The purposes of this study were to investigate the effects of the speeds of rising and of the chair height on the onset timing of the leg muscle electromyography (EMG) activities during the initiation of rising from a chair to stand. Six healthy adults were recruited and performed sit-to-stand movements under three task conditions. The tibialis anterior, gastrocnemius, biceps femoris, rectus femoris and gluteus maximus were all activated prior to the onset of the lift-off of the buttocks from the chair. The TA and RF were usually the first to be recruited. The differences in the individual muscle onset timing between rising from normal chair at the speeds and at fast speeds were nonsignificant. The effects of increases in chair height on the onset timing of each of the five muscles were also nonsignificant. It is concluded that both flexor and extensor muscles in the leg participated in the initiation of rising up from a chair to stand. Suggestions for clinical training of sit-to-stand movement was also proposed.