The Effects of Midsole Bending Stiffness on Ball Speed During Maximum Effort Soccer Kicks

Footwear Science

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The effects of midsole bending stiffness on ball speed during maximum effort soccer kicks

Sasa Cigoja, Jordyn Vienneau, Sandro R. Nigg & Benno M. Nigg

To cite this article: Sasa Cigoja, Jordyn Vienneau, Sandro R. Nigg & Benno M. Nigg (2018): The effects of midsole bending stiffness on ball speed during maximum effort soccer kicks, Footwear Science, DOI: 10.1080/19424280.2018.1538263 To link to this article: https://doi.org/10.1080/19424280.2018.1538263

Published online: 04 Dec 2018.

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The effects of midsole bending stiffness on ball speed during maximum effort soccer kicks Sasa Cigoja* , Jordyn Vienneau, Sandro R. Nigg and Benno M. Nigg Human Performance Laboratory Faculty of Kinesiology, University of Calgary, Calgary, Canada

(Received 7 August 2018; accepted 15 October 2018)

The soccer kick is the most prominent movement in soccer. Kicking performance depends on two major factors: kicking accuracy and ball speed. During the soccer kick, momentum is transferred from the foot/cleat to the ball. This interplay between the foot/cleat and the ball can be modelled as a mixture of ‘impulse-like’ and ‘throwing-like’ components. While kicking the ball, the metatarsophalangeal (MTP) joint and the cleat experience deformation. This deformation can be reduced by increasing the bending stiffness of the cleat’s midsole. Therefore, the purpose of this study was to investigate experimentally the influence of midsole bending stiffness on ball speed during maximum effort soccer kicks. Twenty male subjects (mean ± SD; age: 29.5 ± 5.6 years, height: 175.5 ± 6.4 cm, mass: 74.3 ± 8.4 kg) performed six maximum effort soccer kicks in five stiffness conditions. Kinematic data were collected using an optical motion capture system consisting of eight high-speed cameras, and ball speed was recorded using a radar gun. There was no significant difference in the average ball speed between the five stiffness conditions when all subjects were pooled. Further, there was no significant difference in the change of the MTP joint angle from before ball contact to after ball contact between the stiffness configurations. Thirteen (of the 20) subjects showed the highest ball speed in the two stiffest cleats. The differences between the best and the worst performing stiffness configurations across all subjects ranged from 2.2 km/h to 9.2 km/h. The optimal stiffness condition that resulted in highest ball speed was subject specific, and therefore soccer cleats need to be tuned to individual players. Keywords: footwear; biomechanics; performance; soccer; cleats

Introduction cannot be modelled as an elastic impact only, but rather The soccer kick is the most relevant and most prominent as a mixture of ‘impact-like’ and ‘throwing-like’ compo- movement in soccer (Tsaousidis & Zatsiorsky, 2007). nents due to the relatively long contact time between the Nearly 80% of all goals scored at the 2016 UEFA foot and the ball (Tsaousidis & Zatsiorsky 1996). The European Championship were achieved by playing the interaction between two bodies can be regarded as an ball with the foot (MacKenzie & Sprigings, 2016). Due elastic impact if: 1) the duration of the interaction and to its importance for the sport, the soccer kick has the displacement of the bodies during the collision is received a lot of attention in previous biomechanical small and can be neglected, 2) the external forces are research (Barfield, 1998; Hennig & Sterzing, 2010; small and can be neglected, and 3) the total momentum Hennig & Zulbeck, 1999; Lees & Nolan, 1998; Sterzing, and kinetic energy of the system are conserved. Kroiher, & Hennig, 2008; Tsaousidis & Zatsiorsky, According to the findings by Tsaousidis & Zatsiorsky 1996). The performance of a soccer kick depends on two (1996), none of the above-mentioned assumptions is major factors: kicking accuracy and ball speed (Sterzing valid for the cleat – ball interaction during a soccer kick. et al., 2007). Both factors are influenced by six compo- The authors demonstrated three reasons why the soccer nents of kicking: the approach angle, plant foot forces, kick cannot be modelled solely as an elastic impact: (1) swing limb loading, kinematics of the hip and knee, foot During the contact period, the cleat and ball undergo sub- contact with the ball, and the follow-through after ball stantial displacement together (26.0 ± 2.3 cm). (2) More contact (Barfield, 1998). An optimised interplay of these than 50% of the ball’s speed and at least 30% of its kin- components allows for the most effective transfer of etic energy is imparted to the ball without any contribu- momentum from the foot/cleat to the ball, which ultim- tion of the potential energy of the ball deformation. (3) ately results in a higher ball speed (Hennig & Sterzing, The foot does not decelerate during the contact phase 2010). Previous research has shown that the soccer kick with the ball, despite the fact that forces act from the

