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Original article
Take your shoes off to reduce patellofemoral joint stress during running
  1. Jason Bonacci1,
  2. Bill Vicenzino2,
  3. Wayne Spratford3,
  4. Paul Collins4
  1. 1Centre for Exercise and Sports Science, School of Exercise and Nutrition Sciences, Deakin University, Geelong, Victoria, Australia
  2. 2Division of Physiotherapy, The University of Queensland, Brisbane, Queensland, Australia
  3. 3Department of Movement Science, Biomechanics, Australian Institute of Sport, Canberra, Australia
  4. 4School of Engineering, Deakin University, Geelong, Victoria, Australia
  1. Correspondence to Dr Jason Bonacci, Centre for Exercise and Sports Science, School of Exercise and Nutrition Sciences, Deakin University, Geelong, VIC 3216, Australia; jason.bonacci{at}deakin.edu.au

Abstract

Aim Elevated patellofemoral joint stress is thought to contribute to the development and progression of patellofemoral pain syndrome. The purpose of this study was to determine if running barefoot decreases patellofemoral joint stress in comparison to shod running.

Methods Lower extremity kinematics and ground reaction force data were collected from 22 trained runners during overground running while barefoot and in a neutral running shoe. The kinematic and kinetic data were used as input variables into a previously described mathematical model to determine patellofemoral joint stress. Knee flexion angle, net knee extension moment and the model outputs of contact area, patellofemoral joint reaction force and patellofemoral joint stress were plotted over the stance phase of the gait cycle and peak values compared using paired t tests and standardised mean differences calculated.

Results Running barefoot decreased peak patellofemoral joint stress by 12% (p=0.000) in comparison to shod running. The reduction in patellofemoral joint stress was a result of reduced patellofemoral joint reaction forces (12%, p=0.000) while running barefoot.

Conclusions Elevated patellofemoral joint stress during shod running might contribute to patellofemoral pain. Running barefoot decreases patellofemoral joint stress.

  • Running
  • Knee
  • Biomechanics
  • Gait analysis
  • Running shoes

https://doi.org/10.1136/bjsports-2013-092160

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    Introduction

    Interest in barefoot and minimalist shoe running has surged in recent times. Laboratory studies have shown that barefoot running alters foot-strike landing patterns and the impact peak of the vertical ground reaction force (GRF).1–3 In comparison to shod running, barefoot running also reduces knee flexion excursion during mid-stance4 and the subsequent peak knee extension moment.5 A reduction in the knee extension moment may be advantageous for the patellofemoral joint (PFJ), as PFJ stress is a product of PFJ reaction force per unit of contact area.6 Conversely, because the PFJ contact area increases with knee flexion angles up to 60°,6 ,7 a reduction in the knee flexion angle during barefoot running may be deleterious to the PFJ. During fast walking, modelling studies have demonstrated that elevated PFJ stress in individuals with patellofemoral pain syndrome (PFPS) and patella alta is due to a reduction in the patellofemoral (PF) contact area.8 ,9 Of these two factors, PFJ reaction force is thought to have the greater overall bearing on PFJ stress.10 Reducing PFJ stress in runners is of high importance as the most common musculoskeletal running complaint is PFPS;11 a condition whose aetiology and progression are frequently attributed to excessive PFJ stress.12 ,13

    A recent study showed that wearing shoes with a medium and high heel increases PFJ stress during walking in comparison to a low-heeled condition.14 This increase was a result of an increase in the knee flexion angle that in turn increased the knee extension moment and PFJ reaction force. While this study did not examine running, it is the first to indicate that shoes may influence stress occurring at the PFJ. Given the observed differences in knee kinematics and kinetics with barefoot running, it is timely to examine the effect of running barefoot on PFJ stress. The purpose of this study was to determine if running barefoot alters PFJ stress in comparison to shod running. We hypothesised that running barefoot would decrease PFJ stress.

    Methods

    Study population

    Twenty-two (14 men and 8 women) highly trained runners were recruited for the study. The participants had a mean (SD) age of 29.2 (6.0) years; height of 1.76 (0.07) m and body mass of 65.6 (8.8) kg. Participants were running on average (SD) 105 (33) km/week in 7.6 (2) sessions/week at the time of testing and their personal best 10 km time in the previous 12 months was 33 min 41 s (3 min 43 s). Participants were excluded from the study if they had suffered from any musculoskeletal or neurological condition that prevented them from training in the previous 3 months. Written informed consent was obtained from all participants and ethical approval was granted by the Deakin University and Australian Institute of Sport Human Ethics Committees.

