Reliability and validity of trunk flexor and trunk extensor strength measurements using handheld dynamometry in a healthy athletic population

Objectives: To investigate reliability and validity of handheld dynamometry (HHD) strength tests to accurately measure isometric trunk flexion and extension strength.

Design: Test-retest design and comparative analysis to investigate reliability and validity respectively.

Setting: Isometric trunk flexion and extension strength, measured with a handheld dynamometer, was compared to isometric strength testing on a stationary isokinetic dynamometer. Different test positions were examined for the HHD measurements for trunk flexion and extension strength.

Main outcome measures: Trunk muscle strength was calculated in newton metres (Nm) for both devices. Results of both devices were compared with Pearson correlations and agreement between both methods was explored with Bland-Altman plots. ICC values were calculated to assess HHD intra- and inter-tester reliability.

Results: Reliability was good to excellent for all HHD tests (ICC = 0.67-0.93). High correlations were found between the HHD tests and stationary dynamometer tests (r = 0.65-0.86). Bland-Altman plots showed agreement between both methods for the trunk flexion test, supported in 30° trunk flexion and for the trunk extension test in a prone position in 0° trunk flexion.

Conclusion: Handheld dynamometry is reliable and a clinically applicable valid method to evaluate isometric trunk flexion and extension strength.