A racquet is at a certain tension when it hits a ball. Impact causes the string to elongate and to increase in tension as a result. Peak tension is the original tension plus the increase in tension.

Peak tensions depend on the starting tension (not strung tension, but actual tension at time of play), but given a string strung around 62 pounds, peak tensions will be range from about 75 pounds for gut, 80-90 lbs for nylon, 90-100 lbs for polyester, and 100-160 lbs for Aramid.

Stiffer strings tend to have higher peak tensions.