Henry Gray (18251861). Anatomy of the Human Body. 1918.
this point the turning component gradually decreases and the pressure component changes into a component which tends to draw the two bones apart and which gradually increases as shown in Fig. 370.
When the bone D K is in such a position that the insertion angle φ = 41° 20 the pressure component = 3/4 I m and the turning component 1/4 I m, at 60° the two components are equal, at 90° the pressure component = 0 and the turning component = I M and at 131° 21 the pressure component has been converted into a pulling component = 1/4 I M and the turning component = 3/4 I M.
If, for example, the distance from the origin O to the joint D is less than the distance from the insertion I to the joint D, as in the Brachioradialis muscle, the insertion angle increases with the flexion but never reaches 90°. The turning component gradually increases to a certain point and then slowly decreases as shown in Fig. 371, while the pressure component gradually decreases and then slowly increases. It always remains large and its action is always in the direction of the joint.
Levers.The majority of the muscles of the body act on bones as the power on levers. Levers of the III class are the most common, as the action of the Biceps, and the Brachialis muscles on the forearm bones. Levers of the I Class are found in movements of the head where the occipito-atlantal joint acts as the fulcrum and the muscles on the back of the neck as the power. Another common example is
the foot when one raises the body by contracting the Gastrocnemius and Soleus. Here the ankle-joint acts as the fulcrum and the pressure of the toes on the ground as the weight. This is frequently, though wrongly, considered a lever of the II Class. If one were to stand on ones head with the legs up and with a weight on the plantar surface of the toes, it is easy to see that we would have a lever of the I Class if the weight were raised by contraction of the Gastrocnemius muscle. The confusion has arisen by not considering the fact that the fulcrum and the power in all three classes of levers must have a common basis of action, as shown in Fig. 372.