1 Literature review
Parkour is one of the newest sports practised today, having only been recognised by Sport England in 2017 (Parkour UK, 2017). Parkour is the traversal of an environment and the obstacles within it, but the nature of that traversal can differ between practitioners (known as traceurs) and involve a wide array of movements (O’Loughlin, 2012). While influenced by movements from a broad range of disciplines, including dance and acrobatics (Aggerholm and Højbjerre, 2017), some movements have emerged as identifiably distinct parkour techniques (O’Grady, 2012). The establishment in the UK of accredited coaching qualifications and a national governing body has helped codify some parkour techniques (Sterchele and Ferrero Camoletto, 2017), but data is still sparse on the physical demands of parkour with traceurs often reliant on personal experimentation or anecdotal evidence spread via word of mouth and the internet to guide their training (Gilchrist and Wheaton, 2011). Data on the kinetic and kinematic effects of parkour practice may help coaches and traceurs further develop the sport, as understanding the forces produced by a sporting technique can help improve performance or reduce injury risk (McNitt-Gray, 2008). Analysing the ground reaction forces (GRFs) produced when performing these parkour movements can give an insight into their effect on traceurs.
The level of GRF produced is described as a key indicator of the level of mechanical stress applied to the body during movement (McClay et al., 1994). GRFs are the forces exerted by the ground on a body in contact with it, measured by executing the movement in question on a force plate. Force plates measure in three axes: vertical, anterior-posterior, and medial-lateral (Bartlett, 2007). The harder an athlete pushes against the ground about one of these axes to accelerate or decelerate themselves, then by Newton’s Third Law, the more force is applied back onto the athlete’s body in the same axis (Blazevich, 2007). Vertical (vGRF) and anterior-posterior axes are of the most interest to biomechanists for jumping or running related activities when studying performance or causes of injury (Hunter, Marshall and McNair, 2005). Anterior-posterior GRF is further subdivided into braking and propulsive forces. Braking GRFs (bGRF) occur as the foot first contacts the floor and resists forward motion, with propulsive GRFs following as the centre of mass passes over the midpoint of the foot and the lower limb pushes off the floor to accelerate forward (Cavanagh and Lafortune, 1980). GRFs fluctuate throughout a movement, but the maximum or peak level of GRF produced during execution is often analysed as an important measure of the maximum mechanical stress applied to a body resulting from a movement.
1.2 Parkour research
Research into GRFs in parkour has commonly aimed to compare traceurs with athletes from other sports or the untrained, with a predominant focus on vGRF in jumping and landing tasks. Jumping is often used as a predictor of general athletic ability (McLellan, Lovell and Gass, 2011), with traceurs shown to reach significantly greater heights in drop and countermovement jumps than gymnasts and power athletes (Grosprêtre and Lepers, 2016). Grosprêtre, Gimenez and Martin (2018) attribute this to the large amounts of eccentric lower limb training traceurs undertake, as well as increased neuromuscular coordination for jumping tasks. The forefoot-only landing technique employed by traceurs, known as the precision landing, has also been studied. Forefoot landing is not a new concept, with forefoot landings known to reduce vGRF in basketball players for some time (Gross and Nelson, 1988). However, the focus on keeping the heel raised as opposed to allowing it to lower to the floor may differentiate the parkour precision landing from other traditional landing strategies. This position reduces the contact area of the foot with the floor and consequently requires good postural control to maintain stability after landing (Maldonado et al., 2015), which may explain why it has not seen widespread use before adoption within parkour.
Precision landings have been shown to produce lower vGRF when compared to traditional landings in a study by Puddle and Maulder (2013). However, traceurs also performed the traditional landings in this study, which may not have been habitual to them and may have distorted the results. Standing and Maulder (2015) subsequently found very similar results to Puddle and Maulder, with decreased vGRF in landing tasks by traceurs when comparing landings by traceurs to recreational athletes. Standing and Maulder further speculated that the significantly longer time traceurs took to reach maximum vGRF on landing allowed for improved dissipation of mechanical forces acting upon the body. Increased time to peak vGRF was attributed to high eccentric strength in the lower limb, as the more an athlete can eccentrically prolong a landing, the lower peak vGRF they produce (Cortes et al., 2007). The role of the knee was also emphasised, with greater knee flexion found in landings by traceurs contributing to the majority of energy dissipation compared to the ankle and hip joints.
