The current study suggested a drop-landing task that makes use of drop heights that may be appropriate for rehabilitation purposes and are functional in daily life. This study examined transient knee joint loads during the drop-landing response and the effect of drop height and instruction cues on knee joint loads. Further, this study took steps toward a development of standardized exercise task to examine the effect of acute bouts of mechanical loading on bone and cartilage metabolism in humans, since previous research has been focused on animal models [15, 16]. Consistent with previous literature [24, 30], we found that the knee joint compression force increased with drop height and the instruction to land stiffly. Though sagittal knee joint moment scaled with instruction at the high height, it did not change with instruction set at the low height, suggesting that while instruction can increase knee joint compression at the low height, knee joint moment and perhaps; therefore, joint injury risk are not increased with the instruction set at the low height. Precautions were taken to standardize drop-landing task; demonstrations of step-off and landing techniques were provided. The results suggest relatively low, approximately 7%, variability within subjects in the measure of peak sagittal knee angle across conditions (Table 2).
Knee compression force was modulated by instruction; Fig. 2a suggests that compression force can be reduced by approximately 50% if the soft instruction is used, or increased by approximately 30% if the stiff instruction is used, relative to the natural landing instruction. As compression force has been associated with promoting tissue remodeling , the exercise task we proposed could improve bone and joint health. Since increased joint moment is associated with ligament injury , the absence of increased knee moment suggests that individuals could perform drop-landing task from the low height as opposed to high height without incremental risk of joint injury.
The Effect of Drop Height on Lower Limb Joint Kinetics and Kinematics
The increase in the intensity of the drop-landing task at the greater drop-height resulted in elevated knee flexion angle during landing, vertical GRF, and knee joint compression force. The current findings are in agreement with previous research, which has demonstrated greater peak knee flexion angle and peak vertical GRF when landing from 60 cm compared with 20 cm [37, 38]. The aforementioned biomechanical measures all increase to attenuate impact forces during landing phase. While average frontal and transverse plane moment, which were consistent with previously reported values [39, 40], were found to be statistically different between heights with moderate and small effect size, respectively, the values were smaller than those experienced by young adults in sport setting (abduction moment 1.3 Nm/kg and external rotation moment 0.2 Nm/kg) [41, 42]; and likely not clinically relevant.
The Effect of Instruction on Lower Limb Joint Kinetics and Kinematics
The results revealed the scaling of vertical nGRF, knee joint flexion angle, and compression force with increased landing stiffness. Consistent with the current results, previous research has indicated that instruction to land softly, as opposed to stiffly, from 40 cm resulted in lower vertical nGRF and larger knee joint flexion angle . Additionally, the magnitude and timing of the peak knee compression force (Fig. 1a) were comparable to previously published data . Finally, the force profiles for instruction condition signals were qualitatively comparable between the low and high drop height (Fig. 1a), which was consistent with our expectations.
The Effect of Drop Height and Instruction on Lower Limb Joint Kinetics and Kinematics
The increase in drop height and landing stiffness resulted in increased knee joint compression force. The compression force increased from soft to natural to stiff instruction for both low and high drop heights and was greater for the high drop height (Fig. 2a). Since the increase in landing stiffness produced a linear scaling of the ground reaction force and the knee compression force, it is reasonable to suspect that a similar pattern between drop height and landing stiffness would be observed in the knee flexion moment, i.e., the increase in the flexion moment from soft to natural to stiff for the low and high heights. However, this was not the case. While the instruction to increase landing stiffness from the high height did result in the increase of the knee flexion moment, there was no difference in the moment observed with this instruction during landing from the low height.
