Recommended Weight Limit (RWL)

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Recommended Weight Limit (RWL)

    General Back Education for the Certified Physical Fitness Instructor

Thursday, May 27, 1999

8:30 9:00 A.M. Introduction to Motion Diagnostic Laboratories’ Functional Lift Assessment.

9:00 10:15 A.M. Identifying “At Risk Individuals” using the Functional Lift Assessment.

10:15 10:30 A.M. Break

10:30 12:00 Anatomy and Kinesiology of the Spine

12:00 1:00 P.M. Lunch

1:00- 2:30 P.M. Anatomy and Kinesiology of the Spine and Lower Extremities / Injuries of the


2:30 2:45 P.M. Break

2:45 3:45 P.M. Assessing Strength, Flexibility, Balance and Range of Motion of the Lower

    Extremity and Back in the At Risk Individual.

3:45 4:00 P.M. Day One Wrap-Up

Friday, May 28, 1999

8:15 9:00 A.M. Assessing Strength, Flexibility, Balance and Range of Motion of the Lower

    Extremity and Back in the “At Risk Individual”.

9:00 10:15 A.M. Physical Training for the “At Risk Individual”.

10:15 10:30 A.M. Break

    10:30 12:00 Instruction in Proper Manual Materials Handling.

12:00 1:00 P.M. Lunch

    1:00 2:00 P.M. Instruction in Proper Manual Materials Handling.

2:00 2:30 P.M. Incorporating the “Cognitive Behavioral Program for Patients with Chronic Back


2:30 2:45 P.M. Break

    2:45 3:30 P.M. Incorporating the “Cognitive Behavioral Program for Patients with Chronic

    Back Disorders”.

3:30 4:00 P.M. Review, Questions, and Training Wrap-up.


    Introduction to Motion Diagnostic Laboratories’ Functional Lift Assessment.

    The FLA objectively measures and documents a recruit’s lifting mechanics for the purpose of identifying

    recruits “at risk” of injuring their low back.

    Explanation of Data Tables

1. ROM

    a. Normal ROM

    b. Subject ROM

    c. % of Normal

2. Dynamic Torque (force x ; distance)

    a. Normal Maximum

    b. Subject Maximum

    c. % of Normal

    d. Joint Percent Capable (JPC)

     % of the adult population that would have the static isometric strength to

    perform the task.

    3. L5-S1 Maximum Dynamic Torque and Force Values

    a. Torque

     Torque = force x ; distance from axis of rotation

    b. Compression

     Loading mode in which equal and opposite loads are applied toward the

    surface of the structure, resulting in shortening and widening.

    c. Shear

     loading mode in which a load is applied parallel to the surface of the


4. Horizontal Distance From Load

    a. Normal Maximum

    b. Subject Maximum

    c. % of Normal

5. Knee versus Hip Coordination

    a. Normal Slope

    b. Subject Slope

    c. % Normal

    d. “Knee vs. Hip Angle” Graph

    e. “Hip and Knee Angle” Graph


    6. Calculation of the NIOSH Lifting Equation

    a. Recommended Weight Limit (RWL)

    b. Lifting Index (LI)

    7. Stick Figures

    NIOSH Lifting Equation - 1991

A committee of experts:

    1. reviewed current literature on lifting

    2. recommended criteria for defining lifting capacity

    3. revised the 1981 lifting equation

Three criteria:

    1. biomechanical

     compressive and shear forces and torque on spine (L5-S1)

    2. physiological

     energy expenditure to limit loads for repetitive lifting

    3. psychophysical

     defining workers’ strength and capacity to perform manual lifting at different

    frequencies for different durations

The equation was designed to assist in the identification of ergonomic solutions for reducing the physical

    stresses associated with manual lifting by identifying the features of the lifting task that contribute the most

    to the hazard for low back injuries.

Lifting equation for calculating the Recommended Weight Limit (RWL):

    RWL = LC x HM x VM x DM x AM x FM x CM

    Multiplier Abbreviation Metric U.S.

    Load Constant LC 23 kg 51 lbs.

    Horizontal HM 25/H 10/H

    Vertical VM 1 (.003|V-75|) 1 (.0075|V-30|)

    Distance DM 0.82 (4.5/D) 0.82 (1.8/D)

    Asymmetric AM 1 (.0032 * A) 1 (.0032 * A)

    Frequency FM Table 5 Table 5

    Coupling CM Table 7 Table 7

Lifting Index (LI)

    LI = Load Weight / Recommended Weight Limit

    LI = L / RWL

    Load Weight (L) = weight of object lifted (lbs. or kg).

RWL for a task represents a load value that nearly all healthy workers could perform over a substantial

    period of time (e.g., up to 8 hours) without an increased risk of developing lifting- related LBP


    The Lifting Index is a ratio or comparison between the actual weight lifted (L) and the recommended weight limit (RWL). It is a relative index of physical stress.

