FRIDAY, Sept. 9 (HealthDay News) -- College football players show significant gains in size, strength and power over time, but smaller improvements in speed and agility, a new study says.

Researchers assessed the physical capabilities of 289 Division III college players throughout their four- or five-year careers.

From freshman to senior year, the players' body mass increased an average of 21 pounds, their bench press strength increased by 31 percent, their squat strength increased by 36 percent and their aerobic conditioning improved.

However, their 40-yard sprint time improved by an average of only 0.2 seconds and their vertical jump height increased by about two inches.

While resistance training programs do appear effective in improving players' strength and body mass, genetic factors may limit the ability to significantly improve speed and jumping ability, said lead author Jay R. Hoffman, of the University of Central Florida in Orlando, and colleagues in a journal news release.

"Collegiate football players can become faster and jump higher, but the ability to transform a slow athlete to a fast one appears to be limited," the researchers concluded.

The study appears in the September issue of the The Journal of Strength and Conditioning Research.

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