Abstract | Svrha provedenog istraživanja bila je unaprijediti znanstvenu osnovu treninga sportskog hodanja, temeljem biomehaničke analize kretanja na uzorku od 26 ispitanika – aktivnih natjecatelja i početnika u različitim disciplinama sportskog hodanja, u dobi od 13 do 37 godina, iz četiriju zemalja. Cilj istraživanja bio je utvrditi utjecaje odabranih kinetičkih i kinematičkih varijabli na maksimalnu silu reakcije podloge u prvoj fazi koraka te temeljem toga definirati indikatore kvalitetne tehnike sportskog hodanja. Provedeno je mjerenje odabranih morfoloških karakteristika prema IBP protokolu te raspona pokreta prema standardnom goniometrijskom protokolu zbog prikupljanja osnovnih podataka važnih za analizu kinetičkih i kinematičkih varijabli. Mjerenjem pri pojedinačno specifičnoj natjecateljskoj brzini kretanja tehnikom sportskog hodanja, prikupljeni su kinetički podaci pomoću platforme za mjerenje sila reakcije podloge Kistler, model 9286 i kinematički podaci pomoću triju kamera Panasonic DMC-FZ200 te sustava Optojump Next.
U fokusu analize bila je maksimalna sila reakcije podloge u prvoj fazi koraka te identifikacija sastavnica koje na nju imaju znatan utjecaj. Ne temelju prikupljenih podataka provedeno je testiranje regresijskog utjecaja 48 prediktorskih varijabli na navedenu kriterijsku varijablu te je za 13 varijabli provedena detaljnija analiza. Za 6 najznačajnijih varijabli provedena je višestruka regresijska analiza utjecaja, u zajedničkom djelovanju s relevantnim varijablama koje su s njima imale zadovoljavajuće malu kolinearnost.
U okviru disertacije testirane su dvije osnovne hipoteze. Prva hipoteza - kako postoji statistički značajna povezanost između sile reakcije podloge u drugoj fazi koraka i sile reakcije podloge u prvoj fazi koraka sportskog hodanja - prihvaćena je u potpunosti. U okvirima ograničenja uzorka i primjenjenih analitičkih metoda utvrđeno je da varijabla omjera prosječne relativne sile reakcije podloge u intervalu 70 % - 80 % i prosječne relativne sile reakcije podloge u cjelokupnom trajanju faze kontakta samostalno objašnjava 55 % varijance kriterijske varijable, a u zajedničkom djelovanju s varijablom torakolumbalne ekstenzije objašnjava 66 % varijance kriterijske varijable. Bitan je utjecaj nagiba krivulje relativne sile reakcije podloge u fazi odraza u istom intervalu 70 % do 80 % trajanja kontakta, jer ova varijabla u zajedničkom djelovanju s varijablama zbroja kutova rotacije zdjelice i torakolumbalne lateralne fleksije i pretklona/zaklona trupa objašnjava 71 % varijance kriterijske varijable.
Druga hipoteza - kako postoji statistički značajna povezanost promatranih kinematičkih varijabli i sile reakcije podloge u prvoj fazi koraka sportskog hodanja - prihvaćena je
djelomično s obzirom na to da za kutove stopala prema podlozi fleksiju i ekstenziju kuka i koljena, kao niti za zamah rukama i torakalnu rotaciju nisu nađene značajne statističke veze s maksimalnom silom reakcije podloge u prvoj fazi koraka.
U okvirima ograničenja uzorka i primijenjenih analitičkih metoda utvrđeno je da na sile reakcije podloge u prvoj fazi koraka sportskog hodanja značajno utječe duljinski i vremenski omjer druge i prve faze koraka, ukupna duljina koraka, trajanje faze leta, rotacija zdjelice, torakolumbalna lateralna fleksija i torakolumbalna ekstenzija. Ukupna duljina koraka u zajedničkom djelovanju s varijablama rotacije zdjelice, torakolumbalne lateralne fleksije te pretklona/zaklona zdjelice objašnjava 73 % varijance kriterijske varijable. Trajanje faze leta u zajedničkom djelovanju s varijablama torakolumbalne ekstenzije i rotacije objašnjava 75 % varijance kriterijske varijable. Omjer trajanja dviju faza koraka u zajedničkom djelovanju s varijablama duljine koraka, torakolumbalne ekstenzije i torakolubalne lateralne fleksije objašnjava 64 % varijance kriterijske varijable.
