The Effects of a Carbon Fiber Shoe Insert on Athletic Performance in Collegiate Athletes

Introduction

An athlete has three major strategies available to improve the work-energy balance during training and competition: 1) maximize energy storage and return, 2) minimize the loss of energy, and 3) optimize muscle functions (Nigg, Stefanyshyn, & Denoth, 2000; Nigg, 2010). Athletes often select equipment, such as running shoes, that is designed to maximize energy storage and return. Manufacturers of athletic footwear such as Adidas and Nike often highlight the enhanced energy storage and return provided by their footwear. However, athletic footwear is typically characterized by poor energy storage and return (~1-2%) due to current technological limitations in midsole construction (Nigg, Stefanyshyn, & Denoth, 2000; Nigg, 2010). Consequently, most of the energy (~98-99%) stored in a shoe midsole upon ground contact is lost through heat, friction, and vibrations, decreasing athlete efficiency.

While the effectiveness of foot orthoses/shoe inserts for injury treatment and prevention has been studied extensively (e.g., Hume, Hopkins, Rome, et al., 2008; MacLean, Davis, & Hamill, 2006; Nigg, Nurse, & Stefanyshyn, 1999), limited research has addressed their role in performance improvement via enhanced energy storage and return. Stefanyshyn & Nigg (2000) demonstrated that increasing shoe stiffness (using carbon fiber plates in the midsole) resulted in a 1.7 cm increase in vertical jump height for 25 subjects. These stiff shoes did not increase stored energy but reduced energy loss at the metatarsophalangeal joint, improving performance. Stefanyshyn & Fusco (2004) examined the influence of shoe bending stiffness on sprint performance with 34 athletes using four shoe conditions. On average, increasing stiffness improved sprint performance, though the optimal stiffness was subject-specific. Roy & Stefanyshyn (2006) found approximately a 1% metabolic energy saving in running economy with stiff midsoles. While these studies showed performance improvements with carbon fiber inserts, they used flat designs with uniform stiffness and limited optimization.

VKTRY Gear (formerly ROAR Performance; Milford, CT) has developed the VK (formerly XG4) carbon fiber shoe insert designed to overcome current limitations in athletic footwear and allow for increased energy storage and return. Anecdotal evidence suggests athletes can run faster and jump higher with VK inserts, but no published data supported these claims. Therefore, this study aims to examine the effects of a carbon fiber (CF) shoe insert on athletic performance, specifically assessing speed and power. It is hypothesized that the CF shoe insert will result in increased linear speed (40-yard sprint) and lower-body muscular power (vertical jump).

Methods

Participants

Twenty-eight male collegiate athletes (NCAA Division II skill-position football players) participated. They had a mean age of 19.4 (SD 1.1) yr, mean height of 178.7 (SD 4.5) cm, and mean mass of 85.9 (SD 6.9) kg. All participants provided informed consent according to Southern Connecticut State University's Institutional Review Board policies.

Instruments

Speed was assessed using a 40-yard sprint, and power was assessed using a vertical jump. For the 40-yard sprint, time was measured using a laser timing system (PowerDash 3, Zybek Sports; Broomfield, CO), and start kinetics were measured using two multi-component force plates (Type 9260AA; Kistler Instrument Corp.; Winterthur, SUI). For the vertical jump, height was measured using a Vertec Vertical Jump Training Measurement System (Jump USA; Sunnyvale, CA), and jump kinetics were measured using a portable Quattro Jump force plate (Type 9290CD; Kistler Instrument Corp.; Winterthur, SUI).

Procedures

Potential participants attended an orientation/familiarization session where study procedures and test protocols were explained. An informed consent form was provided. If an individual agreed, they provided written informed consent in the presence of the principal investigator or a co-investigator.

After providing consent, each participant completed a physical activity readiness questionnaire. Participants with pre-existing heart/pulmonary disease or musculoskeletal injuries were excluded. Height, weight, and arterial blood pressure were measured. Participants were fitted in standard footwear (Visaro Control TF; New Balance Athletics, Inc.; Boston, MA) for testing to avoid confounding effects of different bending stiffnesses.

Within one week of consent, participants reported to the Human Performance Laboratory/Moore Field House at Southern Connecticut State University for vertical jump testing (groups of 2-4). One week later, they returned for sprint testing (groups of 2-4). All participants had completed the football team's off-season strength and conditioning program and were familiar with the tests.

Before each session, participants performed a standardized 15-20 minute dynamic warm-up designed by the National Strength and Conditioning Association, including exercises like walking knee to chest, forward lunge, side lunge, toy soldier, high knees, heel ups, carioca, and sprints.