*Corresponding author. Email: [email protected]

ß 2018 Informa UK Limited, trading as Taylor & Francis Group 2 S. Cigoja et al. decompressing ball on the foot. This is only possible due the midsole results in greater metatarsophalangeal to contracting muscles during this phase, which generates (MTP) plantarflexion. One way of decreasing the MTP force and produces mechanical work (Tsaousidis & joint deformation is by increasing the midsole bending Zatsiorsky, 1996). This led to the author’s conclusion stiffness of sports shoes (Madden, Sakaguchi, Wannop, that the soccer kick must be considered a movement that & Stefanyshyn, 2015; Oh & Park, 2017; Roy & consists of ‘impact-like’ and ‘throwing-like’ components. Stefanyshyn, 2006; Stefanyshyn & Nigg, 2000; Similar movements that consist of ‘impact-like’ and Stefanyshyn & Wannop, 2016). A study conducted by ‘throwing-like’ components are the golf swing and the Hennig and Zulbeck, (1999) investigated the influence of ice hockey slap shot. Both of these examples also consist different midsole stiffness on ball speed. The authors of elongated contact times, where momentum is trans- reported differences in specific footwear features, such ferred between the equipment (club/stick) and the ball/ as peak midsole deformation and peak midsole deform- puck. In contrast to golf and hockey, soccer equipment ation velocity when performing maximum effort soccer (the cleat) does not bend and store elastic energy prior to kicks in different soccer cleats. However, no systematic contacting the ball. However, the midsole of the cleat relationship was found between these features and ball does deform during the soccer kick (Hennig & Zulbeck, speed. The authors reported that this might be due to the 1999), which could result in energy storage or dissipa- fact that the cleats differed in multiple footwear features, fi tion. For clari cation, the entire soccer boot or soccer and concluded that future studies should use identical ‘ ’ shoe is referred to as the cleat throughout this manu- shoes, only differing in a single footwear feature (e.g. script. The midsole describes the part of the cleat that bending stiffness) to better understand the influence of a connects the studs or spikes with the upper material. specific cleat characteristic on ball speed. In golf, the shaft of the club is bent at the early part Stefanyshyn and Fusco (2004) introduced the concept of the downswing and then recovers just before impact of an individualised optimal midsole bending stiffness for (Milne & Davis, 1992). Depending on the deflection of sprinting spikes that must be tuned to a given athlete for the shaft, more or less elastic energy is stored, and can maximising performance. They found that most sprinters later be released to accelerate the ball (MacKenzie & showed the best sprint performance in shoes with Sprigings, 2009). The authors showed that clubhead increased stiffness, however, if the stiffness was increased speed was higher in the flexible shaft compared to the beyond a certain threshold the benefits were reversed and stiffest shaft. Greater amount of elastic energy can then resulted in slower sprint times. Similar results were found be stored in the shaft during the downswing and released in distance running (Oh & Park, 2017;Roy& at ball contact. This phenomenon follows the principle Stefanyshyn, 2006) and a 25-minute continuous field- of energy return, where sports equipment stores elastic based soccer protocol (Vienneau, Nigg, Tomaras, Enders, energy during deformation, and then can be used to enhance athletic performance when released at the right &Nigg,2016). Mean oxygen consumption rates changed time, location, and frequency (Nigg, Stefanyshyn & as a function of bending stiffness. Oh and Park (2017) Denoth 2000). suggested that the optimal bending stiffness for a given ’ In hockey, the blade of the stick makes contact with athlete would be close to the individual s MTP joint stiff- the ice prior to the impact with the puck, which initiates ness at maximum deformation of the joint during the stick bending. The stick then recoils when in contact stance phase. Further, they proposed that an increased fi with the puck, which propels the puck forward (Pearsall, bending stiffness is bene cial for maximising perform- Montgomery, Rothsching, & Turcotte, 1999). As with ance, provided it does not restrict the movement of the golf clubs, the authors showed that the most compliant MTP joint. These four studies all concluded that midsole hockey stick resulted in the highest shaft deflection dur- bending stiffness can be tuned or optimised to an athlete ing the contact phase, and highest puck speed at release. in order to maximise performance. However, the mechan- Again, more elastic energy is stored in the more compli- ism for increased ball speed as a function of shoe con- ant sticks, and therefore results in greater puck speed struction is not well understood. upon puck contact, which also follows the principle of Therefore, the purpose of this study was to investi- energy return (Nigg et al., 2000). gate the influence of cleat midsole bending stiffness on Based on these findings for golf and hockey, it can the ball speed during maximum effort soccer kicks, and be speculated that the soccer cleat during a kick can be to investigate whether the change in ball speed is related modelled similarly, where energy is stored in the mid- to the amount of deformation of the MTP joint. It was sole due to deformation, which could affect ball speed. hypothesised that ball speed would be higher in more Previous research has shown that midsoles of soccer compliant cleats as more elastic energy can be stored in cleats experience deformation during maximum effort the midsole of the cleat upon ball contact compared to kicks (Hennig & Zulbeck, 1999). This deformation of the stiff cleats. Further, it was hypothesised that the Footwear Science 3 optimal bending stiffness, i.e. the stiffness in which subjects achieve highest ball speed, would be subject-specific.