    Procedures

    Participants performed 20 overground running trials along a 20 m runway within an indoor 110 m synthetic running track in two randomly ordered conditions (10 trials/condition). The conditions were (1) barefoot; and (2) a neutral control shoe (NIKE LunaRacer). The control shoe had a mean (SD) mass of 184.2 (19.4) g and a rearfoot/forefoot midsole height of 24/18 mm. All running trials were performed at 90% of the participants’ best 10 km time in the previous 12 months. Each participant performed a standardised warm-up that involved five overground running trials within the capture volume. Running velocity was recorded using timing gates (SpeedLight V2 timing gates, Swift Performance Equipment, QLD, Australia) positioned at each end of the 20 m capture volume. A 22-camera VICON motion analysis system (Oxford Metrics Ltd, Oxford, UK) was used to capture three-dimensional joint kinematics of the lower limb. GRF data were collected using eight Kistler force plates (Kistler, Amherst, New York, USA). Marker trajectory and GRF data were sampled at 250 and 1500 Hz, respectively. Trials were accepted if the running velocity was within 5% of the target speed and there was a valid foot strike on a single force plate for the right leg. Kinematic and kinetic data were collected using the University of Western Australia's lower body model.15 Retroflective markers were placed bilaterally over the anterior and posterior iliac spines, lateral and medial femoral condyles, lateral and medial malleoli, calcaneus and the first and fifth metatarsals. In addition, rigid three-marker clusters were placed on the lateral surface of each participant's thigh and leg. After obtaining a static calibration, the markers on the medial femoral condyles and medial malleoli were removed.

    Data analysis

    Kinematics and kinetics

    VICON Nexus (VICON) software was used to reconstruct the joint coordinate data, identify the gait cycle events and filter the raw coordinate and analogue data. Data were processed for the stance phase of the right leg. Sagittal plane marker trajectory data were filtered using a low-pass fourth-order Butterworth filter with a cut-off frequency of 20 Hz. Analogue data were filtered using a low-pass fourth-order Butterworth filter with a cut-off frequency of 50 Hz to calculate kinetic data. The best cut-off frequencies were determined by performing a residual analysis and visual inspection of the resulting kinematic, kinetic and GRF data. A standard Newton-Euler inverse-dynamics approach was used to calculate the net internal knee extension moment. Moment data were normalised by body mass and reported in units of Nm kg. Temporospatial parameters of stride length and stride frequency were calculated from right foot contact events identified by the vertical component of GRF. Data obtained from the 10 trials in each condition were averaged for statistical analysis.

    PFJ kinetics

    A previously described PFJ model (table 1) was used to quantify PFJ reaction force and PFJ stress.8 ,9 This model has been used to determine differences in PFJ stress between those with PFPS and healthy controls, and is sensitive to detect changes when walking in different shoes14 or when wearing a knee brace.16 Input variables for the model algorithm included knee flexion angle, net knee joint moment and PFJ contact area. PFJ contact area was estimated by fitting (R2=0.98) a fourth order polynomial curve algorithm (equation 1) to the seven contact areas (83, 140, 227, 236, 325, 211 and 199 mm2) for seven knee flexion angles (0, 15, 30, 45, 60, 75 and 90°) as reported by Powers et al7 to provide continuous contact areas from 0° to 90° of knee flexion.

    View this table:
    Table 1

    Biomechanical model of the patellofemoral joint

    The effective lever arm for the quadriceps at each knee flexion angle was calculated using a non-linear equation fit to the data of Van Eijden et al17 (equation 2). Quadriceps force (equation 3) was then calculated for each knee flexion angle by dividing the net knee extensor moment by the effective lever arm. PFJ reaction force was calculated by first fitting a non-linear equation to the data of Van Eijden et al18 to determine the coefficient for each knee flexion angle (equation 4). Then the quadriceps force was multiplied by the coefficient for each knee flexion angle to determine the PFJ reaction force (equation 5). Finally, PFJ stress was determined by dividing the PFJ reaction force by the PF contact area for each knee flexion angle (equation 6). The output of the model was PFJ reaction force and PFJ stress, normalised to the stance phase of the gait cycle.

