INTERVENTION AT THE FOOT-SHOE-PEDAL INTERFACE IN COMPETITIVE CYCLISTS

ABSTRACT

Background

Competitive cyclists are susceptible to injury from the highly repetitive nature of pedaling during training and racing. Deviation from an optimal movement pattern is often cited as a factor contributing to tissue stress with specific concern for excessive frontal plane knee motion. Wedges and orthoses are increasingly used at the foot-shoe-pedal-interface (FSPI) in cycling shoes to alter the kinematics of the lower limb while cycling. Determination of the effect of FSPI alteration on cycling kinematics may offer a simple, inexpensive tool to reduce anterior knee pain in recreational and competitive cyclists. There have been a limited number of experimental studies examining the effect of this intervention in cyclists, and there is little agreement upon which FSPI interventions can prevent or treat knee injury. The purpose of this review is to provide a broader review of the literature than has been performed to date, and to critically examine the literature examining the evidence for FSPI intervention in competitive cyclists.

Methods

Current literature examining the kinematic response to intervention at the FSPI while cycling was reviewed. A multi-database search was performed in PubMed, EBSCO, Scopus, CINAHL and SPORTdiscus. Eleven articles were reviewed, and a risk of bias assessment performed according to guidelines developed by the Cochrane Bias Methods Group. Papers with a low risk of bias were selected for review, but two papers with higher risk of bias were included as there were few high quality studies available on this topic.

Results

Seven of the eleven papers had low bias in sequence generation i.e. random allocation to the test condition, only one paper had blinding to group allocation, all papers had detailed but non-standardized methodology, and incomplete data reporting, but were generally free of other bias sources.

Conclusions

Wedges and orthoses at the FSPI alter kinematics of the lower limb while cycling, although conclusions about their efficacy and response to long-term use are limited. Further high quality experimental studies are needed examining cyclists using standardized methodology and products currently used to alter SPFI function.

Level of Evidence

3

INTRODUCTION

Repetitive micro-trauma to the knee complex is one of the most significant musculoskeletal factors in recreational and competitive cycling.¹ Incidence of knee pain is 50% in competitive cyclists, resulting in reduced training and racing in 57% of professional cyclists.² Cycling involves extending and flexing the knee over five million times per year in a competitive cyclist riding in the order of 25-35,000km every year, so that even small inefficiencies in pedaling are likely to contribute to knee pain.^3,4

In seated cycling, there are three main areas of contact between the cyclist and bicycle; the hands, pelvis and feet. Since the foot is the direct point of contact with the drivetrain (pedal-crank-chain-gear system), optimization of this link is of interest to cyclists, clinicians and coaches. This link between foot and pedal influences the kinematics of cycling and is comprised of two elements; the foot-shoe interface (FSI), and the shoe-pedal interface (SPI). (Figures 1a and Figure 1b)

Figure 1.

(a) Relevant bicycle components, (b) Relevant bicycle components

Intervention in footwear alters kinematics of over ground gait,⁵ and it stands to reason that intervention at the foot-shoe-pedal interface (FSPI) might alter axial and frontal plane knee function while cycling. Deviation from optimal frontal plane kinematics has been implicated as a predictive factor for injury in sports such as running, basketball and soccer.^6,7 Excessive, or sub-optimal non-driving moments (knee varus/valgus forces, axial tibial internal or external rotation) are largely considered to be responsible for knee pain in cyclists.^8-10 Multi-planar kinematics can be altered using orthotics in footwear for walking and running,^5,11-13 and increasingly, wedges and orthotics are used to correct aberrant knee motion in cycling.^14,15

Despite a large body of literature on the effect of orthotic intervention in walking and running, there has been limited investigation into the kinematic effects of wedge and orthotic intervention in cyclists. A recent systematic review¹⁶ of the effect of various types of orthoses in cycling concluded that orthoses produce significant changes in pressure and contact area in the foot while not compromising cycling performance. The review by Yeo et al¹⁶examined six articles, only one of which examined cycling kinematics, the majority of the articles examined physiological parameters while cycling with orthoses. The authors determined that orthoses produced a non-systematic, subject-specific effect on some lower extremity variables.¹⁵The study in question did not control for shoe type, cleat type and pedal system, and the methodological design limits firm conclusions on the kinematic effect of orthoses in cycling. The authors concluded that further study in this field is warranted. In response, this updated review of the FSPI includes four additional papers, which examined different pedal systems on cycling kinematics and included EMG responses to changes at the foot-pedal interface in cyclists. The articles reviewed herein, address the kinematic effects of each element of the foot-shoe-pedal-interface: i.e. the effect of a sneaker compared with a cycling shoe, the effect of multi-planar mobility in the pedal-cleat system, and the effect of wedges and orthoses on cycling kinematics. FSPI modifications may be a simple, inexpensive and unobtrusive solution to managing anterior knee pain in cyclists without compromising cycling performance. Cyclists, their coaches and health care professionals, would benefit from research-based guidelines to assist in management of anterior knee pain. To date, there is little agreement upon which FSPI interventions can prevent or treat knee injury, but this review aims to assist the clinician by providing a broader review of the literature than has been performed to date.

