EFFECT OF HEEL LIFTS ON PATELLOFEMORAL JOINT STRESS DURING RUNNING

Abstract

Background

Patellofemoral pain is a debilitating injury for many recreational runners. Excessive patellofemoral joint stress may be the underlying source of pain and interventions often focus on ways to reduce patellofemoral joint stress.

Purpose

Heel lifts have been used as an intervention within Achilles tendon rehabilitation programs and to address leg length discrepancies. The purpose of this study was to examine the effect of running with heel lifts on patellofemoral joint stress, patellofemoral stress impulse, quadriceps force, step length, cadence, and other related kinematic and spatiotemporal variables.

Study Design

A repeated-measures research design

Methods

Sixteen healthy female runners completed five running trials in a controlled laboratory setting with and without 11mm heel lifts inserted in a standard running shoe. Kinetic and kinematic data were used in combination with a static optimization technique to estimate individual muscle forces. These data were inserted into a patellofemoral joint model which was used to estimate patellofemoral joint stress and other variables during running.

Results

When running with heel lifts, peak patellofemoral joint stress and patellofemoral stress impulse were reduced by a 4.2% (p=0.049) and 9.3% (p=0.002). Initial center of pressure was shifted anteriorly 9.1% when running with heel lifts (p<0.001) despite all runners utilizing a heel strike pattern. Dorsiflexion at initial contact was reduced 28% (p=0.016) when heel lifts were donned. No differences in step length and cadence (p>0.05) were shown between conditions.

Conclusions

Heel lift use resulted in decreased patellofemoral joint stress and impulse without associated changes in step length or frequency, or other variables shown to influence patellofemoral joint stress. The center of pressure at initial contact was also more anterior using heel lifts. The use of heel lifts may have therapeutic benefits for runners with patellofemoral pain if the primary goal is to reduce patellofemoral joint stress.

Level of Evidence

3b

INTRODUCTION

Running is a common form of aerobic exercise. However, runners are at risk for developing a variety of lower extremity overuse injuries such as patellofemoral pain (PFP). Patellofemoral joint pain is frequently described as anterior knee pain with an insidious onset, usually exacerbated during exercise involving running or deep knee flexion, often becoming persistent if not appropriately addressed.^1,2 Patellofemoral pain is frequently seen in outpatient orthopedic clinics, with 17% of knee-related and 7.3% all orthopedic visits resulting in the diagnosis of patellofemoral pain disorder. ^3,4 In addition, female runners are at greater risk, with incidence rates being two to three times higher than males.¹

Patellofemoral pain etiology is believed to be multi-factorial where poor patellar alignment, patellar maltracking due to muscular imbalance, decreased vastus medialis oblique muscle mass, weak hip musculature, training errors, and a rearfoot strike pattern have been implicated in development of this condition.^1,5-7 The underlying effects of such factors may result in aberrant patellofemoral joint stresses (PFJS) which are thought to damage the patella and femoral subchondral tissues.^2,8-10

Musculoskeletal modeling studies have investigated PFJS during various movements to characterize this loading.^11-14 Patellofemoral (PF) joint stress is determined by dividing the PF joint reaction force by the PF contact area. Knee flexion angle influences PF joint contact area^2,15 while the interaction of quadriceps force and knee flexion influence the magnitude of PF joint reaction force.^16,17

