Evaluation

Introduction How to evaluate (screen) an individual’s potential ability to perform mechanical tasksHow to evaluate (screen) an individual’s potential ability to perform mechanical tasks reaching to an objectreaching to an object squattingsquatting exerting manual forces on a control or objectexerting manual forces on a control or object liftinglifting pullingpulling pushingpushing

Introduction Interaction with environment depends on two biomechanical properties of the intact musculoskeletal system:Interaction with environment depends on two biomechanical properties of the intact musculoskeletal system: joint motionjoint motion joint mobilityjoint mobility dexteritydexterity flexibilityflexibility limbernesslimberness Range of Motion (ROM)Range of Motion (ROM)

Introduction Interaction with environment depends on two biomechanical properties of the intact musculoskeletal system:Interaction with environment depends on two biomechanical properties of the intact musculoskeletal system: joint motionjoint motion muscle strengthmuscle strength ability of a muscle to produce forceability of a muscle to produce force ability of a muscle group to produce forceability of a muscle group to produce force

Introduction Interaction with environment depends on two biomechanical properties of the intact musculoskeletal system:Interaction with environment depends on two biomechanical properties of the intact musculoskeletal system: joint motionjoint motion muscle strengthmuscle strength Evaluate the Performance of the systemEvaluate the Performance of the system Normative criteria to assess model calculationsNormative criteria to assess model calculations

Planes of motion and axes of rotation Planes:Planes: SagittalSagittal FrontalFrontal Transverse (horizontal)Transverse (horizontal) Axes:Axes: Medio-lateral (frontal)Medio-lateral (frontal) Anterior-posterior (sagittal)Anterior-posterior (sagittal) VerticalVertical

Standard Terminology Raise arms or Arms up???? Standard description of joint motions note errorsnote errors hip/shoulder abduction & adductionhip/shoulder abduction & adduction Know such terms (as if you don’t)Know such terms (as if you don’t) flexion & extensionflexion & extension internal rotation & external rotationinternal rotation & external rotation adduction & abductionadduction & abduction pronation & supinationpronation & supination

Standard Terminology Raise arms or Arms up???? Standard description of joint motions note errorsnote errors hip/shoulder abduction & adductionhip/shoulder abduction & adduction Know such terms (as if you don’t)Know such terms (as if you don’t) flexion & extensionflexion & extension internal rotation & external rotationinternal rotation & external rotation adduction & abductionadduction & abduction pronation & supinationpronation & supination Why is flexibility important?

Standard Terminology Raise arms or Arms up???? Standard description of joint motions note errorsnote errors hip/shoulder abduction & adductionhip/shoulder abduction & adduction Know such terms (as if you don’t)Know such terms (as if you don’t) flexion & extensionflexion & extension internal rotation & external rotationinternal rotation & external rotation adduction & abductionadduction & abduction pronation & supinationpronation & supination Why is flexibility important? 1.Allow for greater choice of movements 2.Improve performance. How? 3.Reduce the risk of injury. How?

Standard Terminology Raise arms or Arms up???? Standard description of joint motions note errorsnote errors hip/shoulder abduction & adductionhip/shoulder abduction & adduction Know such terms (as if you don’t)Know such terms (as if you don’t) flexion & extensionflexion & extension internal rotation & external rotationinternal rotation & external rotation adduction & abductionadduction & abduction pronation & supinationpronation & supination Why is flexibility important? How much do you need?

Standard Terminology Raise arms or Arms up???? Standard description of joint motions note errorsnote errors hip/shoulder abduction & adductionhip/shoulder abduction & adduction Know such terms (as if you don’t)Know such terms (as if you don’t) flexion & extensionflexion & extension internal rotation & external rotationinternal rotation & external rotation adduction & abductionadduction & abduction pronation & supinationpronation & supination Why is flexibility important? How much do you need? What do you want to do?

“Normal” joint ROM Descriptive statistics of ROM at various jointsDescriptive statistics of ROM at various joints Population: young healthy malesPopulation: young healthy males Useful in work design???Useful in work design??? interaction with segment lengthsinteraction with segment lengths Relationship of linear & angular motionRelationship of linear & angular motion S = θrS = θr

Factors affecting ROM Type of joint (shape of surfaces)Type of joint (shape of surfaces) injury, degenerative diseaseinjury, degenerative disease Tissue massTissue mass muscle & fatmuscle & fat Tissue arrangementTissue arrangement tendons & ligamentstendons & ligaments ClothingClothing Past injuryPast injury TemperatureTemperature Age: confounded with disuse, misuse, injury?Age: confounded with disuse, misuse, injury? Sex: confounded with tissue mass?Sex: confounded with tissue mass?

