There are NO proper nouns in the sentence, "The athletes must trudge through thick mud."

A proper noun is the name or title of a specific person, place, or thing. Examples of the same sentence using proper nouns:

• Jack and Jim must trudge through thick mud. (specific people)
• The athletes must trudge through the Mud Hen Marsh.(specific place, in Ohio)

Hen, Ben, den, men, pen, pretend, send, when, Jen, Ken, zen, wren, again, amen, and then, Big Ben, brood hen, but then, cayenne, cow pen, fat hen, gray hen, heath hen, house wren, light pen, marsh hen, marsh wren, mud hen, Phnom Penh, quill pen, rock wren, sage hen, sea pen, sedge wren, sheep pen, UN, wise men, wood hen, ballpoint pen, broody hen, CNN, cactus wren, cattle pen, fill again, fountain pen, guinea hen, jenny wren, jungle hen, LPN, Madeleine, Maori hen, now and then, once again, only when, setting hen, then again, water hen, winter wren, comedienne, ESPN, gambling den, never again, New Zealand wren, now and again, opium den, over again, rock cornish hen, time and again, again and again, Carolina wren, time and time again, every now and then, over and over again, glen, comprehend.

from: rhymezone.com

Send hen pen ken den sin bin men pin Vin win Jen in inn

pen den been Ken Jen

desend,recamend,
ten- hen, win, dean,
Men
Den, zen and when.