The aim of the study: to determine and compare the low frequency fatigue of the legs’ muscles of 12–13 years old boys during the isometric and eccentric-concentric physical exercises. The objectives of the study: 1. To determine the exertion of low frequency fatigue during isometric exercises in boys. 2. To determine the exertion of low frequency fatigue during eccentric-concentric exercises in boys. 3. To compare the exertion of low frequency fatigue among isometric and eccentric-concentric exercises in boys. Subjects. Healthy boys (n = 6) aged 12–13 (12,4 ± 0,6) who do not participate in sports regularly but attend the classes of physical education twice a week. The rest period between the studies was three weeks. The tension was set up at 50 % MVS level. After that the values of the contractions and relaxations of the muscles were registered as follows: 1. The strength of muscle contraction caused by a single electric stimulus (Pt); 2. The strength of muscle contraction (P) caused by the following regiments of electrostimulation: 7 Hz (P7), 10 Hz (P10), 15 Hz (P15), 20 Hz (P20), and 100 Hz (P100) (the duration of the stimulation – 1 s, and the rest intervals between stimulations – 5 s); 3. Maximal voluntary strength (MVS) of muscle contractions (three trials every 3 minutes); 4. The workout (10 minutes of slow pedaling on the ergometer: the heart rate contractions from 110 to 130 times per minute); 5. Isometric workload: 18 times, each lasts 15 s, 40 % MVS contraction... [to full text]