Osnovni je nalaz i znanstveni doprinos istraživanja (u okviru ograničenja veličine i strukture uzorka i metoda istraživanja) da je za smanjenje maksimalne sile u prvoj fazi koraka sportskog hodanja osobito važno smanjiti trajanje faze leta, smanjiti torakolumbalnu ekstenziju, a povećati rotaciju zdjelice i torakolumbalnu lateralnu fleksiju, kao i silu prema podlozi u drugoj fazi koraka. Nužno je osigurati da je brzina kretanja sportaša u granicama u kojima razvoj njihovih motoričkih sposobnosti omogućava način kretanja koji uključuje opisani raspon pokreta. |

Abstract (english) | Racewalking is an integral part of athletics with competitions at the Olympic Games and World Championships, defined by two basic rules: athletes must maintain contact with the ground in such a way that there is no loss of contact (visible to the human eye), and the legs must be straightened (i.e. not bent at the knee) from first contact with the ground to the vertical upright position. For this reason, racewalking coaches consider the extended knee phase as critical in teaching a compliant technique while avoiding excessive ground reaction forces (GRF) that lead to the most common overuse injuries in racewalking (hamstring tears, shin splints and knee problems), as the knees are extended in the first phase of the stride when the peak forces occur. The main problem addressed by this dissertation is therefore how racewalking can be performed without generating excessive forces that can lead to injury. The main objective of the analyses conducted was to identify the main biomechanical indicators that show a statistically significant relationship with the peak GRF immediately after initial ground contact. There are few studies dealing with young and inexperienced novice racewalkers and no scientific recommendations have been defined on the key indicators for high-quality and low-impact racewalking training. Therefore, the scientific contribution of this doctorate is the improvement of the knowledge base of racewalking training by defining the key performance indicators that address the main research problem – the control of excessive ground reaction force in the first phase of the racewalking stride.
The dissertation consists of eight chapters. The first chapter contains an introduction to the research problem, which is based on a descriptive analysis of the kinematics of racewalking in comparison to running over a marathon distance. The main characteristics of racewalking are knee extension in the first phase of the stride, greater pelvic rotation and a smaller range of hip motion with a flight phase of much shorter duration than in running. Hanley et al. (2019) concluded that even at low speeds in racewalking (3.0 ms-1), a short flight phase occurs, while the speed in racewalking at which the visible flight phase occurs (about 0.04 seconds) is about 3.9 ms-1 for male athletes and 3.6 ms-1 for female athletes. The kinetic model of racewalking describes the ground reaction forces with a significantly lower peak than in running and a difference between "N" and "M" shapes of the (vertical) GRF curve. The results of studies on injuries in racewalking are presented, showing that injuries are relatively common and specific,
with the overuse injuries to the hamstrings, shins and knees being attributable to the forces towards the ground in the first phase of the stride.
The second chapter describes the main objective of the study, which is to assess the impact of kinetic variables (GRF in the second phase of the stride) and kinematic variables (stride length, ratio of stride phases, ankle, knee and hip flexion and extension, pelvic and thoracic flexion and rotation, and arm swing) on peak GRF in the first phase of the racewalking gait. In the third chapter, two basic research hypotheses are put forward, namely that in racewalking there is a statistically significant influence of the GRF in the second phase of the stride on the peak GRF in the first phase of the stride (H01) and that there is a statistically significant influence of the observed kinematic variables on the peak GRF in the first phase of the stride (H02).