While acknowledging that training load is a major contributing factor to overuse injuries of the lower limb in cycling,^2,17,18 this review will focus on understanding the kinematics of cycling, including the role of FSPI interventions. The purpose of this review is to provide a broader review of the literature than has been performed to date, and to critically examine the literature examining the evidence for FSPI intervention in competitive cyclists. This information will assist the clinician to use evidence-informed interventions in managing knee pain in cyclists.

KINETICS OF CYCLING

In seated cycling, the knee maximally flexes to 110 ° and extends to 20-35 °. Knee extension produces almost 40% of the total lower limb muscle moments, with the remainder from hip extension (27%), ankle plantar flexion (20%), hip flexion (4%) and knee flexion (10%).¹⁹ Moments in the sagittal plane are propulsive or driving forces, moments in the transverse and frontal planes are considered non-driving forces.^10,20,21 While the majority of lower limb motion during cycling is in a sagittal plane, there are also associated accessory joint movements in transverse and frontal planes. Small amounts of tibial and foot rotation (approximately 10 °) occur in the transverse plane.^9,10,20-23 These rotational moments in the transverse plane have an effect on the patellofemoral contact pressures proximally and ankle/foot pressures distally.^5,21 Frontal plane motion (hip adduction/abduction, tibial abduction/adduction) occurs through the recovery and power phase of cycling, and patterns vary between cyclists, with 2-8 cm movement of the knee center in the frontal plane, creating valgus and varus angles at the knee.^10,14 As the knee extends through the power phase from the top dead center (TDC or 0 °) where the pedal is at its most vertical in the cycle, the knee maintains a varus moment (7N), moving to a valgus moment (1-2N) at bottom dead center (BDC or 180 °)⁹ later in the power phase when pushing down on the pedal, returning to a valgus moment in the recovery phase to the top of the cycle.¹⁰ Some studies examining cyclists with a history of knee pain or injury indicate a valgus or medial positioning of the knee.^3,24,25 Alteration of the knee alignment through the pedaling cycle is achieved by changing inter-pedal stance width using washers between the pedal and crank arm or with longer pedal spindles (altering the “Q factor”)²⁶ as well as by canting the foot medially or laterally relative to the pedal.^15,27 Changing the inter-pedal stance width is limited by fixed bicycle frame dimensions and limited pedal spindle lengths and bottom bracket widths,²⁶ so intervention at the FSPI may be a more feasible method of altering cycling kinematics. Laterally or medially inclined wedges are increasingly recommended in the cycling industry to alter the frontal plane knee angle and position relative to bicycle centerline.^{3,9,15,16,25,27-29} While 10 ° wedges have been demonstrated to alter kinematics of the pedaling limb,^14,20,30 there have been few studies using commercially available products which are in 1 degree increments to a maximum of 4-5 degrees. Inclined wedges are used by coaches and bike-fitters to modify medio-lateral deviation of the knee, as a pedaling pattern with a more vertical shank is considered optimal.^3,31 The hypotheses that a vertical shank is optimal for cycling, and that an inclined wedge at the SPI can alter cycling kinematics, need to be explored in controlled experimental studies, where wedges of consistent size and inclination are examined for effectiveness in altering cycling kinematics.

BICYCLE ADJUSTMENTS

Altering the cycling position and using clipless pedal systems (i.e. no toe clips/straps), rigid soled cycling shoes or shoe inserts can modify forces on the lower limb. Studies have examined the influence of many variables on cycling performance, (saddle height,^32-34 handlebar height,^13,35 cadence,^36,37 workload, foot position on pedal¹⁹ fixed vs. ‘float’ (motion available in both the transverse plane (0 °-15 °) and frontal plane (0-10mm) between the shoe and pedal) vs. toe straps^22,32) but adjustments commonly made at the SPI are largely based on interpretation of cadaveric or biomechanical modeling studies, and historical practice.^19,21,22,32 An outline of most common SPI adjustments, associated kinematic responses, and clinical implications is presented in Table 1.

Table 1.

