Watch out for individuals who stabilize in single leg stance by a laterally flexing their trunk. This trunk compensation occurs when an individual side bends at their spine to achieve a position of center of mass (COM) over their base of support. Preferably there would be more contribution from concomitant hip adduction and posterior shift of COM with slight hip flexion. The hip flexion position is important because it loads the hamstrings. There is significant evidence showing that stiff leg balance strategies and poor hamstring activation relative to quads to be predictors of injury particularly of the ACL. A laterally flexed trunk position over a single support is a risky position in sport as the laterally flexed trunk shifts the COM over the stance limb which directs corresponding ground reaction forces directly at the COM. With a laterally flexed trunk, these forces are lateral to the knee and thus create a external knee abduction moment (a force creating valgus collapse.) If you see your patient/ client balancing with a nearly straight knee and be leaning their body over the stance limb then this must be corrected and coached and should be observed to see if any maladaptive carryover has occurred in sport or practice.