1.3 Landing injuries
The ability of traceurs to minimise vGRF when landing is critical for a sport focused on jumping. Large amounts of vGRF during landings are associated with an increased risk of acute lower limb injury (Elvin, Elvin and Arnoczky, 2007). Most commonly, trauma occurs to the anterior cruciate ligament (ACL) of the knee (Ingram et al., 2008). ACL injuries occur more than any other acute knee injury (Majewski, Susanne and Klaus, 2006) and are caused by excessive anterior translation of the tibia in relation to the femur (Sell et al., 2007) due to high valgus motion and abduction forces at the knee joint (Shin, Chaudhari and Andriacchi, 2009). Due to their proximity and the mechanism of injury, injury to the ACL is also often associated with damage to other ligaments and the menisci of the knee (Stevens and Dragoo, 2006). Good deceleration technique to reduce vGRF is a key component of decreasing the likelihood of ACL injury when landing (Silvers and Mandelbaum, 2011). Further, a study of lower limb biomechanics in stop jump tasks by Yu, Lin and Garrett (2006) found that reducing joint stiffness by increasing active knee flexion during landing is important for reducing the risk of ACL injury. The precision landing results in large amounts of flexion and energy dissipation in the knee (Maldonado, Soueres and Watier, 2018), potentially reducing the risk of acute injury to the structures of the knee when landing (Butler, Crowell and Davis, 2003).
Increased flexion in landings, however, is in turn associated with an increase in the risk of chronic knee injuries (Derrick, 2004). Tendinopathy of the patellar tendon is the most common chronic injury in jumping sports, earning the colloquial name “jumper’s knee” (Myer et al., 2015). Patellar tendinopathy is common in other jumping sports such as volleyball and basketball (Cook et al., 2000), affecting up to 50% of volleyball players (Lian, Engebretsen and Bahr, 2005), and can be difficult enough to rehabilitate that it can end athletic careers (Zwerver, Bredeweg and Akker-Scheek, 2011).
The patellar tendon transmits the muscular force of the quadriceps muscles to the tibia, facilitating knee extension concentrically and resisting knee flexion eccentrically (Tan and Chan, 2008). Muscle force generated by the quadriceps can be up to three times larger in eccentric muscle contraction than concentric (Stanish, Rubinovich and Curwin, 1986) and athletes who exhibit higher knee extensor loads are at increased risk of patellar tendinopathy (Visnes, Aandahl and Bahr, 2012). The demanding eccentric component and high degree of knee flexion in the precision landing could, therefore, cause very high levels of patellar tendon loading (Witvrouw et al., 2000), causing microtrauma to the tendon (Peers and Lysens, 2005).
If not allowed time to repair, the cumulative effect of these microtraumas could mean a high risk of degenerative damage developing within the tendon (Galloway, Lalley and Shearn, 2013). Appropriate eccentric strengthening of the knee extensors may mitigate this (Seynnes et al., 2009), but tendon microtrauma recovery and strength increases take time (Prilutsky, 2008). Experienced traceurs train up to an average to 12 hours per week (Grosprêtre and Lepers, 2016), a volume that may not allow for adequate tendon recovery from microtrauma (Visnes and Bahr, 2013). The precision landing technique, while potentially reducing the risk of acute ACL injury, could instead place a traceur at an increased risk of chronic patellar tendinopathy when training over an extended period.
1.4 Parkour vaults
The precision landing technique is taught and encouraged not just when jumping but throughout all parkour movements. Movements in parkour are commonly taught with the cue “land quietly”, with sound used as a coaching tool in the field to judge landing quality (Standing and Maulder, 2015). This leads to an adoption of the precision landing as the default landing technique for all parkour movement. Consequently, the acute and chronic injury risks of precision landings may be relevant to other parkour techniques or even the sport as a whole, not solely two-legged jumping and landing tasks.
One common area of movement in parkour concerns vaulting an obstacle. Parkour vault (PKV) techniques focus on traversal of an obstacle that is too high to directly jump over but not high enough to require climbing; usually between hip and shoulder height. As an example, indoor parkour training often uses gymnastic vaulting horses or tables with a standard height of 1.35 m (Fédération Internationale de Gymnastique, 2017). PKVs are also often performed outdoors, over and onto solid surfaces such as concrete rather than the padded vaulting horses and crash mats used in gymnastics. As the traceur must clear the height of the obstacle to pass over it, there is potential for numerous repetitions of high GRF landings onto hard surfaces during PKV training and subsequently a high potential for injury.