    Statistical analysis

    Data were plotted over the stance phase of the gait cycle for visual inspection and graphical representation. Peak PFJ stress, PFJ reaction force, knee extension moment, knee flexion angle, ankle dorsiflexion at footstrike and stride length and stride frequency were extracted for statistical analysis. All statistical tests were performed using SPSS statistical software (V.20; SPSS Inc, Chicago, Illinois, USA). Differences between barefoot and shod running were examined using paired t tests. The α level was set at 0.05. For significant findings, the mean difference, 95% CI and standardised mean difference (SMD) were calculated. SMD were calculated to express the magnitude of differences between conditions and interpreted according to the following criteria: SMD ≤0.2, small change; SMD=0.5, moderate change; SMD=0.8, large change.19

    Results

    The mean (SD) running velocity for both conditions was 16.1 (1.6) km/h. The main findings of the study are presented in table 2 and illustrated in figure 1. Stride length was shorter (−2.4%, p=0.000) and stride frequency higher (2%, p=0.000) during barefoot running compared with the shod condition. At footstrike, the ankle was less dorsiflexed during barefoot running compared with shod running (82%, p=0.000). Peak knee flexion during stance was greater in the shod condition (4.2%, p=0.000). The greater knee flexion angle when shod was associated with a slightly greater peak utilised PF contact area compared with barefoot (0.13%, p=0.027). The peak knee extension moment was 9% less during barefoot running than shod running (p=0.000, SMD=0.7). The PFJ reaction force demonstrated a large effect for condition (SMD=0.8), with a 12% reduction in the peak joint reaction force during barefoot running (p=0.000). PFJ stress was also 12% less during barefoot running than shod running (p=0.000), and this change demonstrated a moderate effect (SMD=0.5).

    View this table:
    Table 2

    Group mean (SD) values and the difference between barefoot and shod conditions

    Figure 1
    Figure 1

    Comparison of the group mean patellofemoral joint reaction force (A) and patellofemoral joint stress while running shod (black solid line) and barefoot (grey dashed line). *Indicates a significant difference in peak values (p=0.000).

    Discussion

    The hypothesis that running barefoot would decrease PFJ stress in comparison to shod running was supported by the findings of this study. In comparison to the shod condition, there was a 12% reduction in peak PFJ stress. The predominant influence for the decrease in PFJ stress was a reduction in the PFJ reaction force. The reduction in the PFJ reaction force occurred due to the smaller knee flexion angle during the stance phase of running, which decreases the demand on the quadriceps muscles.20

    Our finding of a smaller peak knee flexion angle and knee extension moment with barefoot running is consistent with previous comparisons of barefoot and shod running.4 ,5 These changes are a function of a reduction in stride length during barefoot running, which places the lower extremity more closely beneath the centre of mass, thus reducing the moment arm of the quadriceps muscles. It had previously been theorised that increased knee extension moments and quadriceps work during shod running may increase the pressure and stress across the PFJ;5 however, this is the first study to show that running shod does increase PFJ stress in comparison to barefoot running. The likely cause of elevated PFJ stress when shod is the cushioning and elevated heel of the modern running shoe as these factors have been show to alter the running mechanics2 ,20 in comparison to barefoot and contribute to increases in knee joint moments.5 ,20 ,21 The observed change in the knee extension moment in the current study (9%) was much smaller than that reported previously (36%),5 which suggests that shoes may have an even greater effect on PFJ stress than what is reported in our study.

    Current running shoes are constructed with cushioning and an elevated heel that reduces the magnitude and rate of loading of the vertical GRF during footstrike, quite likely reducing discomfort at contact.22 However, these factors have little influence at midstance, which is the time at which the GRF, joint moments and contact forces are at their greatest.23 The alterations in footstrike patterns associated with barefoot running decrease or eliminate the impact peak of GRF,2 thereby achieving similar effects as the shoe but with the additional benefit of reducing the peak knee joint moments and PFJ stress. It is noted that the shoes in the current study had no motion control features that may provide some putative beneficial effect at the knee. It is unknown if these features are able to reduce PFJ stress and there is little to no evidence for their capacity to prevent musculoskeletal injury.24

    The peak PFJ reaction force and stress values reported in the current study are higher than those previously reported in walking.9 ,14 ,16 This is expected due to the higher demand of running and the greater GRFs. Others have shown that the peak PFJ reaction force and stress increases with an increased walking velocity.8 ,9 ,16 Our values are also slightly higher than those reported in a running study.25 Again, this is quite likely due to the greater velocity in the current study. Interestingly, the latter study reported that altering the sagittal plane trunk position during running can decrease PFJ stress. Patella bracing is also effective at reducing PFJ stress during free and fast walking,16 though this has not yet been demonstrated in running. Patella taping has been shown to reduce superior translation of the patella,26 which may increase the PF contact area, thereby reducing PFJ stress. Other interventions such as in-shoe orthoses, gait retraining and gluteal and vastus medialis oblique muscle strengthening may theoretically reduce PFJ stress by limiting factors such as peak hip adduction27 ,28 and tibial internal rotation29 which quite likely increase PFJ loading and PFJ stress. Taken together, it appears that both distal and proximal interventions may be effective at reducing PFJ stress, although this has yet to be quantified during running.

