SBS_Data_Plos1.xlsx

Data presented represents kinematic and kinetic data collected from thirty-two participants (16 males and 16 females; mean ± SD age 21.3 ± 2.9 years; stature 1.70 ± 0.08 m; mass 69.9 ± 10.0 kg) during single-leg hopping while attached to an inclined lower body sled-based system (SBS).
Experimental procedures
Participants completed a familiarization session one week prior to initial testing to acquaint them with procedures (Day_0) [31]. Following this, participants underwent testing on four occasions. The first (Day_1), second (Day_2) and third (Day_3) testing bouts took place at the same time of day, spaced three to seven days apart [23,32]. On the final test day, participants completed procedures twice; at their typically scheduled time (Day_3) and six hours prior to or following their scheduled time (Day_3Offset).
For each test, participants wore dark, tight-fitting clothing. Retro-reflective markers (14 mm) were placed at six anatomical locations on participants’ right side (acromion process, greater trochanter, lateral epicondyle of the femur, lateral malleolus, calcaneus and fifth metatarsal). Marker placement was conducted by the same tester throughout for consistency. During all test bouts (Day_0 – Day_3), participants completed two 10 second trials of single-leg hopping at each of 1.5, 2.2 and 3.0 Hz in time with a digital metronome (TempoPerfect Metronome, NCH Software, Greenwood Village, CO, USA) while secured within an SBS. The design of the SBS was described previously. As previous research suggests little effect of leg dominance on kleg or kjoint, participants were instructed to land as close as possible to the force plate center (AMTI OR6-5; AMTI, Watertown, MA, USA) in time with the metronome, using their right leg and natural hopping technique. Trials were accepted for analysis if participants hopped within ± 5% of the target frequency. The order of hopping trials was randomly assigned, and participants received 60 seconds recovery between each trial to limit fatigue effects. An analogue triggering device was used to initiate 3D kinematic and kinetic data acquisition simultaneously. Kinematic data were recorded using three MAC Eagle cameras (MotionAnalysis Corporation, Santa Rosa CA, USA) operating at 200 Hz. Kinetic data were recorded at 1 kHz over the 10 second duration.

Data Processing
Analysis of data revealed an inconsistent delay in the initiation of kinematic and kinetic recordings. Fifth metatarsal vertical coordinate data were subsequently differentiated to jerk [36]. The time of peak jerk was determined, and kinematic data were subsequently aligned with the time at which the vertical force increased above 5 N.
Marker trajectories were digitized using Cortex motion analysis software (version 2.1; MotionAnalysis Corporation, Santa Rosa, CA, USA). 3D kinematic and kinetic data were subsequently exported and analyzed using customized MS Excel macros. Recorded kinematic and kinetic data were concurrently filtered using a fourth-order Butterworth Low-Pass digital filter with an optimal cut-off of 11 Hz determined via residual analysis. 3D coordinate data were subsequently interpolated to 1 kHz using a cubic spline. Filtered kinematic and kinetic data were used to calculate resultant joint moments occurring about ankle (Mankle), knee (Mknee) and hip (Mhip) joints throughout the ground contact phase of each trial using inverse dynamics. Segment inertia and mass characteristics were determined using the standards of Dempster (1955). Having calculated resultant moments, average torsional stiffness of the ankle (kankle), knee (kknee) and hip (khip) were calculated as a ratio of changes in joint moment (ΔMjoint) and angle (Δϴjoint) for respective lower limb joints.
In addition, kleg was recorded throughout the ground contact phase of all hopping trials using the spring-mass model. Thus kleg was calculated as the ratio of Fz max and maximum leg compression (ΔLegL) occurring during ground contact as measured from video records, where leg length represented the distance between the greater trochanter and the center-of-force. In all analyzed trials, the temporal occurrence of discrete events of Fz max and ΔLegL coincided to within 10% of the hop period.