Variability in antagonist muscle activity and peak torque during isometric knee strength testing.

Background & Objective: Strength testing is common in the treatment of people with knee pathology and in research related to knee health. Variability in the magnitude of antagonist muscle activity and peak torque measurements during isometric knee strength testing is not well defined and has potential implications of strength test validity and reliability. The aim of this study was to determine the magnitude and variability (side-to-side, session-to-session) of antagonist muscle activity and peak torque during isometric knee strength testing and to compare and contrast the results of males and females.

Methods: Electromyograms and torque data were collected from 30 active young people (15 males, 15 females) during isometric strength testing of the knee extensors and flexors at two sessions that took place approximately one week apart. The magnitude of antagonist muscle activity and peak torque during isometric knee strength testing was calculated and the variability in these parameters assessed.

Results: Significant side-to-side differences were observed in the magnitude of antagonist muscle activity when the leg with higher antagonist activity was contrasted with the leg with lower antagonist activity (P < 0.001). Significant side-to-side differences were also observed when peak torque measurements were contrasted in a similar manner (P < 0.001). No significant differences were observed in peak torque and antagonist activity measurements between sessions. Significantly higher vastus medialis antagonist activity was observed in females (P < 0.001).

Conclusions: Our findings suggest that significant variability in antagonist muscle activity and peak torque is present during maximal isometric knee strength testing. This variability may reduce the accuracy of knee strength tests, especially when side-to-side comparisons are made as is typical in clinical settings. The results of this study may be helpful when interpreting strength test results and setting criteria for patient progression.