Bicycle adjustments, associated physiological responses, and clinical implications

Variable	Adjustment	Response	Implications
Inter-pedal width	Variable with spacers 1-20 mm. Limited adjustability secondary to frame width.	Alters Q angle in frontal plane. Narrower stance improves efficiency	Too narrow or too wide a stance width can adversely affect frontal plane knee angle, increasing lateral or medial knee strain, creating foot stress in shoes, pressure points in shoes.
Cleat position fore-aft	Adjusts 0-10mm in most cleats.	Alters the point of pedal contact with the shoe sole. Changes location of pedal pressure on the foot in a fore-aft direction.	Too far forward a position could increase stress on metatarsals and increase pressure points in plantar aspects of feet.
Cleat lateral-medial	Adjusts 0-5mm in most cleats, adjusted in combination with pedal stance width.	Alters the point of pedal contact with the shoe sole. Changes location of pedal pressure on the foot in a medio-lateral direction.	Can be used to alter pedal stance width. Could increase lateral/medial knee frontal plane strain.
Cleat internal-external rotation	Adjusts 0-15 °in most cleats.	Alters the transverse plane angle at which the shoe connects to the pedal. Some cleats allow “float” which allows movement in this plane.	Inadequate foot rotation transfers axial load proximally potentially straining meniscal and articular knee tissues. Cleat “float” reduces axial loading at the pedal and knee.
Cleat varus/valgus tilt	Adjustable with use of medially or laterally inclined wedges 1 ° to 5 °	Varus/valgus alters Q angle and foot pressure points.	Potentially increases lateral or medial knee frontal plane stress.

THE FOOT-SHOE-PEDAL INTERFACE

Competitive cyclists constantly seek to improve performance, and equipment is always evolving in the quest for greater power production and cycling efficiency. The modern clipless pedal system was developed in response to technological developments in the ski industry in the 1970s, and a cleat on the undersurface of the cycling shoe now connects directly into the pedal. This allows the shoe to remain in greater contact with the pedal, assisting propulsion during the recovery phase of the pedaling cycle (i.e. from 180 ° to 360/0 °).

Intervention at the FSPI with wedges and in-shoe orthoses in cycling, is purported to contribute to improved kinematics,¹⁵ increased power output,¹⁴ altered plantar pressures at the foot,^38,39 and a reduction of knee joint forces, ^20,22 but there is limited available literature to allow sports performance and health professionals to definitively support their use. The quality of the available literature limits conclusions about the effectiveness of FSPI intervention, as the majority of studies have small sample sizes, and examine mostly competitive, young adult male participants. This restricted sampling limits study power, generalizability of results to females and older cyclists, and impedes the application of findings to cyclists of varying skill and experience. With increasing demand for performance improvement and injury mitigation, there is a need for experimental studies to guide clinicians. Only one known systematic review has been performed on the effect of foot orthoses and wedges on cycling, concluding that further study in this area is warranted.¹⁶ Table 2 presents the most recent experimental studies published in peer-reviewed journals examining FSPI intervention.

Table 2.

