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Original Research

Influence of Initial Foot Dorsal Flexion on Vertical Jump and Running Performance

Faiss, Raphaël1; Terrier, Philippe1; Praz, Manu1; Fuchslocher, Jörg2; Gobelet, Charles1; Deriaz, Olivier1

Author Information
Journal of Strength and Conditioning Research: September 2010 - Volume 24 - Issue 9 - p 2352-2357
doi: 10.1519/JSC.0b013e3181aff2cc
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Abstract

Introduction

The underlying hypothesis in the technical development of athletic footwear is that a particular shoe's characteristic induces a kinematic adaptation that in turn has secondary consequences on kinetics and therefore on performance and injury. For instance (4), it is evident that shoe design, such as sole hardness, influences running kinematics.

In this context, it has been postulated that the position of the foot could influence the performance of a vertical jump. Several studies analyzed whether a higher initial dorsal flexion of the foot (i.e., the heel being lower than the forefoot) may improve jump performance. In fact, the performance of vertical jumps was measured at different angles of dorsiflexion (DF) induced by an adjustable wooden platform with different inclinations (12). Results show that a DF of 3° to 4° was associated with the highest jumps. Moreover, an increased jumping performance was observed in subjects wearing special shoes (prototype) inducing a 5° dorsal flexion of the foot (associated with a reduced flexion of the metatarsal phalange joint) (9). Several mechanisms may explain these results: a) the DF of the ankle may induce a higher torque at the triceps surae level (16,17) or b) the reduced plantar flexion of the metatarsial phalange joint may lose less energy (18).

Furthermore, Arampatzis et al. (1) observed that a low energy cost of running was associated with an increased maximal voluntary strain of the triceps surae (and consequently the Achilles tendon). From these results, it can be postulated that a DF shoe may induce an immediate increase in the triceps surae strength and, consequently, a reduced energy cost of running. However, Arampatzis et al. (1) measured the strength of the contractile component of the muscle, whereas DF shoes may induce an acute increase in strength essentially caused by the stretched elastic components of the calf. Interestingly, muscle activity during running may also have an impact on energy expenditure; for instance, a higher calf electromyographic (EMG) activity observed with DF shoes (6) should increase energy cost of running (11). Conversely, a lower EMG activity of the knee extensors was observed with DF shoes (6) and may, in this case, induce a decrease in energy cost of running. From these results, it is clear that the putative effect of DF shoes on energy cost of running cannot be predicted. Further research is clearly needed to better understand this issue.

To increase the benefit of shoes inducing a moderate DF in sports, we believe that DF shoes must offer the same comfort and ensure the same sensations to the athlete as standard fitness shoes (S shoes). As far as we know, no manufacturer has developed lightweight fitness shoes that would take advantage of DF (enhanced jump performance) without any side effects such as an increase in energy cost of running or discomfort.

The hypothesis of the present study is that DF shoes, with a similar design to S shoes, may present the same advantages as the other experimental DF shoes on jumping performance. Second, compared with S shoes, DF shoes are hypothesized to present similar energy cost of running as well as a better comfort that allows the athletes to wear these shoes without problem during training sessions.

Methods

Experimental Approach to the Problem

To test our hypotheses, we needed to assess whether the type of shoes, that is, comfortable DF shoes versus S shoes have an impact on jump performance and treadmill running economy. Results from the literature suggest that DF shoes may improve the jumping performance (probably by a prestretch of the elastic component of the muscles) with no change in energy cost.

Our goal was therefore to measure, on 2 occasions (i.e., with each type of shoes), the jump performance on a force platform and the energy cost of running on a treadmill by using a crossover design. We decided to proceed with healthy female subjects because the DF shoes were specifically designed for women.

Subjects

Twenty-one healthy female subjects (30 ± 6 yr, 58 ± 6 kg, O2max 45 ± 3 mL·kg−1·min−1, mean ± SD) were included in the study. The experimental risks of the protocol were told to the subjects, who signed an informed consent document before the investigation. The investigation was approved by the ethics committee of the local medical association. Women were recruited in regional fitness clubs. Subjects were in good health and were training an average of 2 or 3 times a week.

Shoes

Standard fitness shoes (Reebok Revent Mid, Reebok, Canton, MA, USA) and DF fitness shoes (Springboost, B-Fit, Scientific Park, EPFL, Lausanne, Switzerland) were used. The DF shoes are characterized by a specific insole inducing 4° of dorsal flexion of the foot; moreover, the shoes were designed to provide a comfortable fit and were not very different than S shoes. To get used to the shoes, subjects were asked to wear the shoes determined for each test at least 1 hour per day during 10 days before the test. However, the sequence of this crossover design was determined randomly to control for any putative training effect.

