We examined the physical demands of small-sided soccer games in untrained middle-age males and muscle adaptations and performance effects over 12 weeks of recreational soccer training in comparison with continuous running. Thirty-eight healthy subjects (20-43 years) were randomized into a soccer (SO), running (RU) and control (CO) group. Two-three weekly 1-h training sessions were performed. Muscle lactate (30.1 +/- 4.1 vs. 15.6 +/- 3.3 mmol/kg d.w.), blood lactate, blood glucose and time above 90% HR(max) (20 +/- 4% vs. 1 +/- 1%) were higher (p < 0.05) during training in SO than in RU. After 12 weeks of training, quadriceps muscle mass and mean muscle fibre area were 9 and 15% larger (p < 0.05) in SO, but unaltered in RU, and in SO, the fraction of FTx fibres was lowered (10.7 +/- 1.8 vs. 17.9 +/- 3.2%). In SO, citrate synthase activity was 10 and 14% higher (p < 0.05) after 4 and 12 weeks, but unaltered in RU. After 4 weeks VO(2max) and Yo-Yo IE2 performance were elevated (p < 0.05) to a similar extent in SO (7 and 37%) and RU (6 and 36%) but increased further (p < 0.05) from 4 to 12 weeks in SO (6 and 23%). In SO, 30-m sprint performance was improved (p < 0.05) by 0.11 +/- 0.02 s. Blood lactate during running at 11 km/h was lowered (p < 0.05) from 0 to 4 and 4 to 12 weeks (2.6 +/- 0.3 vs. 3.8 +/- 0.6 vs. 6.1 +/- 0.9 mM) and from 0 to 12 weeks in RU. No changes occurred for CO. In conclusion, recreational soccer organized as small-sided games stimulates both aerobic and anaerobic energy turnover and is an effective type of training leading to significant cardiovascular and muscular adaptations as well as performance enhancements throughout a 12-week training period.