Specific FSPI adjustments

Author	Participants (n, sex, type)	FSPI intervention	Methods	Outcome	Clinical implications
Anderson et al 41	6 male, 4 females, untrained cyclists.	Posted 4 ° rearfoot, 4 ° forefoot molded orthoses. Mixed rigid shoe and sneakers.	Cycle ergometer, preferred geometry, workload moderate and maximal. Different shoe types and workloads.	No significant differences in O2 consumption and HR different between conditions	Small N might contribute to non-significance. Tendency for higher HR with sneakers/lower HR with rigid shoes at higher load, supporting use of rigid cycling shoes to increase cycling efficiency.
Bousie et al 39	8 male, 4 female cyclists.	Prefabricated orthoses vs. flat inserts.	Cycle ergometer, preferred geometry, 12/20 RPE (Borg) Cyclists own rigid cycling shoe and clipless pedals.	Inc. plantar pressure, inc. contact area, in medial > lateral sole. Orthoses inc. pressure at heel, and inc. perceived support† but not comfort	Prefabricated orthoses increase plantar contact and improve perceived support in rigid cycling shoes.
Boyd et al23	10 male cyclists.	Instrumented pedal, rigid cycling shoe. 2 degrees of freedom in pedal.	Cycle ergometer, preferred geometry, 90rpm, and 250W.	Allowing 2 degrees of freedom at pedal reduces moments at FSPI† but not at the knee	Transverse and frontal plane pedal float does not significantly change force driving the pedal. Pedal float alters moments at the FSPI but not significantly at the knee.
Cruz et al 42	4 male triathletes	Clipless pedal and cycling shoe vs. toe-clip and sneakers.	Cyclists own bicycle, 100rpm. Stationary rollers.	EMG lower in HS, GS for clipless pedals. No statistical analysis.	Muscle activity may change with different FSPI systems.
Dinsdale et al 58	6 male untrained cyclists.	Rigid cycling shoe, clipless pedal, 1-4 ° medial wedge under shoe, forefoot varus matched with appropriate medial wedge.	Cycle ergometer, 30s anaerobic test (Wingate).	Correlation between power increase and FF varus correction† Peak power and mean power not improved with wedges	Custom orthoses for Forefoot Varus may improve anaerobic power.
Gregersen 9	15 male cyclists.	Instrumented pedal with rigid cycling shoe. 5-10 ° medial and lateral pedal inclination.	Cycle ergometer, preferred geometry, 90rpm, and 225W.	Varus wedge (10 °) caused increased EMG VM/VL ratio with reduced TFL activity and reduced varus moment at knee† 38-72% CV for varus-valgus moments. 59% CV for axial moments.	Varus wedge alters Quad ratio, reduces knee moments and may have implications for PFPS patients. Wedge is significantly larger than those used commercially.
Hennig et al40	26 male, 3 female cyclists.	Rigid cycling shoe, toe-clip pedal system vs. sneakers.	Cycle ergometer, preferred geometry, 80rpm 100W, 200W, 300W, 400W.	An increase in workload causes increased medial forefoot and first toe pressure, in cycling shoe vs. sneaker† Greater index of effectiveness with cycling shoe†	Cycling shoes improve applied force to the pedal, and increase pressure in the medial foot.
Jorge et al59	6 male cyclists	Rigid cycling shoe, toe-clip system vs. sneakers	Bicycle on rollers, 80rpm, seat height 100% of trochanter height	Sneakers produced higher EMG in Quads, and Hamstrings, lower in Rectus Femoris and Gastrocnemius	Rigid cycling shoes reduce EMG activity in Quads and Hamstrings during the propulsive phase of the pedal cycle.
Koch, et al 56	18 male cyclists.	Pre-fabricated carbon insoles	Rigid cycling shoe, clipless pedal system. Anaerobic test (Wingate) Cyclists’ own bike, shoe, and pedal system.	No significant difference in mean power, peak power, cadence	Short effort may not reflect longer endurance efforts.
O'Neill 15	9 males, 3 females, cyclists.	Custom orthoses, varied types posted and molded, varied materials.	Rigid cycling shoe, clipless pedal system. Cycle ergometer, 5’ @85% max HR.	Orthoses produced a trend towards increased knee distance from bicycle centerline and reduced tibial IR, but was not statistically significant. Small group numbers may have underpowered the study.	Custom orthoses may alter knee position in frontal plane.
Sanderson et al 31	28 mixed gender cyclists.	10 ° medial or lateral wedge (not commercially available)	Toe-clip pedal system. Own bicycle, 90rpm, standard gearing.	Lateral wedge significantly altered distance of knee from centerline † but wedges did not significantly alter the frontal plane knee angle	Wedges altered knee position in frontal plane. Wedge is significantly larger than those used commercially. There was wide inter-individual variance in response to wedges

^†= Statistically Significant

METHODS

A literature search was performed in October 2015 using the following databases: PubMed, EBSCO, Scopus, CINAHL and SPORTdiscus using the following search terms: (cycling OR bicycling) AND ((orthotic OR orthoses OR orthoses) OR (wedges OR wedge) AND kinematics), 29 articles were returned and when Full-text, English language filters were applied, and 27 articles were examined. Inclusion criteria were as follows: Experienced cyclists, standard bicycle frame or cycle ergometer, experimental design, orthotic device or wedge. Eleven articles were selected for review, seven met criteria for inclusion in the literature review, and four additional papers meeting these criteria were obtained from other reference lists within journal articles.

A risk of bias analysis was performed according to guidelines developed by the Cochrane Bias Methods Group.⁴⁰ Articles were reviewed with an effort to determine bias in selection (randomized allocation), performance (blinding of participants and personnel), detection (blinding of outcomes), attrition (management of incomplete data) and reporting (selectivity of outcome reporting) of data. Table 3 displays the relative risk of bias for each paper under consideration.

Table 3.

