Biomechanics - Biomedical Physics

1. Biomechanics
Force on the spine
Dr. Saed Al Atawneh, PhD
Professor (Assistant) of Nuclear Physics
Faculty of Science/Department of Physics
Zarqa University
21/03/2024

2. Force on the spine
• Let us examine the impact of net
forces on the human spine.
• Assume:
• WT: The weight of the Trunk, its equal
to 0.4w or 0.4 Fg of body weight.
• WH: The head weight, its equal to
0.2w or 0.2 Fg of body weight.
• Fc: Tension Force by erector spine
muscle.
• Fp: The force the pelvis exerts on
spinal column.
• Note: Spine is inclined by 𝜃.

3. • Solution:
• Let’s consider the Translational Equilibrium:
 𝐹𝑥 = 0,
𝐹𝑝𝑥 −𝐹𝑐 cos 12 − 𝑊𝐻 sin 𝜃 − 𝑊𝑇 sin 𝜃 = 0 …… (1)
 𝐹𝑦 = 0,
𝐹𝑝𝑦 +𝐹𝑐 sin 12 − 𝑊𝑇cos(𝜃) −𝑊𝐻 cos(𝜃) = 0 … … .(2)
Force on the spine

4. • Let’s consider the Rotational Equilibrium:
 𝜏 = 0,
= −𝑊𝐻cos(𝜃) ∗ 𝑙 −𝑊𝑇 cos 𝜃 ∗
𝑙
2
+ 𝐹𝑐 𝑠𝑖𝑛 12
2
3
𝑙 = 0 … (3)
= −0.2 Wcos 𝜃 ∗ 𝑙 − 0.4𝑊𝑐𝑜𝑠 𝜃 ∗
𝑙
2
+𝐹𝑐 𝑠𝑖𝑛 12
2
3
𝑙 =0
𝐹𝑐 =
0.4
2
+ 0.2 𝑊𝑐𝑜𝑠 𝜃 ∗ 𝑙
2
3
∗ 𝑙 ∗ sin(12)
Force on the spine
Fc

5. • For rest of calculations, assume 𝜃 = 30°.
𝐹𝑐 =
0.4𝑊𝑐𝑜𝑠 30
2
3 sin(12)
= 2.5W
• Substitute in Eqs. (1) and (2):
𝐹𝑝𝑥 = 2.5𝑊 cos 12 − 0.4 sin 30 − 0.2 sin 30 ……… (1)
𝐹𝑝𝑦 = 2.5𝑊 sin 12 − 0.4 cos 30 − 0.2cos(30) ……… (2)
𝑭𝒑𝒙 = 𝟐. 𝟏𝟒𝑾
𝑭𝒑𝒚 = 𝟎. 𝟎𝟎𝟏𝟔𝑾
Important Note: The force on the spine by the pelvis is over two times larger if the spine is
horizontal than if it is vertical. You really should "lift with your legs (𝜃 = 90º), not with your back
(𝜃 = 0º).
Force on the spine

6. Homework
• Chapter 1.
• Section1.4.
• Problem No. 5, 6 and 8.
Please Submit it by the next lecture
Thank You