Effect of Sex-Specific Running Shoes on Female Recreational Runners

Abstract

Alterations in running shoe design have been studied and used in the prevention of injury and enhancement of performance allowing running shoe companies to market to a variety of runners based on skill level, foot-strike pattern, and even sex. These alterations have been shown to affect biomechanical and physiological variables associated with running. Some shoe companies have designed shoes specifically for biological female runners due to the morphological differences found between male and female feet. The purpose of this study is to determine if sex-specific running shoes can alter female runner biomechanics or physiology. Female runners were asked to run in the male and female models of the Altra Torin 4 Plush shoe to determine if there were differences in ground reaction forces (GRFs), sagittal plane joint angles and moments, oxygen consumption (VO₂), respiratory exchange ratio (RER), and perceived level of comfort while running; There were no significant differences in GRFs, sagittal joint angles and moments, VO₂, RER, or perceived comfort; There were no differences in measured biomechanical or physiological variables between the female and male version of the shoes suggesting that the alterations made to the female-specific shoe do not provide any additional benefit to female recreational runners.

1. Introduction

Running shoe design over the years has evolved, leading to a plethora of running shoes manufactured for injury prevention and performance enhancement. With alterations in heel-to-toe drop, midsole thickness, the addition of carbon fiber plates, and overall shoe form, running shoe companies can now market towards a wide variety of runners based on skill level, foot-strike pattern, and even sex [1,2,3,4].

Previous investigations on running shoes utilized GRF variables such as loading rates, impact durations, and vertical GRFs to determine the effectiveness of shoe alterations on injury prevention. Specific findings of some of these variables show mixed results regarding how they may alter biomechanics of the runner. Higher vertical average loading rates and vertical instantaneous loading rates in individuals with patellofemoral pain and plantar fasciitis have been reported along with increased ankle range of motion (ROM) in individuals with plantar fasciitis [5,6]. Additionally, tibial stress fracture rates have been reported to increase with increased vertical and average loading rates [7,8].

While research on altered shoe design has mixed results, it does suggest that differences in shoes may play a role in specific biomechanical variables. Alterations such as heel-to-toe drop modifications, which is the difference in height of the heel and the forefoot, have been shown to affect vertical loading rates or the time over which the GRF is applied. Richert et al. reported greater vertical loading rates and impact peak durations in shoes with 8 mm and 4 mm heel-to-toe drops, which differ from the typical 10–12 mm heel-to-toe drop [9]. When comparing midsole thickness, Hannigan et al. reported a significant decrease in vertical loading rates when comparing maximal cushioning to minimal cushioning [10]. Comparison of joint moments between conditions in running shoe studies has shown that some shod conditions result in higher mechanical stress and improved mechanical advantage in force generation while others showed increased patellofemoral joint moments lead to greater stress on the joint [11,12]. Finally, a study of plantar heel pain individuals shows that those with pain have stiffer soled shoes than those who do not [13] While these studies cannot specifically link injury to shoe design, they do show the potential to alter biomechanical variables which may lead to pathology in runners.

Physiological variables such as VO₂ and RER have been used to determine how alterations such as adding carbon fiber plates impact running economy and the overall metabolic cost of using such shoes. These studies have shown that the use of carbon fiber plates decreased the VO₂ of participants and increased running economy to allow for more effective running mechanics and performance [14,15].

When comparing biological sex, it has been shown that there are significant differences found in lower limb kinematics while running, which have been attributed to anatomical and morphological differences found between the sexes. These differences in morphology have been seen to influence frontal and sagittal plane joint angles of biological females when compared to biological males while running which can have an impact on running-related injuries [16,17,18,19]. One sex-related anatomical difference in the lower limb, which has been studied extensively, is found in the feet showing a difference between overall length, anatomical ball width, heel width, toe height, and instep height [20,21,22,23].

With these anatomical differences in mind, Altra has specifically designed sex-specific shoes by altering shoe arch as well as heel/midfoot width, and forefoot width in hopes to further help prevent injury or enhance performance specifically in female runners [24]. Typically, running shoe companies scale down the size of the male running shoe mold and then alter the color of the shoe without taking into consideration the morphological differences found in female feet hence the industry-wide phrase “shrink it and pink it” [21].

Although there are morphological differences in male and female foot size and shape, there is little biomechanical and physiological data testing the difference between morphologically unique male and female shoe molds. The purpose of this study was to determine if there are biomechanical, physiological, or perceived comfort differences present in female runners while running in sex-specific running shoes compared to non-sex-specific running shoes. It is hypothesized that there will be no significant difference between the male and female shoe regarding GRFs, sagittal plane joint angles and moments as well as VO₂, RER, or perceived level of comfort.

