Five principal components were extracted from the 48 kinematic parameters of STS and classified into three subgroups (SG) according to their kinematic characteristics. It was suggested that SG2, with kinematic characteristics similar to the momentum transfer strategy shown in previous studies, performed better in PROMs, and in particular, may be significantly closer to obtaining the forgotten joint, which is considered the ultimate goal after TKA. This is the first study to objectively analyze the kinematic evaluation of STS during the 30s-CST, which had previously been performed subjectively by the therapist, using markerless motion capture, and to clarify the relationship with clinical outcomes in patients one year after TKA surgery.
Two challenges need to be overcome for STS. First, shifting the center of mass (COM) from a wide base of support (BOS) created by the hips, thighs, and feet to a narrow BOS of only the feet by anterior shank tilt after buttock release. Second, lifting the COM from sitting height to standing height by generating forward momentum, primarily through trunk flexion, and upward momentum, primarily through lower-extremity extension movements [25]. PC1 was considered a kinematic parameter associated with the exertion of upward propulsion through anterior rotation of the thigh; achievement of STS requires afferent contraction of the hip and knee joint muscle groups that produce vertical propulsive force to lift the body [26]. A higher PC1 score indicates a greater contribution of hip and knee joint function to the upward shift of the center of gravity.
PC2 may indicate the angular velocity of trunk flexion, which generates forward energy in STS. Sufficient muscle strength and coordination to generate upper-body motion before lifting the body is an important factor in achieving STS [26], and it has been reported that older adults and patients with knee OA have a greater trunk flexion angle and a tendency to project COM into the BOS during gluteal release [13, 27]. Higher PC2 scores were associated with a greater contribution of trunk flexion to the forward shift of the center of gravity.
PC3 is considered a kinematic parameter related to STS variability. It is considered to represent the redundancy of STS in repetitive measurements and can be interpreted as less variability in achieving STS with better kinematic efficiency; however, it can also be interpreted as less variability in the ROM of the hip and knee joints due to a narrower ROM of the joints in the movement. In a previous study, at one month post TKA, there was increased co-contraction between the quadriceps and hamstrings during STS [10], and decreased ROM of the knee and hip joints [28]. A higher PC3 score may indicate a greater range of joint motion during STS.
PC4 may indicate the ankle joint plantar-dorsiflexion angular velocity that contributes to the forward and upward energy generation in STS. In the STS motion, as in the hip joint, the shank forward tilt motion affects the forward movement of the COM [29], and the forward energy is absorbed by the shank forward tilt movement and is converted to upward energy by the ankle joint plantar flexion movement [30]. The more the trunk is flexed, the more energy that is absorbed and converted into upward energy. The lower the trunk flexion movement, the more the shank forward tilt movement is required [31]. The faster the STS speed, the greater the dorsiflexion moment of the ankle joint increases [32]. A higher PC4 score is associated with a greater contribution of ankle joint function to the upward and forward movement of the center of gravity.
PC5 is considered a kinematic parameter associated with the action of switching forward propulsion from hip and knee flexion motion to upward propulsion from trunk extension motion. STS, the scaling and timing of momentum generation, are important [26], and the timing of forward momentum generation and the magnitude of antigravity movement to lift the COM upward are important in terms of energy efficiency. A higher PC5 score would indicate a later timing of momentum generation contributing to the forward shift of the center of gravity through forward thigh rotation, and a greater contribution of trunk activity to upward momentum generation. The contribution of trunk activity was considered significant.
Various factors influence the determination of the STS motion strategy, but there is a trade-off between stability and force generation [15]. In healthy young adults, the momentum transfer (MT) strategy is selected as indicated by Hughes et al. [33–35]. The MT strategy is the most efficient method, although it requires instability, because it generates forward momentum through trunk flexion and raises the COM against gravity through coordinated extension of the lower extremities before the projected point of the COM enters the standing BOS. On the other hand, the exaggerated trunk flexion (ETF) strategy, which is characterized by large trunk flexion in the elderly and knee OA patients, projects the COM into the BOS at the time of buttock release and does not utilize the rotational moment due to gravity in the early phase of the movement [25, 27]; or the dominant vertical rise (DVR) strategy, which is characterized by the fact that trunk flexion ceases immediately after hip release, and knee and hip joints are extended to allow vertical dominance of COM movement [35]. Both strategies have been reported to focus on stability rather than motor efficiency [27, 36, 37].
