There are three different energetic resources for any kind of muscular work, including tennis (see Appendix 1):
1
ATP–CP (or, anaerobic alactatic) system, which lasts 10–15 seconds, uses stored ATP and creatine phosphate (CP), with no by–products. This energy source is related to speed and strength. It is essential for the serve and winning shots in tennis.
2
Glycolysis (or, anaerobic lactic) system, which lasts 15 seconds – 3 minutes, uses blood glucose and muscular glycogen to make ATP; its by–product is lactic acid. This energy source represents anaerobic endurance. It is essential in long, exhausting tennis relays.
3
Aerobic system, which lasts from 2–3 minutes to several hours, uses glucose, glycogen, fats, and proteins to make ATP within the aerobic energy pathway; its by–products are carbon dioxide and water. This energy source represents aerobic endurance. It is essential in the fourth and fifth set of any serious tennis match.