Steps in a biomechanical analysis of lifting

  1. Choose a frame of reference
  2. Evaluate gravitational forces during lifting
  3. Evaluate moments associated with the gravitational forces
  4. Evaluate opposing muscular forces and moments
  5. Evaluate forces’ effect at joint surface
  6. Explain how we minimize compressive forces when we lift heavy loads?
  7. Include the effects of modifying intra-abdominal pressure

1. FRAME OF REFERENCE

This analysis focuses on the forces and moments that act in the sagittal plane around the L5-S1 joint. It models those forces for a woman who weighs 100 lbs, and who lifts 45 lbs.

2. GRAVITATIONAL FORCES

With respect to the L5-S1 joint, gravity acts on two important masses:
  1. the mass superincumbent to the L5-S1 joint. This force, equal to roughly half the woman's body weight, is 50 lbs.

  2. the object that the woman lifts, equal to 45 lbs.

3. GRAVITATIONAL MOMENTS

The moments that gravitational forces produce at L5-S1 depend on their moment arms. These depend on:
  1. the size of the mass that the woman lifts, or the mass' distance from the woman's body.

    She can minimize flexor moments on L5-S1 if she holds the mass so that its center of gravity is close to her body as she lifts it (Nordin & Frankel, 1989, Fig.10-20).

  2. the woman's lifting posture

    She can minimize flexor moments on L5-S1 if she bends her knees to lift rather than lifting with straight knees (Nordin & Frankel, 1989, Fig.10-22).

4. MUSCLE FORCES AND MOMENTS

The muscles must produce an extensor moment equal to the sum of the flexor moments that gravity exerts around the L5-S1 joint's lateral axis.

FM * sM = FG * sG

FM = (FG * sG) / sM

If we assume:
  1. The greatest flexion moment she experiences during a bent knee (squat) lift is 1575 in*lbs,

  2. The muscles' average moment arm is 2 inches,

Then she must develop 787.5 lbs. of force in the extensor muscles to perform this lift.

5. EFFECT OF FORCES AT L5-S1 JOINT SURFACE

lateral view of lumbar spine

The reference line in the figure represents the surface of the L5-S1 intervertebral joint.

Vector Fm, depicted in red, is a representative lumbar extensor muscle.

We can use vector resolution to understand the effects of this muscle's force on the L5-S1 joint surface.

lateral view of lumbar spine

After drawing a copy of vector Fm so that its point of application rests on the reference line, we resolve it into two component vectors:

  1. Fc, perpendicular to the reference line, representing joint compression.

  2. Fs, parallel to the reference line, representing joint shear.

(Fm)2 = (Fc)2 + (Fs)2

Further: Fs = Fmsin a

and Fc = Fmcos a

The relative magnitudes of shear and compressive forces produced by a force like Fm depend on the angle its vector forms with the joint surface. In this example, musculoskeletal anatomy make the angle between the muscle's force vector and the L5-S1 joint surface to be constant. Gravitational forces, which are always vertical, have different orientations to a joint's surface depending on the lifting posture.

6. HOW DO WE MINIMIZE COMPRESSIVE FORCE WHEN LIFTING HEAVY LOADS?

We know from the vector resolution that compressive force Fc depends on the angle a as well as the magnitude of various forces:

Fc = Fmcos a

Then, because: FM = (FG * sG) / sM

We summarize what we know about compressive force:

Fc = [(FG * sG)*cos a] / sM

Therefore, we can decrease lumbar compression (Fc) if we:

  1. decrease FG

  2. decrease sG

  3. alter the lumbosacral angle 'a'

  4. increase sM

7. THE ROLE OF INTRA-ABDOMINAL PRESSURE (IAP)

A structure under pressure is relatively stiff; it resists bending (flexion) more than an unpressurized structure.

Coke can

Compare the stiffness of a full drink can with that of an empty one.

We similarly stiffen the trunk by "pressurizing" the abdominal cavity!

'Coca-Cola' is a registered trademark of the The Coca-Cola Company. While neither I nor the University of Oklahoma endorse this product, I appreciate the company's kind permission to use it to illustrate this biomechanical concept.
thoracolumbar fascia

Two muscles whose lines of applications flatten the lower abdominal wall and increase IAP (without producing trunk flexion) are the internal oblique and the transversus abdominus.

These muscles also attach to the thoracolumbar fascia (TLF, also called the lumbodorsal fascia).

The figure is modified from Hertling and Kessler's (1996, Fig. 16-14A, p. 508) text. Bartelink (1957) first proposed increased IAP as a reflexive mechanism that spares the lumbar spine from excessive compression. While more recent researchers (Sullivan, 1994) question the accuracy of Bartelink's original formulation, the phenomenon of increased IAP during lifting is real, whatever its usefulness.

References

Bartelink, D.L. (1957). The role of abdominal pressure on the lumbar intervertebral discs. Journal of Bone and Joint Surgery [Br], 39, 718-736.

Hertling, D., & Kessler, R.M. (1996). Management of common musculoskeletal disorders: Physical therapy principles and methods. (3rd ed.). Philadelphia: J.B. Lippincott.

Nordin, M., & Frankel, V.H. (1989). Basic biomechanics of the musculoskeletal system (2nd ed.). Philadelphia: Lea and Febiger.

Sullivan, M.S. (1994). Lifting and back pain. In Twomey, L.T., & Taylor, J.R. Physical Therapy of the Low Back. (2nd ed.). New York: Churchill Livingstone, 329-356.
Consult AHRQ's link to clinical practice guidelines for acute low back problems in adults (Guideline #14)

Last updated 10-24-01 Dave Thompson PT
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