## 2 Materials and Methods  
Twelve healthy individuals (6M, 6F; age 29.3 \(\pm\) 9.8 years; height 171.1 \(\pm\) 0.1 cm; body mass 68.7 \(\pm\) 13.3 kg) participated in this study. Inclusion criteria included the following: (1) being between 20 and 50 years of age, (2) being able to walk on a treadmill, and (3) being able to understand spoken English at the level needed to (a) understand and follow instructions for equipment setup, testing, and task performance, (b) answer questions related to effort and preference, and (c) be able to understand consent document. Exclusion criteria included the following: (1) significant health conditions that could limit the ability of the individual to exercise and (2) history of cardiovascular incident or other known risk factor for stroke or heart attack. This study was conducted in accordance with the Declaration of Helsinki and approved by the University Institutional Review Board. Participants were informed of the benefits and risks of the investigation before providing written consent. Each participant performed one bout of one minute of cycling and one bout of one minute of HFgait on two different days to prevent the onset of fatigue. For cycling, resistance was set at 1.6 W/kg while cadence held at 80 rpm. For HFgait, the treadmill (AMTI Inc., Watertown, MA, USA) speed was set at 1.2 mph. The selected cycling resistance and treadmill speed corresponded to exercise intensities that represent typical intensities used in clinical populations at the introduction of an exercise program. For cycling trials, participants were first fit on a stationary cycle ergometer. Saddle height was set such that the knee achieved 25-30\({}^{\circ}\) knee flexion, measured with a hand-held goniometer, when the pedal was placed at bottom dead center. The saddle fore/aft position was set such that the participant's knee was aligned with the pedal spindle when the  
Figure 1: Illustration of the setup and HFgait performed during the study. In this frame, subject is increasing left maximum hip flexion to meet the target maximum hip flexion shown in the display (blue line) while walking on the treadmill. crank was in the horizontal forward position. This was verified by hanging a plumb-bob from the inferior pole of the patella and manually adjusting saddle position [15]. Finally, the handlebar position of the ergometer was adjusted so the angle formed between the participant's trunk at thigh was 90\({}^{\circ}\) when the crank was set at the 90\({}^{\circ}\) position [15]. During HFgait, participants used a biofeedback system that uses markerless motion tracking for hip flexion angle calculation [13; 14]. This system displays the real-time hip flexion angle and the target peak hip flexion angle while participants walk on the treadmill. This ensured that participants maintained a consistent peak hip flexion angle during the test. Previously reported error between the target hip flexion and the actual hip flexion is 8% which corresponds to approximately 6.9\({}^{\circ}\)[14]. Before the test, participants performed a dynamic calibration trial where they were asked to walk on the treadmill at the selected speed while flexing their hips as much as possible. This trial set the target hip flexion to be used during the test. Participants were then fitted with retroreflective spherical markers. These markers were used to create a 7-segment lower-extremity biomechanical model of the pelvis, thighs, shanks, and feet [16]. The pelvis was defined with reflective markers placed bilaterally on anatomic landmarks of interest, specifically the most superior aspect of the iliac crest and the greater trochanter of the femur. Pelvis motion was tracked during testing with a tracking 'cluster' of 4 markers secured to a semi-rigid thermoplastic shell attached to the posterior pelvis. Anatomic markers were then placed on the medial and lateral femoral epicondyles, medial and lateral malleoli, base of the 1 st and 5 th metatarsal heads, and on the shoe of the participant directly over the tip of the second toe.