Objective: To investigate the reliability of knee extensor strength measurements using a supine hand-held dynamometer (HHD) anchoring frame in patients with limited physical activity. Although an HHD is suitable for bedside use, its inter-rater reliability is low because measurements can be influenced by tester strength. Methods: Maximal knee extensor isometric strength was measured using an HHD anchored to the supine frame. Three trials of three maximal contractions were assessed by two raters. Results: A total of 33 inpatients who were non-ambulatory due to acute illness participated in the study. The intraclass correlation coefficients were 0.974 (inter-rater) and 0.959 (intra-rater). The minimal detectable changes in intra- and inter-observer measurements were 29.46 N (24.10%) and 36.73 N (29.26%), respectively. The limits of agreement ranged from -19.79% to 24.81% for intra-rater agreement and from -21.45% to 37.07% for inter-rater agreement. Conclusion The portable dynamometer anchoring system can measure the isometric strength of the knee extensor reliably in the supine position, and could be used for measurements in patients who have difficulty visiting the laboratory and maintaining a seated posture.