In this section, we will discuss evidence, such as studies and biomechanical analyses, which illustrate the contribution lifting has on lower back pain. We have also designed a case study based on our research material and the information learned in lecture to further explain the effects of lifting on the spine in a biomechanical manner.
how does lifting contribute to Lower back pain?
What stresses does lifting place on the lower back and why?
When lifting, a forward bending moment is caused by the weight force of the lifter’s center of gravity, as well as the weight force of the load being lifted. The erector spinae muscles of the back must therefore produce a moment, Mmuscle, to counteract this forward bending moment.
When lifting, a forward bending moment is caused by the weight force of the lifter’s center of gravity, as well as the weight force of the load being lifted. The erector spinae muscles of the back must therefore produce a moment, Mmuscle, to counteract this forward bending moment.
![Picture](/uploads/4/2/5/1/42510819/7200990.png?213)
A larger forward bending moment requires a larger muscle moment to counteract it.
Mmuscle = Fmuscle * d
Therefore, a increasing the muscle moment required will increase the muscle force.
Why is this important?
This muscle force contributes to the resultant compression force. In a study done by the National Institute for Occupational Safety and Health (as referenced in the Spinal Mechanics lecture), compressive force on vertebral discs is a predictor of lower back pain, and a significant cause of vertebral end-plate fracture and disc herniation. This study also recommends maintainting compressive stress in the lower back to a maximum of 3.4 kN, as anything greater may increase risk of back problems.
Mmuscle = Fmuscle * d
Therefore, a increasing the muscle moment required will increase the muscle force.
Why is this important?
This muscle force contributes to the resultant compression force. In a study done by the National Institute for Occupational Safety and Health (as referenced in the Spinal Mechanics lecture), compressive force on vertebral discs is a predictor of lower back pain, and a significant cause of vertebral end-plate fracture and disc herniation. This study also recommends maintainting compressive stress in the lower back to a maximum of 3.4 kN, as anything greater may increase risk of back problems.
Case Study: Let us consider Tom, our 50th percentile male. Tom is a construction worker and lifts loads of up to 50 kg, multiple times a day. What stresses will this place on his spine?
Case 1: Tom lifts with a straight back (neutral spine), bent knees and is holding the load close to his body, with a small forward bend of only 10° (80° from right horizontal).
Case 1: Tom lifts with a straight back (neutral spine), bent knees and is holding the load close to his body, with a small forward bend of only 10° (80° from right horizontal).
Total forward bending moment in this situation:
Mtotal = MCOG + Mload = -(401.22N * 0.046m) + -(490.5N * 0.15m) = -92.03 Nm
The muscle must therefore produce a moment of 92.03 Nm to maintain static equilibrium.
Muscle force required for this: (assuming an erector spinae muscle moment arm of 0.05m)
Mmuscle = Fmuscle * Lmuscle
92.03 Nm = Fmuscle * 0.05m
92.03 Nm/ 0.05m = Fmuscle
Fmuscle = 1840.6 N
Mtotal = MCOG + Mload = -(401.22N * 0.046m) + -(490.5N * 0.15m) = -92.03 Nm
The muscle must therefore produce a moment of 92.03 Nm to maintain static equilibrium.
Muscle force required for this: (assuming an erector spinae muscle moment arm of 0.05m)
Mmuscle = Fmuscle * Lmuscle
92.03 Nm = Fmuscle * 0.05m
92.03 Nm/ 0.05m = Fmuscle
Fmuscle = 1840.6 N
![Picture](/uploads/4/2/5/1/42510819/748688_orig.png)
Compression and shear forces when lifting with a 10° forward bend:
Fcomp = all forces acting parallel to the spine (green)
= Fmuscle + FCOGsin80 + Floadsin80
=(1840.6) + (401.22sin80) + (490.5sin80)
=2718.77 N
Fshear = all forces acting perpendicular to the spine (purple)
= FCOGcos80 + Floadcos80
=(401.22cos80) + (490.5cos80)
=154.85 N
Fcomp = all forces acting parallel to the spine (green)
= Fmuscle + FCOGsin80 + Floadsin80
=(1840.6) + (401.22sin80) + (490.5sin80)
=2718.77 N
Fshear = all forces acting perpendicular to the spine (purple)
= FCOGcos80 + Floadcos80
=(401.22cos80) + (490.5cos80)
=154.85 N
Case 2: What if, instead, Tom decides to bend over, decreasing his trunk angle to 50 degrees, and hold the object farther away, increasing the moment arm of his load to 0.6 m?
