The length of the moments arms will determine this. Your lab partner's extensor muscles must produce a moment that equals the sum of the flexor moments that gravity exerts around the L5-S1 joint's lateral axis. Solve for the muscle force using the equation for rotational equilibrium . Use a laboratory skeleton to estimate an average moment arm for the extensor muscles. Consider how your lab partner might substitute ligamentous force to produce a moment that resists gravity's effect on flexing the trunk.
Ligaments develop force when they are elongated.
Consider the ligaments that restrain intervertebral motion . Consider a brief demonstration. Palpate your lab partner's erector spinae as he or she bends forward. Soon after the motion begins, the erector spinae become active. As your partner continues to bend forward slowly, you may detect a cessation in muscle activity.