Arch Support and Ankle Pronation/Supination Correcting the Vertical Pedaling Trajectory of the Knee Joint: A Complete Mechanical Analysis from Subtalar Joint Kinematics to Cleat Wedge Adjustment

1. Introduction and Cutting-Edge Research Background: The Kinetic Chain Revolution from Foot Contact to Knee Joint Trajectory

In cycling, the knee joint is widely regarded as the “central hub” of power transmission, yet training science has long focused primarily on thigh muscle groups and cardiorespiratory capacity, overlooking the fact that the “foot”—the most distal segment, closest to the pedal—is actually the critical starting point that determines knee joint movement trajectory. In recent years, through the interdisciplinary integration of sports biomechanics and foot and ankle medicine, the International Cycling Union (UCI) and the motion capture laboratories of top professional teams have gradually confirmed a paradigm-shifting view: the knee valgus and knee varus trajectories are not simply caused by strength imbalances between the quadriceps and gluteus medius, but rather originate from a “cascading compensatory effect” triggered by excessive pronation or supination of the subtalar joint during the pedaling cycle.

This discovery has fundamentally transformed the scientific content of bike fitting (geometry setup). Over the past two decades, most bike fitters have focused on saddle height, knee-over-pedal-spindle (KOPS) position, and crank arm length, yet few have delved into the arch support morphology inside cycling shoes or cleat wedge angles. However, since 2020, multiple studies—including those published in the Journal of Sports Sciences and Foot & Ankle International—have indicated that when the foot undergoes excessive pronation near bottom dead center (BDC) during pedaling, the subtalar joint rotates at an average angular velocity of 8 to 12 degrees per second. This motion directly drives the tibia into coupled internal rotation, which in turn causes the knee joint to deviate from its original vertical trajectory in the frontal plane, producing a significant valgus angle.

For Taiwan’s local riding environment—whether it’s the 87-kilometer continuous climb of Eastbound Wuling, or the frequent steep out-of-saddle efforts on the Yangmingshan Fengzhongjian route—stable contact between the foot and pedal is absolutely critical for maintaining pedaling efficiency. Particularly on steep sections exceeding 10% grade, riders unconsciously drop their heels and supinate their feet to increase pedaling torque, causing the subtalar joint to momentarily withstand reaction forces exceeding 2.5 times body weight. Under such conditions, without adequate arch support, the magnitude of knee valgus is dramatically amplified. Over the long term, this not only leads to iliotibial band friction syndrome but may also trigger patellofemoral pain syndrome.

This article adopts a rigorous biomechanical perspective, starting from the kinematic foundations of the subtalar joint, to systematically deconstruct how arch collapse cascades through tibial internal rotation to induce knee valgus. Furthermore, through comparative analysis of measured data, we evaluate the empirical effectiveness of custom heat-molded insoles and cleat wedges in correcting the vertical pedaling trajectory of the knee joint. Additionally, this article provides a complete periodized adaptation training plan and practical adjustment guide, helping riders achieve true “foot-to-knee” kinetic chain optimization on a scientific basis.

2. Core Mechanisms of Exercise Physiology and Biomechanics: The Cascading Effects of Subtalar Joint Kinematics and Knee Joint Trajectory

2.1 Kinematic Foundations of the Subtalar Joint: A Pivotal Role in Three-Dimensional Space

The subtalar joint is a compound joint formed by the talus and calcaneus, with its articular axis oriented approximately 42 degrees upward and 16 degrees medially in space. This unique oblique axis means that subtalar joint motion is not simply frontal-plane inversion and eversion, but is simultaneously accompanied by transverse-plane internal and external rotation. During pedaling, the reaction force from the foot relative to the pedal is transmitted upward along the long axis of the tibia. When the foot pronates, the talus undergoes internal rotation and anterior glide relative to the calcaneus. Through the coupled mechanics of the talocrural joint, this motion directly drives the tibia into coupled internal rotation.

2.2 The Cascading Effects of Arch Collapse: Kinetic Chain Disruption from the Longitudinal Arch to the Knee Joint

The medial longitudinal arch is a crucial shock-absorbing structure that dissipates ground reaction forces during standing and locomotion. When the arch collapses (pes planus) due to congenital structural factors, overtraining, or prolonged use of improper footwear, the subtalar joint is forced into excessive pronation during the stance phase. In the cyclic motion of cycling pedaling, this excessive pronation reaches its peak near bottom dead center (BDC), because at this moment the quadriceps and posterior calf muscles are simultaneously generating maximum downward thrust, and the vertical reaction force on the foot reaches its highest point.

