Biomechanical Analysis of Lifting

-Nidhi Chhabra

Lifting

• Lifting is an activity that is an essential part of everyday life.

• Unfortunately, it has been implicated as a contributing factor in the development of a variety of musculoskeletal injuries, particularly those that involve the lumbar spine.

• So, in order to know how much load is being imposed on joints in day today life, biomechanical analysis is important.

Characteristics of Lifting Tasks

• Lifting involves movement of an object from one location to another location, generally traversing both vertical and horizontal distances.

• This, can be subdivided into three stages:– Access– Movement– Placement

Lifting Techniques

Stoop Lifting

Lifting Techniques

Squat Lifting

Lifting Techniques

Semi- squat Lifting

Squat Lifting v/s Stoop Lifting

Stoop lift• Trunk flexion achieved by

thoracolumbar flexion and there is no knee flexion.

• The extensor muscles are at a disadvantage , because of shorter moment arms in this position, a change in the line of pull and the possibility of passive insufficiency resulting from elongated state of muscles.

Squat lift• Spine remains as erect as

possible and trunk flexion achieved primarily by hip and knee flexion.

• The deep layer of erector spinae is capable of producing posterior shear, which helps to offset anterior shear that occurs with trunk flexion, in case of carrying additional load.

Squat Lifting v/s Stoop Lifting

Stoop lift• Intradiskal pressure higher. • Knee-High quadriceps

femoris group activity at the beginning and at the end of the lift.

• Moderate use of hamstrings occurs in the middle third of the lift.

Squat lift• Intradiskal pressure

comparatively lower.• Knee-The flexion moment at

the beginning of the lift changes to an extension moment.

• Medium to high activity of vastus lateralis, less activity of rectus femoris and low to medium activity of biceps femoris.

Squat Lifting v/s Stoop Lifting

Stoop liftPhases

Muscles Beginning Middle End

Quads *** * ***

Hams * ** *

Squat liftPhases

Muscles Beginning Middle End

Quads *** *** ***

Hams * * *

* = Low activity** = Moderate activity*** = High activity

Squat Lifting v/s Stoop Lifting

Stoop lift• FRR in lumbar erector

spinae is evident and the peak EMG response was delayed towards the middle of the lift. 1

• This FRR results in the flexed spinal motion segment resisting the flexor moment by the posteriorly placed passive structures.

Squat lift

• Earlier peak EMG readings in erector spinae were evident.1

1. McGorry et al 2001.

Squat Lifting v/s Stoop Lifting

Stoop lift• The erector spinae in flexed

posture have changed line of actions relative to the motion segment and are therefore less able to resist the anterior shear forces seen to cause damage to the spine in full flexion.

Squat lift• Contraction of erector

spinae muscles results in the development of a posterior shear force on the superior vertebrae. This has the potential effect of reducing the effect of anterior shear forces generated by the weight of the upper trunk and load.

Load

Stability of the load • Keep the load close :

– Documented reduction in lumbar stress.• Ensure the placement of a secure hand couple:

– Minimizes trunk instability during a lift involving asymmetric handling and load shift.

• Use the lifting technique that is most applicable to the situation:– Semi squat lift- This would be the ideal lift for heavy loads performed on

an occasional basis.– Squat lift- To be used as an alternative to the semi-squat when space is

limited and load size does not allow for foot placement to the side of the object to be lifted.

– Stoop lift- Lifting scenarios requiring light loads (20 pounds and below) on a frequent basis are more efficiently managed using this technique.

Stability of the Load• When lifting on an uneven-sloped surface, face down the slope to

negotiate the lift.• When lifting, do so as much as is possible in the sagittal plane.

Role of Intra-abdominal Pressure(IAP)

• An increase in IAP is frequently demonstrated during lifting tasks.• Bartelink suggested that an increase in IAP decreases spinal compressive

loads by pushing it on the rib cage.• McGill and Norman challenged this theory by arguing that the large

compressive loads caused by contraction of the abdominal muscles negate any potential unloading effect and that a net increase in compression through the lumbar spine would result from increased IAP.

• Cholewicki et al suggested that the increase in IAP has more to do with providing stability to the lumbar region and less to do with generating extensor torque.

• Hodges et al suggested that an increase in IAP may in fact facilitate an extensor torque if the IAP is generated through selective muscle recruitment, in particular of the diaphragm, pelvic floor muscles and transversus abdominis.

Manual lifting analysis• NIOSH developed the Work Practices Guide for Manual Lifting

– the first comprehensive tool to assist in the process of risk factor identification and subsequent ergonomic abatement to correct those factors identified as being potential problems.

• Revised NIOSH lifting equation RWL= LC x HM x VM x DM x AM x FM x CMHere, RWL= Recommended Weight Limit LC = Load Constant HM = Horizontal Multiplier VM = Vertical Multiplier DM = Distance Multiplier AM = Asymmetry Multiplier FM = Frequency Multiplier CM = Coupling Multiplier

Take home message

References• Joint structure and function (Fourth edition)- Pamela K. Levangie,

Cynthia C. Norkin• Ergonomics for therapists• Biomechanical Basis of Human Movement- Hamill, Joe• Ergonomics in Sport and Physical Activity- Thomas Reilly• Stoop or squat: a review of biomechanical studies on lifting

technique -Jaap H. van Dieen , Marco J.M. Hoozemans, Huub M. Toussaint

• How to lift a box that is too large to fit between the knees -Idsart Kingma, Gert S. Faber and Jaap H. van Diee

• Biomechanics of Lifting and Lower Back Pain- S.N. Robinovitch • http://www.cdc.gov/niosh/