Materials and methods Twenty male subjects (mean ± SD; age: 29.5 ± 5.6 years, height: 175.5 ± 6.4 cm, body mass: 74.3 ± 8.4 kg) partici- pated in this study. Biomechanical testing was performed at the Human Performance Laboratory at the University of Calgary. The study was approved by the University of Calgary Conjoint Health Research Ethics Board: REB16-2046. A commercially available soccer cleat was Figure 1. Soccer cleats used for the experiment. modified to five different levels of bending stiffness (Figure 1). The bending stiffness was altered by injecting different amounts of plastic material into the midsole Table 1. Soccer cleat characteristics as reported by the during the production process (Table 1). manufacturer. The kicking foot of each subject was equipped with Stiffness configuration C1 C2 C3 C4 C5 nine spherical light-reflecting markers with a diameter of 12 mm to capture MTP joint kinematics (Figure 2). The Midsole bending stiffness [%C1] 100 110 204 426 447 markers were applied by the same researcher throughout the study, and were mounted on the following anatom- ical landmarks: (1): distal phalanx of the great toe, (2): lateral forefoot, (3): distal head of the 1st MTP, (4): distal head of the 5th MTP, (5): lateral heel, (6): lower heel, (7): upper heel, (8): medial malleolus, (9): lateral malleolus. The measurement setup (Figure 3) consisted of eight high-speed video cameras (Motion Analysis Corporation, Santa Rosa, USA) version 3.6.1.1315, which collected kinematic data at a sampling rate of 240 Hz. Ball speed was measured using a StalkerVR Sports radar gun (Applied Concepts Inc., Richardson, USA). Kinematic data were smoothed using a lowpass filter with a cut-off frequency of 8 Hz. The power spec- trum for marker trajectories of the forefoot (locations 2, 3 and 4; Figure 2) for 10 randomly selected trials were determined. The cut-off frequency for each trial was determined using the residual analysis (Winter, 2009). The highest frequency of the 10 trials was selected for all kicking trials to reduce the risk of losing information due to a low cut-off frequency. This allowed for an objective method of determining the cut-off frequency. Subjects were instructed to perform six maximum effort instep kicks in each stiffness condition. The num- Figure 2. Marker locations on the kicking foot from the lateral ber of trials per cleat was determined using pilot data (a) and medial (b) perspectives. where subjects performed ten maximum effort soccer kicks. After six trials the mean ball speed did not change more than approximately 2% of the overall mean. The For each kick, the change in MTP dorsi-/plantarflexion cleat order was randomised to mitigate the risk of fatigue angle from before ball contact to after ball contact was influencing ball speed. The investigators and the subjects computed using a custom-made MATLAB script (The were blinded to the stiffness condition. Reflective tape MathWorks, Inc., Natick, USA; Version: R2018a). The was affixed to the ball, and ball contact was determined change in MTP joint angle in the sagittal plane was as the point in time when the reflection changed position determined as the difference between the joint angle in the anterior direction for the first time during the kick. from one frame before ball contact and the maximum 4 S. Cigoja et al.