Cochrane risk of bias assessment tool

Paper	Sequence generation		Allocation concealment		Blinding		Incomplete data		Selective outcome reporting		Other validity threats
Anderson et al41	L	Randomly assigned to orthosis test condition	U	No indication of allocation concealment process	U	No indication of blinding process for participants or assessors	U	The study did not address this outcome	L	Protocol is available and reports include expected outcomes	H	Males and females did not complete the same protocol, flaw in study design
Bousie et al39	L	Randomly assigned to orthosis test condition	U	No indication of allocation concealment process	U	No indication of blinding process for participants or assessors	U	The study did not address this outcome	L	Protocol is available and reports include expected outcomes	L	The study appears to be free of other sources of bias
Boyd et al23	L	Randomly assigned to pedal test condition	U	No indication of allocation concealment process	U	No indication of blinding process for participants or assessors	U	The study did not address this outcome	L	Protocol is available and reports include expected outcomes	L	The study appears to be free of other sources of bias
Cruz et al42	H	No random allocation to pedal test condition	U	No indication of allocation concealment process	U	No indication of blinding process for participants or assessors	U	The study did not address this outcome	L	Protocol is available and reports include expected outcomes, although with limited methodology and data analysis details	L	The study appears to be free of other sources of bias
Dinsdale et al58	L	Randomly assigned to insole test condition	U	No indication of allocation concealment process	U	No indication of blinding process for participants or assessors	U	The study did not address this outcome	L	Protocol is available and reports include expected outcomes	L	The study appears to be free of other sources of bias
Gregersen et al9	L	Randomly assigned to pedal test condition	U	No indication of allocation concealment process	U	No indication of blinding process for participants or assessors	U	The study did not address this outcome	L	Protocol is available and reports include expected outcomes	L	The study appears to be free of other sources of bias
Hennig et al40	H	No random allocation to shoe test condition	U	No indication of allocation concealment process	U	No indication of blinding process for participants or assessors	U	The study did not address this outcome	-1	Protocol is available and reports include expected outcomes	L	The study appears to be free of other sources of bias
Jorge et al59	H	No random allocation to shoe test condition	U	No indication of allocation concealment process	U	No indication of blinding process for participants or assessors	U	The study did not address this outcome	L	Protocol is available and reports include expected outcomes	L	The study appears to be free of other sources of bias
Koch et al56	L	Randomly assigned to orthoses test condition	L	Participants were blinded to the allocation	L	Participants were blinded to the group allocation	U	The study did not address this outcome	L	Protocol is available and reports include expected outcomes	L	The study appears to be free of other sources of bias
O’Neill et al15	H	No random allocation to orthoses test condition	U	No indication of allocation concealment process	U	No indication of blinding process for participants or assessors	U	The study did not address this outcome	L	Protocol is available and reports include expected outcomes	L	The study appears to be free of other sources of bias
Sanderson et al31	L	Randomly assigned to pedal test condition	U	No indication of allocation concealment process	U	No indication of blinding process for participants or assessors	U	The study did not address this outcome	L	Protocol is available and reports include expected outcomes	L	The study appears to be free of other sources of bias

L = Low bias risk, U = Unclear bias risk, H = High bias risk

Seven of the eleven papers had low bias in sequence generation i.e. random allocation to the test condition, only one paper had blinding to group allocation, and no papers described blinding of outcome assessors or other personnel. All papers had detailed methodology sections, but had incomplete data reporting, with lack of detail about missing data, missing outcomes, and how such data were handled during analysis. Per Cochrane guidelines, all of the papers were generally free of other bias sources such as sample homogeneity, unequal variance or other threats to validity.

RESULTS

Shoe-type

Competitive cyclists did not universally and simultaneously adopt the now ubiquitous rigid shoe and clipless pedal systems, so both types of footwear were often included in earlier studies. Two studies^39,41 examined the kinematic effects of FSPI intervention using both rigid cycling shoes and sneakers, but only one study directly compared the effect of two different shoe types on cycling kinematics. Cruz⁴² examined the EMG response in four cyclists when pedaling with sneakers with toe clip pedals or rigid soled cycling shoes with clipless pedals, plotted data for each subject, and visualized that there was reduced EMG activity in the biceps femoris and gastrocnemius muscles when cycling using a rigid cycling shoe. The authors concluded that use of the rigid shoe with clipless pedals was more efficient. While they did not directly study cardiovascular efficiency, they inferred from EMG trace data that it was more efficient, as it has a reduced amplitude compared with prior recordings from participants cycling with sneakers and flat pedals. While the raw EMG signal was normalized to the signal average, no statistical analyses were reported, limiting support for their conclusions. The observations from their study were supported by Andersen et al,⁴¹ who demonstrated a lower heart rate while cycling in rigid cycling shoes when compared with sneakers. Hennig et al³⁹ compared sneakers and rigid shoes with resultant higher medial foot pressure and greater cycling effectiveness when pedaling using rigid shoes. Their observations of greater pressures in the medial foot when using a rigid cycling shoe were supported in a later study by Bousie et al.³⁸ While earlier studies on FPSI intervention examined both sneakers and cycling shoes, use of a rigid cycling shoe has now become standard in experimental studies, increasing internal validity by reducing confounding variables.

