Difference of stability between two elite boxing groups: a preliminary study

1

Keywords:

• Elite boxing
• stability
• IMU
• performance

1. Introduction

Boxing is a very physical combat sport, which requires a great coordination to succeed in the strikes while avoiding the attacks of the opponent. Boxing is also a non-symmetrical sport, the right and left sides can have a role of attack or defense according to the action Punch performance can be seen as the ability to deliver a fast, strong attack while maintaining balance and accuracy. There are three standard boxing techniques (Baitel et al. 2015; Kimm and Thiel 2015): the cross, the hook and the uppercut. The first technique is the most common and the first that is taught. Cross consists of a stroke in the anteroposterior axis that takes place essentially at distance and in the sagittal plane. It starts with the support of the foot on the same side of the punching arm and ends with an extension of the elbow to transfer energy to the fist. Up to now, performance in boxing is mainly associated to the punch efficiency which is studied by 2 indicators: the means of striking force and its striking speed. However, a more general study of Martial Arts shows that other parameters can influence the performance. The work of Cesari and Bertucco (2008) on the biomechanical study of karate, shows that expert populations had better control over the displacement of their center of pressure and thus showed greater stability. Performance in boxing would then be a combination of strength, speed and stability to create maximum impact (Loturco et al. 2016).

Several studies have investigated performance through a comparison between novice and elite groups. These studies show differences in terms of performance, level or skill (Leal and Spaniol 2016). However, comparison between high level groups is less clear. Nonetheless, the understanding of performance mechanisms a key point in high level sport in order to provide insight for junior athletes to improve skills.

The aim of this work is then to investigate stability as a descriptor of performance through a comparative study between two elite boxing groups.

2. Methods

2.1. Participants

In order to study the effect of stability on high level population, two elite boxer groups were recruited. The first one was composed 10 senior elite potential Olympic medallist boxers (SE) (age: 21.1 ± 3.0 years; height: 1.76 ± 0.10 m; body mass: 67.2 ± 19.7 kg) considered to be the performance guarantor. The second one was composed of 10 high level junior boxers (JE) (age: 16.1 ± 0.7 years; body height: 1.75 ± 0.05 m; body mass: 61.3 ± 9.7 kg) representing a team from a regional boxing academy. This study was approved by the French Boxing Federation and carried out in accordance with the Declaration of Helsinki.

2.2. Protocol

After a standardized warm-up organized by the coaches, participants were asked to perform 3 crosses standardized technique. The instructions were to complete a precise motion in the punching bag with maximal possible strength. Each participant was equipped with a MVN Biomech Link suit (XSens Technologies BV, Enschede, The Netherlands) composed of 17 inertial measurement units (IMU) strapped onto the body with a sampling frequency of 240 Hz (Figure 1). Only the fastest cross was kept for the analysis, based on the hand IMU recording

Figure 1. Positioning of the MVN Biomech suit during a cross standardized technique.

2.3. Data processing

First, based on de Leva anthropometrical model (De Leva 1996), and segments positions (computed from the IMU by XSens software), the center of mass (COM) position was determined by the sum of the centers of mass of each body segment. Then, according to Newton’s 2^nd law, Vertical Ground Reaction Forces (VGRF) were estimated from the COM vertical acceleration and the participants mass.

The VGRF distribution between the right and the left leg was computed based on a proportional distribution on the feet. First, the COM position was projected on the ground. Then the distance between the projected COM and the left and the right foot were measured with the help of kinematic data acquired by the foot IMU. The VGRF distributions on the right and left foot were computed as proportional to the distance between the projection and the position of the feet. The VGRF distribution on the front dominant leg was used to reckon the stability of the boxer and was presented as a percentage of VGRF. A stability more than 50% represented a greater support on the front dominant foot, contrariwise a stability less than 50% represented a greater support on the rear non-dominant foot. The mean, the standard deviation (SD) and the coefficient of variation (CV) of the two groups were computed. A Wilcoxon statistical test was used to study significant difference of the stability for the two groups. The null hypothesis is rejected for p-value < 0.05.

3. Results and discussion

Stability is presented in Table 1. Results show that the stability is closer to 50% for SE (51.7 ± 5.6%) than for JE (42.8 ± 6.1%). Furthermore, the CV is lower for SE (10.8%) than for JE (14.3%). Looking at the mean values, it is possible to observe that the SE group is closer to a stability value of 50%. This means that at impact the boxers had a better distribution of their weight on both legs and do not present an imbalance. In a different way, the JE population had a stability lower than 50% meaning that they remained on their rear non-dominant foot at impact. This may mean they had a lower center of pressure movement, which would confirm the tendency measured by Cesari and Bertucco (2008) for karate punches.

Table 1. Stability (%) of the 10 subjects for the senior (SE) and junior (JE) for the cross.

Subject	SE	JE
1	58	35
2	50	42
3	49	40
4	47	50
5	58	37
6	57	42
7	43	48
8	50	35
9	47	51
10	58	48
Mean	51.7	42.8
SD	5.6	6.1
CV (%)	10.8	14.3

However, the results of the standard deviations show that the two groups present a large variation (even more so for the JE group). This may be explained by the heterogeneity of the two groups. These observations were confirmed by the results of the statistical test which shows no significant difference between the two groups (p-value = 0.032). In view of the results, it is impossible to conclude on the effect of the stability, because the level of performance of these two groups may be very close. Nevertheless, a more in-depth study with separate groups seems necessary to study stability as a descriptor of performance The results of this preliminary study underline that stability remains an original criterion for studying the boxing performance. This indicator could be determined simply on elite athletes and respecting the conditions of boxing. However, besides the difficulties presented by heterogeneous populations, several points remain to be improved. Indeed, at this stage stability is an indicator estimated by means of inertial units and is based on strong hypotheses that need to be validated. Moreover, the holding method used for the sensors should be validated for high intensity sports as it could be a source of error.

4. Conclusions

Boxing performance should be study through more than classical force and speed parameters. Athletes’ stability presents different trend between two elite boxer groups, potentially reflecting the performance difference. Thus, even if this trend remains to be confirmed, this may help to evaluate performance for in situ studies by the means of inertial measurement units.