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Validity and Reliability of the 3-Dimensional Printed PhysioCode Handheld Dynamometer for Knee Muscle Strength
George Schayer Sabino1,2, Diego Henrique Antunes Nascimento3, Fernanda Oliveira Madaleno1
1Department of Physical Therapy, Graduate Program in Rehabilitation Sciences, School of Physical Education, Physical Therapy and Occupational Therapy, Universidade Federal de Minas Gerais (UFMG), Belo Horizonte, MG, Brazil.
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.
