The seat-pan to backrest angle was kept fixed in the most upright position possible (104 deg), where the knee angle was defined as the angle formed by the projected longitudinal axes of the femur and the tibia. Here, the flexed position referred to a knee angle of 90 deg, with 0 deg considered fully extended. The System Prowas calibrated for each individual to allow the determination of the subject-specific anatomical knee angles. Calibration was performed by having each subject position undergo knee flexion at specified angles (0 deg, 30 deg, 45 deg, 60 deg, 90 deg) while the System Prorecorded the associated machine angle for the encoder. During the exercise, a visual interface provided the subject with real-time feedback pertaining to target and actual (interpolated) knee angles. Potential angular deviations arising from plantarflexion were minimized by allowing the footplates to freely pivot about a hinge situated at their respective centers.

Adjustable lead screw subassemblies enabled prominences that produced variable backrest support depths and positions (height from seat-pan). This setup permitted backrest profile continuity for various back support geometries, which were defined by the vertical position and depth of the center of the curved prominence. A constant support slope was maintained by setting the adjacent adjustment points 0.7 cm less than the depth of the central prominence. This resulted in linearly decreasing depths, beginning from the trial support depth, and ending at the reference depth of the backrest.

A total of 15 different backrest configurations were tested (Position, P = 4.6, 9.4, 19.0, 28.4, or 33.2 cm; Depth, D = 0.7, 1.4, or 2.1 cm), with all test sessions additionally including the neutral geometry (flat backrest profile, D = 0 cm). The order in the backrest configurations were tested was computer-randomized for each participant. A protocol comprising of a set of 10 repetitions, consisting of 4 s of eccentric and 2 s of concentric exertion, resulted in 60 s trials. Trials were performed using 50% of the subject's body weight per leg. At least 3 min of rest was provided between trials, and test sessions were constrained to avoid muscular fatigue. Generally, not all backrest configurations could be investigated within a single session, with subjects typically achieving 9–10 configurations per session, since the neutral geometry was also tested five times to obtain a reliable control and quantify intrasubject variability. Eight subjects (4m/4f) made themselves available on multiple occasions to allow the complete range of configurations to be investigated (termed “repeat subjects”).

Subjects performed two unrecorded practice sets to allow accustomisation. During testing, the subject was instructed to be seated, firmly press their lower back against the backrest, place their hands in a comfortable position (where they would remain), and perform each trial repetition in a single, fluid motion. Here, the trial began and ended in the near-extended position (∼5 deg) to avoid subject's locking their knees, and involved flexing to a knee angle of 90 deg. Loading was kept constant throughout the protocol such that muscular tension was maintained for the duration of the trial.