Conservative intervention strategies of PFP often involve attempts to alter running mechanics.^12,13,16 Decreasing step length by 10% results in a 15-20% decrease in patellofemoral joint (PFJ) kinetics,¹⁶ while increasing the runner's cadence by 10% was successful in reducing PFJ forces by 14%.¹² Teng and Powers reported that a 7 degree increase in trunk flexion led to significant decreases in knee extensor energy absorption and generation.¹³ Another strategy utilized to alter PFJS is adopting a forefoot strike (FFS) pattern during running. A forefoot strike pattern occurs when a runner contacts the ground with the anterior third of their shoe compared to a rearfoot striker (RFS) who contacts the ground with the posterior third.¹⁸ Biomechanical variations that occur between the foot strike patterns may account for the differences in PFJS, as FFS runners initially land with a plantarflexed ankle, flexed knee, and lower vertical loading rates.^19-21 Overall, mild-moderate repetitive stress injuries (ie patellofemoral pain syndrome [PFPS], medial tibial stress reaction, IT band syndrome) were 2.5 times higher in RFS runners²² and multiple authors have reported that runners who adopt a FFS pattern experience significant reduction (13-27%) in peak PFJS.^12,14,19 One recent case series examined the efficacy of converting runners from a RFS to FFS pattern.²³ These patients were able to decrease their PFP and maintain changes in foot strike pattern during follow up session.²³ However, adopting a FFS running technique may be difficult, requiring extensive time and specialized equipment.

The use of heel lifts has traditionally been used in Achilles tendon rehabilitation or to correct leg length discrepancies.^24-26 Insertion of heel lifts into a standard running shoe is both convenient and affordable, however their effects at more proximal joints have yet to be analyzed. Of particular interest is the PJF, which has been shown to be influenced by foot position during running.^19-21

The purpose of this study was to examine the effect of running with heel lifts on patellofemoral joint stress, patellofemoral stress impulse, quadriceps force, step length, cadence, and other related kinematic and spatiotemporal variables. Secondary analyses aimed to discern if kinematic or spatiotemporal differences occurred between running in heel lift conditions, most notably: peak knee flexion angle, peak dorsiflexion angle, peak plantarflexion angle, dorsiflexion at initial contact, knee angle at initial contact, step length, stride cadence, and initial center of pressure. The working hypothesis was that heel lifts would decrease peak PFJS, quadriceps force, PF stress impulse, and dorsiflexion at initial contact, as well as increase initial knee flexion angle and shift the initial center of pressure anteriorly but would have no effect on peak plantarflexion angle, step length, or stride cadence.

METHODS

Subjects

Sixteen healthy active females (Age: 21.7 ± 1.6 yrs; height: 169.8 ± 5.8 cm; mass: 61.3 ± 9.6kg) were recruited from October 2015 to December 2015. Sample size was based off previous repeated measure studies that focused on PF kinetics and kinematics.^12-14,19 All participants included were rearfoot strikers, as determined by center of pressure at initial contact occurring in the rear most third of the foot during testing.¹⁸ Additional inclusion criteria included five years of recreational running experience, self-reported running routine of >10 miles per week, and a score of at least level five on the Tegner Activity Level Scale (a questionnaire of regular participation with recreational sports which require running). Exclusion criteria consisted of current lower extremity injury or pain, or surgery within the last year. Informed consent to the testing protocol was obtained from all subjects prior to participation; all methods were approved by the University of Wisconsin-La Crosse Institutional Review Board.

Protocol

Prior to the running trials, participants were fitted with the same model of footwear (Model 625SB, New Balance, Boston, MA), reflective markers, and tight-fitting clothing. All participants completed a five-minute treadmill warm-up walking at a self-selected speed. After several practice trials, participants ran five successful trials down a 20-m runway under two randomized conditions: 1) No heel lift and 2) With 11mm heel lift. A trial was deemed successful when the participant demonstrated a rearfoot strike pattern, proper speed, and was observed to not target the force plate. A predetermined speed of 3.46 m/s ± 2.5% was selected and monitored via two photoelectric timers placed 2.3 m apart.

Instrumentation

Forty-seven reflective markers were placed at predetermined anatomical landmarks using a modified Helen Hayes type marker set for three-dimensional (3D) data collection.¹⁴ A static, neutral standing calibration was collected prior to both conditions. The calcaneal reflective markers were raised 11mm during heel lift conditions, to correspond with the associated raising of the heel within the shoe. Kinematic data were recorded at 180 Hz with 15 motion analysis cameras (Motion Analysis Corporation, Santa Rosa, CA, USA) surrounding the runway. Kinetic data were simultaneously collected using a force platform flush with the runway, synchronized with the cameras, and sampled at 1800 Hz (Model 4080, Bertec Corporation, Columbus, OH, USA).