![Picture](/uploads/4/2/5/1/42510819/9397034.png?250)
LCOG (Lw): 0.266m cos 50
= 0.171m
FCOG: mgHNT + 2mg(upper arm + forearm + hand) = 328.64 + 2(20.6+11.77+3.92)
= 401.22 N
Lload (Lp): = 0.6m
Fload: mg = 9.81 * 50kg
= 490.5 N
= 0.171m
FCOG: mgHNT + 2mg(upper arm + forearm + hand) = 328.64 + 2(20.6+11.77+3.92)
= 401.22 N
Lload (Lp): = 0.6m
Fload: mg = 9.81 * 50kg
= 490.5 N
Total forward bending moment in this situation:
Mtotal = MCOG + Mload = -(401.22N * 0.171m) + -(490.5N * 0.6m) = -362.91 Nm
The muscle must therefore produce a moment of 362.91 Nm to maintain static equilibrium.
Muscle force required for this: (assuming an erector spinae muscle moment arm of 0.05m)
Mmuscle = Fmuscle * Lmuscle
362.91 Nm = Fmuscle * 0.05m
362.91 Nm/ 0.05m = Fmuscle
Fmuscle = 7258.17 N
Mtotal = MCOG + Mload = -(401.22N * 0.171m) + -(490.5N * 0.6m) = -362.91 Nm
The muscle must therefore produce a moment of 362.91 Nm to maintain static equilibrium.
Muscle force required for this: (assuming an erector spinae muscle moment arm of 0.05m)
Mmuscle = Fmuscle * Lmuscle
362.91 Nm = Fmuscle * 0.05m
362.91 Nm/ 0.05m = Fmuscle
Fmuscle = 7258.17 N
![Picture](/uploads/4/2/5/1/42510819/7342939.png?250)
Compression and shear forces when lifting with a 40° forward bend:
Fcomp = all forces acting parallel to the spine (green)
= Fmuscle + FCOGsin50 + Floadsin50
=(7258.17) + (401.22sin50) + (490.5sin50)
=7941.3 N
Fshear = all forces acting perpendicular to the spine (purple)
= FCOGcos50 + Floadcos50
=(401.22cos50) + (490.5cos50)
=573.2 N
Fcomp = all forces acting parallel to the spine (green)
= Fmuscle + FCOGsin50 + Floadsin50
=(7258.17) + (401.22sin50) + (490.5sin50)
=7941.3 N
Fshear = all forces acting perpendicular to the spine (purple)
= FCOGcos50 + Floadcos50
=(401.22cos50) + (490.5cos50)
=573.2 N
Although the situations above are an exaggeration, they emphasize how forward bending while lifting and increased object distance (case 2) from body dramatically increase the compression and shear stresses on the lower back. As a result, lifting with such a stance can have very damaging effects on the lower back of the lifter, and significantly increase lower back pain.
Therefore, an individual with an occupation like Tom, in which regular heavy lifting is required, should try to maintain a stance more similar to that shown in Case 1. By maintaining a more upright stance, and holding the weight closer to the body, individuals can reduce the moment requirements about the lower back, and thus reduce the total compressive stress acting on the spine.
Therefore, an individual with an occupation like Tom, in which regular heavy lifting is required, should try to maintain a stance more similar to that shown in Case 1. By maintaining a more upright stance, and holding the weight closer to the body, individuals can reduce the moment requirements about the lower back, and thus reduce the total compressive stress acting on the spine.