Asymmetry of Contractile Characteristics of Knee Extensors and Flexors in Elite Badminton Players

Summary

Repetitions of unilateral movements puts the sport at risk of imbalance, i.e. – asymmetry between the same muscle groups of opposite side of body (lateral asymmetry) and interfunctional opposite structures between opposing parts of the body (functional asymmetry). Tensiomyography, a non-invasive method of assessing the contractile characteristics of muscle fibers, allows insight into the current state of sports and provides a huge opportunity for improvement both in the current and in the process of maintaining sports. The aim of this work is to examine the lateral and functional asymmetries of the muscles surrounding the knee joint in elite badminton players and compare them with the standard recommended values in practice.

The sample of respondents who were tested and analyzed the contractile characteristics of muscles using the TMG method consisted of elite badminton players of the Serbian national team, with many years of experience in that sport. The assumption was that the pronounced asymmetry is the result of constant engagement of the musculature of the dominant side during the specific technique of moving and hitting the ball in badminton. An independent t test was used to process the results, and the estimated values indicate the support of the hypothesis about the present imbalance.

Functional asymmetry, i.e. imbalance of the extensor muscles in relation to the flexors of the knee joint, is represented at the level of 25%±1, which does not represent a great risk and danger for athletes, and is present to a greater extent in the deficiency of the legs. The size of the radial deformation of the biceps femoris and vastus lateralis is higher in the non-dominant leg than in the dominant leg. The contraction time of the knee extensor muscles of the dominant leg is shorter than that of the non-dominant leg. The contraction time of the knee flexor muscles of the non-dominant leg is not shorter than that of the dominant leg.

Introduction

Badminton is a sport in which the objective is to hit a shuttlecock over the net into the opponent’s court and score points in this way. It is extremely popular as a recreational activity and is favored by people of all ages. The game is played on a court of specific dimensions and requires a particular technique of movement and shuttle hitting using a racket. The game involving a feathered shuttlecock is very old. In India, there are cave paintings over two thousand years old that depict this game. In China, it was known as di jian-zi and was especially popular among girls and women (Wikipedia).

The Inca and Aztec civilizations in the Americas were also familiar with a shuttlecock-based game, and it was not uncommon in European royal courts during feudalism. A painting by the French artist Jean-Baptiste-Siméon Chardin (1699–1779), currently housed in the Uffizi Gallery, depicts a girl holding a racket in her right hand and a feathered shuttlecock in her left. The painting is titled La fillette au volant. Badminton as a modern sport developed in the early 14th century from the ancient Indian game poona, which was brought to England by British officers in 1873. They demonstrated it at a castle in southern England on the estate of the Duke of Beaufort, after which the game was named. In 1877, the first badminton club was established (Bath Badminton Club), and in 1893, the first national association was founded—the Badminton Association of England—at which time the first official rules were published. These rules closely resemble the rules in force today. The International Badminton Federation (IBF) was established in 1934 and continues to govern the sport and organize competitions. Badminton was later included in the Olympic Games (Wikipedia).

At the elite level, badminton is characterized by dynamics, speed, and agility. Timely positioning of the athlete’s body into the most efficient stance for a technically precise and tactically planned shot is crucial for gaining an advantage over the opponent and winning points, matches, and ultimately medals at championship tournaments. A shot in badminton is performed through simultaneous movement toward the shuttle’s trajectory with a lunge of the dominant leg—on the same side as the racket-holding arm (Figures 2–6). Repeated unilateral movement places the athlete at risk for muscular imbalances, leading to lateral and functional asymmetry of the body.

Theoretical Framework

Definition of Basic Concepts

The primary goal of any training process and the fundamental prerequisite for enhancing human abilities in any form of physical activity should be the balance and synchronization of neuromuscular function.

