Elsevier

Journal of Biomechanics

Volume 111, 9 October 2020, 110008
Journal of Biomechanics

Center of pressure predicts Intra-limb compensatory patterns that shift demands away from knee extensors during squatting

https://doi.org/10.1016/j.jbiomech.2020.110008Get rights and content

Abstract

The persistence of knee extensor moments deficits following anterior cruciate ligament reconstruction (ACLr) may be attributed to difficulty quantifying inter- and intra-limb compensations clinically. Force plate derived center of pressure (CoPpos) and vertical force (vGRF) may provide valuable information regarding limb and joint loading impairments in this group. This study aimed to determine the: 1) relationship between measures CoPpos and intra-limb extensor moment distribution during a squat, and 2) utility of using CoPpos and vGRF to estimate knee extensor moment deficits post-ACLr. Twenty-four individuals, 142 ± 22.5 days post-ACLr, performed bilateral squats. Ankle (aEXT), knee (kEXT) and hip (hEXT) extensor moments were calculated using three-dimensional kinematics and GRF. Moments, CoPpos and vGRF were identified at peak kEXT. Intra-limb moment distribution was characterized using hEXT/kEXT and aEXT/kEXT ratios. Linear regressions analyzed relationships between CoPpos and hEXT/kEXT and aEXT/kEXT. Stepwise regressions determined if between-limb CoPpos ratio predicted between-limb ratios of hEXT/kEXT and aEXT/kEXT. Stepwise regression determined if between-limb CoPpos and vGRF ratios predicted between-limb kEXT ratio. Results found that CoPpos predicts intra-limb moment distribution (hEXT/kEXT and aEXT/kEXT); more anterior CoPpos related to higher moments at the hip and ankle relative to the knee. In addition, between-limb CoPpos ratio predicts between-limb ratio of hEXT/kEXT and aEXT/kEXT ratios. Together between-limb CoPpos (52%) and vGRF (18%) ratios explained 70% of the variance in between-limb kEXT ratios (R2 = 0.70, p < 0.001). These data suggest that force plate derived CoPpos and vGRF may be important for detection of knee extensor moment deficits in individuals post-ACLr.

Introduction

Despite an emphasis on loading the surgical knee in rehabilitation following anterior cruciate ligament reconstruction (ACLr) (Adams et al., 2012, Baumgart et al., 2015) the presence of reduced knee extensor moments are commonly reported across tasks (Baumgart et al 2015.; Labanca et al., 2016, Paterno et al., 2007, Roos et al., 2014, Sigward et al., 2018). Persistence of underloading strategies greater than a year after surgery is concerning as asymmetrical loading during bilateral tasks is thought to contribute to increased risk of a second ACL injury (Paterno et al. 2007). Previous studies demonstrate that individuals utilize strategies that underload the surgical limb and knee extensors during submaximal tasks even in later rehabilitation. Individuals 13 months post-ACLr exhibited a 17% reduction in knee extensor moment in the surgical limb compared to the non-surgical limb during a bilateral squat (Roos et al., 2014). Asymmetry of this magnitude would not be expected at this point of recovery during a squat as individuals are progressed on to more challenging tasks, and are often returned to sports and more demanding activities (Adams et al., 2012, Bizzini et al., 2012, Myer et al., 2006, Waters, 2012).

Difficulty visualizing altered loading patterns may underlie their persistence. A recent study found that individuals 3 months post-ACLr who exhibited a 38% decrease in knee extensor moment in the surgical limb during a bilateral squat had on average a 2 degree differences in knee flexion and no difference in hip flexion angle (Sigward et al., 2018). Underloading of the knee extensors during the squat was accomplished with inter-limb and intra-limb loading compensations that were not reflected in obvious alterations in joint angles. An inter-limb shift in loading (ground reactions forces) to the non-surgical limb explained 62% of the variance in knee extensor deficits in the surgical limb at 3 months post-ACLr. While this compensation could ultimately be detected by measuring under foot forces, it would not capture compensations occurring within the surgical limb. This is important because an intra-limb compensation that shifted the demand away from the knee to the hip explained an additional 23% of the variance in knee extensor deficits at 3 months post-ACLr. Moreover, when these same participants were evaluated 2 months later they continued to exhibit a 30% deficit in knee extensor moment that was only explained by an intra-limb shift to the hip; again in absence of between limb differences in joint angles. The inability to detect altered knee joint loading during this fundamental rehabilitation exercise limits clinician’s ability to address it.