Data Processing

Using the Human Body Model (Motek ForceLink, Amsterdam, the Netherlands), muscle forces were calculated using a musculoskeletal model with 16 rigid segments, 44 degrees of freedom (DOF), and 300 muscle tendon units.²⁷ Estimates of muscle force were based on joint moments by minimizing a static cost function at each time step where the sum of the squared muscle activation was related to the maximum muscle strengths.^28,29 Muscle forces of the rectus femoris, vastus medialis, vastus lateralis, and vastus intermedius were summed to determine total quadriceps force. To determine PFJ reaction force, the following equation derived by Brechter and Powers⁸ was used to calculate the k constant:

$$\begin{array}{l}k\left(x\right)=\left(4.62e^{-01}+1.47e^{-03}x-3.84e^{-05}{x}^2\right)/\\ \hspace{1em}\hspace{1em}\hspace{1em}\left(1-1.62e^{-02}x+1.55e^{-04}-6.98e^{-02}{x}^3\right)\end{array}$$

where x is the knee joint angle in the sagittal plane. The constant k signifies the amount of the quadriceps force that is directly imposed on the PF joint from the knee joint angle and quadriceps muscle orientation as described by van Eijden et al.³⁰ Therefore,

$$\begin{array}{c}\text{PF}\hspace{0.17em}\text{joint}\hspace{0.17em}\text{reaction}\hspace{0.17em}\text{force}\left(x\right)=\\ k\left(x\right)\times \text{quadriceps}\hspace{0.17em}\text{force}\left(x\right)\end{array}$$

Data from Connolly et al¹⁵ were used to determine PF joint contact area as a function of knee angle:

$$\text{PF}\hspace{0.17em}\text{contact}\hspace{0.17em}\text{area}\left(x\right)=0.0781x^2+0.6763x+151.75$$

PF joint stress was calculated by dividing PF joint reaction force by the contact area:

$$\begin{array}{l}\text{PF}\hspace{0.17em}\text{joint}\hspace{0.17em}\text{stress}\left(x\right)=\text{PF}\hspace{0.17em}\text{joint}\hspace{0.17em}\text{reaction}\hspace{0.17em}\text{force}\left(x\right)/\\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\text{PF}\hspace{0.17em}\text{contact}\hspace{0.17em}\text{area}\left(x\right)\end{array}$$

Average of each of the kinematic and kinetic parameters were examined during from five performance trials for each condition during the stance phase of the running cycle. This was determined when using a 10 N threshold from force platform measurements.

Statistical Analysis

Multivariate analysis of variance (MANOVA) statistics with repeated measures were performed to compare within subject differences between heel lift and no lift conditions. All statistics were calculated using SPSS Version 23 (IBM, Armonk, NY) with an a-priori significance of p<0.05. Cohen's d was calculated to determine effect size for each dependent measure.

RESULTS

All subjects landed with a RFS pattern with an initial center of pressure location of 31.21% ± 11.10% of foot length during heel lift trials and 22.13% ± 8.55% with the no heel lift inserted, which indicated a 9.1% anterior shift when running with heel lifts (p<0.001). During heel lift trials, a 4.2% (p=0.049) and 9.3% (p=0.002) decrease was observed in peak PFJS and PF stress impulse, respectively (Table 1). Figures 1 and 2 depict these reductions in both PFJS and PF stress impulse throughout the stance phase during heel lift trials. Peak quadriceps force decreased 3.8% when heel lifts were inserted, however this was not significant (p=0.053). When donning the heel lift, peak ankle dorsiflexion was reduced 20% (p=0.006), while ankle dorsiflexion at initial contact was reduced 28% (p=0.016). No differences were found for key spatiotemporal data such as step length and cadence and other kinematic variables, such as peak ankle plantarflexion, peak knee flexion and knee flexion at initial contact (p>0.05).

