Validity and Reliability of the 3-Dimensional Printed PhysioCode Handheld Dynamometer for Knee Muscle Strength Assessment.

CONTEXT: Handheld dynamometry is common for isometric strength testing, but the high cost and import barriers of commercial devices (eg, MicroFET II) limit access. Three-dimensional (3D) printing enables low-cost, customizable instruments. We developed the 3D-printed PhysioCode and examined its measurement properties for knee strength.

DESIGN

Methods and Technical Report evaluating concurrent validity and test-retest reliability versus a reference handheld dynamometer.

METHODS

Forty-nine healthy adults (23.2 [3.9] y; body mass index 21.9 [2.8] kg/m2) performed 3 isometric trials of dominant-limb knee extension and flexion with both devices in randomized order, assessed by 2 trained novice raters under blinded analysis. Concurrent validity was evaluated using Pearson correlation coefficients (r) and root mean square error, and agreement between devices was assessed using Bland-Altman analyses. Reliability was assessed using intraclass correlation coefficients (ICC[2,1] and ICC[2,k]), along with the standard error of measurement and minimal detectable change at 95% confidence. We compared single and repeated test strategies.

RESULTS

All mean values differed by ≤21 N between devices across knee extensors and flexors. Concurrent validity was strong-very strong (r = .88-.95; P < .001), with lower root mean square error for averaged repetitions. Bland-Altman showed small, direction-specific bias: PhysioCode slightly overestimated extensor force values (-17 to 21 N) and underestimated flexor force values (-12 N). Reliability was excellent: PhysioCode ICC(2,1) = .93 to .95 and ICC(2,k) = .98 to .98; MicroFET II ICC(2,1) = .89 to .94 and ICC(2,k) = .96 to .98. Averaging 2 to 3 trials reduced standard error of measurement and minimal detectable change at 95% confidence relative to single trials or maxima.

CONCLUSIONS

The 3D-printed PhysioCode showed adequate concurrent validity and excellent test-retest reliability for knee extensor/flexor strength, with performance comparable to the MicroFET II in this sample of young, healthy adults. Averaging 2 or 3 repetitions improved precision. These findings suggest that PhysioCode may represent an option for strength assessment, warranting further evaluation in target clinical populations.