Based on the text the word “that” (l. 10) refers to

Backpacs” (title) - a large bag carried on the back.

“strap” (l. 2) - a strip of leather, cloth or other flexible material.

“lugging” (l. 5) - carrying something with great effort. “prof” (l. 8) - professor.

“sprains” (l. 18) - injuries.

“strains” (l. 18) - severe demands on physical strengh.