From a macroscopic point of view, sprint running is a very simple task. Realizing optimally large steps at a high stride frequency will result in a high maximal running velocity. To improve running velocity, one of these parameters has to be increased to a greater extent than the other is decreased because they are not mutually independent. Whether step length or step rate has a greater impact on maximal running velocity is still debated and will probably not be resolved in the near future. There is always a trade-off between step length and step rate, and the optimal ratio for peak performance seems to be highly individual (32), which is also supported by data of the actual world's fastest sprinters who differ strongly in terms of step length or step rate reliance (33).
In contrast, there is scientific consensus that the ability to realize short ground contacts is a requirement for elite sprint performance (15,24,26,29,35,36,38). Apart from neuromuscular aspects (26,31), ground contact time is modulated to a great extent by coordinative factors such as sprint technique and the optimal coordination of muscles and limbs (6,22,24). In this context, the concept of “Front Side Mechanics” (23,24) is well-known by sprint coaches and receives substantial focus in the development of elite sprinters. Based on the work of Mann (23,24), who first demonstrated that the best athletes in the world tended to shorten their ground contact by using adequate sprint mechanics, the effects of front side mechanics have also been discussed in several other studies (6,11,36,38). Front side mechanics are typically characterized by greater maximal hip flexion (high knee position), less maximal hip extension (i.e., back swing), and less hip and knee extension at take-off (24). Executing the stride cycle with a focus on leg action in front of the body has positive effects on the touchdown-distance of the foot (i.e., less horizontal distance of the foot to the sprinter's center of mass) by contributing to early stance ground force application (6) and better synchronized foot speed before touchdown (2,24). Thus, sprint athletes manage to produce the majority of applied ground forces in the first half of stance (6,24) facilitating an early shift to the swing phase. A poor sprint technique decreases the effectiveness of the theoretically optimal sprint performance and should be avoided. Therefore, it should be considered that with the implementation of different sprint training methods changes of sprint kinematics might occur.
In light of front side mechanics, the use of training means which shift the leg action towards the back of the body should be minimized or even avoided. Regarding overload-methods, Cross et al. (9) state that running mechanics should be minimally affected and that the disruption to normal sprint kinematics increases with the extent of external loading. Majumdar and Robergs (22) point out that every motor movement impacts the nervous system in a positive or negative manner, and neural pathways shall be trained to behave accurately with the desired movement pattern. Modern training regimes consist of various sprint training methods, such as resisted or assisted sprints, which have shown to distinctly change kinematic and kinetic parameters of the sprint cycle (1,5,8,14,16,25,27,30). The group of resisted sprint training methods particularly addresses the “power-aspects” of sprinting by overloading the required musculature and, therefore, serves as specific strength training. However, assisted methods aim for the “speed-aspects” of sprint running by facilitating the demands of a movement and teaching the nervous system to produce the required forces in less time. Both methodologies are believed to create a positive transfer into the competition movement (i.e., free sprint). However, it is well-documented that performance enhancement is exercise specific and a transfer to related movements cannot necessarily be expected (19,39). To avoid unexpected negative effects, caution is recommended when applying these methods. Therefore, it is essential to know about possible kinematic and kinetic changes based on the applied training interventions and make adjustments towards clear defined aims of a training session. Recently, our research group (17,18) presented a body-weight supporting kite combined with slight towing assistance that reduced ground contact time during full-effort sprints of well-trained sprinters. However, due to the novelty of this device, it remains unclear whether sprinting with this kite implies any adverse effects concerning front side mechanics. If this uncertainty can be abolished, the recommendation for sprint coaches to include kite sprints into their training regime may be strengthened.
Thus, the aim of this study was to clarify the effects of a body-weight supporting kite on full-effort sprint kinematics in general and on front side mechanics in particular. Based on the discussion of possible negative effects on front side mechanics during kite sprints in a previous study (18), we hypothesized that kite sprints will not lead to (a) an increased hip joint extension at take-off, (b) an increased maximal hip joint extension, (c) a decreased maximal hip joint flexion, (d) an increased knee joint extension at take-off, or (e) an increased horizontal distance of the foot to the center of mass at touchdown.
Experimental Approach to the Problem
A 3D-analysis of the high-speed phase of well-trained sprinters during full-effort sprints was used to determine kinematic changes provoked by a body-weight supporting kite in comparison with free sprinting. To achieve equal maximal velocity, as in free sprinting, this kite was combined with a slight towing assistance to counteract its retarding force. Kinematic-dependent measures were ground contact time, air time, horizontal distance of the foot to the center of mass at touchdown and take-off, height of the center of mass at touchdown and take-off, minimal knee angle during stance phase, knee and hip angle at take-off, maximal hip extension, and maximal hip flexion.