However, a PKV does not necessarily equate to a direct drop from the height of the obstacle. Using the hands or feet as support on the obstacle may reduce vertical drop velocity, but different levels of support provided by different PKV techniques could result in an equally varied range of GRFs. PKVs also emphasise the maintenance of horizontal speed throughout, rather than converting horizontal speed to vertical height as in a gymnastics vault (Koh et al., 2003). Given an emphasis on horizontal motion, the traceur’s centre of mass may not follow a path directly up and down on either side of the obstacle. It may instead take a much shallower projection arc, like those seen in long jumping (Linthorne, Guzman and Bridgett, 2005), reducing the resulting drop height on the far side of an obstacle.
Three common PKVs are the step vault, the kong vault, and the dash vault. The step vault involves using the hands and a foot on the obstacle to pass over it, while the kong and dash vaults only involve the use of the hands on the obstacle. Step vaults are commonly the first vault taught to beginners due to their relative safety and increased ability to control the movement throughout an extended contact time with the top of the obstacle (Gerling, Pach and Witfeld, 2013). The landing strategies used to exit a PKV can also vary. A traceur may come to a complete stop on two feet (precision landing style), or land with a single foot and keep moving (running landing style), often used to transition into a run or link with another technique. Even when landing on a single foot, the forefoot style landing of the precision technique is encouraged by coaches, but not exclusively depending on the preceding technique and desired outcome.
Running style landings may increase the risk of acute lower limb injury in PKVs. vGRF significantly increases when switching from a two-legged to single-legged landing in drop landing tasks, with an associated increase in the risk of acute injury (Yeow, Lee and Goh, 2011). Endeavouring to maintain horizontal velocity in a running style landing may also lead to a conscious decrease in lower limb joint flexion to avoid excessive downward travel of the body. Reductions in lower limb joint flexion have been found to increase vGRF in other jumping sports such as volleyball (Bisseling et al., 2007) and potentially contribute to lower limb injuries in gymnasts (Seegmiller and McCaw, 2003). Contact time between the foot and the floor also decreases as horizontal velocity increases (Grabowski and Kram, 2008). A short ground contact time would not allow traceurs to fully apply the long eccentric phase of the precision landing technique, resulting in higher vGRFs. This could lead to a reversal of the injury scenarios posited for the precision style landing, with increased vGRFs and reduced knee flexion in running style landings increasing the risk of acute injury (Kulig, Fietzer and Popovich Jr, 2011).
As well as vGRF, landings also produce bGRF, particularly in jumping exercises involving horizontal motion (Kossow and Ebben, 2018). The level of bGRF produced varies according to the movement executed prior to landing, with varied angular momentums in the air producing different bGRFs in models for gymnastic techniques (Mills, Pain and Yeadon, 2009). bGRF has been found to increase with greater vertical travel in a movement (Gottschall and Kram, 2005), a stiffer knee joint, (Milner et al., 2006), and an increase in horizontal speed (Gutekunst, Frykman and Seay, 2010). While bGRF is less commonly associated with acute injury in jumping sports, increases in bGRFs have been linked to stress fractures in runners (Zadpoor and Nikooyan, 2011) and with patellar tendinopathy in dancers (Fietzer, Chang and Kulig, 2012). Maldonado, Soueres and Watier (2018) recorded bGRF in the precision landing and found that it was lower in traceurs than an untrained subject in drop landings, but still increased at greater drop heights. This indicates that frequent repetitions of high bGRFs may also be a risk factor for chronic injuries in traceurs.
Such a wide variety of contributing factors mean that it is currently unclear how GRFs may vary between PKVs and landing styles. As PKVs are some of the most frequent movements traceurs perform, the forces exerted upon the body during their execution need to be understood to effectively allow safe and appropriate training. Some variable elements in PKV execution (support on the obstacle and shallow projectile arc) may reduce GRFs, while others (single leg landings, obstacle height, and reduced ground contact time) may have the inverse effect.
The aim of this study is to examine the vGRF and bGRF experienced by a single limb during three common parkour vault techniques and a drop jump of an equivalent height, performed with both two-legged precision and one-legged running style landings. The research hypothesis is that vGRF and bGRF will differ between the four movements and between landing styles.