The research methods used are presented in the fourth chapter. The laboratory tests were conducted with 33 participants, of which the results of 26 participants were used for the detailed analyses, while the research protocol could not determine representative GRF curves for 7 participants. The 26 participants were 16 women and 10 men aged 13 to 37 years from 4 different countries. Seven participants were elite senior athletes, another 3 participants were junior elite athletes, 8 were national competitive athletes, and 8 participants were novices who had competed in racewalking. All tests were conducted bilaterally, increasing the sample size to 52. The athletes racewalked within +/- 5% of their individually determined pace, which was based on the previous season's best competition results at the distances at which the athletes primarily compete. Each participant had to perform at least 12 correct trials by positioning each foot six times on the Kistler force plate (model 9286, Winthertur, Switzerland) with a recording frequency of 1,000 Hz. Of these 6 trials, 3 were selected as representative using the least squares method. The problem of different speeds of the participants was solved by defining speed as one of the independent variables in the regressions. Kinematic data were collected with Panasonic cameras (DMC-FZ200, Japan) in all three planes and with the Optojump Next system. A total of 10 kinetic and 38 kinematic variables were analysed in a simple linear regression with peak GRF as the dependent variable. As the coefficients of determination were below 0.20, 29 variables were excluded, while a further 6 variables were not included due to redundancy. The remaining 3 kinetic and 10 kinematic variables were considered relevant for the detailed analyses. Of these 13 variables, 6 were selected as the most important, i.e. as leading variables with coefficients of determination greater than 0.40. For these variables, 6 multiple regression models were tested linking the leading variables to the variables that were not highly
correlated with them. The multicollinearity test was performed based on the variance inflation factor, while the G*Power programme was used to calculate the adequacy of the effect size and the statistical significance of the variables in the multiple regressions, considering the number of independent variables.
The fifth chapter presents the most important results of the analyses conducted. Of the 26 participants, 15 participants had predominantly M-shaped GRF curves with two distinct maxima, while 7 participants had N-shaped GRF curves with a single maximum and 4 participants had atypical curve shapes. The regression analysis showed that the variance of the peak GRF in the first phase of the stride can be explained by the indicators of GRF before toe-off in the interval 70-90% of the contact phase with a large coefficient of determination (R2 = 0.56). Racewalking speed is also an important factor in explaining the variance of the peak GRF (R2 = 0.59). Among the kinematic variables not included in a more detailed analysis due to low value of coefficient of determination in a simple regression were the indicators of arm swing, thoracic rotation, hip and knee flexion and foot angle to the ground. Of the remaining indicators, the highest coefficients of determination were calculated in a simple regression for the duration of the flight phase (R2 = 0.64), stride length (R2 = 0.59), the sum of pelvic rotation and lateral thoracolumbar flexion (R2 = 0.42) and thoracolumbar extension (R2 = 0.40). Multiple regression analysis was performed for these 6 leading variables to evaluate the combined influence with other relevant variables on the peak GRF in the first phase of the stride, to address the basic problem and test the hypotheses of this dissertation. The results of this analysis represent important scientific contributions of this dissertation. Namely, they show that a reduction of the GRF in the first phase of the racewalking stride is possible due to the simultaneous influence of the following variables:
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by reducing the duration of the flight phase as well as the thoracolumbar extension, with greater pelvic rotation (R2 = 0.75);
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by shortening the stride and reducing the forward tilt of the pelvis, with a greater sum of the angles of pelvic rotation and thoracolumbar lateral flexion (R2 = 0.73);
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by reducing the racewalking speed as well as the backward tilt of the trunk, with increased pelvic rotation (R2 = 0.72);
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by increasing the sum of the angles of pelvic rotation and thoracolumbar lateral flexion, with decreased backward trunk tilt and decreased slope of the GRF curve in the second phase of the stride (R2 = 0.71);
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by increasing the GRF in the second phase of the stride, with decreased thoracolumbar extension (R2 = 0.66);
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by decreasing the ratio between the second and first phase of the stride and shortening the stride, with decreased thoracolumbar extension and increased thoracolumbar lateral flexion (R2 = 0.64).