For SG1, the fast angular velocities of trunk flexion and extension and slow angular velocities of hip and knee extension were the most characteristic. The elderly tend to flex their trunk more significantly before gluteal release, bringing the COM closer to the BOS and obtaining higher locomotion [38]. This strategy increases the hip flexion moment and decreases the knee flexion moment, thus decreasing the lower-extremity muscle strength required to lift the body upward [25]. The trunk flexion angle and knee joint extension muscle strength were inversely correlated during standing movements [39]. The trunk flexion angle and knee joint extension muscle strength were inversely correlated during standing movements. In addition, STS of patients one year after TKA surgery showed an increase in the angular velocity of knee extension compared to the preoperative level, but a decrease in the angular velocity of knee joint extension compared to healthy subjects [40], and increased hip flexion angle and extension moment on the operative side [8]. However, in comparison to healthy subjects, the postoperative exercise strategy of TTS is different from that of healthy subjects, even one year after the surgery. This exercise strategy has been interpreted as a compensation strategy to reduce the demand on knee extensor muscles, such as the quadriceps, but the characteristics of SG1 with high angular velocity of trunk movement and low angular velocity of lower limb movement were thought to be similar to the ETF strategy shown in previous studies. The lower movement velocity was due to the emphasis on stability rather than efficiency, suggesting a lower functional reserve [38]. In particular, the present study considered that the highest speed is required for STS task execution, which strongly reflects a reduced functional reserve and is most impaired by the inability of the SG to select the optimal strategy for the task.
For SG2, the maximum angular velocity of hip and knee flexion was reached early in STS, characterized by fast maximum plantar and dorsiflexion angular velocities of the ankle joints and slow maximum angular velocity of trunk extension. In healthy subjects, the upper body is exercised before the body is lifted upward, and a motor strategy is chosen to convert the high forward momentum due to trunk flexion into upward work of the center of gravity toward the standing position [41]. In SG2, as in the able-bodied subject, forward rotation of the thighs by hip and knee joint motion early in STS generates sufficient forward momentum for STS, and ankle joint motion controls COM to convert forward momentum into vertical momentum, thus not relying on trunk extension angular velocity, which may generate upward momentum without relying on trunk extension angular velocity. The fact that many of the items had PC scores in the middle compared to other SGs also suggests that this is a balanced strategy for trunk and limb locomotion in terms of speed and timing. This strategy is similar to the MT strategy shown in previous studies. In the MT strategy, the generation and conversion of momentum in the upper body and the body as a whole reduces the load on lower-extremity muscle strength and trunk movements, but body balance becomes unstable during the transition period when momentum is converted. This is because the COM is often located behind the trailing edge of the foot-only BOS immediately after the foot is released, and the section of the BOS transition after release is the most challenging for COM control owing to the backward rotational moment that occurs [25]. This is why STS at the maximum speed is the most difficult for COM control. Based on the above, the MT strategy, which emphasizes force generation rather than stability, is considered the optimal locomotor strategy for this research task, which requires STS tasks at maximum speed, and SG2, which is able to select the MT strategy, may indicate a high level of functional reserve capacity.
For SG3, the characteristics are generally in contrast to those of SG1, with less trunk and ankle motion, and higher hip and knee extension angular velocities. In STS, the trunk acts to generate and control momentum, but as the body ages, the maximum trunk angle becomes smaller. In addition, patients with knee OA are unable to fully transfer the forward momentum generated by trunk flexion to the lower extremities, resulting in inefficient movement strategies that utilize energy flow [36]. To compensate for the above, a movement strategy that relies on hip and knee joint muscle output was selected [27], which was considered to be similar to the DVR strategy in previous studies; at the maximum speed required for the 30s-CST. It is considered to be a more desirable movement strategy than the ETF strategy, but it should be noted that PC1 is very high and PC2 is very low compared to other SGs. In other words, the COG is lifted upward with the trunk relatively vertical, and the forward momentum cannot be converted to upward momentum, resulting in significantly larger knee joint maximal torque values in STS than in the other movement strategies [27]. Therefore, it is considered to be a strategy that places a greater load on the lower limb for the generation of upward momentum and is an exercise strategy that is inferior to the MT strategy in terms of kinetic efficiency of force generation.