Table 1.

Means and standard deviations (SD) for peak patellofemoral joint stress (PFJS), patellofemoral (PF) joint stress time integral, peak quadriceps force, initial contact ankle dorsiflexion, peak ankle dorsiflexion, initial contact knee angle, peak knee flexion, initial contact center of pressure location, step length and cadence when running with and without a heel lift.

	Run No Heel Lift	Run+Heel Lift	Mean Difference	p-value	Effect size (Cohen's d)
	Mean (SD)	Mean (SD)
Peak PFJS (MPa)	12.04 (1.92)	11.56 (2.01)	0.48	0.049*	0.24
PF Stress Time Integral (MPa*s)	21.78 (3.33)	19.93 (3.56)	1.85	0.002*	0.54
Peak Quadriceps Force (N/kg)	4969.28 (775.54)	4788.57 (860.26)	180.71	0.053	0.22
Ankle Dorsiflexion at Initial Contact (degrees)	3.13 (5.56)	1.37 (5.18)	1.76	0.016*	0.33
Peak Ankle Dorsiflexion (degrees)	8.95 (6.34)	5.74 (5.01)	3.21	0.006*	0.56
Peak Ankle Plantarflexion (degrees)	13.00 (11.76)	13.30 (9.70)	−0.3	0.933	0.03
Knee Angle at Initial Contact (degrees)	13.08 (5.08)	13.08 (5.23)	0	0.999	0.00
Peak Knee Flexion (degrees)	48.57 (3.18)	48.46 (4.20)	0.11	0.852	0.03
Initial Contact Center of Pressure location as %total foot length	22.13 (8.55)	31.21 (11.10)	−9.08	<0.001*	0.91
Step Length (meters)	0.75(0.044)	0.76 (0.051)	−0.01	0.52	0.21
Cadence (strides/minute)	80.16 (4.54)	79.74 (5.13)	0.42	0.56	0.09

^*p<0.005

Figure 1.

Ensemble average Patellofemoral joint stress (MPa) during running with heel lift (Run+Lift) and running without heel lift (Run No Lift).

Figure 2.

Ensemble average center of pressure during running with heel lift (Run+Lift) and running without heel lift (Run No Lift).

DISCUSSION

The primary purpose was to compare the effects of heel lifts on peak PFJS, PF stress impulse, and quadriceps force during running. The study findings partially support the hypotheses that heel lift use would decrease peak PFJS and PF stress impulse, however no difference in peak quadriceps force was found between conditions. Secondary analyses examining kinematics and spatiotemporal variables also partially supported our hypothesis, as indicated by significant differences in dorsiflexion and center of pressure but not knee flexion at initial contact, step length, or stride cadence.

This may be the first study examining how the use of heel lifts influence PFJS in running. The justification for raising the heel of RFS runners is to alter the kinematics and kinetics to more closely resemble a FFS runner, a running pattern that has been shown to decrease PFJS.^12,14,19 Overall, the results of heel lift insert trials tended to trend towards FFS kinetics and kinematics, as demonstrated by an anterior shift in initial center of pressure, and decreased PFJS, PF stress impulse, and dorsiflexion at initial contact. High PF joint stress and strain have been associated with PFPS^2,10 therefore strategies to reduce PFJS appear warranted. Previous studies have reported FFS runners experience a 13-27% reduction in peak PFJS.^12,14,19 Running with 11 mm heel lift had a 4.2% decrease in peak PFJS (p=0.049) and small effect size (ES=0.24). This study found a 9.3% decrease (p=0.002, ES=0.54) in PF stress impulse, which factors in both the magnitude and duration the joint is under stress during each step. With an average step length of 0.76 m, each of the participants would have taken approximately 1322 steps over one kilometer. When the per step reductions are summed over a one kilometer distance, the cumulative effect is a 2445 Mpa*s reduction in total PF stress impulse. Similarly, peak quadriceps force trended downward (p=0.053) during heel lift insert trials. This may be related to the center of pressure being more anterior on the foot thus bringing the resultant ground reaction force vector closer to the knee.

