When a man wants to stop running and check the impetus he is forced
to hang back and take short quick steps. [Footnote: Lines 5-31 refer
to the two upper figures, and the lower figure to the right is
explained by the last part of the chapter.] The centre of gravity of
a man who lifts one of his feet from the ground always rests on the
centre of the sole of the foot [he stands on].

A man, in going up stairs involuntarily throws so much weight
forward and on the side of the upper foot as to be a counterpoise to
the lower leg, so that the labour of this lower leg is limited to
moving itself.

The first thing a man does in mounting steps is to relieve the leg
he is about to lift of the weight of the body which was resting on
that leg; and besides this, he gives to the opposite leg all the
rest of the bulk of the whole man, including [the weight of] the
other leg; he then raises the other leg and sets the foot upon the
step to which he wishes to raise himself. Having done this he
restores to the upper foot all the weight of the body and of the leg
itself, and places his hand on his thigh and throws his head forward
and repeats the movement towards the point of the upper foot,
quickly lifting the heel of the lower one; and with this impetus he
lifts himself up and at the same time extends the arm which rested
on his knee; and this extension of the arm carries up the body and
the head, and so straightens the spine which was curved.

[32] The higher the step is which a man has to mount, the farther
forward will he place his head in advance of his upper foot, so as
to weigh more on _a_ than on _b_; this man will not be on the step
_m_. As is shown by the line _g f_.

[Footnote: See Pl. XXIII, No. 1. The lower sketch to the left
belongs to the four first lines.]