Validity of a portable hanging scale to assess the isometric muscle strength during knee movement

Inexpensive and practical equipment to assess muscle strength can disseminate objective measures, which can provide valid information to implement effective treatment and exercise training. The purpose of the study was to examine the instrumental validity of the hanging scale (HS) to assess the muscle strength during knee flexion and extension by comparing the peak force values to a standard equipment (laboratory-grade load cell [LLC]). Knee isometric strength was assessed on thirty-two subjects (16 women and 16 men, 22 [5] years, 171 [9] cm, 69 [15] kg, 23 [4] kg/m²). The knee flexion was tested by placing the volunteer's body in prone with the knee flexed at 90°. Knee extension was assessed with the volunteer seated on a chair with the feet resting on the floor, knees, and hips flexed at 90°. No differences were observed comparing the isometric peak forces between a laboratory-grade load cell and a HS (p > 0.05). The intraclass correlation coefficient (ICC)_1,1 returned values above 0.90. The Cronbach's α test also returned values above 0.90 for all comparisons. Bland-Altman results showed high levels of agreement with low risk of bias. The HS appears to be a valid method to assess the knee isometric strength. The portability, the cost-effectiveness and the simple user-friendly system provides an effective way to assess the knee isometric strength.