Standing and locking out your knees activates not only the quadriceps but also the hamstring, there is a synergistic engagement that occurs between the two; it also draws our attention to the imbalance from right to left. So in walking, working at your Pilates, trekking up the fells etc. we do not fully engage our legs properly! Standing for any length of time like this it also tightens up your lower back therefore restricts your stability muscles doing their job!! Awareness is always the key keep checking in to your alignment.