Vertical Jump Test Procedures

1. The tester adjusted the height of movable color-coded plastic vanes to the athlete's standing reach height. The highest vane reachable determined the standing touch height.
2. The vane stack was raised by a measured distance so the athlete would not jump higher or lower than the set vanes. Corrections could be made on a second attempt.
3. Without a preparatory step, the athlete performed a countermovement by flexing knees and hips, moving the trunk forward and downward, and swinging arms backward. During the jump, the dominant arm reached upward, and the nondominant arm moved downward.
4. At the highest point, the athlete tapped the highest possible vane with the dominant hand. The score was the vertical distance between standing reach height and the highest tapped vane.
5. The best of three trials was recorded to the nearest 0.5 inches or 1 cm. The Vertec was used as a visual target; actual jump height was measured using take-off velocity from the Quattro Jump force plate.

Participants completed a vertical jump test for the control condition and three different carbon fiber (CF) shoe insert stiffnesses: medium (VK 4 flex; 4F), stiff (VK 5 flex; 5F), and extra stiff (VK 6 flex; 6F). Conditions were tested in random order, with 5-7 minutes of rest between conditions.

40-Yard Sprint Test Procedures

1. The athlete warmed up and dynamically stretched for 15-20 minutes.
2. The athlete was allowed three practice runs at submaximal speed.
3. The athlete assumed a starting position using a four-point stance.
4. On an auditory signal, the athlete sprinted the 40-yard distance at maximal speed.
5. The best 10-, 20-, and 40-yard split times from two trials were recorded to the nearest 0.01 second (Figure 2).
6. At least 2 minutes of active recovery or rest were allowed between trials.

Participants completed a sprint test for the control condition and two different carbon fiber (CF) shoe insert stiffnesses: medium (VK 4 flex; 4F) and extra stiff (VK 6 flex; 6F). Conditions were tested in random order, with 5-7 minutes of rest between conditions.

Design and Analysis

Performance and biomechanical data were measured during both vertical jump and sprint tests. For the vertical jump, dependent variables included: jump height, peak force (absolute and relative), impulse (absolute and relative), rate of force development (absolute and relative), total work, average power, and peak power. For the 40-yard sprint, dependent variables included: 10-yard split time, 20-40 yard split time, 40-yard total time, peak propulsive force (absolute and relative for front and back feet), rate of propulsive force development (absolute and relative for front and back feet), and peak propulsive power (absolute and relative). Biomechanical variables for the sprint start (peak force and rate of force development) were analyzed separately for front and rear feet due to sequential ground reaction force patterns.

Kinetic analyses for both tests began when propulsive ground reaction force exceeded participant weight by 10 N and ended when force platforms no longer measured propulsive ground reaction force. Peak force was the maximum force on the force-time curve during the propulsive phase. Impulse was calculated by trapezoidal integration of the area under the force-time curve. Rate of force development was peak force divided by the time to reach peak force. Velocity was calculated from the integration of the force-time curve, multiplied by force, to yield power. Force-time data were sampled at 1,000 Hz and filtered using a fourth-order Butterworth low-pass filter (25 Hz cutoff).

To determine if CF shoe inserts improved performance, a repeated measures ANOVA compared four insert conditions (control vs. 4F/5F/6F) for the vertical jump and three conditions (control vs. 4F/6F) for the sprint. A dependent t-test compared the optimal CF insert condition to the control for all variables. Analysis was performed using Excel software. The significance level was set at p < 0.10, acknowledging that minor expenses from false positives are outweighed by the benefits of performance improvement.

Results

Vertical Jump

Jump Height

No difference in vertical jump height was found across the four shoe insert conditions. However, comparing the control condition to the optimal CF flex condition showed a significant (p < 0.0005) increase in jump height from 62.8 ± 9.4 cm to 65.4 ± 13.0 cm (a 4.1% increase).

Maximal Force

Relative peak force was significantly greater (p < 0.05) in the control and 5F conditions compared to 4F and 6F conditions. Comparing the control condition to the optimal CF flex condition showed a significant (p < 0.05) increase in peak force from 3.12 ± 0.37 BW to 3.20 ± 0.40 BW (a 2.6% increase).

Impulse

No difference in relative impulse was found across the four shoe insert conditions. However, comparing the control condition to the optimal CF flex condition showed a significant (p < 0.05) increase in relative impulse from 0.355 ± 0.027 BW·s to 0.362 ± 0.034 BW·s (a 2.0% increase).

Rate of Force Development

No difference in relative rate of force development was found across the four shoe insert conditions. However, comparing the control condition to the optimal CF flex condition showed a significant (p < 0.005) increase in relative rate of force development from 23.6 ± 4.5 BW/s to 25.4 ± 5.7 BW/s (a 7.6% increase).