Pedal system

Authors have compared different cleat-pedal systems (Figure 2) and resultant cycling kinematics. Reduced axial (internal rotation) knee moments have been demonstrated effectively with clipless float pedal design without changing pedal loads or moments.²² Combined axial (internal rotation) and frontal plane (valgus) moments applied at the pedal have been demonstrated to increase patellofemoral contact loading area by 29%, and loading force by 28%, with greatest load on the patellofemoral joint obtained at 90 ° flexion.²¹ This is the point in the downstroke where maximal propulsion, muscular forces and joint moments occur,³ so alleviation of potential excessive patellofemoral pressure via use of pedals that allow transverse plane mobility may be desirable. However, not all cyclists respond to pedal-float with predictable kinematic responses. ^22,23,29 Wheeler et al made direct comparisons of knee joint moments using clipless, toe clip and flat pedals while cycling at several different work rates (150W to 350W).³² Increasing power output from 150W to 350W resulted in an almost linear increase in knee moments with a fixed pedal system. Clipless fixed pedals produced the greatest knee axial and varus moments, which were attenuated by use of a clipless system allowing transverse rotation, with 50% reduction in internal rotation moment at 250W power output. Ruby and Hull had previously demonstrated most significant reductions in pedal and knee moments when using a multi-axial instrumented pedal which allowed rotational mobility in the transverse plane.²³ Boyd et al²² used a custom multi-axial pedal platform to introduce two degrees of freedom to the SPI resulting in a significant reduction of axial and transverse plane moments at both the pedal and the knee, and pedal systems offering this mobility have been widely adopted by competitive and recreational cyclists. Alteration of cleat angle in order to modify an individual's lower limb alignment (tibial torsion, tibial varum etc.) might be a consideration in preventing or managing injury to the knee in cyclists.²⁹ Supported by these findings, there are many recommendations to use cleat rotation to reduce potentially adverse moments on the knee while cycling, ^1,22,31 and the majority of competitive cyclists currently use clipless pedal systems.⁴²

Figure 2.

Position of foot and shoe relative to pedal and cleat interface. Used with permission of BikeFit™

In addition to reducing stress at the knee joint, it is thought that clipless pedals produce more efficient cycling patterns than toe-clip pedals. Most studies examining the effect of SPI intervention in cycling have examined joint kinetics; few have examined resultant changes in muscle activity with different degrees of pedal constraint. Cruz et al⁴² compared visual plots of EMG RMS data from lower limb when cycling (n = 4) with combinations of clipless pedal/cycling shoe and toe-clip/sneaker and flat pedal. Cyclists using clipless pedals demonstrated lower activation levels of hamstring muscles. The authors concluded that a reduction in EMG represented a more efficient pedaling action, but did not address the influence of shoe type on the measured EMG. Since only four cyclists were examined, normalized EMG data were not processed for statistical analysis. Without a larger group of subjects, removal of confounding factors (different shoe types) and appropriate statistical analysis, it is not possible to determine the influence of pedal type on EMG pattern. Patterns of muscle activity in cyclists using clipless pedal systems have been documented ^43-45 with noted difference between competitive and novice cyclists,^13,44 but further studies of EMG responses to FSPI intervention are needed.

Wedges/Insoles

A coupled linkage between the talo-crural joint of the ankle, and the joints of the foot has been examined extensively in over-ground walking and provides the basis for much of the orthoses-based interventions used during human gait.⁵ Rigid modern cycling shoes can have medially or laterally inclined wedges bolted to the undersurface of the sole influencing cycling kinematics. Changes in cycling kinematics have been observed in biomechanical modeling studies using instrumented multi-axial pedals ²² and in experimental studies using wedges at the SPI.³⁰ Sanderson used a 10 ° wedge and measured frontal plane knee angle in 28 cyclists, with wedges altering the knee distance from centerline.³⁰ There was no significant difference between the neutral condition and a 10 ° medial wedge or the neutral condition and a 10 ° lateral wedge. Participants exhibited a significant response to the SPI intervention only between extreme wedge angles, i.e. between 10 ° valgus and 10 ° varus wedges, with a small magnitude of frontal plane knee motion of approximately10mm. The study used two dimensional (2D) video analysis, which has much less accuracy and reliability than current gold-standard three dimensional (3D) motion analysis systems^46,47 and the recently available handheld mobile devices.⁴⁸.

