Dr.Biol. A.V. Voronov¹
Dr.Hab., Professor P.V. Kvashuk¹
R.V. Malkin¹
PhD Ya.V. Golub^2
1 Federal Scientific Center for Physical Culture and Sports, Moscow
² St. Petersburg Scientific Research Institute of Physical Culture, St. Petersburg

Keywords: freestyle, jumping, technical elements, muscle activity.

Background. Freestyle as a variation of BMX racing has been actively developed since the end of the last century. The first freestyle competitions will take place at the Tokyo 2020 Olympic Games. Bicycle acrobatics includes various tricks (stunts): rotations relative to the longitudinal and vertical body axes, flips, barspins. All the technical elements are performed in the air during the 1.5-sec flight. The body orientation in the airborne phase imposes special requirements on the athletes' vestibular and neuromuscular systems, as regardless of the complexity of the acrobatic figures, they must land on the back wheel of the bicycle. The physiological basis for the successful execution of the technical elements in freestyle is formed by the intra- and intermuscular coordination skills. The athletes' muscular system functionality was rated by the method of bipolar telemetric electromyography.

Objective of the study was to determine the electromyographic cost of technical elements in freestyle depending on the difficulty level of the jump performed.

Methods and structure of the study. Electromyographic activity of the muscles was recorded using the hardware and software complex "SportLab" (Russia), consisting of the 8-channel telemetric electromyography, video camera, and synchronization device. The technical characteristics of "SportLab" and recording procedure are presented in the study [1]. We registered the surface electromyographic activity of the muscles on the right side of the body:

– upper limb: m. brachioradialis (1)[1]; m. biceps brahii (2); m. deltoideus (3);

– upper limb: m. brachioradialis (1); m. biceps brahii (2); m. deltoideus (3);

– core: m. trapesius (4); m. latissimus dorsi (5) и m. rectus abdominis (6);

– lower limb: m. vastus lateralis (7) и m. biceps femoris caput longus (8);

Sampled for the study was a highly-skilled athlete, who performed 11 technical elements (Table 1).

Table 1. Technical elements in freestyle BMХ

Fig. 1. Phase composition of technical elements on the example of "Flair tailwhip". Repulsion phase: beginning (1), end (2). Flight phase: start (2), end (5). Landing phase: beginning (5), end (6)

Processing of the study results. The EMG signal was inverted and smoothed by the method of a moving average (50 ms averaging window) [2]. The following indicators were calculated:

- electromiographic cost () of technical elements:

,                                                                             (1)

where  –  smoothed EMG; j – technical element (j=1-11); K – muscle (К=1-8); t=0– beginning of the repulsion phase (position 1 in Fig. 1); t=T – end of the landing phase (position 6 in Fig. 1).

Results and discussion. The EMG amplitude depends on the functioning and anatomical position of the muscles. The lower limb and core muscles are to resist gravitational and inertial forces and thus are in constant tension. The upper limb muscles perform complex coordination and highly-accurate movements with little effort: for example, writing or typing, picking up different objects and similar motor actions. Accurate movements require rapid recruitment and synchronization of motor units, which affects the amplitude of the myogram. If the amplitudes of the upper and lower limb muscles are compared, the upper limb muscle amplitude turns out to be higher than that of the lower limb ones at the same effort. Therefore, it is incorrect to evaluate the role of muscles in the execution of technical elements only by the myographic amplitude (or electromechanical work). Technical elements 2-11, presented in the table, were normalized by of "Bunny Hop" as the "simplest" element, i.e. we assessed the influence of the level of difficulty of jump on the changes in the electromyographic cost () of the upper limb, lower limb and core muscle work. The calculations were based on the formula:

,                                   (2)

where the upper index 1 relates to the 1st technical element (see the table),  j=2-11; the upper index  K relates to the muscle K=1-8.