From a biomechanical derivation perspective, there is a high positive correlation between the knee valgus angle (θ_knee) and the subtalar pronation angle (θ_subtalar). According to the coupling theory of the kinetic chain, the tibial internal rotation angle (θ_tibial_rotation) is approximately 0.6 to 0.8 times the subtalar pronation angle, and the knee valgus angle in the frontal plane is linearly correlated with the tibial internal rotation angle. In other words, when the subtalar joint undergoes 10 degrees of excessive pronation, the tibia will correspondingly produce approximately 6 to 8 degrees of internal rotation, causing the knee joint to deviate from its vertical trajectory by 5 to 7 degrees during pedaling.

2.3 Mechanical Formulas and Numerical Models of Vertical Pedaling Trajectory

To more precisely quantify the impact of arch support on knee joint trajectory, we can establish a simplified two-dimensional rigid body model. At bottom dead center (BDC) of the pedal stroke, the pedal reaction force (F_pedal) acts vertically upward on the plantar surface of the foot. Its moment arm relative to the knee joint center can be expressed as:

[
M_{knee} = F_{pedal} \times d_{knee}
]

where ( d_{knee} ) is the horizontal distance from the knee joint center to the line of action of the pedal reaction force. When the foot undergoes excessive pronation, the rotation of the subtalar joint causes the tibia to shift medially, increasing ( d_{knee} ), which in turn elevates the valgus moment experienced by the knee joint. According to electromyography (EMG) research, when the knee valgus moment increases by 15%, the activation timing difference between the vastus medialis oblique (VMO) and vastus lateralis (VL) significantly widens, leading to abnormal patellar tracking and subsequently causing anterior knee pain.

2.4 Intervention Mechanisms of Cleat Wedges and Arch Support Insoles

The primary function of custom arch support heat-molded insoles is to provide a stable “tray-like support” for the arch, maintaining the subtalar joint in a neutral position and preventing it from entering excessive pronation at bottom dead center. From a mechanical perspective, the arch support structure of the insole redistributes pressure originally concentrated on the first metatarsal head to the heel and fifth metatarsal head, forming a stable three-point support structure. Cleat wedges, on the other hand, involve inserting a wedge shim that is thicker on the medial side between the cleat and the cycling shoe. Their function is to directly alter the angle of the foot relative to the pedal, positioning the subtalar joint in a slightly supinated position at the start of the pedal stroke, thereby reserving adequate pronation range throughout the pedaling cycle and preventing excessive pronation.

3. Key Parameter Measurements and Comparative Analysis: Data Validation of Insole and Wedge Interventions

To concretely illustrate the effects of different interventions on knee joint trajectory, two sets of measured data are compiled below. The first dataset comes from a 2023 motion capture analysis of 32 amateur cyclists, comparing knee valgus angles and pedaling efficiency when using flat insoles versus custom arch support heat-molded insoles. The second dataset comes from laboratory testing of cleat wedges of different thicknesses (0mm, 2mm, 4mm).

Table 1: Comparison of Knee Joint Trajectory and Pedaling Parameters: Flat Insoles vs. Custom Arch Support Heat-Molded Insoles

The data in this table show that after the intervention of custom arch support heat-molded insoles, the riders’ maximum knee valgus angle significantly decreased from an average of 8.4 degrees to 4.2 degrees, a reduction of 50%, with a proportional decrease in tibial internal rotation angle. Notably, pedaling power did not decrease after the insole intervention; instead, it slightly increased by 3.1%, indicating that arch support indeed optimizes force transmission efficiency rather than merely restricting joint motion. The improvement in pedaling smoothness (+6.9%) reflects increased stability of the kinetic chain along the vertical trajectory, reducing unnecessary lateral energy dissipation.

Table 2: Effects of Cleat Wedge Thickness on Knee Joint Trajectory and Pedaling Stiffness

From Table 2, it can be observed that increasing wedge thickness further reduces the knee valgus angle and decreases the angular velocity of tibial internal rotation, indicating improved dynamic stability of the knee joint during pedaling. The pedaling stiffness index increased from 18.6 N/mm to 25.7 N/mm, demonstrating reduced energy loss caused by joint instability during force transmission from the foot to the pedal. However, when the wedge thickness reached 4mm, riders’ subjective comfort ratings significantly dropped to 6.2, with some riders reporting noticeable pressure on the lateral aspect of the foot. This indicates that excessive wedge intervention may lead to new biomechanical imbalances, and intervention must follow the principle of progressive adjustment.

4. Periodized Training Plan and Equipment Adjustment Guide: Phased Adaptation and Intensity Planning

During the process of introducing arch support insoles or wedges, the neuromuscular system requires time to adapt to new joint alignment and muscle activation patterns. If high-intensity training is undertaken immediately after the intervention, proprioceptive disruption can lead to motor control dysfunction and even compensatory muscle strains. Therefore, the following provides a four-week periodized adaptation training plan, using heart rate zones and power zones as intensity control references.