Figure 3. Measurement set-up including eight high-speed cameras and a radar gun. Subjects kicked the soccer ball into a net.

Figure 5. Number of subjects who performed best in each Figure 4. Change in MTP dorsi-/plantarflexion joint angle for stiffness configuration. C1 is the most compliant condition and 20 subjects in all stiffness configurations. C1 is the most com- C5 is the stiffest condition. pliant condition and C5 is the stiffest condition. Grey squares represent individual subjects, and black circles represent mean and standard error of all individuals in a given stiffness joint angle and (2) the ball speed between stiffness configuration. conditions. The Spearman rank correlation coefficient was determined for each subject individually in order to investigate the relationship between the MTP joint plantarflexion angle within the first 10 frames (approxi- deformation and ball speed. The significance level a mately 40 ms) after ball contact. This 40 ms period was set to 0.05. included the whole contact phase between foot and ball, which has been shown to be approximately 9 ms long (Shinkai, Nunome, Isokawa, & Ikegami, 2009). Results \Friedman’s test for non-parametric data was used There was no significant difference (p ¼ .57) in the to investigate differences in (1) the change in MTP change in MTP joint angle in the sagittal plane between Footwear Science 5

Figure 8. MTP joint dorsi-/plantarflexion angle of one subject during one maximum effort soccer kick. Larger values repre- Figure 6. Ball speed for 20 subjects in all stiffness configura- sent dorsiflexion, smaller values represent plantarflexion. tions. C1 is the most compliant condition and C5 is the stiffest condition. Grey squares represent each individual’s average; black circles represent average and standard error of all individ- performing better in more compliant cleats and others in uals in a specific stiffness configuration. stiffer cleats (Figure 7). The correlation coefficients between the change in MTP joint dorsi-/plantarflexion angle from before ball contact to after ball contact and ball speed for each individual ranged from 0.8 to 0.4, indicating no systematic relationship across all subjects. Ten subjects showed a negative relationship, six showed a positive relationship, and four showed no relationship between change in MTP joint dorsi-/plantarflexion angle and ball speed.