Eleven (10 male, 1 female) healthy, Austrian sprinters (mean ± SD, age: 20.5 ± 2.5 years, range: 17–26; height: 1.77 ± 0.07 m, range: 1.64–1.86; mass: 72.2 ± 8.2 kg, range: 55.5–84.0; 100-m personal best (PB): 11.01 ± 0.50 seconds, range: 10.41–12.29) participated in this study. The study was approved by the University institutional review board, and all the subjects provided written informed consent, including parental consent for subjects under 18 years of age.
The study was conducted in the early outdoor season when participants were used to performing full-effort sprints. Four maximal flying 20-m sprints were executed on a synthetic indoor track (Mondo, Alba, Italy) from a standing start with a 40-m approach. The warm-up procedure was performed individually as before a competition and consisted of jogging, mobility exercises, sprint drills, free sprints, and kite sprints. Testing order was as follows: (a) “free sprint,” (b) “kite sprint,” (c) “kite sprint,” (d) “free sprint” with recovery intervals of 25 minutes between running conditions. Intraclass correlation coefficient (ICC) was used to control for test reliability of both free sprints. Results showed excellent ICC values (34) ranging between 0.92 and 0.99 for running velocity, ground contact time, air time, stride length, and stride frequency.
The body-weight supporting kite (NPW-150, Sieger's Vliegers, Harlingen, the Netherlands) was attached to a chest harness (Edelweiss, Saint Chamond, France) beneath the pectoral muscles, allowing normal arm movements during the sprint. Previous studies showed a high lift effect of this kite accompanied by a distinct ground contact time reduction compared with the free sprint (17,18).
The retarding force of the kite in the body-weight supported condition was neutralized by a rope and pulley-towing device to ensure equal running velocity for the kite sprint and the free sprint condition (for details, see Ref. 18). The ICC value for sprinting velocity in both running situations was 0.95, indicating comparable experimental conditions to examine the effects of the kite on kinematic parameters.
Running velocity, ground contact time, air time, stride length, and stride frequency were determined by Optojump-next (Microgate, Bolzano, Italy), which was set up in the high-speed section from 32 to 57 m. The last 2 steps were excluded from analysis, and data of the previous 8 steps were averaged.
Figures 1A–E show the 5 positions during a sprint cycle at which selected parameters were determined. These positions are high knee position (Figure 1A) (defined as the instant of maximal hip joint flexion during late swing), touchdown (Figure 1B), minimal knee joint angle during stance (Figure 1C), take-off (Figure 1D) (with knee and hip joint angles being of great importance), and maximal hip joint extension (Figure 1E). Horizontal distance of the foot to the center of mass at touchdown (d-TD) and take-off (d-TO), height of the center of mass at touchdown (h-TD) and take-off (h-TO), hip angle at take-off (hip-TO), maximal hip extension (hip-ex), maximal hip flexion (hip-flex), minimal knee angle during stance phase (k-stancemin), and knee angle at take-off (k-TO) were captured by a Vicon MX motion capture system (Vicon Peak, Oxford, United Kingdom). These 16 infrared cameras sampling at 400 frames per second were arranged around the measurement sector to record ∼8 meters of the high-speed phase (∼43–51 m). For kinematic analysis, 39 retroreflective markers 14 mm in diameter were attached on anatomical landmarks of the head, thorax, pelvis, right and left humerus, radius, hand, femur, shank, and foot according to the Plug-in-Gait template outlined in Davis et al. (10). Additionally, a digital 912.5 Hz high-speed camera (HCC-1000, VDS Vosskühler, Osnabrück, Germany) was incorporated in the experimental set-up to retrospectively ensure a nonbiased estimation of the time of touchdown and take-off. A custom-made trigger apparatus allowed synchronization of all kinematic devices.
After marker reconstruction, labeling, and in case gap interpolation, random noise in resultant 3-dimensional trajectories was eliminated through a quintic spline filter (Mean Square Error (MSE) = 10) by using Vicon Nexus software version 1.7.1 (Vicon, Oxford, United Kingdom). Joint angles were derived by means of Euler rotations by comparing the relative orientation of the proximal and distal segments (10,13), whereas the computation of the center of mass included mathematical procedures and parameters described in detail by Winter (37).
Data reliability was assessed by calculating ICC (2,1) for the sprint velocity and ICC (2,8) for all Optojump parameters of both free sprints. Bonferroni–Holm-corrected paired sample t-tests were performed to determine differences between both sprint conditions. Cohen's d was used to assess the effect size and classified as small ≥0.20, medium ≥0.50, and large ≥0.80 (7). Statistical significance was set at alpha <0.05. All statistical analyses were computed with PASW 18.0 (SPSS, Chicago, IL, USA).