Regardless of advances in surgical techniques, one in five patients feel unsatisfied after TKA [42, 43], and the factors necessary to achieve good satisfaction after TKA are unclear. Surgical techniques (implant design [44, 45], postoperative alignment [46], and surgical approach [47]) had no influence and were significantly correlated with postoperative knee function, and [48] postoperative motion function may influence clinical outcomes.
In this study, more items were significantly lower in SG1 than in SG2 with respect to PROMs; however, SG1 and SG3 did not show significant differences. SG1 chose the ETF strategy for STS in the 30s-CST compared to SG2, who chose the MT strategy. We hypothesized that SG1, who selected the ETF strategy for STS at the 30s-CST, showed decreased functional reserve capacity, which may have manifested itself as difficulties experienced by the patient in activities of daily living. In addition, SG2 and SG3 did not show significant differences in the 30s-CST scores, but SG2 scored significantly higher on the FJS-12 and KOOS ADL. The difference between SG2 and SG3 may be due to the fact that SG2 chose a strategy that placed a greater burden on the hip and knee joints.
The most important point of this study is that SG2 was significantly higher than the other SGs in FJS-12, which is an index for achieving the ultimate goal of the “forgotten joint” in post-TKA patients, which is living without joint awareness. The MT strategy is the preferred STS strategy because the ETF and DVR strategies require extra motion and joint torque [27]. SG2, who were able to choose the MT strategy for STS in the 30s-CST, may have had a better ability to choose the appropriate exercise strategy for the environment and tasks in activities of daily living other than STS, resulting in better subjective patient assessment performance. Thus, we hypothesized that STS kinematics of the 30s-CST may capture physical functions that cannot be captured by the 30s-CST score alone. Our results suggest that kinematic differences in STS may be expressed as differences in FJS-12, which is an important finding for rethinking physical therapy to improve knee function and satisfaction after TKA. We believe that the results of this study are useful because the responsibility of physical therapy is not only to enable or disable movement but also to increase freedom to adapt to any environment, prevent secondary disability, and return to society with reacquired efficient posture and movement strategies.
This study had some limitations. It should be noted that STS during the 30s-CST was cut out and evaluated in this study and does not reflect the STS performed in daily life activities. It has been reported that STS during the 5-chair stand test and STS performed in daily life have different motor strategies [49]. However, it has not been clarified how the exercise objective, an important factor in the selection of movement strategy, is optimized in daily life. The exercise objective is determined based on the weighting of various factors such as energy cost, safety, and pain avoidance, and the exercise strategy is selected accordingly [16]. The following is a list of some of the most important factors in exercise selection. Since the goal required for the 30s-CST is to achieve STS at maximal velocity, we believe that the results of this study are useful to assess whether the patient has the ability to choose a velocity-weighted exercise strategy. Therefore, it is important to interpret the results of this study as an evaluation of physical function in TKA patients rather than focusing solely on STS improvement. Furthermore, it is important to consider the fact that the study is based on sagittal plane movement strategies alone; therefore, it is only a limited representation of the STS movements employed by one-year postoperative TKA patients when performing the 30s-CST. Another issue is the low cumulative contribution ratio of 55.7% resulting from the principal component analysis, which means that the extracted principal components may not provide a good overview of the kinematic data. Although a higher cumulative contribution ratio is desirable because it more strongly reflects the kinematic data information used in the analysis, the criteria for the contribution ratio are not clearly defined. One way to address this is to select many principal components to increase the cumulative contribution ratio. However, we considered that this not only makes it difficult to capture features when a cluster analysis is performed but also runs the risk of biasing the classification due to overlearning. Furthermore, reducing the data on which principal component analysis was performed might also be considered. However, since no previous study has kinematically analyzed the STS of the 30s-CST by markerless motion capture, we considered it difficult to clearly determine which kinematic data were unnecessary. Although it should be noted that only approximately half of the kinematic parameters used were reflected in this study, we believe that the content extracted supports many previous studies using 3D motion analysis devices. This is an important finding that demonstrates the usefulness of markerless motion capture, which can be easily used in clinical practice. Finally, this study was only a kinematic assessment and did not identify the interventions that were effective. Although the results of this study showed that SG2, who selected a movement strategy similar to the MT strategy, performed better on clinical outcomes, this does not mean that it is preferable to aim for acquisition of the movement strategy of SG2 as normal movement, as movement strategies vary depending on the environment and task. Because movement is redundant, future research should clarify that, when considering treatment content, the underlying physical function of individuals who are able to select MT strategies should be considered in the STS of the 30s-CST.