Discussion In golf and hockey, more compliant golf shafts and hockey sticks experience greater deformation when swings or shots are performed (MacKenzie & Sprigings, 2009; Pearsall et al., 1999). Further, the compliant shafts and sticks result in higher shooting speeds compared to Figure 7. Change in ball speed between most compliant and the stiffer equipment. A possible explanation is that stiffest cleat configuration for 20 subjects. Each line represents more elastic energy can be stored in the compliant one individual. equipment when bent and then released upon contact with the ball or puck, thus resulting in higher shooting speeds. Similarly, it was speculated that more compliant the five stiffness conditions (Figure 4). On average, the cleats would deform more, and therefore store and highest deformation of the MTP joint during the soccer release more elastic energy when performing maximum kick was found in the moderately stiff cleat (C3). effort soccer kicks. This would in return, transfer greater Thirteen (out of 20) subjects had the highest ball energy to the ball, resulting in higher ball speeds, com- speed in the two stiffest cleats, C4 and C5 (Figure 5). pared to kicks in stiffer cleats. However, this hypothesis Four subjects performed best in the second most compli- was not confirmed because first, the MTP deformation in ant cleat (C2) and no subject performed best in the most the compliant cleats was not significantly larger than in compliant cleat (C1). stiff cleats, and second, average ball speed did not differ When all subjects were pooled, the ball speed did between stiffness conditions. A possible explanation is not change significantly (p ¼ .14) between stiffness con- that the elastic energy stored in the bent midsole cannot ditions (Figure 6). However, individual differences in be used to propel the ball because ball release is hypoth- ball speed were observed with some individuals esised to occur prior to the recoil of the midsole. 6 S. Cigoja et al.

