12.1.1 Force associated with intra-abdominal pressure

In the model, intra-abdominal pressure is represented by an equivalent force Fₐ whose line of action lies anterior to the lumbar spine. The moment produced by this force helps balance the flexion-extension component of the moment at L4/L5 and therefore modifies the moment that the trunk muscles must counteract.

The force generated by intra-abdominal pressure can be estimated73F¹ using Equation 53, where F_A is the force generated by intra-abdominal pressure in newtons, θ_t is the angle in degrees between the trunk axis and the extension of the thigh axis (as shown in Figure 63), and M_L5/S1 is the component perpendicular to the sagittal plane of the moment caused by handled loads and the weight of body segments above the joint (in N·m). An average diaphragm area74F² of 465 cm² is used to determine F_A.

\[F_{A} = 6.199473 \bullet 10^{- 4}\ \ \left( 43\ - \ 0.36\ \theta_{t}\ \right){(M_{L5/S1})}^{1.8}\ \]

Equation 53

Determining the moment generated by this force about the lumbar joint requires its moment arm (D_FA in Figure 63). The moment arm ranges from approximately 7 to 15 cm75F³ depending on the sine of angle θ_t; its value in meters can be estimated using Equation 54.

\[D_{FA} = 0.066 + 0.082\ ABS(SEN(\theta_{t} \cdot \ \pi\ /\ 180))\ \]

Equation 54

Once the force generated by intra-abdominal pressure F_A and its moment arm D_FA are known, their product gives the moment associated with intra-abdominal pressure, which is incorporated into the flexion-extension component of equilibrium at L4/L5.