The lower limb power analysis revealed that at the peak knee flexion moment, both the ankle and the knee showed negative power (Fig. 3a and b) and flexion joint moments (Fig. 2b; Table 2), indicating energy absorption and eccentric activation of the associated musculature, which is consistent with previous research . In contrast to previous reports , however, the current study revealed positive hip joint power (Fig. 3c) and extension moment (Table 2), indicating concentric activation rather than eccentric. The concentric activation about the hip likely served to rotate the trunk forward and bring the body’s center of mass closer to the knee joint center in order to reduce the flexion moment at the knee by decreasing the effective length of the moment arm. A shorter moment arm, given that the ground reaction force magnitude was unchanged, would produce a lesser flexion moment about the knee joint. Importantly, in the low height condition, all three joints showed no difference in power magnitude across all levels of instruction, which suggests an equal rate of energy transfer between levels of instruction observed for each of the three joints (Fig. 3). In contrast, at the high height, the rate of energy absorption (the ankle and knee; Fig. 3a and b) and energy generation (the hip; Fig. 3c) increased across instruction levels in all three joints. The difference in patterns between heights as seen across instruction levels in the knee flexion moment and power measures may be driven by the kinetic energy absorption demand. Kinetic energy absorption demand was larger during landing from the high height than from the low; hence, the instruction condition produced a more pronounced energy absorption response at the high drop height than at the low. Considering that the hip showed positive power and that there was a significant interaction effect between the drop height and instruction condition with positive hip power increasing across instruction levels at the high height, but not at the low, it appears as though neuromuscular control of the hip at the low height reduced the loading effect at the knee. However, despite the increase in the positive hip power at the high height, it appears, the capacity of the hip joint to attenuate or obviate the development of additional moment at the knee became relatively less pronounced, as the hip joint became unable to efficiently reduce knee joint flexion moment. The dissimilarity between the previously reported data and the current findings with respect to hip power is likely because previous studies often reported peak joint power, while the current paper reported joint power values measured at the peak knee flexion moment. The peak values of measures of interest may often be temporally misaligned with one another and the event of interest, which in this paper was defined as the peak knee flexion moment. Hence, in order to explain why the peak knee flexion moment did not change with instruction at the low height, when it has increased significantly at the high height, we investigated joint power measured specifically at the peak knee flexion moment.
Studies agree that the majority of soft tissue knee injuries are non-contact and occur during sudden deceleration and/or landing maneuvers . The literature suggests that loading through the quadriceps may be one of the mechanisms leading to knee joint ligament injury, as the quadriceps muscle activity producing sagittal moment has been shown to generate large shear force pulling tibia anteriorly on femur [20, 44]. In this study, knee compression forces as well as sagittal moments were larger in the high drop height. In the low height, however, while the knee compression force increased with instruction, the instruction did not influence the knee flexion moment. Since an increase in the knee flexion moment may increase the risk of joint injury, it appears that the knee joint compression force, and thus, perhaps tissue stimulation toward cartilage formation, during landing from a lower 22 cm height (consistent with a household stair) can be increased using an instruction to land more stiffly without suggestion of increase in the risk of knee joint injury as the knee flexion moment did not increase with landing stiffness.
The current study is limited by inclusion of male participants only. Future research should include female participants, as due to the anatomical differences, the kinetics and kinematics of the lower limb may differ between sexes. Further, the current study is limited by the use of an inverse dynamics approach. The authors recognize that the bone-on-bone force does not correspond directly to joint compression force calculated using inverse dynamics approach . The inverse dynamics approach does not consider muscle activation force, which adds to the compression force in the knee. Previous research has shown comparable knee loads during stair climb to the current findings during the low-soft condition, while the tibiofemoral bone-on-bone force was 3.5-fold larger when compared to joint reaction force . Nevertheless, the inverse dynamics approach is a feasible and simpler alternative to techniques used to quantify joint bone-on-bone loading . The future work should include muscle force modeling to improve joint compression force estimation. Lastly, a relatively large number of trials collected in this study may have resulted in participant fatigue despite that participants received rest/recovery breaks. Fatigue can affect measures obtained using the inverse dynamics approach resulting in the order effect. We mitigated the effect of fatigue by counterbalancing the first two blocks of trials across participants and randomly assigning the last four trial blocks. With the follow-up analysis, we showed no difference between the earlier and later trial means which suggests that fatigue, if present, did not affect the measures of interest.