Figure 8 shows the MTP joint angle in the sagittal plane of one subject during one maximum effort soccer kick. In preparation for ball contact, the subject plantarflexed the MTP joint. Elastic energy was stored in the cleat during phase I because of midsole deformation resulting from contact with the ball. However, this energy cannot be used to accelerate the ball because ball release occurred before the midsole recoiled and had the potential to transfer the stored energy to the ball. Ball release was estimated to occur approximately 10 ms after ball contact, which has been determined by previous researchers measuring foot/ball interaction during the ball impact phase using two ultrahigh-speed cameras (Shinkai et al., 2009). It is speculated that optimal ball release would occur when the MTP joint angle reaches the angle at ball contact (end of phase II), which would allow the midsole to recoil and return the stored elastic energy to the ball. In golf and hockey, the principle of energy return is applied, where elastic energy is stored in the sports equipment due to bending prior to contact with the ball/puck. In soccer, however, the rationale is not the same as the midsole of the cleat is not bent sub- stantially prior to ball contact, but rather during and after ball contact (Figure 8). Therefore, the principle of energy return cannot be applied to the soccer kick. Instead, the principle of minimising energy loss (Nigg et al., 2000) could be more applicable to the soccer kick. Due to the fact that the recoil of the midsole occurred after ball release, the elastic energy stored in the cleat dissipated fi and was not bene cial to performance. Instead, greater Figure 9. Subject specific differences in ball speed between the cleat deformation (i.e. C1) was disadvantageous and stiffness configurations. C1 is the most compliant condition and resulted in lower ball speed. Therefore, the deformation C5 is the stiffest condition. of the MTP joint should be restricted by the footwear, e.g. with increased midsole bending stiffness, in order to Stefanyshyn, 2006; Stefanyshyn & Fusco, 2004; achieve higher ball speeds. This was partially supported Vienneau et al., 2016 ). This study showed that optimal by the findings. Thirteen out of 20 subjects performed midsole bending stiffness, i.e. the bending stiffness that best in the two stiffest cleats, although there was no sig- resulted in the highest ball speed, was subject specific nificant decrease in MTP joint deformation compared to (Figure 9). However, MTP joint deformation was not in the compliant cleats. systematically associated with midsole bending stiffness As stated by Barfield (1998), the soccer kick and its or ball speed. Therefore, future studies should investigate performance are influenced by many different factors. the individual characteristics (e.g. anthropometric or This study tried to isolate one of these factors, the mid- neuromuscular) that relate to improved performance sole bending stiffness, and investigate its direct effect on in specific stiffness conditions in order to better under- ball speed. However, there are multiple other variables, stand how to optimise footwear bending stiffness e.g. swing limb loading or plant foot forces, which could across athletes. have affected the outcomes of this study. Those factors Hennig and Zulbeck (1999) have shown that differ- have been controlled for as good as possible but it can- ent shoe models influence ball speed. However, because not be ruled out that they have played a major role in the soccer cleats used in their study were different on- why no systematic relationship was identified between the-market shoe model brands, they differed in several midsole bending stiffness of soccer cleats and ball speed footwear features, such as midsole stiffness or upper during maximum effort soccer kicks. material. Therefore, it is difficult to conclude whether Previous research showed that different individuals the changes in ball speed occurred due to differences in perform best in different levels of midsole bending stiff- a specific footwear feature or a combination of multiple ness of sports shoes (Oh & Park, 2017; Roy & footwear characteristics. The current study was the first Footwear Science 7 to systematically change one property of a soccer cleat, point in time. For this reason, MTP joint deformations the midsole bending stiffness, and investigate its effects were determined starting from one frame before ball on ball speed, thereby isolating the effects of a single contact (i.e. 4.2 ms before the ball moves for the first independent variable on ball speed. time). This allowed to measure MTP joint angles one The differences in ball speed between best and worst frame before ball contact, approximately two frames dur- performing cleats ranged from 2.2 km/h to 9.2 km/h. ing ball contact (assuming contact times of 9 ms), and Only three out of 20 subjects showed statistically signifi- eight frames after ball contact. cant differences in ball speed between stiffness condi- Further, impact position was not accounted for. A tions. Although the number of significant differences kick closer to the toes could result in greater deformation was low, an increased ball speed of 2.2 km/h could be a of the MTP joint due to impact forces acting further dis- determining factor for whether a goalkeeper is able to tally to the joint centre. Future studies should try to con- reach the ball before it crosses the line (Hennig & trol for impact position using ultrahigh-speed Sterzing, 2010). The individuals that showed a signifi- video analysis. cant difference in ball speed between stiffness conditions had lower within-cleat variability. A major limitation of this study was the low sam- Conclusion pling frequency of 240 Hz of the motion capture system The midsole bending stiffness of soccer cleats influenced resulting in a resolution of 4.2 ms. Shinkai et al. (2009) ball speed. Some individuals achieved higher kicking showed that contact times between foot/cleat and ball velocities in stiffer cleats while others benefited most are approximately 9 ms using ultrahigh-speed cameras from more compliant cleats. This indicates the existence with a sampling frequency of 5000 Hz. This means that of an optimal midsole bending stiffness that results in the system used in this study would record two data higher ball speeds, and therefore needs to be tuned for points during the contact phase between foot/cleat and each individual. The concept of increased energy stor- ball. Therefore, it cannot be guaranteed that maximum age/release in compliant sports equipment (e.g. golf MTP joint deformation or the points in time where peak clubs and hockey sticks) was not supported by the find- ball reaction forces occur during the contact phase ings of this study, which could be due to the complex were observed. interaction of the foot/cleat with the ball. Another limitation is the use of a radar gun to determine ball speed. All radar guns need to clock objects moving directly at or away from the gun. Clocking at an Disclosure statement angle results in erroneous measurements of ball speed. fl According to the manufacturer of the radar gun that has No potential con ict of interest was reported by been used in this study, the measurement error ranges the authors. between 0.4% and 29.3% when clocking at an angle of 5 degrees to 45 degrees, respectively. In the current study, the radar gun was placed directly behind the ball, ORCID minimising the error. Furthermore, the reflective tape on Sasa Cigoja http://orcid.org/0000-0003-0794-834X the ball that was used to determine ball contact was tracked for ten frames after ball contact to calculate ball speed using the motion capture system. The correlation References coefficient was determined for 561 kicks (20 subjects 5 conditions 6 kicks; 39 trials had to be excluded due to Barfield, W.R. 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