Objectives: The aim of this experiment was to investigate the recruitment of a medial and a lateral component of the quadriceps muscles during voluntary tasks that challenge the stability of the knee and thus to evaluate whether the central nervous system (CNS) recruits these muscles simultaneously or selectively to control the patella in people without knee pathology. Methods and Measures: Nine subjects (five males and four females) without knee pathology were recruited for the study. Recordings of electromyographic (EMG) activity of vastus medialis obliquus (VMO), vastus lateralis (VL), tibialis anterior (TA) and soleus were made using surface electrodes. Subjects were instructed to rise onto their toes by contracting their triceps surae muscle or to rock onto their heels by lifting their toes and contracting their TA muscle. The trial was performed as a choice reaction time task. The onsets of EMG activity were expressed relative to the EMG onset of the prime mover (i.e. TA or soleus) for the trial. The relative difference in time of onset of EMG activity of VMO and VL was quantified by subtracting the onset of VMO from that of VL. Results: The results of the present study indicate that activity of the quadriceps muscles is initiated in an anticipatory manner either before or at the same time as the muscle responsible for movement of the ankle in standing. Furthermore, the onset of activity of the medial and lateral components of the quadriceps group are initiated simultaneously. Conclusions: These findings indicate that when the CNS can predict that a movement will challenge the stability of the knee an anticipatory postural adjustment is initiated that co-activates the medial and lateral muscles that guide the movement of the patella in the femoral groove. Further research is required to determine whether the temporal relationship differs in a symptomatic population such as one with patellofemoral pain syndrome (PFPS).