2. Materials and Methods

Sixteen female recreational runners (body mass, 61.9 ± 6.1 kg; height, 169.1 ± 10.3 cm; age, 26.8 ± 7.9; average weekly mileage, 24.1 ± 18.0 miles) the laboratory for a one-time data collection. This study was approved by the Utah Valley University Institutional Review Board (IRB log number 439). Participants were required to read and sign the approved informed consent before participation in the study.

Upon arrival, height, weight, age, weekly mileage, regular running pace, and shoe size were recorded after which a 5-min warm-up was performed on an instrumented treadmill (Bertec Inc., Columbus, OH, USA). Two different footwear conditions were measured, a female shoe (Altra Torin Plush 4), designated as condition 1, and the corresponding male shoe (Altra Torin Plush 4), designated as condition 2. Ten pairs of shoes were utilized in this research project. The male shoe sizes ranged from 7.0–9.5 US (40–43 Europe) and the female shoe size ranges were from 8.5–11 US (39–42 Europe). We ensured that each female shoe size had a corresponding male shoe size to make accurate comparisons. From industry standard sizing charts, the corresponding female shoe to the male shoe is identical in shape and size. Tread patterns were identical between the two shoes. Regarding design differences, the Altra female shoe is marketed as being specifically designed for women with their trademarked “FIT4HER” technology. According to Altra, this technology specifically addresses the unique sex in the female foot as stated on their website [25]. The female shoe has a narrower heel, longer arch, and an altered angle of the footpad under the metatarsal heads compared to the male shoe. In addition, the male and female shoe molds differ in that the male shoe width is built on a D width from the heel through midfoot and a 2E toe box. The female shoe is built on a B width from the heel through midfoot and with a D-E toe box. The insole (5 mm) and stack height (28 mm) are completely identical for both shoes according to the Altra website. Lastly, the female shoe weighs 241 g and the male shoe weighs 286 g [25,26]. Furthermore, footwear characteristics which were applicable were evaluated and compared via previously conducted research [26]. Fit of the two shoes were identical for length, width, and depth. The weight/length ratio was slightly different with the female shoes being about 0.5 to 0.75 smaller than the male shoes. Heel height and forefoot height were confirmed to be the same, all with a zero drop between heel and toe. The forefoot sole flexion point was altered on the female shoe, representing the altered angle of the footpad under the metatarsal heads. All other components of the shoe design in terms of stiffness, stability and cushioning were identical between shoe conditions. A counterbalanced randomization method was used to randomize the shoe conditions. The selected condition was then placed on the participant along with reflective markers on the feet. Marker placement included the distal first or second phalanx, first and fifth metatarsal heads, a four-marker cluster on the posterior aspect of the shoe, medial and lateral malleoli, shank and thigh clusters, medial and lateral epicondyles of the knee joint, posterior superior iliac spine, and anterior superior iliac spine. One static motion capture was taken for each pair of shoes for data analysis before markers on the knee, ankle, and toes were removed. A self-selected running speed was determined for each participant based on the pace at which they were most comfortable running. Participants ran for three minutes to enter a normal gait pattern, following which three 10-s trials of biomechanical and force data were collected using an instrumented treadmill with embedded force plates (Bertec, Inc., Columbus, OH, USA) and a 16-camera motion capture system (Vicon Nexus, Inc., Denver, CO, USA). Physiological data were then collected using indirect calorimetry (Parvo Medics TrueOne Metabolic Measurement System, Sandy, UT, USA) and heart rate data (Polar Electro Inc., Lake Success, NY, USA) for an additional 2 min. Relative VO₂ was determined by taking the average value obtained over the last 2 min. Perceived comfort was assessed by utilizing a visual analog scale (VAS) from 1 to 10. This same protocol was performed for both conditions with up to a 10-min recovery period between each condition. This study was performed and analyzed using a within-subjects design. A paired two-tailed t-test (α = 0.05) was performed to compare discrete variables between conditions. The dominant leg variables were compared between subjects to produce all tables and graphs.

3. Results

Comparison of discrete biomechanical variables showed no significant results as seen in

Table 1. Average dominant leg discrete biomechanical values. Data are presented as mean ± standard deviations with their associated p-values from the paired t-test (α = 0.05).

