= Solution
A <lever> is a body free to rotate about a <fulcrum>. Its usefulness comes from a balance of <torques>, not from producing <energy>. For a stationary ideal <lever>, let $d_E$ and $d_L$ be the perpendicular <lever arms> of the effort $E$ and load $L$. Then
$$
E d_E=L d_L,\qquad \boxed{\mathrm{MA}=\frac{L}{E}=\frac{d_E}{d_L}.}
$$
The <mechanical advantage> uses perpendicular distances to the lines of action, which need not equal distances measured along a bent or obliquely loaded arm. In an ideal slow motion, the effort moves through the larger distance when its <force> is the smaller: <work> is conserved.
For the three familiar straight arrangements, the <fulcrum> lies between effort and load in a first-class <lever>, the load lies between <fulcrum> and effort in a second-class <lever>, and the effort lies between <fulcrum> and load in a third-class <lever>. The second class has ideal <mechanical advantage> greater than one; the third has it less than one. The first can have either, depending on the <lever arms>.
The forearm bends by <elbow flexion>. The <biceps brachii> and <brachialis> pull through short <tendons> close to the <elbow>, while a load in the hand acts farther away. This is predominantly a third-class arrangement: a large <muscle force> buys a comparatively large and rapid hand displacement. The <biceps brachii> also contributes to <forearm supination>. <Elbow extension> is produced chiefly by the <triceps brachii>, whose <tendon> acts on the <olecranon> behind the <elbow>. The extensor insertion and forearm load lie on opposite sides of the effective pivot, giving the corresponding first-class analogy. Actual <anatomical joint> motion has changing <muscle moment arms>, and the <joint reaction force> also enters the full <force> balance; the body is not a set of frictionless pins. <Anatomy and Physiology 2e by J. Gordon Betts and others>[§11.1, lever systems and antagonistic muscle action; §11.5, arm muscles]{ref}
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