The purpose of this study is to explore the possible effects of MPH use on physiological functions among ADHD\ADD patients, by performing a set of physical trials to assess aerobic and anaerobic capacity, to characterize the tendency for muscle break down while performing monitored moderate physical effort and to assess the physiological strain while performing moderate exercise in heat load conditions by using the heat tolerance test.

20 male, non-combat soldiers with a pre-existing childhood diagnosis of ADHD\ADD, who are chronically treated with MPH will participate in the study. If necessary, civilian volunteers with a childhood diagnosis of ADHD\ADD may also be recruited to complete the study population (up to 50%, i.e. 10 participants).

1. st encounter: the subjects will receive an explanation of the study and sign an informed consent form, complete a medical questionnaire and undergo physical examination by a physician, including ECG.
2. nd-9th encounter: the subjects will be requested to undergo 10 tests on 8 examination days, which will include the: maximal oxygen consumption test, wingate test, heat tolerance test and step test. Each subject will perform each test twice: once while taking MPH and once with placebo.

Inclusion Criteria:

• Civilian volunteers aged 18-25 years.
• BMI range of 17-25.
• Diagnosed with attention deficit disorder (ADD or ADHD) since childhood.
• Routine use of Methylphenidate (at least 5 days a week).
• Without known medical illness or medication use.
• Report of performing physical exercise (twice or more a week).
• Without history of heat injury.

Exclusion Criteria:

• The existence or suspicion of existing cardiac or respiratory disease.
• Infectious disease 3 days prior to the experiment.