	Female Shoe (C1)	Male Shoe (C2)	t-Test (α = 0.05)
Kinetics
First Peak Vertical GRF (BW)	971.99 ± 177.99	938.32 ± 202.19	0.1843
Peak Anterior GRF (BW)	135.70 ± 22.35	135.89 ± 22.11	0.8175
Peak Posterior GRF (BW)	156.12 ± 21.57	153.92 ± 20.29	0.1983
Peak VGRF (BW)	1396.59 ± 167.41	1394.31 ± 174.89	0.6825
Angles
Hip
Sagittal Contact (°)	34.16 ± 4.80	34.29 ± 5.01	0.8500
Sagittal Off (°)	−4.63 ± 4.94	−4.51 ± 4.78	0.7646
Peak Adduction (°)	14.09 ± 4.28	14.34 ± 4.10	0.4901
Sagittal ROM (°)	−38.79 ± 3.86	−38.81 ± 3.74	0.6761
Knee
Sagittal Contact (°)	−10.175 ± 3.99	−9.92 ± 4.31	0.6671
Flexion (°)	−37.28 ± 4.59	−37.31 ± 4.40	0.8168
Sagittal Off (°)	−15.24 ± 4.60	−15.35 ± 5.20	0.4563
Adduction (°)	0.63 ± 3.12	0.45 ± 2.83	0.8206
Loading ROM (°)	−27.10 ± 5.32	−27.39 ± 5.30	0.5294
Propulsion ROM (°)	18.40 ± 14.85	18.01 ± 15.44	0.4816
Ankle
Foot Strike (°)	6.00 ± 5.80	6.86 ± 7.03	0.2561
Peak Dorsiflexion (°)	17.27 ± 4.39	17.60 ± 5.00	0.5208
Peak Plantarflexion (°)	−18.77 ± 4.34	−18.97 ± 4.87	0.8619
Sagittal ROM (°)	−32.41 ± 14.86	−32.63 ± 15.69	0.1747
Moments
Hip
Peak Extension (Nm/kg)	−0.93 ± 0.19	−0.93 ± 0.17	0.8976
Peak Flexion (Nm/kg)	0.88 ± 0.14	0.89 ± 0.13	0.4914
Peak Abduction (Nm/kg)	−1.81 ± 0.24	−1.81 ± 0.21	0.9474
Knee
Peak Extension (Nm/kg)	1.91 ± 0.32	1.90 ± 0.36	0.7806
Peak Abduction (Nm/kg)	−0.71 ± 0.22	−0.72 ± 0.24	0.3796
Ankle
Peak Plantarflexion (Nm/kg)	−3.47 ± 063	−3.45 ± 0.63	0.5620
Peak Inversion (Nm/kg)	0.55 ± 0.25	0.53 ± 0.27	0.3352
Peak Eversion (Nm/kg)	−0.08 ± 0.06	−0.08 ± 0.06	0.6444

.

Analysis of vertical ground reaction force (VGRF) variables when normalized by body weight over the stance phase revealed no significant results between conditions. As seen in Figure 1, both shoe conditions produced VGRF curves which appear to be nearly identical.

When comparing sagittal plane joint angles in the ankle, knee, and hip normalized over stance, there appeared to be no significant difference between shoe conditions as seen in Figure 2.

The comparison of sagittal plane joint moments shows no significant difference between shoe conditions as seen in Figure 3.

A comparison of VO₂, RER, and minute ventilation (VE) between conditions showed no significant results as seen in

Table 2. Table showing the comparison of VO₂, gas exchange, RER, and VE between shoe conditions. Data are presented as mean ± standard deviation with their associated p-values from the paired t-test (α = 0.05).

	Female Shoe (C1)	Male Shoe (C2)	p-Value (α = 0.05)
VO2/kg (mL/kg/m)	34.88 ± 4.43	34.94 ± 4.78	0.7090
VO2 (L/min)	2.13 ± 0.27	2.13 ± 0.30	0.6437
VCO2 (L/min)	2.00 ± 0.28	1.99 ± 0.30	0.4741
RER	0.94 ± 0.08	0.94 ± 0.09	0.5115
VE (L/min)	59.21 ± 7.85	58.71 ± 7.76	0.3384

.

Comparison of subject comfort perception showed no significant difference as seen in

Table 3. Comparison of shoe comfort perception between shoe conditions. Data are presented as mean ± standard deviation with their associated p-values from the paired t-test (α = 0.05).

	Female Shoe (C1)	Male Shoe (C2)	p-Value (α = 0.05)
Shoe Comfort Perception	7.37 ± 1.11	7.10 ± 1.44	0.5579

.

4. Discussion

The current study investigated the effect of sex-specific running shoes on female recreational runners by comparing participants’ GRFs, sagittal plane joint angles and moments, VO₂, RER, and comfort perception while running in the female and male versions of the Altra Torin 4 plush shoe. The study found that there were no significant differences in the previously mentioned variables confirming the hypothesis that there would be no significant differences in GRFs, sagittal plane joint angles and moments, VO₂, RER, and comfort perception between shoe conditions. These findings suggest that the alterations made in the sex-specific shoes in this study, were not distinct enough to elicit changes in running biomechanics or physiology.

