Error associated with antagonist muscle activity in isometric knee strength testing

Abstract

The purpose of this study was to determine the measurement error associated with antagonist muscle activity in isometric knee strength testing at 60 degrees of knee flexion in both sexes. Muscle specific EMG-contraction intensity relationships were obtained from 22 young people by having them match moment targets ranging from 10% to 100% peak moment. The moments attributed to each of the quadriceps and hamstrings muscles were partitioned using a practical mathematical model. Subject specific EMG-moment relationships were defined for each muscle using second-order polynomial equations. These equations were subsequently used to predict the countermoment associated with antagonist muscle activity. Error during strength testing was calculated by expressing net antagonist moments as a percentage of net agonist moments. The net antagonist moments associated with quadriceps and hamstrings muscle activity were 11.0% and 8.7% of the peak moment values recorded when the same muscle groups were acting as agonists. The error associated with antagonist activity was significantly higher in knee flexion (20.1%) than in knee extension (4.5%). Females displayed significantly higher error in knee flexor testing (P < 0.001). Limb symmetry indices did not change significantly when the countermoments generated by the antagonist muscles were accounted for (P > 0.05). The results of this study indicate that the error associated with antagonist activity in knee extensor testing is relatively small, whereas the error in knee flexor testing is larger. This is due to the quadriceps being much stronger than the hamstrings muscles while displaying similar levels of antagonist activity.

Conflict of interest statement

Conflict of interest statement The authors declare that they have no conflict of interest.

Figures

Fig. 1

A subject positioned on the test system for an isometric knee strength test (a) and EMG–contraction intensity testing (b)

Fig. 2

EMG–moment curves for the five knee muscles tested. The values on the x-axis represent EMG activity normalized as a percentage of maximum. The values on the y-axis represent predicted agonist moment (i.e., the partitioned moment for each muscle based on Eq. 1) normalized as a percentage of the peak agonist net moment measured during strength testing. Note that the maximum values on the y-axes of the quadriceps muscles (VL, RF, & VM) approximate relative volume data used in Eq. 1. This is because the individual quadriceps muscles have the same relative activation and moment arms during maximal contractions. Males and females had similar EMG–moment curves

Fig. 3

Box plots representing the coefficients of determination (R2) obtained from the subject-specific EMG–moment curves, which were fitted using second-order polynomial equations. The boundary of the box closest to zero indicates the 25th percentile, the black line within the box marks the median, the dashed line within the box marks the mean, and the boundary of the box farthest from zero indicates the 75th percentile. Whiskers (error bars) above and below the box indicate the 90th and 10th percentiles, respectively. Data points outside the whiskers represent values that were outside the 95th and 5th percentile values. VL vastus lateralis, RF rectus femoris, VM vastus medialis, ST semitendinosus, BFL long head of the biceps femoris

Fig. 4

Stacked bars representing the mean antagonist moment associated with the quadriceps (a) and hamstrings (b) muscle antagonist activity of males and females during isometric knee strength testing. The net antagonist moment was not significantly different between the sexes. VL vastus lateralis, RF rectus femoris, VM vastus medialis, ST semitendinosus, BFL long head of the biceps femoris

Fig. 5

Females had significantly higher quadriceps moments when mean antagonist moments were expressed as a percentage of same muscle groups’ agonist moment (a). Error during knee flexor strength testing was significantly higher in females when compared to males, whereas error during knee extensor strength testing was similar (b)