Mean towing force in the body-weight supported condition was 34 ± 5.2 N (4.8% ± 0.4 BW), leading to an equal running velocity in the high-speed section of the kite sprint compared with the free sprint (9.69 m·s−1 ± 0.43 vs. 9.72 m·s−1 ± 0.50; p = 0.52; d = 0.20). Compared with the free sprint, stride length (SLfree sprint = 217.9 cm vs. SLkitesprint = 217.7 cm; p = 0.86; d = 0.05) and stride frequency (SFfree sprint = 4.46 seconds vs. SFkitesprint = 4.45 seconds; p = 0.56; d = 0.18) remained unchanged. The effects of body-weight supported sprinting on essential kinematic variables and the results from inferential statistics are presented in Table 1.
The objective of this study was to investigate the effects of body-weight support on kinematic stride parameters of well-trained sprinters in the high-speed phase of sprinting during full-effort sprints. The hip joint extension at take-off and maximal hip joint extension in the early swing showed no increase, and the maximal hip joint flexion in the late swing was not reduced during kite sprints compared with free sprinting. Contrary to the hypothesis, we found an increased knee joint extension at take-off because of the elevated center of mass during the stance phase. Moreover, kite sprints distinctly decreased the horizontal touchdown-distance of the foot.
A variety of factors contributes to sprint performance. Proper individually adapted sprint mechanics are one big part of a complex composition, which is based on the individual physical preconditions of an athlete. Training with the body-weight supporting kite clearly aims for improvements in neural motor control with a special focus on decreased ground contact times. The nervous system should be trained to generate high ground forces in less time. Previous studies showed promising results concerning the stride cycle parameters, revealing decreased ground contact time and increased air time while running velocity, stride length, and rate remained unchanged (18). The changes in stride cycle parameters found in this study with −5.6% ground contact time, +5.5% air time, and unchanged stride length and rate confirm the results of the previous study by Kratky and Müller (18), indicating high reproducibility of these effects. To our knowledge, no data of other research groups exist concerning kinematic and kinetic effects of body-weight supported sprint running. Research on body-weight support has only been performed in slower locomotion velocities, like walking or running (for references, see Refs. 17, 18). Results of running studies can hardly be transferred to sprint running because motion techniques differ distinctly. Accordingly, Mann (24) notes that sprinting is not a natural action because of the marked focus on leg action in front of the body. Clark and Weyand (6) reported clear differences in ground force production patterns between runners and sprinters, showing an asymmetric shape of the force curve of sprinters with the major portion of forces generated in the first half of ground contact. One possibility to embed our findings into current scientific knowledge is to compare our data to results of weighted vest sprints, which is the opposite of body-weight supported sprint running. Therefore, opposing effects on stride kinematics might be expected, which is partly confirmed by a recently published study that showed increased ground contact times and decreased stride lengths and air times during vest sprints (9). However, stride length in kite sprints did not differ to the free sprint, which was mainly attributed to the slight towing support (3), whereas decreased stride length in the vest sprints most likely resulted from decreased running velocity. Cross et al. (9) reported no changes of hip joint and knee joint angles at touchdown and take-off in the acceleration phase of vest-sprinting. Our results indicated no negative changes of hip joint angles concerning front side mechanics. Because leg action during kite-sprints tended toward the front side of the body (i.e., decreased hip joint angles at take-off and in the early swing, and increased maximal hip joint flexion in the late swing), we assume a facilitated preparation of an effective stance phase. Only the knee joint extension at take-off was increased indicating a negative effect compared with data of Mann (24). Ito et al. (11) also discussed positive consequences of the “knee-flexing” take-off they found in World-class sprinters and stated that by doing so, the drive velocity of the leg is increased. With respect to the lifting effect of the kite, the greater extension angles in the knee joint at take-off are not surprising. Our data indicate a significant elevated center of mass during touchdown and take-off, and also an increased knee joint extension during stance (i.e., greater minimal knee joint angle of the stance leg). Greater knee joint angles during stance previously have been discussed in association with increased stiffness of the muscle–tendon units, which has been shown to have positive effects on sprint performance (21,26,31). In a study with sprinters, stiffer knee joint levels during stance have been reported to appear with growing sprint velocity (20). A stiffer system contributes to an increased rate of force development at ground contact, causing decreased ground contact times and higher peak forces (31), which might explain the reported impact of kite sprints on ground contact times (shorter stance phase). However, ground reaction forces have not been measured in this study and statements about stiffness remain uncertain.