When comparing the GRF variables produced between shoe conditions, the current study found no significant difference. Most studies differed from the current study in that there were significant differences in the shoe alterations that were being tested. Some examples of alterations that influenced GRFs were varying heel-to-toe drops and midsole thickness. Richert et al. reported a greater vertical loading rate in a 4 mm heel-to-toe drop when compared to 8 mm, 12 mm, and barefoot running [9]. Similarly, when comparing the effect of midsole thickness on GRF variables, it was found that a minimal amount of cushioning produced greater vertical loading rates when compared to maximal cushioning and traditional cushioning [10]. Additionally, when comparing 15 mm, 10 mm, 5 mm and 0 mm drops, Zhang et al. found that patellofemoral joint force was significantly higher in shoes with 15 mm and 10 mm drops when compared to shoes with a 0 mm drop [12]. GRF variables were used in these studies to determine the potential lessening of impact-related forces on lower limb joints when comparing shoe conditions, all of which showed alterations that were then related to the potential of having a decreasing effect on injury in the patellofemoral joint and ankle joint regions as these are areas where injuries most frequently occur in runners [5,6,9,10,12]. When considering female runners, Sinclair et al. reported significantly greater patellofemoral knee loading rates when compared to males, as such women may benefit from a gender specific shoe that contains adaptations to try and reduce these forces [19]. The alterations of the sex specific shoes in this study did not contain any heel-to-toe drop modifications or other midsole differences that have been shown to reduce these variables. If sex-specific shoes continue to be investigated, more substantial changes need to be made to influence GRFs.

Results regarding lower limb joint angles and moments in the current study showed no significant differences when comparing shoe conditions which differ from results used in previous studies. Knee extension moments and knee flexion angles were significantly greater in 15 mm, 10 mm and 5 mm drops when compared to 0 mm drops according to Zhang et al. resulting in conclusions being drawn about potential ways of manipulating patellofemoral joint stress [12]. Similarly, Richert et al. reported that 12 mm, 8 mm and 4 mm drops all had greater ROMs in the knee and ankle when compared to barefoot running conditions while there was no difference in hip ROM between conditions [9]. From these studies, it is apparent that sagittal plane joint moments and angles can be used to determine the effect that shoes have on the lower limb. The current study reported no significant differences in lower limb sagittal plane joint angles and ROM. At the ankle level, there did appear to be a difference in ROM and some discrete variables showing a smaller ROM in the female shoe condition when compared to the male shoe condition. When comparing female recreational runners with plantar fasciitis to healthy controls Pohl et al. reported a significantly greater ROM at the ankle joint in the injured group in comparison to the healthy group, which may be slightly supported by the results of the current study [6]. However, because the results are not significant, it can only be said that there may be a possibility of some benefit at the ankle joint when running in these sex-specific shoes.

The use of physiological variables, specifically VO₂ and RER for performance enhancement in shoe conditions has shown to be effective in comparing marathon racing shoes with and without carbon fiber plating [13]. The study found that carbon fiber plating in the shoe insole decreased VO₂ showing an overall improvement in running economy [14]. In a similar study performed by Whiting et al. when comparing the Nike Vaporfly 4% to conventional racing shoes on varying path grades, they found that overall carbon-fiber plating had greater metabolic savings when using VO₂ and carbon dioxide production as mediating variables in determining the effectiveness of shoe conditions [15]. In comparison to the current study, the alterations made to the insole of the sex-specific shoe when compared to the male shoe showed no significant difference in VO₂ leading to the conclusion that the sex-specific shoe did not provide any metabolic benefit to the user.

One of the main limitations of the study was the assessment of comfort perception. A single-question VAS was used to determine participant level of comfort; however, this method did not provide a full view of participant comfort. Future use of Mündermann’s multi-faceted approach would provide greater insight into running shoe comfort [27].

5. Conclusions

Seeing that there was no significant difference between conditions when comparing GRF, joint moments, and angles, there may not be a biomechanical or physiological advantage to the wearer of the shoe. The alterations in these sex-specific shoes, namely a narrower heel pad, longer arch and altered angle of the footpad under the metatarsal heads, were not distinct enough to change running biomechanics or physiology. Personal preference may warrant the use of sex-specific shoes, but perceived comfort did not show any significant difference between both male and female running shoes when runners were asked about the comfort level of the shoes.

The significance of this research shows that wearing a sex-specific shoe does not alter the biomechanics or physiology of the runner significantly. The purpose behind designing these shoes was to target sex-specific differences in morphology, with no specific claims that these differences would lead to altered biomechanics and/or physiology. The findings of this study suggest that while the shoes were altered to try and meet the morphological differences in feet, the differences were not significant enough to produce any meaningful changes in running biomechanics or physiology. Further research should be conducted on more substantial shoe alterations that are tailored for female feet. Potential methods that target morphological differences found between the sexes at different joints could be another potential avenue to investigate. In addition, studies assessing the use of sex-specific shoes for longer runs or long training periods may provide greater insight into the potential benefits that could come from the shoes presented in the current study.