The calculation data are presented in Figure 2. Compared to the 1st technical element, the 50-90% increase in the electromyographic cost of m. brachioradialis and m. biceps brahii were observed during such jumps as "Cork 720", "Frontflip" and "Flair tailwhip» that are associated with rotational movements of the body. When performing Type 9 technical element ("Frontflip"), the athlete performs a forward flip in the midair while holding on the handle bars; when performing Type 10 jump 10 ("Flair tailwhip"), the rider rotates the bike in the air and, as a consequence, the work of the arm muscles increases. When performing "Cork 720 no hander" (6), the rider does not hold on the handle bars for some time when in the air, and the work of the arm and core muscles consequently does not differ much from that during the first jump, which indicates a correct registration of EMG (as presented in the upper graph of Fig. 3). A similar downward trend was observed for Types 2, 5, 8, 10, and 11 jumps, respectfully. It should be noted that in the execution of some technical elements related to the performance of a flip (7, 8) or rotation of the handle bars or bike itself (3, 10), there was a significant increase of  in m. rectus abdominis (Fig. 3). The increase in the electromyographic cost during Types 3, 5, 7, 9, and 10 jumps (see the table) was 50-240% (see Figure 3). In the airborne position, the abdominal muscles (estimated by the EMG activity of m. rectus abdominis) help hold the torso in the inclined position parallel to the bicycle frame, which reduces the moment of body inertia and enables to perform rotational movements. Given the flight time, m. rectus abdominis has to contract very quickly, which is reflected in the shape of the EMG spectrum. Figure 3 illustrates the smoothed spectra of m. rectus abdomis in the following exercises: a frontflip (9), a frontflip while simulating a jump into water², sit-ups. The abdominal muscles are actively involved in work during these exercises. The first spike in the spectrum amplitude within the 14-27 Hz band indicates the recruitment of motor units. During the air tricks, the spectral width of m. rectus abdomis did not exceed 125 Hz; during the sit-ups, the spectral width of m. rectus abdomis increases to 200 Hz. The more narrow the EMG spectrum, the faster the recruitment of motor units with simultaneous synchronization of their contraction.

Fig. 2. Normalized electromyographic cost of technical elements

Fig. 3. Smoothed spectrum of m. rectus abdomis

Consequently, to successfully perform the complex elements of air tricks it is necessary to develop the speed-strength capabilities of the abdominal muscles.

Conclusions. Depending on the level of difficulty of the freestyle jump performed,  the electromyographic cost in m. brachioradialis, m. biceps brahii, and m. rectus abdominis increased.
An increase in the activity of m. rectus abdominis during the execution of complex technical elements led to a decrease in the activity of the antagonist muscles m. trapesius and m. latissimus dorsi.
The electromyographic cost of work of m. vastus lateralis and m. biceps femoris caput longus was not affected by the element’s complexity, but by the jump height, as these muscles perform a shock-absorbing function on landing.

References

1. Voronova A.A., Voronov A.V., Kvashuk P.V. Electromyographic methods to determine muscle groups to affect sports results in speed climbing. Teoriya i praktika fiz. kultury. 2019. No. 12. pp. 24-26.
2. Voronov A.V. Anatomical structure and speed-strength qualities of human lower extremity muscles. Doct. Diss. Abstract (Hab.). M., 2004. 50 p.

Corresponding author: rudra54@yandex.ru

Abstract

Freestyle as a variation of BMX racing has been actively developed since the end of the last century. The first freestyle competitions will take place at the Tokyo 2020 Olympic Games. Bicycle acrobatics includes various tricks (stunts): rotations relative to the longitudinal and vertical body axes, flips, barspins. All the technical elements are performed in the air during the 1.5-sec flight. The body orientation in the airborne phase imposes special requirements on the athletes' vestibular and neuromuscular systems, as regardless of the complexity of the acrobatic figures, they must land on the back wheel of the bicycle. The physiological basis for the successful execution of the technical elements in freestyle is formed by the intra- and intermuscular coordination skills. The athletes' muscular system functionality was rated by the method of bipolar telemetric electromyography.

Objective of the study was to determine the electromyographic cost of technical elements in freestyle depending on the level of difficulty of the jump performed.

Methods and structure of the study. Electromyographic activity of the muscles was recorded using the hardware and software complex "SportLab" (Russia), consisting of the 8-channel telemetric electromyography, video camera, and a synchronization device. We registered the surface electromyographic activity of the muscles on the right side of the body. Sampled for the study was a highly-skilled athlete, who performed 11 technical elements.

Results and conclusions. Depending on the level of difficulty of the freestyle jump performed,  the electromyographic cost in m. brachioradialis, m. biceps brahii, and m. rectus abdominis increased.

An increase in the activity of m. rectus abdominis during the execution of complex technical elements led to a decrease in the activity of the antagonist muscles m. trapesius and m. latissimus dorsi.
The electromyographic cost of work of m. vastus lateralis and m. biceps femoris caput longus was not affected by the element’s complexity, but by the jump height, as these muscles perform a shock-absorbing function on landing.