Changes in Biceps Femoris Long Head Fascicle Length after 10-d Bed Rest Assessed with Different Ultrasound Methods

PURPOSE: This study aimed to investigate the changes in fascicle length (Lf) of biceps femoris long head (BFlh) after 10 d of bed rest (BR) by comparing four different ultrasound (US) methods. METHODS: Ten healthy men participated in 10-d BR. Before (BR0) and after (BR10) the BR period, BFlh Lf values were obtained using 1) extended-field-of-view (EFOV) technique, 2) the manual linear extrapolation (MLE) method, and 3) two trigonometric equations (equations A and B) from a single US image. RESULTS: After BR10, decreased Lf values were observed by EFOV (P < 0.001; Hedges’ g = 0.29) and MLE (P = 0.0082; g = 0.22) methods, but not with equations A and B. Differences between equation A and the other US methods were detected at both time points. The percentage of changes in Lf between BR0 and BR10 was influenced by the US methods applied, with difference detected between the changes measured by EFOV and the ones estimated by equation A (P = 0.04; g = 0.53). Bland–Altman analyses revealed relevant average absolute biases in Lf between EFOV and other methods at both time points (range BR0–BR10: MLE, 0.3–0.37 cm (3.4%–4.32%); equation B, 0.3–0.48 cm (3.24%–5.41%); equation A, 2.44–2.97 cm (24.05%–29.2%)). A significant correlation (r = 0.83) in percentage of change in Lf values was observed only between EFOV and MLE. CONCLUSIONS: We showed that four distinct US methods lead to different results in the assessment of BFlh Lf changes after a short-term period of unloading. The implementation of EFOV technique (or alternatively MLE) to assess Lf changes in BFlh during longitudinal studies is warranted.