    While this study cannot determine the relationship between changes in pain and PFJ stress, it is plausible that the decrease in PFJ stress with barefoot running may reduce symptoms in those with PFPS and/or assist in the prevention of this condition in runners. The aetiology of PFPS is frequently attributed to repetitive overloading of the PFJ,12 ,13 and runners are highly susceptible to this condition due to the repetitive nature of the activity. Those suffering from PFPS have been shown to exhibit elevated PFJ stress during fast walking,8 thus a decrease in PFJ stress may be beneficial for these individuals. Powers et al16 reported that a knee brace, which immediately reduced pain, was capable of reducing peak PFJ stress by 1 MPa during fast walking. The magnitude of change in the current study was 2.4 MPa, which suggests that the change may be clinically relevant. Further research is required to examine the relationship between changes in PFJ stress and pain and to determine if running barefoot can reduce symptoms in those with PFPS.

    Musculoskeletal overuse injuries such as PFPS are influenced by both stress magnitude and loading exposure. Although barefoot running decreased the PFJ stress magnitude, the corresponding increase in stride frequency will increase loading cycles for a given amount of mileage, which may offset any potential benefit. However, a probabilistic model for tibial stress fractures demonstrated that reducing the magnitude of loading outweighs the detriments of increased loading cycles.30 It is unknown if this same beneficial effect occurs at the PFJ, though increases in stride frequency during running have been shown to decrease the peak hip adduction angle and knee joint loading31 and increase gluteal muscle activity,32 which may have therapeutic benefits for the PFJ.

    This study has a number of limitations that warrant consideration. The model utilised to estimate PFJ stress was a simplified planar model and thus does not take into consideration the individual three-dimensional patella kinematics and patella geometry. It is possible that the individual alterations in patella geometry and kinematics may affect the magnitude of contact area and thus the PFJ stress. However, the current model has previously been able to detect differences in stress when walking in different shoes14 and also detect differences between those suffering from PFPS and healthy controls.8 Our study compared within participant changes and patella geometry would remain constant between conditions, so more sophisticated measures may provide improved accuracy of actual joint stress, but they are less likely to change the outcomes of this study. A further limitation is that the runners recruited for the study were novice barefoot runners, and thus their running kinematics may not be representative of habitual barefoot runners and caution is warranted in generalising these results to habitual barefoot runners. Finally, care must be taken in extrapolating our results to a symptomatic population as people suffering from PFPS have been shown to exhibit slightly different running kinematics33 and PF contact area34 than healthy controls.

    Conclusion

    Running barefoot significantly decreases PFJ stress compared with shod running. The observed decrease in PFJ stress was the result of a decrease in the PFJ reaction force. The greater loading of the PFJ during shod running indicates that shoes may be one of many factors that contribute to the development of PFPS in runners. Further research is required to examine the relationship between shoes, PFJ stress and pain in runners.

    What are the new findings?

    • Running barefoot decreases peak patellofemoral joint (PFJ) stress.

    • This study confirms previous postulations that greater knee joint moments during shod running elevate loading of the PFJ.

    How might it impact on clinical outcomes in the near future?

    • Running barefoot may be utilised as a means to unload the PFJ as part of a rehabilitation plan.

    • Careful consideration to shoe prescription may be warranted for those suffering from PFJ overload.

    Acknowledgments

    We are grateful to the School of Sport Science, Exercise and Health at the University of Western Australia for use of the lower limb kinematic and kinetic model. We thank NIKE for providing the footwear used in this study. We also thank Philo Saunders and Amy Hicks for assisting with participant recruitment and data collection.

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    View Abstract

    Footnotes

    • Contributors JB was involved in the concept and design of the study, conducted the data collection and analysis and prepared the manuscript. BV was involved in the concept and design of the study, data analysis and interpretation and preparation of the manuscript. WS conducted the data collection and was involved in interpretation of the data and preparation of the manuscript. PC conducted the data analysis and was involved in interpretation of the data and preparation of the manuscript.

    • Competing interests None.

    • Ethics approval Deakin University and Australian Institute of Sport Human Ethics Committees.

    • Provenance and peer review Not commissioned; externally peer reviewed.

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