Muscle and Muscle Group Imbalance

The term muscle imbalance in skeletal muscles may refer to asymmetry in the following contexts:

Bilateral muscle pairs, i.e., imbalance between muscles on one side of the body compared to the corresponding muscle on the opposite side (left vs. right, and vice versa)

Lateral asymmetry

• Agonistic (synergistic) muscle pairs, i.e., imbalance between muscles that act from different angles to perform the same joint movement or fulfill the same function (e.g., the internal and external heads of the quadriceps femoris muscle, which together perform knee extension)

Functional asymmetry

• Antagonistic muscle pairs or groups, i.e., imbalance between muscles located on opposite sides of the same joint that are involved in the same movement, but in opposite directions (e.g., the anterior vs. posterior thigh muscles, which perform knee flexion and extension, respectively)

Functional asymmetry

• Muscle imbalances between different muscle pairs or muscle groups may be observed when describing various properties of skeletal musculature:

Figure 1 – Sourced from the dynamicmedicalfitness.com

Bilateral, agonistic, or antagonistic muscle pairs in imbalance can result in asymmetry in technique execution, asynchrony in isolated or combined movements, and consequently lead to reduced movement efficiency or increased risk of injury to the locomotor system. When assessing healthy individuals, an imbalance of less than 10% can be considered normal, between 10% and 20% possibly abnormal, and more than 20% likely abnormal. However, in individuals with an injury, damage, illness, or post-operative condition—where imbalance is expected—an asymmetry of 10% to 20% can already be regarded as likely abnormal, and over 20% as almost certainly abnormal. The most commonly applied criterion of 80% to 90% capacity of the injured, impaired, affected, or operated limb compared to the healthy limb is considered the minimum standard for returning an individual to sport (Baltzopoulos & Brodie, 1989; Kannus, 1994). By applying selective strengthening techniques or various recovery methods (e.g., stretching, cryotherapy, physical therapy, etc.), potential imbalances or dysfunctions can be corrected for any parameter of an individual TMG signal, whether relative to average, minimum, or absolute values for a given population, or compared to another selected muscle or muscle head.

Due to the characteristics of the method itself, TMG can be used at any point during a training or competitive cycle. Without testing and detailed analysis of results, it is impossible to accurately identify critical muscles and muscle groups, and thus determine an adequate corrective training program to address the identified imbalances. Properly conducted testing with the TMG system does not induce any form of fatigue that could affect maximal human performance.A large amount of practically applicable data is obtained by comparing test results with reference values determined by sex, age, and occupation. In other words, the larger the database (number of conducted tests), the higher the quality of the individual test report.

Figure 2 – Viktor Petrović performing a net shot

Figures 3–6: Viktor Petrović in situational play on court

Application and Significance of the TMG Method

Tensiomyography isolates the muscle of interest by excluding confounding central variables that may influence the measurement. It allows for the assessment of muscle contraction via electrical stimulation alone. The level of contraction induced is relatively low (in the case of knee flexors, only about 10% of maximum voluntary contraction). Electrical stimulation may be applied either to the motor nerve or to the muscle surface at the point of motor nerve entry. In both cases, the stimulation travels through the nerve fiber and reaches the muscle fiber. A muscle twitch can be defined as the contractile response to a single electrical impulse. Factors affecting the muscular response depending on the intensity of the electrical stimulus include: motor unit recruitment threshold, skin conductivity, subcutaneous fat thickness, water retention level, and temperature.

Tensiomyography can provide insight into muscle composition, functional muscle characteristics, local muscle fatigue, atrophy, muscle inhibition, spasticity, tone, and more. For these reasons, TMG is applied in monitoring mechanical adaptations of muscles, fatigue and damage, recovery, injury prevention, assessment of muscle characteristics regarding the proportion of fast- and slow-twitch fibers or muscle stiffness, and in determining balance (symmetry by various parameters) between muscles and muscle groups that may disrupt movement or sports technique in any type of activity. It is also useful in tracking the effects of rehabilitation processes. Because of all these applications, the TMG method is equally used for the assessment and/or monitoring of both athletic and non-athletic populations. It is based on recording different indicators of muscle fiber functional capacity in response to stimulation.

Through adapted software, test results can be easily applied in the processes of training control, monitoring, design, and evaluation, with the aim of injury prevention and performance enhancement. The number of institutions applying this method for effective individualization of training activities is increasing globally year by year.