Anderson et al⁴¹ examined metabolic responses to use of custom orthoses while cycling. Rearfoot and forefoot posting of 4 ° did not significantly alter cycling efficiency, but shoe type was related to changes in heart rate (HR). Using a rigid cycling shoe was associated with a lower HR when compared with a soft running sneaker, both with and without orthoses (mean change 22 bpm for males and 33 bpm for females). A mean change in HR of 26bpm was not statistically significant, but would be clinically significant in most physiologic studies^49,50, indicating that the study (n = 10) was likely underpowered.

Bousie et al³⁸ compared contoured pre-fabricated orthoses with flat inserts in cycling shoes of 12 experienced cyclists. Plantar pressure, contact area and perceived support were significantly increased with contoured orthoses (p < 0.001) but the intervention was limited to 20 minutes of seated, steady state cycling. Replication of this study using long-duration or fatigue protocols would produce information about cycling performance closer to typical racing conditions, as well as more realistic information about the relationship between pressure and comfort at the FSPI over timeframes approximating training and competition.

Boyd et al²² used an instrumented pedal platform with freedom from SPI constraint in two planes (x and y, approximating sagittal and transverse planes). Transverse motion ranged from 6.3 ° to 17.3 °, and caused reduction of moments at both pedal and knee, although the changes in knee moments were not statistically significant.²² This study was unique as it relied on custom-built dynamometer pedals. This capability is likely to become increasingly available as technological advances have led to commercially available pedals and sensors that can measure power output, shear forces and pedal forces orthogonal to the pedal.⁵¹ The ability to use the cyclist's own bike with these instrumented pedals will provide for improved implementation of research findings into clinical practice.

More closely approximating normal practice in the cycling athlete, O’Neill¹⁵ examined the kinematic responses to orthoses of different types. The distance of the knee to the bicycle centerline and the frontal plane knee angle was reduced when using orthoses (37.2 ° to 34.8 ° p < 0.001) with significant responses demonstrable within-subjects. While the orthoses produced reduced tibial axial internal rotation in many participants (mean change from 16.8 ° to 15.2 °), there were no significant systematic effects (p = 0.072). The study was possibly underpowered, with only 12 cyclists, but, there was a tendency towards kinematic changes similar to those in gait studies,⁵² with tibial rotation reduced in the order of 2-3 ° when using orthoses while cycling. As knee loads in seated cycling are <50% lower than walking and stair climbing, ^5,19 it remains to be seen whether this small reduction in tibial rotation is important in cycling.

Gregersen et al monitored the response of thigh muscle activation ratios when laterally inclined wedges were placed at the SPI while cycling. There was a significant reduction in tensor fascia lata (TFL) EMG activity, improved vastus lateralis / vastus medialis (VL/VM) ratios (i.e. increased VM activation correlated with reduction in knee varus (r² = 0.65-81, p < 0.0001), and reduced knee varus moments (53-55%, p < 0.001) with a lateral wedge.⁹ Varus loading (4-10N) at the knee occurs in the power phase of cycling at 80-100 ° ^9,22 and is considered to be a potential source of pain and injury when excessive. An imbalance between the ratios of quadriceps and hamstring muscle activity has been implicated in contributing to excessive patella-femoral loading^53-55 and can contribute to excessive repetitive loading in regions of the patella-femoral articular surfaces, but variance in the Gregersen et al⁹ study is large (tibial varus-valgus coefficient of variation (CV) = 38-72%, tibial rotation CV = 59%) warranting caution in extrapolation and inferences from this data. The 5-10 ° wedge used in this study is larger than the 1-4 ° wedges most commonly used in competitive cycling, so it remains to be determined if the smaller wedges in current use have similar effects on muscle activation.

Wedges and insoles have been examined for their effect on power output while cycling. Koch et al ⁵⁶ examined the effect of prefabricated carbon insoles on power output during an anaerobic test, with non-significant results (p = 0.76 Mean Power and p = 0.53 Peak Power) similar to a prior study.¹⁴ There was no standardization of bicycle, pedal or shoe system, possibly introducing excessive variance in measurements. In an effort to match participants’ forefoot anatomy with appropriate wedge intervention, Dinsdale¹⁴ used 1 ° to 4 ° varus wedges inside the cycling shoe to correct for forefoot varus, and examined subsequent power output in a 30s anaerobic test. While the results were not statistically significant for changes in mean power or peak power, there was strong correlation between the degree of varus correction and improved power (r = 0.957) indicating that cyclists with the greatest forefoot varus benefitted the most from varus corrective wedges. The sample was very small (n = 6) and the authors express caution about the study being underpowered. Untrained athletes were used in the study, yielding less important information than if competitive athletes had been used, since there are known EMG and kinematic differences between untrained and trained cyclists.^13,14 While this is a small study, it is the one of the few published studies using commercially available wedges, which are in current use by cyclists, bike-fitters and coaches, and thus, merits attention.