4.1 Week 1: Proprioceptive Adaptation Phase (Total Riding Time: 4-6 Hours)

The goal of this phase is to allow the feet and knee joints to gradually adapt to the new support structure. Training intensity should be strictly controlled within Zone 1 to Zone 2 (power zone < 60% FTP, heart rate zone < 75% HRmax), focusing on low-intensity, high-cadence (90-100 rpm) flat-road riding. Daily riding time should not exceed 90 minutes, and during each ride, single-leg drills should be performed every 15 minutes, each lasting 30 seconds, to strengthen neuromuscular proprioceptive feedback for the new knee joint trajectory.

4.2 Week 2: Muscle Activation Phase (Total Riding Time: 6-8 Hours)

Entering the second week, moderate-intensity climbing training can be introduced, with intensity elevated to Zone 3 (power zone 76-90% FTP, heart rate zone 76-85% HRmax). It is recommended to schedule two sessions of climbing intervals, each consisting of 3 sets of 8 minutes, with gradients controlled at 5-8% and cadence maintained at 80-90 rpm. During this phase, special attention should be paid to knee joint stability at bottom dead center. Video analysis can be used to confirm whether the knee remains on the vertical projection line above the second toe.

4.3 Week 3: Pedaling Stiffness Strengthening Phase (Total Riding Time: 8-10 Hours)

In this phase, training intensity is further elevated to Zone 4 (power zone 91-105% FTP), incorporating high-cadence (110-120 rpm) sprint training and low-cadence (50-60 rpm) heavy-load climbing training to comprehensively enhance pedaling stiffness. It is recommended to schedule one “Wuling simulation training” session, selecting a continuous climb with gradients between 8-12% (such as the Yangmingshan Zhonghu Combat Readiness Runway), performing 3 sets of 6-minute high-intensity climbs with 5-minute recovery between sets.

4.4 Week 4: Race Integration Phase (Total Riding Time: 10-12 Hours)

The final week should simulate real race scenarios, integrating intensity and riding patterns. It is recommended to schedule one long-distance ride (120-150 km), including multiple 5-minute Zone 5 high-intensity attacks (power zone > 106% FTP), as well as long-distance cruising (Zone 2) simulating common scenarios in events like the One-Day Taipei-Kaohsiung or Twin Towers races. During this phase, closely monitor for any knee discomfort. If pain or abnormal swelling occurs, immediately cease training and seek professional sports medicine evaluation.

5. Race Nutrition, Environmental Adaptation, and Race Strategies: Full-Scenario Applications from Wuling to KONA

5.1 Carbohydrate Intake and Muscle Glycogen Optimization

Whether it’s the long-distance climb of Eastbound Wuling or the cycling leg of the KONA Ironman, a stable energy supply is fundamental to maintaining pedaling technique and knee joint stability. It is recommended to perform carbohydrate loading in the 3 days prior to the race, increasing daily intake to 8-10 grams per kilogram of body weight. On race day, consume 60-90 grams of carbohydrates per hour (using 6-8% concentration sports drinks combined with energy gels), along with 500-750 milliliters of fluid per hour. Particularly in the Wuling race, where altitudes exceeding 3,000 meters accelerate respiratory fluid loss, it is recommended to replenish 150-200 milliliters of fluid every 15 minutes to maintain blood volume and muscular oxygen delivery efficiency.

5.2 Climate Adaptation and Foot Microenvironment Management

Taiwan’s hot and humid summer environment can easily cause foot swelling, which in turn affects pressure distribution inside cycling shoes and the effectiveness of arch support. It is recommended to undergo heat acclimatization training before the race, performing 5-7 days of low-intensity riding in environments above 30°C, 60-90 minutes daily, to promote plasma volume expansion and adaptation of sweat electrolyte concentrations. On race day, choose breathable, lightweight cycling socks and use anti-chafing powder inside the shoes to reduce friction. If the race environment involves high altitude and low temperatures (such as Wuling), pay attention to foot warmth to avoid plantar muscle stiffness caused by cold, which could compromise the active support capability of the arch.

5.3 Race Strategies: Switching Pedaling Techniques Between Climbs and Flats

Under different terrain conditions, strategies for arch support and knee joint trajectory should be adjusted accordingly. On steep sections exceeding 10% grade (such as the Kunyang section of Wuling), it is recommended to adopt a strategy alternating between seated and standing pedaling. When seated, shift the center of gravity slightly rearward to reduce forefoot pressure and ensure the knee remains on its vertical trajectory; when standing, distribute body weight evenly across both feet to avoid excessive pronation moment on one side. On flat or gentle gradient sections, maintain a high cadence (90-100 rpm) with smooth pedaling, focusing on the vertical up-and-down motion of the knee and minimizing lateral sway.