Review of Previous Research

As noted by Wiewelhove et al. (2017), tensiomyography is an indirect method of measuring contractile properties of muscles and holds potential as a technique for detecting exercise-induced skeletal muscle fatigue. The aim of their study was to assess the sensitivity of tensiomyographic markers in identifying reduced muscle performance in elite young athletes. Fourteen male junior tennis players (age: 14.9 ± 1.2 years), ranked on the International Tennis Federation list, participated in the study. Muscle assessment was performed using the TMG method before and after a four-day high-intensity interval training (HIIT) microcycle consisting of seven sessions. TMG parameters included maximum radial displacement of the rectus femoris (Dm), contraction time between 10% and 90% of Dm (Tc), and deformation velocity at 10% (V10) and 90% of Dm (V90), respectively. Diagnostic characteristics were evaluated using receiver operating characteristic (ROC) curve analysis and contingency table analysis. The data indicated that none of the TMG markers proved to be effective diagnostic tools for identifying impaired muscle performance in elite young athletes.

Based on these findings, the authors concluded that the TMG parameters assessed in this study were not sufficiently sensitive to detect performance changes in elite youth athletes. However, given the preliminary nature of the study, further research is required to evaluate the sensitivity of TMG in this population (Wiewelhove et al., 2017).

The objective of the study published by García-García et al. (2017) was to determine the mechanical and neuromuscular profiles of knee extensors and flexors in professional football players at the start of the preseason, to calculate symmetry percentages, and to examine differences by playing position. The muscles measured included the vastus medialis (VM), vastus lateralis (VL), rectus femoris (RF), and biceps femoris (BF). Sixteen professional football players were assessed using tensiomyography. A paired-samples t-test (p < 0.05) was used to compare dominant and non-dominant lower limbs. One-way ANOVA was applied, with playing position as the independent variable. No significant differences were found between dominant and non-dominant legs. The highest degree of symmetry was recorded for VM (92.5 ± 2.7%), and the lowest for BF (80.7 ± 10.9%). Playing position was associated with significant differences in some variables for BF, RF, and VM; however, only the half-relaxation time in BF and sustain time in VM differed across all considered playing positions. The results indicated that TMG is a useful tool for assessing neuromuscular characteristics of football players at the beginning of the preseason and for identifying players' baseline functional muscle profiles (Macgregor et al., 2018).

Park (2020) focused on describing tensiomyography as a tool for detecting injuries caused by imbalances in muscle groups and/or asymmetry between sides. TMG was presented as a device capable of detecting and measuring contractile properties and mechanical response based on muscle belly displacement. Although previous studies proposed well-established protocols for evaluating several muscle groups using TMG, methods for detecting imbalances in the lower back muscles have not been thoroughly explored, and systematic reviews on this topic are lacking. Therefore, this study aimed to synthesize the theoretical findings of previous TMG research and briefly summarize its utility by conducting simple experiments on the left and right erector spinae muscles.

Figure 8 – Lateral Symmetry in a Basketball Player (96%, left) and Functional Symmetry of the Knee Joint between the Vastus Lateralis (VL) and Biceps Femoris (BF) (right), measured in a basketball player.

Lateral symmetry of the vastus lateralis in a basketball player, as reported by Simunic et al. (2022), showed a near-perfect symmetry ratio of 96% (Figure 8, left chart). Functional symmetry of the knee joint between the vastus lateralis (VL) and biceps femoris (BF) in the same population (Figure 8, right chart) revealed a symmetry rate of 58%, with contraction times of 23 ms for VL and 38 ms for BF (Simunic et al., 2022). Additional asymmetry was observed in the sustain time.

Calculating lateral symmetry using all five TMG parameters allows for comparison of the same muscle across different sides of the body. Estimating lateral symmetry with a single value simplifies interpretation, as the relevance of each of the five parameters may vary depending on the sport or health status of the subject. The typical threshold for acceptable lateral symmetry is above 85% (Figure 8), regardless of sport. In inherently asymmetric sports (e.g., golf, hockey), lower lateral symmetry is also present, requiring a decision whether to accept the asymmetry or address it through targeted training programs (Park, 2020).

Functional symmetry can be calculated by comparing antagonistic muscle pairs (e.g., biceps femoris and quadriceps femoris) or synergistic muscles (e.g., vastus lateralis and vastus medialis). Maximum displacement depends not only on muscle tone but also on muscle volume, and therefore this parameter is excluded from functional symmetry calculations. Functional symmetry ratios typically exceed 65%; lower values may indicate increased injury risk, reflect sport-specific asymmetry, or indicate a history of prior injury (Figure 8). Athletes engaged in explosive sports require not only powerful but also fast muscles, and no individual muscle should deviate significantly from this balance (Simunic et al., 2022).