Total Work

No difference in relative total work was found across the four shoe insert conditions. However, comparing the control condition to the optimal CF flex condition showed a significant (p < 0.05) increase in relative total work from 11.3 ± 1.2 J/kg to 11.7 ± 1.6 J/kg (a 3.8% increase).

Average Power

No difference in average power was found across the four shoe insert conditions. However, comparing the control condition to the optimal CF flex condition showed a significant (p < 0.01) increase in average power from 3,656.8 ± 531.5 W to 3,795.4 ± 626.4 W (a 3.8% increase).

Maximal Power

No difference in relative peak power was found across the four shoe insert conditions. However, comparing the control condition to the optimal CF flex condition showed a significant (p < 0.005) increase in relative peak power from 85.7 ± 12.1 W/kg to 89.6 ± 14.8 W/kg (a 4.6% increase).

40-Yard Sprint

0-10 Yard Split Time

No difference in 0-10 yard split time was found across the three shoe insert conditions. However, comparing the control condition to the optimal CF flex condition showed a significant (p < 0.05) improvement in 0-10 yard split time from 1.72 ± 0.07 s to 1.69 ± 0.6 s (a 1.6% decrease).

40-Yard Sprint Time

No difference in 40-yard sprint time was found across the three shoe insert conditions. While there was an improvement from 5.06 ± 0.12 s to 5.05 ± 0.12 s when comparing the control to the optimal CF flex condition, this difference was not statistically significant (p = 0.26).

20-40 Yard Split Time

No difference in 20-40 yard split time was found across the three shoe insert conditions. However, comparing the control condition to the optimal CF flex condition showed a significant (p < 0.10) improvement in 20-40 yard split time from 2.18 ± 0.08 s to 2.15 ± 0.12 s (a 1.3% decrease).

Maximal Force (Front Foot)

No difference in front foot relative peak force was found across the three shoe insert conditions. However, comparing the control condition to the optimal CF flex condition showed a strong trend (p = 0.11) for an increase in front foot peak force from 1.03 ± 0.09 BW to 1.05 ± 0.13 BW (a 1.6% increase).

Maximal Force (Rear Foot)

No difference in rear foot relative peak force was found across the three shoe insert conditions. However, comparing the control condition to the optimal CF flex condition showed a significant (p < 0.05) increase in maximal force from 0.762 ± 0.186 BW to 0.824 ± 0.149 BW (an 8.1% increase).

Rate of Force Development (Front Foot)

No difference in front foot relative rate of force development was found across the three shoe insert conditions. However, comparing the control condition to the optimal CF flex condition showed a significant (p < 0.05) increase in front foot relative rate of force development from 11.1 ± 4.2 BW/s to 12.1 ± 3.8 BW/s (an 8.9% increase).

Rate of Force Development (Rear Foot)

No difference in rear foot relative rate of force development was found across the three shoe insert conditions. However, comparing the control condition to the optimal CF flex condition showed a significant (p < 0.10) increase in front foot relative rate of force development from 12.2 ± 3.6 BW/s to 13.3 ± 4.4 BW/s (a 9.3% increase).

Maximal Power

No difference in relative peak power was found across the three shoe insert conditions. However, comparing the control condition to the optimal CF flex condition showed a significant (p < 0.10) increase in relative peak power from 25.1 ± 10.4 W/kg to 27.2 ± 11.3 W/kg (an 8.1% increase).

Summary

The use of a carbon fiber shoe insert can improve performance in the 40-yard sprint and vertical jump. In the vertical jump, jump height increased by 2.6 cm (4.1%) in a group of 28 participants, similar to findings by Stefanyshyn & Nigg (2000). Biomechanical variables measured during the vertical jump (maximal force, impulse, rate of force development, total work, average power, and maximal power) increased between 2.0-7.6% with a CF shoe insert compared to a standard insert, observed in the optimal CF shoe insert condition for each individual. For the 40-yard sprint, 0-10 yard and 20-40 yard split times improved by 1.6% and 1.3%, respectively, similar to a 1.2% improvement in 20-40 yard split times observed by Stefanyshyn & Fusco (2004). Biomechanical variables during the 40-yard sprint (maximal force, rate of force development, and maximal power) increased between 1.6-9.3% with a CF shoe insert compared to a standard insert.

It is imperative that athletes choose the correct CF shoe insert flex to benefit from these improvements. An athlete cannot use an insert of random stiffness and expect improvement; the correct flex must match the athlete's body weight and movement biomechanics. Individual differences in technique and the length-tension and force-velocity relationships of calf musculature may influence the appropriate CF shoe insert stiffness for each athlete to achieve maximal performance.

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