6. Common Operational Misconceptions and Scientific Myth-Busting

6.1 Myth 1: The Harder and Higher the Arch Support, the More Effective It Is

Many cyclists mistakenly believe that the harder and higher the arch support structure of an insole, the better the stability it provides. However, from a biomechanical perspective, excessively rigid arch support restricts the necessary micro-motion of the subtalar joint during pedaling (approximately 4-6 degrees), paradoxically causing the tibia to bear additional torsional stress. Research shows that optimal arch support should maintain the subtalar joint in a neutral position while still allowing approximately 2-3 degrees of pronation cushioning during the pedal stroke.

6.2 Myth 2: Adding Wedges Will Immediately Improve Knee Valgus

The effects of wedge intervention are not immediate; the neuromuscular system requires at least 2-3 weeks to re-establish motor control patterns. If high-intensity training is undertaken immediately after the intervention, the brain will produce defensive muscle co-contraction due to altered proprioceptive input, paradoxically leading to decreased pedaling efficiency. Wedge thickness should be increased progressively, with each adjustment not exceeding 1-2mm, followed by at least one week of adaptation observation after each adjustment.

6.3 Myth 3: Knee Valgus Is Entirely a Foot Problem, Unrelated to the Hip Joint

Although this article emphasizes the importance of arch support, knee joint trajectory is the result of the entire kinetic chain working together. Weakness in the gluteus medius causes the thigh to adduct and internally rotate during pedaling, exacerbating knee valgus. Therefore, the intervention of arch support insoles and wedges should be conducted in conjunction with gluteus medius strengthening exercises (such as side leg raises and glute bridges with abduction) to achieve optimal trajectory correction.

6.4 Myth 4: Insoles and Wedges Can Only Be Used One at a Time

Custom arch support heat-molded insoles and cleat wedges have different mechanisms of action: the former provides static support for the plantar surface, while the latter adjusts the angle of the foot relative to the pedal. They are not mutually exclusive but can be used in combination based on individual rider needs. For riders with more severe arch collapse, it is recommended to first establish arch support with custom insoles, then determine whether additional wedges are needed based on improvements in knee joint trajectory.

7. Expert FAQ

Q1: How can I determine if I have arch collapse and need professional foot biomechanical assessment?

A1: The simplest preliminary assessment method is the “wet footprint test”: wet both feet and step onto dry ground or paper, then observe the imprint in the arch area. If the imprint in the arch area exceeds one-third of the foot width, there may be a tendency toward arch collapse. However, static arch height does not fully reflect dynamic behavior during pedaling. It is recommended to seek a professional fitter with a sports biomechanics background for dynamic plantar pressure analysis and gait observation to obtain more precise assessment results.

Q2: What is the difference between custom arch support heat-molded insoles and commercially available generic arch supports?

A2: Generic arch supports are designed based on average foot morphology, and their arch support position and height cannot precisely correspond to an individual’s subtalar joint axis, potentially leading to misaligned support. Custom heat-molded insoles use heat-forming technology to precisely match the arch support structure to an individual’s arch morphology and subtalar joint neutral position, providing more uniform pressure distribution and more effective joint alignment correction.

Q3: How should I choose the thickness of wedges? Is thicker always better?

A3: The selection of wedge thickness should follow the principle of “minimum effective dose.” Based on experimental data, a 2mm wedge already significantly reduces the knee valgus angle (from 7.8 degrees to 5.1 degrees), while maintaining rider comfort at 7.8. Increasing to 4mm further reduces the knee valgus angle to 3.5 degrees, but comfort noticeably decreases. It is recommended to start with 1-2mm, observe for one week; if there is no discomfort and knee joint trajectory improves, maintain that thickness; if improvement is insufficient, consider increasing by 1mm.

Q4: After wearing arch support insoles, I feel soreness and swelling in my feet while riding. Is this normal?

A4: Mild soreness and swelling during initial wear is a normal physiological adaptation phenomenon, as the plantar fascia and small muscle groups adapt to the new tension distribution. This sensation typically diminishes after 3-5 rides. However, if the soreness persists for more than one week, or is accompanied by severe plantar pain or numbness, it may indicate that the arch support height of the insole is too high or the position is incorrect. Immediately discontinue use and undergo re-evaluation.

Q5: After arch support intervention, is supplementary ankle and knee joint training still necessary?

A5: Absolutely necessary. Insoles and wedges are passive structural interventions whose function is to provide the “initial conditions” for joint alignment, but dynamic joint stability during pedaling still relies on active neuromuscular control. It is recommended to perform 2-3 sessions per week of ankle stability training (such as single-leg balance and barefoot arch gripping exercises) and knee joint surrounding muscle strengthening (such as squats and split squats) to enhance the active stability of the kinetic chain. Only by advancing both passive support and active control in tandem can long-term optimization of the knee joint’s vertical pedaling trajectory be truly achieved.