Pay Attention to Muscles
To draw a believable human figure, it's good to have a working knowledge of musculature. For comic book artists, exaggerated muscles will help define a character (for other cartoonists, a broader approach is better). In understanding how muscles wrap around the skeleton, enabling the body to move, an artist is able to draw the figure in motion much more convincingly.
This is especially important for cartoonists aspiring to illustrate superhero comics. Where other cartoonists will use general musculature to give their characters a more lifelike appearance, comic book artists will amplify and overstate specific muscles to give their characters a powerful look.
The upper arm has two major muscle groups to be aware of. The deltoids drape over the top of the shoulder. Under the deltoids is the bicep muscle. The forearm has several long muscles that extend down the length of the bone and attach at the wrist.
Notice how these muscles are all fairly oblong. Some comic book artists — eager to draw arms that wield tremendous power — tend to overemphasize these shapes until they become almost spherical. The result looks more like peas in a pod than a muscular arm.
There are two muscle groups on the legs to pay particular attention to. The quadriceps are made up of muscles that stretch down from the pelvis to the knee. Notice how they angle inward. They are attached along the outside of the leg near the pelvis and along the inside and top of the knee. Of course, the other muscle that bears mention on the leg is the calf, which protrudes on the back of the lower leg, below the knee. From the front, calves also bow out along the outside more than they do along the inside.
Of course, the major muscles on the chest are the pectorals. Pay particular attention to where they attach to the deltoids on the outside of the shape. When the arms are outreached, both the deltoids and pectorals are flexed, and therefore the lines from one will flow into the other.
The lower line of the rib cage will be evident in characters who are not carrying extra weight on their lower abdomen. Pronounced abdomen muscles will also make your character look buff. But resist the urge to go overboard here. It's colloquially called a six-pack for a reason: there are three pairs of muscles in this muscle group — no more.
No body feature is more consistently overexaggerated than female breasts. In spite of the importance placed on this part of the anatomy by culture and commercials, you'll do well to remember that breasts are simple glands consisting of soft tissue. They are not generally spherical in shape.
Furthermore, breasts are connected to the pectoral muscles. Too often, beginners draw breasts that seem to connect to the collarbone. Draw the rib cage first. Next, add the breasts as extensions of the lower pectoral muscles.