Procedures

Each subject undertook the testing protocol twice, once with the DF shoes and once with the S shoes, in a random sequence (crossover design). For all tests, subjects were asked to refrain from intense training within the 48 hours preceding the measurements. Subjects were told not to change training and dietary habits during the protocol. Mean body weights of the subjects were not different between tests. Subjects were divided into 2 different groups, 1 group starting the first test with the S shoes and the other with the DF shoes. Both tests were scheduled at the same time of the day to control for circadian variations. A 4-week washout period was chosen between the 2 tests to perform measurements in the same phase of the subjects' menstrual cycle. The whole protocol was completed during Spring in a 4-month period between March and June.

Vertical Jump Performance

A Kistler QuattroJump Type 9290AD force platform (Kistler, Winterthur, Switzerland) (5) was used to measure vertical displacement of the center of gravity, maximal power, and maximal velocity while subjects performed a squat jump (SJ), a countermovement (CMJ) jump, and continuous jumps (CJ) during 15 seconds. All jumps were performed with the hands on the hips. Each subject performed the jumps 3 times, with best jump recorded. During CJ, the 10 best jumps were recorded. This test has a good reliability and reproducibility (8,15).

O2max Measurements

A first visit allowed us to measure subjects' maximum oxygen uptake (O2max) and determine each individual speed for the treadmill running measurements. A Quark b2 indirect calorimeter (Cosmed, Ltd., Rome, Italy) was used to measure ventilation, oxygen uptake, and carbon dioxide values (2). The O2max was determined on a treadmill (RunRace, Technogym, Gambettola, Italy) by using exercise steps of 3 minutes (increase of 1.8 km·h−1 from 5.4 km·h−1 to exhaustion). The O2 data from the last 30 seconds of each step were used for analysis. At the end of each step, capillary blood lactate was determined by taking a blood drop at the fingertip (Lactate Pro, Arkray, Japan). A blood lactate concentration of 4 mmol/L was used as an indicator of the running speed corresponding to the lactate threshold.

Oxygen uptake measurements were made during running after 4 minutes warm-up at 5.4 km·h−1. Subjects were asked to run on a treadmill (RunRace, Technogym) at 95% of the speed corresponding to their individual lactate threshold for 15 minutes. A 7-minute steady state period was then used for calculation. The oxygen uptake values were used to evaluate the energy cost of running (13).

Statistical Analyses

Means and SD are presented. Results were analysed with a 2-factor (treatment, sequence) analysis of variance (ANOVA) for repeated measurements (α = 0.05) (JMP software, version 4.04, SAS institute, Cary, NC, USA).

In parallel, we quantified the effect size (14) between the 2 conditions to better delineate the importance of the difference. Following Cohen's (7) recommendations, we considered that a d (effect size) between 0.2 and 0.5 signified a small effect, d between 0.5 and 0.8 signified a medium effect, and d larger than 0.8 a large effect. The root mean square error of the ANOVA divided by the mean value, which represents the coefficient of variation (CV) (10), has been calculated for variables that did not change significantly with the treatment.

Results

The average results for the jump tests are summarized in Table 1. For the CJ, the height was 22.0 ± 6.0 cm and 17.5 ± 4.2 cm (p = 0.0001), and the mean power was 22.7 ± 7.5 W/kg and 16.3 ± 6 W/kg (p = 0.0001) for the DF shoes and the S shoes, respectively. The DF shoes increased the height of CMJs (+2.2 cm), SJs (+2.9 cm), and CJ (+4.5 cm) as compared with S shoes. The normalized effect size was medium for both CMJ (d = 0.56) and SJ (d = 0.71) but large for CJ (d = 0.81).

Table 1
Table 1:
Results of vertical jumps for countermovement jumps (CMJ) and squat jumps (SJ); comparison between standard (S) and dorsiflexion (DF) shoes.

Moreover, no sequence effect was found for any variable (i.e., results of subjects who began the protocol with DF shoes were not different from those who began with the S shoes). There is either a decrease in performance with time for the height of the SJ (−5%, p = 0.0017), the maximal speed of the SJ (−3%, p = 0.0017), the mean height of the CJ (−6%, p = 0.049), and the mean power of the CJ (−10%, p = 0.03) or an increase in performance for the mean power of the SJ (+4%, p = 0.026). For all remaining variables, no significant differences with time were observed (i.e., for the CMJ and O2 measurements). The CV was 4.5% and 5% for the O2 and height of the CMJ, respectively.