As an intra-limb shift in extensor demands is not accomplished through overt alterations in joint kinematics, it is conceivable that manipulation of the position of the center of pressure (CoPpos) may play a role in modulating joint moments. Center of pressure, the point at which the average location of the ground reaction forces (GRF) act on the foot, can dictate the effect of the vGRF on the joint moments (Gruben and Boehm, 2012). Previous studies support the relationship between medial–lateral and anterior-posterior CoPpos and knee joint moments in the fontal and sagittal planes, respectively (Ferrigno et al., 2016, Kernozek et al., 2018). Theoretically, an increased anterior CoPpos could shift the vGRF vector closer to the knee and further from the hip and ankle joint centers (Fig. 1). This is supported by a recent study that reported increased hamstring and gastrocnemius muscle activation with decreased quadriceps activation when healthy participants were asked to shift their center of pressure forward in static squat positons (Kitamura et al., 2019). However, it is not known if individuals following ACLr alter CoPpos in the surgical limb to shift the demands away from the knee extensors toward the hip extensors and ankle plantar flexors. If unilateral adjustments in CoPpos reflect an intra-limb compensation to reduce knee extensor demands in the surgical limb this information could be combined with ground reaction force magnitude to identify knee extensor moment deficits during squatting. Therefore, the purposes of this study were to determine: 1) the relationship between CoPpos and sagittal plane joint moment distribution across the hip, knee and ankle, 2) if between-limb difference in CoPpos were reflective of between-limb difference in moment distribution, and 3) if between-limb CoPpos and ground reaction forces difference could predict between-limb knee extensor moment differences in individuals 4–5 months post-ACLr. It is hypothesized that more anterior CoPpos during a squat will be related to smaller knee extensor moments relative to both hip extensor and ankle plantar flexor moments. When COPpos is considered relative to the non-surgical limb, it will be a strong predictor of between-limb differences in sagittal plane moment distribution. As a result of these relationships, between-limb COPpos ratio combine with between-limb GRF ratio will be a strong predictor to between-limb differences in knee extensor moments.

Section snippets

Participants

Twenty-four individuals (age: 28.9 ± 11.7 years; sex: 14 females, 10 males; height: 171.0 ± 10.8 cm; weight: 69.7 ± 11.8 kg) 4–5 months (142 ± 22.5 days) following ACLr participated in this study. Allograft (n = 9) and bone–patellar tendon–bone (n = 11) and hamstring (n = 4) autograft procedures were performed by four different surgeons. Participants were included in the study if they were between the ages of 14–55, status-post ACLr and currently participating in physical therapy and excluded

Results

Descriptive data are found in Table 1. CoPpos differed between surgical and non-surgical limbs (p = 0.015, ES = 0.37; Table 1). On average, the CoPpos in the surgical limb was 4% more anterior than the non-surgical limb. The hEXT/kEXT was greater in the surgical limb (p = 0.02, ES = 1.28; Table 1) and the aEXT/kEXT was greater in the surgical limb (p < 0.001, ES = 0.088; Table 1).

The linear regression analyses indicated that the CoPpos predicted hEXT/kEXT ratio in the surgical (R2 = 0.51,

Discussion

The results of this study indicate that anterior/posterior position of the CoP is related to sagittal plane joint moment distribution during a squat in individuals following ACLr. As theorized, a more anterior position of the CoPpos is related to a greater demand at the ankle and hip relative to the knee in the sagittal plane (Fig. 1). Center of pressure position is more predictive of the within limb hEXT/kEXT ratio, explaining up to 51% of the variance versus explaining only 38% variance for

Conclusions

During a bilateral squat task, center of pressure position is predictive of intra-limb sagittal plane moment distribution. A more anterior center of pressure position is related to relatively higher demands at the hip and ankle compared to the knee. This relationship is strong when considering intra-limb compensations between the knee and hip in individuals following ACLr. Between-limb differences in center of pressure location explain 62% of the variance in between-limb differences in hip to

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This research was supported in part by grant K12 HD0055929 from the National Center Medical Rehabilitation Research (NICHD) and the National Institute Neurological Disorders and 5R24HD065688-05 from the Eunice Kennedy Shriver National Institute of Child Health and Human Development, as part of the Medical Rehabilitation Research Infrastructure Network of the National Institutes of Health (NIH). Its contents are solely the responsibility of the authors and do not necessarily represent the

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