Validity and reliability of strain gauge measurement of volitional quadriceps force in patients with COPD

This study investigated the validity and reliability of fixed strain gauge measurements of isometric quadriceps force in patients with chronic obstructive pulmonary disease (COPD). A total cohort of 138 patients with COPD were assessed. To determine validity, maximal volitional quadriceps force was evaluated during isometric maximal voluntary contraction (MVC) manoeuvre via a fixed strain gauge dynamometer and compared to (a) potentiated non-volitional quadriceps force obtained via magnetic stimulation of the femoral nerve (twitch (Tw); n = 92) and (b) volitional computerized dynamometry (Biodex; n = 46) and analysed via correlation coefficients. Test–retest and absolute reliability were determined via calculations of intra-class correlation coefficients (ICCs), smallest real differences (SRDs) and standard errors of measurement (SEMs). For this, MVC recordings in each device were performed across two test sessions separated by a period of 7 days (n = 46). Strain gauge measures of MVC demonstrated very large correlation with Tw and Biodex results (r = 0.86 and 0.88, respectively, both p < 0.0001). ICC, SEM and SRD were numerically comparable between strain gauge and Biodex devices (ICC = 0.96 vs. 0.93; SEM = 8.50 vs. 10.54 N·m and SRD = 23.59 vs. 29.22 N·m, respectively). The results support that strain gauge measures of quadriceps force are valid and reliable in patients with COPD.