The day-to-day reliability of residual force enhancement during voluntary and electrically stimulated contractions.

Residual force enhancement (rFE) is characterized by increased steady-state isometric force following active muscle lengthening compared with a fixed-end isometric contraction at the same muscle length and level of neuromuscular activation. Many studies have characterized rFE in humans; however, the day-to-day reliability of rFE is unclear. We aimed to examine day-to-day reliability of rFE across various contraction types in the dorsiflexors in males and females. Twenty-five recreationally active young adults completed two visits, 1 week apart. Following determination of maximum voluntary contraction (MVC) strength, rFE was assessed during maximal voluntary effort, 20% MVC electrically stimulated, and 20% MVC torque-matching conditions. Each rFE condition was completed at two joint excursions: 0°-20° plantar flexion (PF) and 0°-40° PF. Intraclass correlation coefficients (ICC) assessed relative reliability and typical error of measurement (TEM), and the correlation variability of TEM (CV) assessed absolute reliability. Electrically stimulated contractions demonstrated the highest reliability at 40° PF (ICC: 0.9; CV: 22.8%) and 20° PF (ICC: 0.8; CV: 34.3%), followed by maximal voluntary contractions at 40° PF (ICC: 0.7; CV: 55.1%) and 20° PF (ICC: 0.1; CV: 81.1%). The torque-matching trials showed poor reliability for 20° and 40° PF (ICC: -0.1 to 0.3; CV: 118.1%-155.2%). Our results demonstrate higher reliability of rFE when stretching to the descending limb of the torque-angle relationship compared with the plateau region, and in electrically stimulated compared with voluntary contractions in the dorsiflexors for both males and females.