Comparison of Antagonist Muscle Activity During Walking Between Total Knee Replacement and Control Subjects Using Unnormalized Electromyography

Abstract

Background: Although satisfactory outcomes have been reported after total knee replacement (TKR), full recovery of muscle strength and physical function is rare. We developed a relative activation index (RAI) to compare leg muscle activity from unnormalized surface electromyography (sEMG) between TKR and control subjects.

Methods: Nineteen TKR and 19 control subjects underwent gait analysis and sEMG. RAIs were calculated by dividing the average sEMG for 2 consecutive subphases of stance defined by the direction of the external sagittal plane moment (flexion or extension).

Results: RAIs and external moments indicate TKR subjects have less initial stance antagonist rectus femoris activity (P = .004), greater middle stance antagonist biceps femoris activity (P < .001), and less late stance agonist biceps femoris activity (P < .001) than control subjects. Individuals with TKR demonstrate increased flexor muscle activation during weight bearing, potentially contributing to altered gait patterns found during the stance phase of gait.

Conclusion: The RAI helps detail whether decreased external moments correspond to less agonist or more antagonist muscle activity to determine true muscle activity differences between subject groups. Identifying the mechanisms underlying altered muscle function both before and after TKR is critical for developing rehabilitation strategies to address functional deficits and disability found in this patient population.

Keywords: gait analysis; muscle co-contraction; surface electromyography; total knee replacement.

Figure 1

Example relative activation index (RAI) calculation. A. Subphases of stance are first defined according to the sign of the external sagittal plane extension moment. B. The RAI between two consecutive subphases of stance is equal to the average sEMG activity during the first subphase divided by the average sEMG activity during the second subphase. Average sEMG activity during each subphase is shown as the dotted line. In this example RAIs equal 0.7 for subphase 1-subphase 2, 1.4 for subphase 2-subphase 3, and 2.6 for subphase 3-subphase 4. EMG data are shown as %RVC (relative voluntary contraction). Such data normalization is optional.

Figure 2

Moment balance about the knee during gait. The ground reaction force results in an external moment about the knee joint that must be balanced by the action of muscles. When the ground reaction force is posterior to the knee joint (left), it causes an external flexion moment which is balanced by a muscle extension moment created by the net action of the knee extensor muscles. The knee extensors are agonists because they help balance the external moment. When the ground reaction force is anterior to the knee joint (right), it causes an external extension moment which is balanced by a muscle flexion moment created by the net action of the knee flexor muscles. The knee flexors are agonists because they help balance the external moment.

Figure 3

Kinematics and kinetics measured with marker-based gait analysis during level walking for 19 TKR and 19 control subjects. Numbers represent locations of gait parameters compared between the TKR and control subjects. A. Flexion angle gait parameters included: 1) heel strike flexion angle, 2) mid stance maximum flexion angle, 3) terminal stance minimum flexion angle, and 4) maximum flexion angle during swing. B. External knee moment gait parameters included: 1) 1^st maximum extension moment, 2) 1^st maximum flexion moment, and 3) 2^nd maximum extension moment.

Figure 4

Percentage of TKR and control subjects with active muscles at each percentage of the gait cycle. Darkest color indicates instances during gait where all 19 of the subjects had muscles considered “on” and white indicates instances during gait where all 19 of the subjects had muscle considered “off”. Muscles were designated as “on” when their sEMG voltages were at least three standard deviations greater than the voltage with the highest frequency of occurrence. The locations of the darkest and lightest areas are different for the TKR and control subjects, indicating differences in the timings of muscle activation.

Figure 5

Relative activation index (RAI) between subphases of stance for 19 TKR and 19 control subjects. A RAI value of one (dashed horizontal lines) means that the muscle activity in the earlier subphase is equal to the muscle activity in the later subphase of stance. P-values correspond to comparisons between the TKR and control subjects (Mann-Whitney U-tests).