Several authors show that on the individual level, vertical ground reaction force grows with increasing running velocity (25,26,35); however, it tends to level-off above 70% of maximal velocity (20,29). This seems plausible, as previous studies report a decreased vertical center of mass displacement during air time with growing running velocity (4,26). Because improvements in maximal velocity do not require dramatically higher vertical forces, the required vertical impulse for the next air phase does have to be generated in a shorter time window (35). Taylor and Beneke (33) estimated that some world-class athletes realize ground contact times as short as 70 milliseconds. Such short ground contact times require an extraordinary high developed rate of force development, rather than the ability to generate superior maximal forces (35). The unloading of the body-weight supporting kite can be estimated by applying regression equations that were recently presented by Kratky and Müller (17). The extent of body-weight support was dependent on running velocity and kite size and resulted in a calculated mean lifting force of the kite of 102 Newton (∼10.4 kg). This is the portion of the vertical ground forces that does not have to be produced by the sprinter.
The importance of the horizontal ground reaction force for the 100-m performance was highlighted by Morin et al. (28), who showed that horizontal force production plays a major role, especially in the acceleration phase. However, the generation of horizontal forces during ground contact certainly is also of great relevance in the phase of maximal velocity. The generated horizontal forces are clearly smaller when compared to the acceleration phase; however, they have to be at least as great as the opposing forces that emerge during the amortization phase of stance (24). Realizing the touchdown of the foot horizontally closer to the center of mass will decrease the horizontal breaking force in the first eccentric part of ground contact. A number of studies have reported a positive correlation between a small touchdown-distance and running velocity (24,26,29). The body-weight supporting kite allowed the sprinters to place the foot distinctly closer to the body at touchdown, which may be explained by the elevated center of mass to some extent. However, the enhanced front side mechanics (e.g., greater maximal hip joint flexion in the late swing) allows a better synchronization of the foot speed with the ground at touchdown (24) that facilitates a smaller touchdown-distance. No ground reaction forces were measured in this study, but we speculate that the decreased touchdown-distance in combination with the more extended knee joint will affect breaking phase in a positive way. Less horizontal breaking forces would tilt the resultant ground reaction force vector in a more vertical direction, which would be in line with a demand of Mero et al. (26), who postulated to direct the resultant force as vertical as possible in the breaking phase and as horizontal as possible in the propulsion phase.
In sprint training, the nervous system must be stimulated to act specifically to fast movements (22). It can be assumed that offering these specific stimuli (i.e., shorter ground contacts during full-effort kite sprints) repeatedly to the neuronal system would lead to an adaptation of motor control during free sprints, although some uncertainty remains. A study by Kristensen et al. (19) revealed velocity-specific improvements after 6 weeks of training mainly in the practiced sprint training form (resisted vs. normal vs. assisted sprint), indicating transfer effects being rather small. Interestingly, the group that practiced at the highest movement velocity (assisted sprint group) was the only group that demonstrated improved sprint times in the free sprint as well. A recently published training study (12) reported improvements in the 40-y sprint time after a 6-week acceleration program on a body-weight supporting treadmill; however, the study was conducted with youth female soccer players and the applied training protocols differ distinctly from those usually implemented in sprint training. A longitudinal study of sprinters using the kite on a regular basis is strongly required to eliminate the uncertainty about its long-term effects on sprint performance, and also on sprint cycle parameters.
In summary, we confirmed our previous results, which demonstrated that sprinting with a body-weight supporting kite reduces ground contact time during full-effort sprints. Concerning front side mechanics in sprint running, we found positive effects on hip joint movements resulting in less extension at take-off and subsequent swing, and greater maximal flexion. The elevated center of mass during stance caused a greater extension in the knee joint during take-off, which is discussed as a negative effect. However, increased knee joint angles during stance might indicate a higher stiffness of the system, facilitating the production of the required ground forces in a shorter period; shorter horizontal touchdown-distances of the foot could be realized during kite sprints, which have been shown to be beneficial for sprint performance.
The body-weight supporting kite (in combination with a slight towing support to erase the retarding force) reduces ground contact time during full-effort sprints of well-trained sprinters and can be applied to familiarize the nervous system to the shorter activation pattern with potentially positive transfer effects to the free sprint. Furthermore, kite sprints could be useful in situations, when athletes have problems mastering the requirements of proper sprint mechanics. Increased air time reduces the time pressure to reposition the lower limbs for the next step and might help to execute the sprint technique with an emphasis on front side mechanics.
We strongly recommend using the kite in combination with other training methods and free sprints because monotone stimuli rarely have been shown to be advantageous for 100-m sprint performance.
This study was funded by the Christian Doppler Foundation, Austria. The authors thank Stefan Lindinger, Thomas Stöggl, and Donna Kennedy for critically reading the manuscript and their valuable feedback. There are no conflicts of interest.
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