Fig 1.
Components of the novel device.
The left figure shows the configuration of different parts of the novel device, as described above. The device can be anchored under the bottom of the steel frame of any commercially available weight machine (right bottom figure). The top of the strut has an eyebolt through which the weight selector pin passes and is set at the highest weight stack (right top figure). This locks down the weight stack, converting it into an isometric system and allowing measurement of the forces applied by the individual to lift the weight stack. The load cell registers the application of a tension force to the steel pin. The force is sensed by the digital-to-analog microcontroller attached opposite the load cell. Real-time force data, including the force-time graph from the microcontroller, can be wirelessly displayed and recorded by associated custom software.
Fig 2.
This is a schematic of the two-session testing protocol, with an indication of the datasets used for each analysis.
Fig 3.
Photo of the set-up for measuring isometric knee extensor strength on the dominant side using a commercially available HHD (Lafayette Instruments, US, model no. - 01165). The participant was positioned in high sitting at the end of a treatment table with legs hanging off the side at 90⁰ of knee flexion and posterior thigh in complete contact with the treatment table. The proximal thigh was fixed to the treatment table by a mobilization belt. To ensure the angle of the knee joint was at 90⁰ before each trial, a universal goniometer was attached using Velcro with the axis at the lateral femoral condyle, fixed arm directed towards the greater trochanter, and movable arm towards the lateral malleolus. The HHD units were set to Newtons (N) and measurement time to 5 seconds before placing the device on the participant’s tibia. The HHD is strapped to the tester’s dominant hand so that the tester can place the measurement side of the HHD on the previously marked point of the participant’s tibia. The plinth height was raised to allow the tester to stabilize their elbow at 90 degrees of flexion against their body when providing resistance perpendicular to the tibia against the participant’s knee extensor force.
Fig 4.
Photo of the set-up testing knee extensor strength with the ND (left top panel) and the accompanying generated force-time curve (left bottom panel; X-axis is time, and Y-axis is the force in newtons). Participants were seated in the knee extension weight stack machine (Life Fitness, Illinois, USA; right panel). The axis of the participant’s knee joint (tip of lateral femoral condyle) and the machine axis were aligned. Participants were positioned so that the seat fully supported their thighs, and pillows were used when necessary to ensure a trunk-neutral position. The strapping, belt fixation, and goniometer placement procedures were the same as the HHD protocol. Knee flexion was aligned to 90⁰, and participants were asked to hold the side handles of the chair similar to the HHD protocol to maintain the neutral trunk alignment. The tibial pad was located at the mark on the tibia. The ND was inserted into the weight stack, leveled, and adjusted so no slack was present in the cable system.
Fig 5.
Photo of the set-up for testing knee extensor strength using the IKD. The participants were seated in the IKD machine chair (Cybex, Humac Norm, US), with the lateral epicondyle of the tested side femur centered with the IKD’s input arm axis by adjusting the chair’s position. The hip knee adapter stabilizing pad was kept against the participants’ tibia at the tibial marking by adjusting the knee/hip adapter and stabilizing pad until the bar was at the correct height. The investigators strapped the torso, thigh, and tibial harness to provide stability during the test. The knee angle (90⁰ of flexion) was manually adjusted using the software and confirmed with a universal goniometer.
Table 1.
Demographics and clinical characteristics of all participants and separated by group.
Table 2.
Test-retest reliability of novel device (within-day, intra-rater) of all participants and separated by group.
Table 3.
Within-day and between-days interrater reliability of the hand-held and novel devices for all participants and separated by group.
Fig 6.
Representative time-force curves, averaged across three trials, for two participants (time on the X-axis and force on the Y-axis). The force-time curve mean and standard deviation (shadow) of the ND (red) are similar to those of the IKD (purple). However, the HHD force-time curve (green) differs from those of ND and IKD.
Table 4.
Statistical procedures and outcomes for concurrent validity testing all participants.
Fig 7.
These represent the mean difference, p-value, limits of agreement (LoA), and bias between each pair of devices. The upper row plots include all participants, the middle row the HVs, and the bottom row knee OA participants. Each graph represents the mean difference at the center (solid line), the upper and lower limits of agreement, (dashed lines), and the dots represent the between device difference in Fmax for each participant.