Studies of leg muscle activities in haemophilic patients during upright standing proved altered time domain parameters (amplitudes, coordination patterns, and co-activation indices). The purpose of this study was to evaluate if frequency parameters of knee muscles during upright standing in haemophilic patients would be altered too. For each of the 6 left- and right-sided muscles (rectus femoris, vastus medialis, vastus lateralis, biceps femoris, and both heads of gastrocnemius muscles) the median power frequency (MDPF) from the spectrum of surface electromyography (SEMG) was calculated (Short- Term Fourier Transform, boxcar window, 30 seconds) in 26 haemophilic patients (H, 24 Haemophilia A, 2 B, 23 severe, 3 moderate, age: 42 ± 10, mean ± SD in years) compared to 25 healthy control subjects (C, age: 43 ± 12) during a standing period of 30 seconds. Prior to calculations of MDPF patient's extremities were classified according to major (MA, 6.7 ± 3.8) and minor (MI, 2.8 ± 3.2) affected joints (Gilbert- Score), and both sides of C were pooled. The comparisons of MDPF of SEMG signals showed no statistical differences between groups and within H (p > .05; e.g. biceps femoris: H-MA 92.5 ± 19.4, H-MI 90.2 ± 18.0, C 96.3 ± 9.8 Hz). Independent from joint arthropathy and group- affiliation MDPF of extensors ranged from 73.2 to 80.9 Hz whereas flexors ranged within 90.2 and 109.8 Hz. Amplitude parameters are reflecting load, whereas spectral parameters are often used to monitor muscle fatigue. Time domain data in H already could prove higher loads during upright standing compared to C, especially for the extensor apparatus. However, the applied low level contractions were not able to provoke changes of MDPF in H. Even if the variability of patients' data in the time domain is high, MDPF and thereby muscle fatigue seem to remain unaffected by the disease compared to C, although arthropathy, muscle atrophy and strength deficits are notably present in H.