Possible Clues:

• They're subject to the starts of 17-, 26-, 46- and 59-Across
• Sinews
• Mr. Olympia's pride
• Eg, biceps and abs
• Nickname for a bodybuilder
• He-man's nickname
• Stretchable things stretched across 17-, 24-, 49-, and 62-Across
• Parts of the body that may be ripped
• Discussed shellfish -- they give us beef
• What weightlifting builds