    Theoretically, the magnitude of the LI may be used as a gauge to estimate the percentage of the workforce that is likely to be at risk for developing lifting-related LBP.

Identifying hazardous lifting jobs/tasks using the Lifting Index:

    It is likely that lifting tasks with a LI > 1 pose an increase risk for lifting-related LBP for some fraction of the workforce. Therefore, the LI may be used to identify potentially hazardous lifting jobs or compare the relative severity of two jobs for the purpose of evaluating and redesigning them.

    The 1991 committee believed that the combination of using a multiplicative model and the practice of using the most conservative criterion or data values when faced with uncertainty served to provide a final lifting equation which is more likely to protect healthy workers for a wider variety of lifting tasks than methods which rely on a single task factor (e.g., weight) or single criterion (e.g., intradiscal pressure).


    Waters TR, Putz-Anderson V, Garg A, Fine LJ 1993, Revised NIOSH equation for the design and evaluation of manual lifting tasks. Ergonomics, 36:7, 749-776.

    Waters TR, Putz-Anderson V, Garg A 1994, Application Manual for the Revised NIOSH Lifting Equation. U.S. Department of Health and Human Services.

    Work Practices Guide for Manual Lifting, 193, American Industrial Hygiene Association.

    Identifying “At Risk Individuals” using the Functional Lift Assessment

1. Range of Motion



2. Dynamic Torque



3. Joint Percent Capable (psychophysical)



     99% for males / 75% for females

4. L5-S1 Maximum Torque and Force Values


     Compression *

     Shear *

5. Horizontal Distance form Load

6. Knee versus Hip Coordination

7. NIOSH Lifting Equation




    Anatomy of the Spine

33 vertebra w/ 23 IV disks

     7 cervical

     12 thoracic

     5 lumbar

     5 sacral (fused)

     4 coccygeal

    Spinal Nerves from the Spinal Cord

     Cervical - emerge from above the corresponding vertebrae rdth and 4 cervical vertebrae E.g. spinal nerve C4 is between the 3

     Thoracic and Lumbar - emerge from below the corresponding vertebrae. thth E.g. spinal nerve L4 is between 4 and 5 lumbar vertebrae.

Structure of a Typical Vertebrae:

     Vertebral body anterior

     Vertebral arch posterior

     2 transverse processes (divides arch into anterior/posterior portions)

     anterior portion ; pedicle

     posterior portion ; laminae

     1 spinous process

     4 articular processes

     2 superior articular facets

     2 inferior articular facets

     par (inter-)articularis

     posterior portion of laminae between superior & inferior processes

     Cartilagenous end plates

     Lie on the verterbral body

Intervertebral Disk

Structure of IV Disks

     Annulus fibrosus

     Outer portion of disk

     Annular fibers (tree-like)

     Composed of collagen, fibrocartilage, H0 2

     Nucleus Pulposus

     Inner portion of disk


     Composed of H0 and collagen 2

    Facet Joints (Zygapophyseal or Apophyseal Joints)

     Articulations between the superior and inferior facets/articular processes


Ligaments and Joint Capsules

     Anterior longitudinal ligament

     Axis to sacrum

     Posterior longitudinal ligament

     Axis to sacrum

     Ligamentum flavum

     Axis to sacrum

     Posterior surface of vertebral canal connecting adjacent lamina


     Thoracic and lumbar spines

     Ligamentum nuchae

     Continuation of supraspinous ligament in cervical spine


     Primarily lumbar


     Primarily lumbar

     Facet Joint Capsule

     Encapsulate the facet joints

Functional Anatomy of the Spine

Functions of the Spine

    The spine is needed for both stability and mobility!!

    1. Base of support for head & internal organs 2. Base of attachment for muscles, ligaments, and bones of the extremities (axial vs. appendicular

    skeleton), rib cage, and pelvis

    3. Protect SC

    4. Transfer loads between head and neck and pelvis

Curves of the Spine

     Primary curves (convex kyphotic throughout life)

    1. Thoracic

    2. Sacral

     Secondary curves (initially convex ; concave lordotic) *

    1. Cervical

    2. Lumbar

    * develop as an accommodation to the upright posture

     c/s curve develops as child begins to hold head up

     l/s curve develops as child begins to stand and walk

     curves cease developing between 12 and 17


Function of curves:

    1. Attenuate Ground Reaction Forces (GRF)

Joints of the Spine

    1. cartilagenous joints between vertebral bodies and IV disk

    2. synovial joints called zygapophyseal/apophyseal/facet joints between articular processes

Intervertebral Disks

     in length, width, and thickness from c/s to l/s distribute loads and restrains motion

Nucleus Pulposus

     Composed of water and collagen

     Acts hydrostatically ; uniform pressure to store energy and distribute loads

     Disk is hydrophilic (proteoglycans) however age and degeneration reduce this

     resists tensile forces and compressive forces (Type II collagen)

     Dry disk is inelastic ; ability to store energy and distribute loads.