Kinematic comparisons of running with heel lift and a FFS pattern revealed some similarities. Similar to FFS runners, participants running with heel lift inserts contacted the ground with a relative increase in plantarflexion.^19,20 The relative 1.8 degree increase in initial plantarflexion during lift conditions is considerably less than the 16.1 degree weighted mean increase reported from the meta-analysis of three studies by Almeida et al. which compared FFS to RFS runners.²⁰ This meta-analysis also depicted a 3.1% increase in knee flexion angle at initial contact in FFS runners.²⁰ This contrasts to our study which depicted no differences between the no lift and lift conditions. Despite the center of pressure being more anterior (31.21% ± 11.10% with lift vs. 22.13% ± 8.55% without lift), subjects were still utilizing a RFS. This may explain similarities in knee flexion angle at initial contact to those previously reported of RFS runners.^14,20,21 As hypothesized, step length and cadence were similar between lift conditions, which suggests they did not play a significant role in altering PFJS. This is important to note, as previous studies have found altering these variables can reduce PFJS.^12,16 This further strengthens the case that the anterior shift in center of pressure is the primary driving force for decreased PFJS and PF stress impulse observed in our study.

Running with a FFS pattern may not be without its drawbacks, placing runners at an increased risk for injuries involving the Achilles tendon, metatarsals, or other plantar structures.^31,32 Because of this, it is widely accepted that when transitioning from one foot strike pattern to another, a gradual transition is indicated.^14,22,32 Implementation of heel lifts may help to serve as an intermediate way to decrease PFJS since it appears to move the center of pressure more anteriorly thereby reducing both PFJS and PF stress impulse during stance phase of running. Perhaps even the use of differing amounts of lift may serve to promote such a transition.

Both the practical and cost-effective nature of this intervention may be of benefit. One pair of heel lifts are relatively inexpensive, typically less than $10/pair. This is in stark contrast to studies that have implemented programs aimed at altering foot strike patterns. These studies have required specialized equipment in addition to large amounts of time and effort from both the participants and clinical researchers.^23,33 Furthermore, the heel lifts are simple to utilize in a variety of shoe sizes and styles. Lastly, participants in this study were not given specific instructions to run in a certain manner yet still experienced decreased PFJS and PF stress impulse. One may be able to surmise from this investigation that runners using this intervention will not need any certain coaching or feedback upon implementation as none was provided in this study.

Future studies may build from these works in several ways. For starters, monitoring trunk flexion and hip extensor force generation with and without heel lifts may help to determine what influence hip musculature may have at altering PFJS when heel lifts are donned. Analyzing the relationship between heel lift size and/or running speed would also be warranted, as only one lift size and running speed were examined in the current study. Lastly, recruiting runners actively suffering from PFPS may be warranted in determining the efficacy of using heel lifts in reducing patient symptoms.

Limitations

Results of this study must be analyzed within the context of certain limitations. First, the PF joint model was generalized from previous research and failed to take into account each subjects individual anatomical and musculoskeletal differences. This model was two dimensional in nature and did not account for loading due to joint motion or loading from other planes. While an improved way to estimate muscle forces, static optimization is still an estimation of muscle force production during dynamic activities. This study contained a relatively small number of subjects and therefore may not be generalizable to a larger sample of females, male subjects or clinical population of runners seeking treatment for patellofemoral pain. Finally, force plate targeting was determined via qualitative visual analysis and may have been susceptible to error. However, the use of a repeated measures design does minimize the influence of many of these study limitations.

CONCLUSION

These results demonstrate that running with heel lifts reduce peak PFJS and PF stress impulse with a trend for decreased peak quadriceps force. Additionally, runners impacted the ground with the ankle in a less dorsiflexed position with a center of pressure more anterior at initial contact. There was no difference in step length, cadence, knee flexion at initial contact or at peak during stance. The utilization of heel lifts may be beneficial in reducing PFJS variables in running.