Workplace considerations IndividualIndividual joint motion reflects lifestylejoint motion reflects lifestyle “tightens up” with extreme postures“tightens up” with extreme postures joint motion reflects injury historyjoint motion reflects injury history Joints have a “memory”Joints have a “memory” EnvironmentEnvironment can be accommodated for reduced ROMcan be accommodated for reduced ROM TaskTask can be assigned according to ROMcan be assigned according to ROM

Why is it important to understand and evaluate strength in the workplace? Many jobs require high levels of exertionMany jobs require high levels of exertion This has a direct impact on safety & performanceThis has a direct impact on safety & performance

Boeing Example Thanks to Rush Green, Boeing Commercial Airplane Group May, 1994

Muscle Strength a muscle’s ability to generate forcea muscle’s ability to generate force Maximum voluntary exertionMaximum voluntary exertion Maximum voluntary contractionMaximum voluntary contraction Maximum voluntary strengthMaximum voluntary strength Affected by...Affected by...

Muscle Strength a muscle’s ability to generate forcea muscle’s ability to generate force Neural factors.Neural factors. a. synchronization.a. synchronization. b. recruitment.b. recruitment. c. summation.c. summation. d. neural quality (aging factors)d. neural quality (aging factors) trainable

Muscle Strength a muscle’s ability to generate forcea muscle’s ability to generate force Neural factors.Neural factors. Psychological factors.Psychological factors. a. motivationa. motivation ie rehab: desire to return to workie rehab: desire to return to work b. hormonal releaseb. hormonal release

Muscle Strength a muscle’s ability to generate forcea muscle’s ability to generate force Neural factors.Neural factors. Psychological factorsPsychological factors Physiological factors.Physiological factors. a. fatigue.a. fatigue. b. available energyb. available energy trainable & diet

Muscle Strength a muscle’s ability to generate forcea muscle’s ability to generate force Neural factors.Neural factors. Psychological factorsPsychological factors Physiological factorsPhysiological factors Mechanical factors.Mechanical factors. a. length-tension relationshipa. length-tension relationship b. force-velocity relationshipb. force-velocity relationship c. angle (posture) ==> Torquec. angle (posture) ==> Torque

Torque (moment of force) Tendency of a force to cause rotation Torque = Force x moment armTorque = Force x moment arm moment arm: perpendicular distance from line of action of the force to axis of rotationmoment arm: perpendicular distance from line of action of the force to axis of rotation

Torque (moment of force) Tendency of a force to cause rotation Torque = F * MATorque = F * MA MA varies through ROMMA varies through ROM Feel it with your fingers.Feel it with your fingers. Torque potential varies throughout ROMTorque potential varies throughout ROM change in moment armchange in moment arm change in muscle force production (L/T)change in muscle force production (L/T)

Moment arms of the arm From Houk et al, 2000

Ankle moment arms

Finger flexion effects on tendons

Finger flexion effects: tendon force on other structures

Torque (moment of force) Tendency of a force to cause rotation Torque = F * MATorque = F * MA MA varies through ROMMA varies through ROM Feel it with your fingers.Feel it with your fingers. Torque potential varies throughout ROMTorque potential varies throughout ROM change in moment armchange in moment arm change in muscle force production (L/T)change in muscle force production (L/T) Change in muscle force (F/V)Change in muscle force (F/V) Fuglevand, 1987

Reality of the situation Not a single muscle, but the muscle groupNot a single muscle, but the muscle group ie elbow flexion strengthie elbow flexion strength biceps brachiibiceps brachii brachialisbrachialis brachioradialisbrachioradialis “wrist flexor muscles”“wrist flexor muscles” Individual muscles (tension and moment arms) interact as a group to produce torque at a jointIndividual muscles (tension and moment arms) interact as a group to produce torque at a joint

What we know about strength: 1. Maximum force (torque) producing capability varies considerably between people and between tasks

Another way to look at muscle force

What we know about strength: 1. Maximum force (torque) producing capability varies considerably between people and between tasks differences in training leveldifferences in training level differences in age (aging workforce; downsizing of workforce (union priority))differences in age (aging workforce; downsizing of workforce (union priority))

What we know about strength: 1. Maximum force (torque) producing capability varies considerably between people and between tasks differences in training leveldifferences in training level differences in age (aging workforce; downsizing of workforce (union priority))differences in age (aging workforce; downsizing of workforce (union priority)) differences in gender/ anthropometricsdifferences in gender/ anthropometrics difference in motivation, etcdifference in motivation, etc strongest 6-8 times stronger than weakest.strongest 6-8 times stronger than weakest.

