Validity of two clinical knee strength assessments compared to the reference standard

Abstract

Objectives: Patellofemoral Pain (PFP) is the most common lower limb condition encountered in clinical practice. It recently emerged as the third highest ranked topic out of 185 in the Chartered Society of Physiotherapy Musculoskeletal Research Priority Project. For a valid assessment of knee strength during rehabilitation, the isometric test using the isokinetic dynamometer (Cybex) and the hand-held dynamometer (HHD) is not well received with clinical practice. An alternative way is through functional hop tests which clinicians have used to assess their patients' lower extremity muscular strength. This study investigates the validity of knee strength assessments conducted with the HHD and the hoptest compared to the isokinetic dynamometer and also explores differences between genders of the three assessments. Design: Each assessment included one submaximal and three recorded maximal efforts of the dominant leg. Quadriceps muscle strength was displayed in normalized torques. Correlation coefficients and box-and-whisker plots was used to analyze the data. Setting: Movement analysis laboratory Participants: Sixteen males (age 23.5 ± 4.23 years, height 1.79 ± 0.08 m and body mass 76.21 ± 10.58 kg, BMI 23.82 ± 2.38 kg/m²) and 16 females (age 25.38 ± 5.49 years, height 1.67 ± 0.08 m and body mass 71.99 ± 16.05 kg, BMI 25.83 ± 4.74 kg/m²) between 18 and 40 years old without any musculoskeletal injuries participated. Main outcome measures: Knee moments normalized for bodyweight (Nm/kg) for dynamometers and distance jumped for single legged hoptest. Results: The strongest significant correlation was found for the comparison between the HHD and Cybex (r=0.71, r²=0.504, p=0.001). Correlation between the HHD and hoptest (r=0.4, r²=0.19, p=0.013), and Cybex and hoptest (r=0.53, r²=0.295, p=0.001) were poor. Comparing genders, the normalized knee extension moment on the Cybex was 28.8% lower and with the HHD 22.3% lower for females. Conclusions: Single legged hop test appears not to provide a suitable alternative for strength measurement in a clinical setting. Differences in hop performances, especially the use of the arms, seem to be important. Gender differences exist in knee strength assessments. The use of EMG-Analysis in further research might identify differences in muscle recruiting during all three tests.