Effects of fatigue produced by a maintained 50% isometric loading on electromyographic (EMG) and mechanical characteristics of voluntary and reflex contractions of human skeletal muscles were studied in 14 males. A continuous isometric loading of the knee extensors caused significant (P less than 0.001) increase in integrated EMG (IEMG) and decrease (P less than 0.001) in mean power frequency (MPF) of the EMG spectrum. The decrease in MPF was related (P less than 0.05) to percentage fast twitch (FT) fibre area of the vastus lateralis muscle. In reflexly induced contractions no changes were observed during fatigue in reflex latency (LAT) but electromechanical delay (EMD) increased significantly (P less than 0.01). The decrease in voluntary MPF and increase in reflex EMD were interrelated (P less than 0.05). Fatigue loading increased (P less than 0.05) the peak-to-peak amplitude of EMG of the reflex contraction but decreased (P less than 0.01) the corresponding force amplitude. The increase (P less than 0.01) in this reflex EMG/force ratio was related (P less than 0.05) to the corresponding increase observed in IEMG/force ratio of the voluntary contraction. The present findings support the differential fatiguing properties of fast twitch and slow twitch muscle fibres. In addition the fatigue effects on reflexly induced contractions show that fatigue took place primarily in the contractile processes and that muscle spindle sensitivity was increased during fatigue loading.