According to Garcia-Garcia et al. (2017), the most reliable and frequently used variables in previous research are Dm and Tc. It is recommended that TMG assessments focus on these stable parameters. In contrast, the half-relaxation time is consistently the least reliable. In previous studies, the intraclass correlation coefficient (ICC) for all variables was 0.86 or higher when assessing test-retest reliability with the same or different examiner. The coefficient of variation was below 5%, except for the Tr variable. When measuring between-day reliability, Dm and Tc showed ICCs of no less than 0.98 and variation coefficients below 5%. When comparing previously activated or fatigued muscle versus resting muscle, reliability was higher in the former. Thus, TMG shows good to excellent reliability both within and between sessions, with minimal measurement error across all variables except Tr and Ts, whose use is currently not recommended.

The symmetry calculations mentioned above rely on variables such as Td, Tc, Ts, and Dm.

Research Problem, Subject, and Objective

Research Problem and Subject

The research problem concerns muscular imbalances caused by long-term adaptation to sports activity.
The subject of the research is the examination of muscular imbalances in knee extensors and flexors among elite badminton players, resulting from long-term adaptation to sport-specific movement patterns.

Research Objective

The objective of this study is to assess lateral and functional asymmetries of the muscles surrounding the knee joint in elite badminton players and to compare them with standard recommended values in practice.

In accordance with the general objective, the following specific objectives were defined:

1. To determine the percentage of muscle imbalance at the knee joint level in elite badminton players.
2. To determine the percentage of lateral asymmetry, i.e., imbalance in the knee flexor muscles of the dominant versus non-dominant leg.
3. To determine the percentage of lateral asymmetry, i.e., imbalance in the knee extensor muscles of the dominant versus non-dominant leg.
4. To determine the percentage of functional asymmetry, i.e., imbalance between the extensor and flexor muscles of the knee joint.
5. To determine whether the percentage of functional asymmetry is more pronounced in the muscles of the dominant compared to the non-dominant leg.

Research Hypotheses

In accordance with previous research, as well as the aim, purpose, and objectives of this study, one general hypothesis (Hg) and four specific hypotheses (H1–H4) were formulated:

• Hg – There is a muscle imbalance in the knee joint among elite badminton players.
• H1 – There is a lateral asymmetry, i.e., a muscle imbalance of the knee flexors of the dominant leg compared to the non-dominant leg.
• H2 – There is a lateral asymmetry, i.e., a muscle imbalance of the knee extensors of the dominant leg compared to the non-dominant leg.
• H3 – There is a functional asymmetry, i.e., a muscle imbalance between the knee extensors and flexors.
• H4 – The percentage of functional asymmetry is more pronounced in the muscles of the dominant leg compared to the non-dominant leg.

Research Methodology

The research was conducted using a transversal experimental method. Additionally, an empirical-bibliographic method was used to construct the theoretical framework and review previous research, while statistical methods were applied to process the collected data. The assessment of muscle fiber contractile properties was performed using a tensiomyography (TMG) device.

Sample of Participants

The study included 15 male athletes from the Serbian national badminton team (age 19.5 ± 3.3 years), each with over 8 years of training experience. All participants were healthy, without any acute or chronic musculoskeletal injuries, and actively engaged in training at least four times per week.

Before participation, all subjects were provided with detailed information and answers to relevant questions regarding the study. They then read and signed an informed consent form outlining the experimental procedures, written in accordance with the Declaration of Helsinki. The document contained a detailed description of all procedures, including potential risks and benefits. For underage participants, the consent form was signed by a parent or legal guardian.

Table 1 presents the descriptive statistics of participants’ basic age, anthropometric, and morphological characteristics, including age, body height, body mass, muscle mass, fat mass, and BMI (Body Mass Index).

Research Procedure

All participants had their basic morphological characteristics measured using an InBody 770 device. Participants were instructed to abstain from food and drink for at least one hour prior to arrival to ensure accurate data collection for body mass and BMI. Body height was then measured, followed by the TMG procedure.