Figure 1 presents the individual jump heights obtained by the subjects under both conditions. A large majority of subjects improved their performance with DF shoes.

Figure 1
Figure 1:
Jump performance. Maximal jump height reached with countermovement jump (CMJ) and squat jump (SJ). Comparison between standard shoes (S shoes) and dorsiflexion shoes (DF shoes). Box plots present extent of data (median and quartiles). Numbers identify subjects.

Figure 2 shows the results for the oxygen uptake. Average of all individual speeds at lactate threshold was 8.7 ± 1.4 km/h. Oxygen uptake was 2.2 ± 0.3 L.min−1 and 2.1 ± 0.3 L.min−1, which represents 84% ± 7% and 83% ± 6% of the O2max for S shoes and DF shoes, respectively (p = 0.468). No significant difference was found between DF and S shoes.

Figure 2
Figure 2:
Oxygen consumption at 95% of speed at lactate threshold during 7-minute steady-state period. Comparison between standard shoes (S shoes) and dorsiflexion shoes (DF shoes). Box plots present extent of data (median and quartiles). Numbers identify subjects.

Discussion

The main finding of this study is that DF shoes are associated with an increase in the jump performance measured on a force platform (Table 1). These results are in accordance with those of Larkins et al. (12), who reported that a 3° to 5° DF of the ankle induced higher jumps. As discussed above, this positive effect may be partly caused by higher torque induced by the muscle triceps surae in a DF position (17). In other words, it can be postulated that an initial ankle DF allows starting the jump with a more stretched tendinous structure and, consequently, an increased stored elastic energy (3), which may be used during the pushing phase of the jump.

We observed no significant difference in energy cost of running between shoes. As mentioned above, DF shoes may act through several mechanisms (i.e., stretching the muscle or inducing better muscle fiber recruitment) that may either increase or decrease energy cost of running. The present study was exploratory and not designed to investigate the factors that may influence energy cost.

From these results, the slight decrease in energy cost observed with DF shoes (1.4%, p = 0.47, very low effect size, d = 0.09) strongly suggest that these shoes do not impair running performance. However, whether DF shoes may actually have a beneficial effect on running performance remains to be investigated more thoroughly. For instance, it is possible that slight changes in energy cost were not observed in our study because of a lack of power in our protocol. For instance, it can be calculated that the present protocol design may only detect changes in energy cost of over 4% (power = 0.8). It is therefore not excluded that DF shoes may influence performance through slight changes in energy cost of running (i.e., of several percentage points).

In addition to energy cost considerations, it should be noted that during the 10 days of the pretesting period, wearing DF shoes was well accepted by the subjects. They did not report pain or discomfort, and neither were they hindered in their sport activities. It appears therefore that a moderate DF can be obtained without sacrificing comfort and without diminishing endurance performance. Further studies are needed to analyze the impact of DF shoes on the risk of injuries and to delineate the effect on different aspects of sport performance. Moreover, our study consisted in jumps with healthy but rather inexperienced women; results may therefore differ with elite players, who certainly present better coordination and jumping experience.

It can be concluded that the DF shoes improve the height of vertical jumps. This effect is probably caused by an increase in the force of the triceps surae exerted during the push-off period. Finally, DF appears to have little influence on energy cost of running and, consequently, may not impair performance of endurance running.

Practical Applications

Our results strongly suggest that wearing DF shoes increases jump height without impairing running performance. In several sports, repeated high, vertical jumps are needed to improve performance (as in volleyball). Wearing DF shoes could lead to a better global performance of the team even though multiple technical and tactical factors are still needed to win. If our results are confirmed by others, sports federations may take them into consideration to highlight any unequal advantage in the game provided by DF shoes.

Because wearing DF shoes did not require more energy during running compared with S shoes, players in sports where both jumps and running are involved (e.g., basketball) should consider DF shoes as a good alternative. On the other hand, fitness enthusiasts will not spend more energy while running with DF shoes. Finally, the concept of dorsal flexion of the foot appears to allow for immediately higher jumps and, consequently, better performance.

Acknowledgments

This research was supported by a subvention of Springboost.

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Keywords:

ankle; fitness shoes; energy cost; force platform

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