The sixth chapter contains the discussion. The results of the conducted research indicate that the peak GRF in the first phase of the stride is smaller when the mode of locomotion is more similar to normal walking than to running, i.e. when the shape of the GRF curve is of the "M" type rather than the "N" type, which is consistent with the results of previously published research (Pavei et al., 2019). The research conducted confirmed that speed is a significant factor in the regression with GRF, confirming previously published research findings. When a racewalker retains a significant percentage of force towards the ground prior to toe-off, the contralateral leg is less loaded at anterior contact, which has also been found in previous research (Hanley and Bissas, 2013), but without formal evaluation of the effects, which is one of the scientific contributions of this dissertation. The analysis conducted also found that the peak GRF is smaller when the stride is shorter and when the ratio between the duration of the second and first phases of the stride is smaller, which is also one of the scientific contributions of this dissertation. Published research did not address the effects of stride length on GRF, but indicated that stride length and the ratio between the duration of the second and first phases of the stride are related to pelvic rotation (Gravestock, Tucker, & Hanley, 2019). The study conducted showed a significant influence of the duration of the flight phase on the peak GRF (R2 = 0.64), confirming the results of a previous study (Hanley and Bissas, 2016), which also found a strong regression between the two variables (R2 = 0.47). Due to ground contact rules, pelvic rotation is a very important way to increase stride length (Cairns et al., 1986; Murray et al., 1983). This dissertation formally demonstrated that peak GRF is significantly dependent on the sum of the angles of pelvic rotation and thoracolumbar lateral flexion. Previous research (Gravestock et al., 2019) indicated a large thoracolumbar extension with 11 degrees of anterior pelvic tilt and posterior thoracic tilt. This dissertation found similar angles with an average pelvic tilt of 7.4 degrees and an average thoracic tilt of 10.2 degrees, while the sum of these two angles (thoracolumbar extension) is a strong predictor of peak GRF, which is also the scientific contribution of this dissertation.
The seventh chapter contains conclusions on the acceptance or rejection of the hypotheses put forward. The first hypothesis on the statistically significant relationship between
the GRF in the second phase of the stride and the peak GRF in the first phase is fully accepted, since the indicator of the average GRF in the interval of 70 % - 80 % of the duration of ground contact in multiple correlation with the thoracolumbar extension explains 66 % of the variance of the dependent variable, while there is also a significant influence of the slope of the GRF curve in the same interval. The second hypothesis about the statistically significant relationship between the observed kinematic variables and the ground reaction force in the first phase of the racewalking stride was partially accepted, as the influence of the indicators of stride length, duration of the flight phase, ratio of the two stride phases, thoracolumbar lateral flexion and extension, as well as pelvic rotation on the peak GRF in the first stride was demonstrated. This hypothesis is only partially accepted due to no significant statistical relationships found between the peak GRF in the first phase of the stride and the arm swing, thoracic rotation, flexion and extension of the hips and knees as well as the angles of the feet to the ground.
The eighth chapter contains a description of the scientific contribution and the possible practical use of the research. The scientific contribution of the research carried out is demonstrated by the identification of indicators of racewalking technique that significantly influence the peak GRF in the first phase of the racewalking stride. The research conducted also has a pragmatic benefit for racewalking training as it defines technical elements for smooth and low impact racewalking while providing tools for risk assessment of the occurrence of the overuse injuries. However, given the size and structure of the sample and the limitations of the analytical methods used, the research findings should be viewed with caution and the author therefore urges further analysis on these research topics.
In conclusion, the research conducted has formally demonstrated that the reduction in peak ground reaction force in racewalking can be achieved by increasing the force towards the ground in the second phase of the stride, reducing the duration of the flight phase, reducing thoracolumbar extension while increasing thoracolumbar lateral flexion and pelvic rotation, and reducing racewalking speed in accordance with the athletes' fitness level and their technical ability to sustain the movement described by the aforementioned variables. |