## 2 Materials and Methods and Results
### Deadweight Calibrations  
#### 2.2.1 General Calibration Procedures  
Digital measurement devices merely output voltage; thus, calibration procedures must be performed to determine the relationship between output voltage and the measure of interest. In this case, the relationship between load applied to a load cell and output voltage was determined using progressively heavier loads that capture at least the range of loads expected in testing (termed \"deadweight calibration\"). The added loads and output voltages were then used to create a linear calibration equation to convert the measured voltage to force (in newtons (N)) created by the applied load during normal use. Calibration equations are used under the critical assumption that the load-voltage relationship remains the same from the time of calibration to testing. Changes in the load-voltage relationship create error by decreasing the accuracy with which a previous calibration equation estimates load. Therefore, we used repeated deadweight calibrations under different conditions to detect the influence of these conditions on the load-voltage relationship. For all calibration procedures, we used 10 weight plates weighing approximately 20 kg each. The weight plates were numbered to ensure that each was loaded in the same order every time deadweight calibration was performed throughout the study. Prior to calibration of the load cell, we weighed each weight plate on a Bertec Jump force plate (Bertec Corporation, Columbus, OH, USA) to determine the exact mass of each weight plate; each weight plate was labeled for future consistent use and its exact mass recorded. This created our independent variable used throughout the calibration phases of external load. For each S-type load-cell calibration in every phase, we followed these procedures: bolt 1 of the aforementioned steel plates to the bottom and 1 to the top of each load cell (the steel plates used were labeled and used for all calibrations), then place on top of a 10 kg weight plate so that the bottom bolt sit through the hole in the 10 kg weight plate, thus creating a stable and level platform for the deadweight calibrations (Figure 3). Turn all components of the load cell and computer on and let run for the warm-up time specified in each phase of testing (this is described in detail within each phase). After the allotted warm-up time, place each of the 20 kg weight plates on top in turn in order of their number labels. After placement of each weight plate, wait for at least 20 s for the reading to stabilize (software outputted a 20 s running average of instantaneous volts as the average voltage reading, hereafter referred to as the \"voltage\"), then record the voltage at that load. Repeat  
Figure 2: (**A**) Whole instrumented pad mounted on sled. (**B**) We drilled a shallow recess into the wooden block of the pad to accommodate the bolt head. until all 10 weight plates are stacked on top (Figure 4). Unfiltered voltage values were obtained using a custom software program [16] written in Labview (National Instruments Corporation, Austin, TX, USA). The final block diagram that represents all the components of our instrumented scrum machine is presented in Figure 5.  
During deadweight calibrations, we used a Perception II weather station (Davis Instruments Corporation, Hayward, CA, USA) to measure temperature, humidity, and barometric pressure in the laboratory. In each phase of research, calibrations were performed at the same time of day to increase the probability of similar environmental conditions. All testing was completed under consistent ambient conditions (~21 \({}^{\circ}\)C, 55% humidity). Environmental conditions were found to have no influence on the relationship between load and voltage and so are not reported in the manuscript. Analysis of potential sources of error (e.g., measurement of load cell/ADC temperature, analysis of noise frequencies) was not conducted as a part of this study.