Biomedical Engineering Reference
In-Depth Information
Fig. 6.3 Relation between the mass that a person lifts from the floor with both hands, incorrectly,
without bending the knees and curving the column 30 to the horizontal, and the force of the spinal
erector muscle and the contact compressive force
conditions for the equilibrium. The weights W 1 and W 2 considered are of a standard
Caucasian adult of 1.70 m height and 70 kg, and the column length of 0.70 m.
Figure 6.3 shows the obtained results.
From Fig. 6.3 we verify that even if any object is lifted ( W 2 corresponds to the
weight of the head plus two arms/forearms/hands), the simple fact of bending the
column at an angle 30 with the horizontal, requires the muscle to exert a force of
1,874 N, which is equivalent to supporting a weight 2.6 times larger than his own
body weight. This force increases linearly with the mass lifted, obeying an equation
of a straight line:
Muscle force F (N)
mass (kg).
To lift a mass of 50 kg, the force exerted by the spinal erector muscles reaches
the spectacular value of 4,999 N which is equivalent to more than 7.1 times the
body weight.
In the case of the contact force C , its intensity is always somewhat greater than
that of the muscle force and can be calculated for this person in this situation by the
equation of a straight line:
Contact force C (N)
¼
1,874.2 + 62.5
¼
2,058 + 66.2
mass (kg).
of the contact force C with the column varies little with the mass that
a person lifts; it is very small and the force is downward, as can be seen in Fig. 6.4 .
The above equations are obtained by solving Example 6.5.
The angle
α
Exercise 6.6 Discuss what happens when the person of Example 6.5 adopts the
correct posture bending the knees to lift an object from the ground. Try to crouch
correctly, bending the knees to see what changes. Solve the exercise quantitatively,
considering that the angle of the column with the horizontal is 70 , and observe
what changes.
 
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