Lifting

Each lab group needs:

• tape measure
• access to a bathroom scale or other device to estimate weights
• objects to lift
• textbooks (1 copy of each)

  • Kendall, F.P., McCreary, E.K., & Provance, P.G. (1993). Muscles: Testing and Function (4th ed.). Baltimore: Williams & Wilkins.

  • Smith, L.K., Weiss, E.L. & Lehmkuhl, L.D. (1995). Brunnstrom's Clinical Kinesiology (5th ed.). Philadelphia: F.A. Davis.
    Read pp. 390-394 to prepare for this week's lectures and lab.

  • University of Oklahoma Department of Physical Therapy. (2000, October 30). Steps in a biomechanical analysis of lifting. Oklahoma City, OK: Author. Retrieved October 19, 2001, from the World Wide Web: http://coph.ouhsc.edu/dthompso/web/namics/lift.htm

  • Anatomy text of your choice

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1. Analyze the forces and moments around the L5-S1 intervertebral joint as one or two members of your lab group perform a lift. You may model your analysis on the diagram below, which depicts a woman performing a lift in different postures. Note that the total flexion moment at the L5-S1 intervertebral joint depends on the woman's lifting posture.

   Your lab partners may lift any convenient object. Compare two lifting strategies

   1. a "knees straight" strategy
   2. a "knees bent" or squat strategy

   in terms of the maximum flexion moments that they produce around the L5-S1 intervertebral joint (IVJ). The total flexion moment is the sum of the moments produced at the L5-S1 IVJ by (1) the body mass superincumbent to the joint and (2) the mass of the object that the person lifts.

   SM_flexion = M_{superincumbent mass} + M_{lifted mass}

   SM_flexion = (F_{superincumbent mass}*d_{superincumbent mass}) + (F_{lifted mass}*d_{lifted mass})

   Use your tape measure to estimate the moment arms (d). Determine the gravitational forces (F), otherwise known as weights, by estimating or by using a bathroom scale. Because the body's center of gravity is located at the level of the second sacral vertebra, you may assume that 50 percent of your lab partner's mass is superincumbent to the L5-S1 joint.

   Compare the two strategies by finding the point during the lift where gravity's moment arms, with respect to the L5-S1 joint, are the longest. At these points, the flexion moment that gravity produces on the L5-S1 intervertebral joint is the largest. Estimate gravity's maximal flexion moment for both types of lift.

   	"knees straight"	"knees bent"
   weight of body mass superincumbent to L5-S1 intervertebral joint (lbs.)	___________
   weight of lifted mass (lbs.)	__________
   moment arm of body mass superincumbent to L5-S1 intervertebral joint (in.)	__________	_________
   moment arm of lifted mass (in.)	__________	_________
   total flexion moment at L5-S1 intervertebral joint (in*lbs.)	__________	_________

2. In either lifting strategy, to achieve rotational equilibrium, your lab partner's trunk extensor muscles must produce a moment around the L5-S1 IVJ that equals the total flexion moment that you just calculated.

   SM = 0 = M_{superincumbent mass} + M_{lifted mass} + M_{extension muscles}

   (M_{superincumbent mass} + M_{lifted mass}) = -M_{extensor muscles}

   (the negative sign only means that the moments produce opposite motions at the joint. As long as we remember this fact, we can ignore the negative sign.)

   SM_flexion = SM_extension = M_{extensor muscle}

   SM_flexion = (F_{extensor muscle} *d_{extensor muscle})

   F_{extensor muscle} = SM_flexion / d_{extensor muscle}

   Use the last equation to calculate the extensor muscle force. Use the figure for total flexion moment from the previous table, and use a laboratory skeleton to estimate an average moment arm for the extensor muscles.

   	"knees straight"	"knees bent"
   maximal flexion moment during lift (from table above) (in*lbs.)	__________	_________
   average moment arm of extensor muscles (in.)	__________
   estimated force shared among trunk extensor muscles (lbs.)	__________	_________

3. Consider how your lab partners might substitute ligamentous force for some of the muscle force that resists gravity's effect on flexing the trunk.
   Ligaments develop force when they are elongated. Force developed in the ligaments is passive force; it requires no expenditure of energy.

   What ligaments, when elongated, produce a force that resists spinal flexion?

   • Consider the ligaments that restrain intervertebral motion. Your text (Smith, Weiss, & Lehmkuhl, 1996, Fig.11-2, p. 371) illustrates these.

   • Consider a brief demonstration.
     • Palpate your lab partner's erector spinae as he or she bends forward, flexing the trunk while maintaining extended knees. Soon after the motion begins, the erector spinae become active. What is their purpose?

     • As your partner continues to bend forward slowly, you may detect a cessation in muscle activity. In "an unusual phenomenon of sudden muscle inhibition" (Smith, Weiss, & Lehmkuhl, 1996, p. 390), the erector spinae relax. Test several people to see whether this occurs. How can the spinal extensors cease working while gravity exerts a flexion moment on the spine? This phenomena is a reflection of the existence of ligamentous strategy for lifting.
   After you have identified ligaments that, when elongated, resist gravity's flexion moment, use a laboratory skeleton to estimate their moment arms with respect to the L5-S1 intervertebral joint. Which ligament has the longest moment arm?

   Decide whether your partners activate the anterior abdominal muscles during a lift. What purpose might abdominal activity serve? Similarly, decide if your partners activate the latissimus dorsi, and consider the muscles' possible purpose during lifting.

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Last updated 10-23-01 Dave Thompson PT
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