DISCUSSION

Since the foot is attached to the pedal while cycling, two components of this attachment that influence lower limb kinematics are the foot-shoe interface (FSI) and shoe-pedal interface (SPI). A recent systematic review by Yeo et al¹⁶ highlighted a dearth of high quality experimental studies of the effect of interventions at the FSI on kinematics of cycling. The results of the Cochrane risk of bias assessment completed for the current paper concur with Yeo's findings; the studies examined for this review are relatively low in bias, but the majority of the reviewed studies do not report adequately on assessor and participant blinding or data management. Few studies use standardized methodology for fitting the cyclist on the bicycle, despite the existence of many standardized bike-fitting protocols in the literature. ^57-59 No studies used the same testing protocol or products, limiting conclusions drawn about their comparative efficacy.

Despite these shortcomings, the studies reviewed herein have provided some insight into possible effects in lower limb biomechanics during cycling. Orthoses have been shown to improve perceived support,³⁸ improve peak power in sprints,¹⁴ and alter knee position.¹⁵ A rigid cycling shoe reduces EMG activity in the biceps femoris and gastrocnemius muscles, and is more efficient than a soft-soled sneaker or soft cycling shoe.^41,42 Studies examining the specific influence of inclined wedges used at the SPI on the kinematics of cycling have shown that they induce changes in knee position,^9,30 moments at the knee,⁹ and alter EMG ratios in thigh and hip muscles.⁹ At the SPI, the choice of pedal system (clipless float vs. no float, clipless vs. platform pedal) also influences moments at the pedal and knee.²² Wedges at the SPI and orthoses at the FSI are thought to influence the knee position via the talo-crural coupling mechanism.⁶⁰ To date, there have been no published studies examining the effect of such commercially used wedges on the kinematics of cycling. Determining the relative influences of interventions at the FSI and SPI on cycling kinematics will likely improve guidelines for management of cyclists with knee pain, but there are insufficient experimental data to determine whether such a difference exists. Future study should compare the kinematic effect of wedges and orthoses in cyclists.

Many kinetic and kinematic studies investigating intervention at the SPI^{9,22,30,38,39,42} and FSI in cyclists.^14,15,41,56 have been conducted. Two SPI studies demonstrate significant changes in lower limb moments and EMG activity.^9,22 One SPI study demonstrated significant response to changes in the frontal plane knee angle.³⁰ One FSI study also demonstrated changes in the frontal plane,¹⁴ but there are no peer-reviewed studies examining the effect of FSI intervention on lower limb EMG or knee moments. Studies examining FSI intervention on cycling kinematics have been fewer in number. Short-term power output may be improved with FSI correction of forefoot varus.¹⁴ Perceived comfort is improved with FSI intervention³⁸ which may be important in studies which examine performance over long periods of cycling. While the current literature indicates some specific kinematic responses to FSPI intervention, the applications for clinicians and coaches remain unclear. In order to determine the kinematic responses to FSPI and to create specific guidelines for clinicians and coaches, studies with improved methodologies are needed. Standardizing the position of the cyclist on the bicycle for seat height, knee angle at the bottom of the pedaling cycle and foot position on pedal is critical to determine if the intervention is effective, or the variability in cyclist positioning limits correct interpretation of the data. Standardizing cycling equipment such as footwear, pedal system, and the wedges or orthoses being tested, will reduce systematic bias and errors in the data. Controlling for sex and experience, and testing larger numbers of participants will reduce variance, increase homogeneity, and improve statistical power. In the interim, with limited supporting evidence for intervention at the FSPI to manage cyclists with anterior knee pain, it is the authors belief that such intervention should be individualized, methodically monitored, and modified based on the cyclist's feedback and performance. In summary, and in agreement with Yeo et al,¹⁶ while there is theoretical plausibility that orthoses and wedges could influence cycling kinematics, there is a need for further high quality studies in this field.

CONCLUSIONS

Excessive frontal plane loading is a significant contributor to knee dysfunction but can be effectively changed with cueing and movement retraining. Because injured cyclists demonstrate kinematic patterns than uninjured cyclists, strategies to correct such aberrant kinematics might be useful to coaches, clinicians and cyclists. Wedges and orthoses at the FSPI appear to alter kinematics of the lower limb while cycling, although conclusions about their efficacy are limited. Further studies are needed examining cyclists using standardized experimental conditions and equipment used to alter FSPI function. Determination of the effect of SFPI alteration on cycling kinematics may offer a simple, inexpensive tool to reduce anterior knee pain.