The purpose of this investigation was to determine the relationships among simple methods for measuring effort in below-knee amputee (BKA) and able-bodied (AB) children. Ten BKA children and 13 AB children walked on a treadmill and selected a freely chosen walking speed (CWS). Children then walked for 2 minutes at each of three speeds: CWS, 20% above CWS, and 20% below CWS. Oxygen uptake, heart rate, physiological cost index, percent maximum heart rate, and vertical displacement of a surface marker on the sacrum were determined for each subject and speed. Linear regression with repeated measures was used to determine correlations between oxygen uptake and the four variables (p < 0.05). To evaluate the effectiveness of the regression equations, two male children not part of the AB group were tested. The proportion of explained variance arising from the significant correlations between oxygen uptake and the four measured variables were all between 0.91 and 0.92. It was concluded that the vertical displacement of a marker on the sacrum is a simple and convenient measure for a biomechanics gait laboratory to estimate effort because only standard biomechanics laboratory equipment is required. Further, in a clinical setting and/or where the necessary equipment is available heart rate, physiological cost index, and percent maximum heart rate are also adequate.