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Using the Valk knemometer, lower leg length (LLL) was assessed relative to changes in the positioning of the upper leg. Lowering the chair height of the knemometer resulted in a more acute angle between the upper and lower leg and a decrease in LLL. This decrease in measurement was attributed to changes in the anatomical surface of the knee underlying the measuring platform as a result of increasing the acuity of the leg angle. Based on four different leg positions, the average change in LLL per centimeter change in chair height was 0.607 mm in a child sample of 50, and 0.655 mm in an adult sample of 20. The difference in chair height with the leg angle at 90° and the lowest chair height possible, ranged from 12.3 to 30.3 mm, relative to lower leg length. This meant the longest leg in the study had a LLL measurement differing by 19.8 mm between these two positions. Due to the effect of leg position, we advised the use of a standard method of measuring LLL with respect to leg angle. Given the difficulties in accurately measuring leg angle with current available tools, we advise the most acute angle.