Show Muscles Only in Straining Limbs; Skin Over Muscle Hollows
From the Treatise on Painting: muscles gain force only in exertion, and spongy fat fills the angles the skin cannot reach.
This page of Leonardo's Treatise on Painting teaches that muscles should be made prominent only in limbs under real force or effort; a body drawn with every muscle bulging resembles a sack of nuts rather than a human figure. A chapter 'Of the muscles of animals' explains that the hollows between muscles must not look like skin over bare sticks, but should rest on spongy, air-filled fat lodged in the angle m n o where muscles meet. A lettered diagram of curved profiles (with labels c, f, i and the angle m n o) illustrates the point, and a further chapter opens the argument that a nude showing all its muscles at once cannot appear to move.
On this page
Muscles show only where a limb is under strain
Muscles are to be given prominence only in limbs under force or effort, while idle parts should show no muscular relief. A body drawn with every muscle displayed looks like a sack of nuts rather than a human figure.
Spongy, air-filled fat fills the angle between muscles
The skin must not seem to clothe two bare sticks in contact, nor sag emptily in a wide curve as at f, but rest on spongy fat as at i, lodged in the angle m n o where two muscles meet. Because the skin cannot sink into that angle, nature fills it with tiny air-filled bladders of fat that condense or rarefy as the muscles swell or shrink.
A nude showing all its muscles cannot appear to move
The chapter [336] states that a nude rendered with every muscle made evident will be motionless, since it cannot move once all its muscles are shown taut at once.
Diagram of skin profiles over the muscle hollows
A row of undulating profile curves illustrates how the skin lies over the hollows between muscles, carrying letter labels including c, f and i and the angle m n o named in the text.