Motion Segment ; Functional Unit of the Spine

     2 adjacent vertebra and the interposing vertebral disk

Orientation of the facet joints determines the direction of movement in the motion segments


     horizontal/transverse plane

     Permits mostly rotation

C3-C7 0 angle to the transverse plane 45

     Flexion/extension, lateral bending, and rotation

Thoracic 0 60 to transverse

     lateral flexion, rotation, and some flexion/extension

     lateral flexion limited by ribs

Lumbar 0 90 to transverse

     flexion/extension, lateral bending, almost no rotation

Passive restraints (ligaments) of the spine

     Anterior longitudinal ligament

     Limits extension reinforces anterior disk

     Posterior longitudinal ligament


     Limits flexion reinforces posterior disk

Ligamentum flavum

     Limits flexion (esp. in L/S)

     highest elastin content allowing it to contract during extension and elongate during

    flexion (always under tension even in neutral)


     Limits flexion

     Vulnerable to hyperflexion injury

Ligamentum nuchae

     Limits flexion

     Vulnerable to hyperflexion injury


     Limits flexion

     Vulnerable to hyperflexion injury


     Limits flexion

Facet Joint Capsule

     Limits flexion

     Vulnerable to hyperflexion injury

    Musculature of the Spine


Rectus Abdominis

Internal Obliques

External Obliques

Iliopsoas flex the trunk but extend the lumbar spine


    Erector spinae (sacrospinalis)

    1. Spinalis (medial) attach to spinous processes

     capitis, cervicis, thoracis

2. Longissimus (middle)

     capitis, cervicis, thoracis

    3. Iliocostalis (lateral) slips to angle of rib

     cervicis, thoracis, lumborum


    Rotators and Lateral Flexors

    Anterior Musculature


     Contralateral external obliques

     Ipsilateral internal obliques

     Lateral Bending

     Ipsilateral obliques

    Posterior Musculature

     Rotation to same side

     Erector spinae

     Quadratus lumborum

     Rotation to opposite side


     Lateral bending to same side

     Quadratus lumborum


    Transversospinalis deepest layer

     responsible for finer movements of the spine

     assist in extension, rotation, and lateral bending

    1. Semispinalis (span about 5 interspaces)

     capitis, cervicis, thoracis

    2. Multifidus (span about 3 interspaces)

     Sacrum ; axis spine

    3. Rotatores (span about 1-2 interspaces)

     Well-developed in thoracic region

    Types of forces incurred in the spine

    1. axial compression 0: thru long axis of spine ; to the IV disks 2


     muscular contraction

     ligament restriction ligamentum flavum

     resisted by IV disks and vertebral bodies

     facet joints also assist in resisting compression

     Greatest in extension ; 30% of total load

     Also high during coupled flexion and rotation


    2. Bending

     Combination of compression and tension

     Flexion ; compression of anterior structures and tension in posterior structures

     Tensile forces resisted by ALL, PLL, etc.

    3. Torsion 0 axial rotation which often occurs with coupled motions 2

     resisted by outer layers vertebral body, IV disk, orientation of facets

    4. Shear

     Movement in the same plane as the IV disk and vertebral body

     Resisted by facet joints/vertebral arches and IV disks/joints

     Spondylolysis and spondylolithesis

    Restriction of ROM at one motion segment results in increased ROM at adjacent segments.

Loading of spine in various positions see Nachemsons’s work

Forces on the Spine during lifting:

Forces on the lumbar spine during lifting are highly dependent on the distance the object is from the body.

Produces a shear and compression component.

What forces are acting on the spine when lifting an object?

    1. weight of load

    2. weight of upper body

    3. forces produced by musculature - erector spinae

Other contributing factors?

     Intra-abdominal pressure

     Force produce by abominal musculature

    In general, the musculature creates large compressive loads on the spine.

Which lifting position/technique will create a greater load/compressive forces on the lower back?

Straight back or rounded back

Rounded back WHY? Paradoxical

Forces are actually produced by ligaments (resisting flexion) muscle in a lengthened/weakened


     Ligamentum flavum highest elastin content allowing it to contract during extension and elongate

    during flexion (always under tension even in neutral)

To protect ligaments - want muscles to contract

    IMPT. static analyses tend to underestimate the forces placed on joints/spine.

Statics do NOT consider the velocity/acceleration involved in lifting ; high accelerations during

    lifting dynamic forces ;


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