Additional gender issues with strength Difference in upper body greater than lower body (?)Difference in upper body greater than lower body (?) smaller muscle moment arms of average women?smaller muscle moment arms of average women? smaller muscle mass?smaller muscle mass? social influence?social influence? Type of measurement?Type of measurement? Differences almost entirely explained by differences in muscle sizeDifferences almost entirely explained by differences in muscle size

What we know about strength: 1. Maximum force producing capability varies considerably between people and between tasks.1. Maximum force producing capability varies considerably between people and between tasks. 2. Static strength is not necessarily correlated with dynamic strength2. Static strength is not necessarily correlated with dynamic strength static strength: measured isometricallystatic strength: measured isometrically What position to measure in? (position specific)What position to measure in? (position specific) dynamic strength: body segments movedynamic strength: body segments move which point to measure at?which point to measure at? effect of techniqueeffect of technique

What we know about strength: 1. Maximum force producing capability varies considerably between people and between tasks.1. Maximum force producing capability varies considerably between people and between tasks. 2. Static strength is not necessarily correlated with dynamic strength2. Static strength is not necessarily correlated with dynamic strength 3. Use of strength data in screening must be done cautiously3. Use of strength data in screening must be done cautiously poor association with injury or performancepoor association with injury or performance

Psychophysical strength method for screening employees Requires simulating specific taskRequires simulating specific task subjects are allowed to adjust the load (unknown amount) after each attempted performance.subjects are allowed to adjust the load (unknown amount) after each attempted performance minutes to simulate workday30-45 minutes to simulate workday ends with subjective maximumends with subjective maximum involves cooperation and motivation to obtain valid results.involves cooperation and motivation to obtain valid results.

Psychophysical Limits note voluntary decrease in max with increased dimensionsnote voluntary decrease in max with increased dimensions note decrease with increased heightnote decrease with increased height note female - male differencenote female - male difference

Workplace considerations IndividualIndividual screen people for sufficient strength to a task (workplace rotation)screen people for sufficient strength to a task (workplace rotation) load should not exceed capacity of least capable employee.load should not exceed capacity of least capable employee. EnvironmentEnvironment reduce package wt, alter layout, new equipment, glove usereduce package wt, alter layout, new equipment, glove use TaskTask redesign, additional workersredesign, additional workers "When I put a glove on my hand to protect me from the cold, I've now changed the grip posture of my hand," says Budnick, who is also president of the Board of Certification in Professional Ergonomics. "In most cases, I'm not getting good tactile feedback, so I don't have the proper closed loop in my musculature system to know how hard I'm gripping. In other words, I'm trying to grip as hard as I can, yet the actual result is probably lower grip strength against whatever I'm grasping because of the intervening bulk of the glove. It depends on the fit and type of the glove, but some research suggests that up to a 40 percent reduction in grip strength is common from glove use, and as far as a risk factor for working in cold environments, that's probably the biggest effect I see.”

Thesis Topic IndividualIndividual screen people for sufficient strength to a task (workplace rotation)screen people for sufficient strength to a task (workplace rotation) load should not exceed capacity of least capable employee.load should not exceed capacity of least capable employee. EnvironmentEnvironment reduce package wt, alter layout, new equipment, glove usereduce package wt, alter layout, new equipment, glove use TaskTask redesign, additional workersredesign, additional workers "When I put a glove on my hand to protect me from the cold, I've now changed the grip posture of my hand," says Budnick, who is also president of the Board of Certification in Professional Ergonomics. "In most cases, I'm not getting good tactile feedback, so I don't have the proper closed loop in my musculature system to know how hard I'm gripping. In other words, I'm trying to grip as hard as I can, yet the actual result is probably lower grip strength against whatever I'm grasping because of the intervening bulk of the glove. It depends on the fit and type of the glove, but some research suggests that up to a 40 percent reduction in grip strength is common from glove use, and as far as a risk factor for working in cold environments, that's probably the biggest effect I see.” The Complexities of Cold By Ronnie Rittenberry · February 1,