The current study is arguably the first to perform a detailed 3D dynamic analysis of the lower extremities during EE and to compare the results to those during level walking. Although ET are capable of simulating the motion of human gait, differences in the lower-limb kinematics and kinetics were found between these two activities.
The closed kinetic chain motion of the locomotor system with the constrained pedal trajectory during EE seemed to be the main reason for the observed differences in the kinematics and kinetics compared to level walking. During level walking, the swing limb is free of constraints for limb advancement, whereas changes of the joint kinematics of the lower limbs were needed during EE to ensure that the swing foot followed the trajectory of the swing pedal. Even with the considerable range of motion of all the lower-limb joints, all the subjects successfully reached a consistent kinematic pattern of the joints that kept the COM excursion within a region that was much smaller than allowed. The 25-cm mediolateral center-to-center distance between the two pedals suggests that the COM could shift between the two feet by the same amount. However, the continuously abducted hip and knee and the adducted and internally rotated ankle of the stance limb helped to reduce the COM's mediolateral displacements to an average of 6.0 cm (Figure 4). Even though the ET has a vertical pedal displacement of 17 cm and a step length of 50 cm, the vertical and anteroposterior displacements of the COM were kept at about 5.1 and 1.7 cm, respectively. This was achieved by a greater flexion of the hip and knee, as well as by greater dorsiflexion at the ankle during the EE cycle. This more flexed posture of both the lower limbs seemed to be useful for the reduction of the COM displacements. The swing limb also contributed in reducing the vertical PRF by assisting in supporting the body weight during the exercise (Figure 2). This is also related to the reduction of the loading rate during EE. Although smaller vertical PRF helped reduce joint moments in the frontal and transverse planes, the specific kinematic changes increased the sagittal components of the lever arms available to the PRF with the lines of action of the PRF passing away from the joint centers (Figure 3), increasing the sagittal components of the joint moments ( Table 2 ).
Although a small loading rate at heelstrike during EE may reduce the risk of developing tibiofemoral joint osteoarthritis, the knee joint sustained a continuously greater extensor moment at a greater flexion angle during EE, with the peak moments approximately three times greater than those during walking. A greater knee extensor moment indicates increased loading for the quadriceps muscles and this loading would be further increased with increased knee flexion as the effective lever arm lengths decreased with increasing knee flexion. This increase may lead to early fatigue of the muscles, which could in turn limit the duration, and consequently the effects, of fitness training. Greater loading of the quadriceps at greater knee flexion during EE may be important with respect to the disadvantageous loads within the patellofemoral compartment. Previous theoretical and experimental studies have shown that patellofemoral joint contact force increased with increased quadriceps force and knee flexion angle.[1,30] It seems that the use of ET for athletic and rehabilitative training would have to consider users' knee joint function and muscle strength to avoid any unnecessary injuries.
Reducing the slope of the pedal ellipse may be helpful for the reduction of the knee joint extensor moment. In the present study the pedal moved along an oblique ellipse with the major axis tilted anteriorly. This downslope inclination may be associated with the observed greater posterior shear force and greater knee flexion (Figure 2, Figure 3 and Figure 4). The increased posterior shear force shifted the line of action of the PRF more posterior to the hip and knee joint centers with increased lever-arm lengths (Figure 3), contributing to the greater hip flexor and knee extensor moments ( Table 2 ). Similar phenomena were also reported in previous comparative studies on level and slope walking.[17,26] Lay et al. showed that flexion angles and extensor moments at the knee increased with increasing downward slopes. Because Lay et al. studied slopes steeper than 15°, whereas the slope of the pedal ellipse in the present study was about 6°, further study is needed for determining to what extent the reduction of the slope of the pedal ellipse can affect the joint angles and moments.
Improvement of the mobility of the pedal system of the ET may present a possibility of reducing hip and knee moments. Apart from the greater magnitudes, the observed motion patterns of the hip and knee during EE were similar to those during walking. However, the ankle had very different motion patterns, especially during late stance and swing phase (Figure 4). During walking, the heel, ankle, and forefoot serve as rockers to reduce the vertical excursion of the COM while in progression, resulting in alternate arcs of dorsiflexion and plantarflexion at the ankle. During the late stance of EE, although the forefoot rocker existed, the excessive flexion at the hip and knee caused the tibia to rotate more than the foot such that the ankle joint continued to dorsiflex instead of plantarflex as in level walking (Figure 4). These kinematic differences change the positions of the joint centers, lines of action of the PRF, and, thus, the moments at the lower-limb joints (Figure 3). A reduced ankle plantarflexor moment, less than 15% of the peak moment during walking ( Table 2 ), was accompanied by increased moments at the hip and knee during EE. If the ankle motion could be brought back to patterns similar to level walking, such as through the change of the mobility of the pedal system, the differences in lower-limb-joint moments between EE and walking may be reduced. Pierson-Carey et al. have shown that mobility of the ankle affects the magnitude and orientation of the resultant PRF during pedaling. Previous studies on cycling also suggest that increasing the mobility of the pedal system helps improve the efficiency of the knee and hip power transfer,[15,18] alleviating the loads transmitted through the knee joint, thus reducing overuse knee injuries.[5,20] Ruby and Hull have revealed that permitting relative motion of the pedal would eliminate constraint loads at the pedal and, thus, decrease intersegmental loads at the knee joint, whereas Boyd et al. report that the valgus knee moment was significantly reduced by a dual-rotation platform (both adduction/abduction and inversion/eversion) compared with the fixed or inversion/eversion-only design. Increasing the mobility of the pedal system increases the degrees of freedom of the closed kinetic chain of the EE and reduces the constraints to the feet. Because of the similarities between EE and standing cycling, it seems that appropriate mobility of the pedal system of the ET may be helpful for a reduction of the potentially harmful loading at the knee.
Med Sci Sports Exerc. 2007;39(9):1651-1658. © 2007 American College of Sports Medicine
Cite this: Joint Loading in the Lower Extremities During Elliptical Exercise - Medscape - Sep 01, 2007.