The water-based exercise group will be held twice a week, 45 min per session for 8 weeks. Supervised exercise will be led by the experienced physiotherapist and water-based exercise by physiotherapist with aquatic exercise expertise.Each session will consist of warm up (10 min), main part (30 min) and cool-down (5 min). During warm up children will perform lower and upper limbs aerobics and breathing exercises. The main part will consist of endurance exercises of upper and lower limbs with focus on breathing pattern. During cool-down focus will be on upper limb and thoracic cage stretches and breathing control.

The land-based exercise group will be held twice a week, 45 min per session for 8 weeks. Supervised exercise will be led by the experienced physiotherapist and water-based exercise by physiotherapist with aquatic exercise expertise.Each session will consist of warm up (10 min), main part (30 min) and cool-down (5 min). During warm up children will perform lower and upper limbs aerobics and breathing exercises. The main part will consist of endurance exercises of upper and lower limbs with focus on breathing pattern. During cool-down focus will be on upper limb and thoracic cage stretches and breathing control.

The control group will be asked not to change their physical activity for 8 weeks. After that period and post-test the control group will receive the exercise.

8 weeks x 2 times per week for 45 min

Exercise capacity will be measured with the use of modification of the Balke treadmill protocol previously validated on a large population-based sample of children ages 7-18 years. Breathing parameters will be measured continuously by the "breath-to breath" method using an ergospirometer Cortex MetaMax 3B system (Biophysik GmbH, Leipzig, Germany). Output from the gas analyses will be sampled at 15 s intervals and stored for use in the calculation of the Oxygen-Uptake Efficiency Slope (OUES).

The fatigue will be assessed with the Polish adaptation of the Chronic Fatigue Syndrome Questionnaire. The questionnaire allows to determine the level of chronic fatigue and severity of individual symptoms: general fatigue, decreased vitality, mental overload, somatic symptoms, anxiety and discouragement. It includes 59 statements with one of the possible answers: "never", "sometimes", "often" - the answers are assigned the values 0, 1, 2, respectively. The higher the score, the greater the severity of the symptoms of chronic fatigue.

Health-related quality of life will be assessed with the PedsQL - a brief, standardized, generic assessment instrument that systematically assesses perception of HRQOL in healthy children and adolescents and those with acute and chronic health conditions.