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Patellar Tendon Stiffness in Elite Breakdancers Assessed by Myotonometric Measurement.

Breakdancing is a dance form created in 1970 based in the Hip Hop subculture. It is characterized by high physical demand, such as muscular strength, energy utilization, agility, coordination, and motor control. (1) Breakdance has been classified as a collision activity, where splits, spins, and handstands contact the ground with great force, which elevates the risk of injury. (2) Previous studies have found that 95% of breakdancers experience musculoskeletal injuries, presumably as a result of the nature of the movements performed. (2) Attention has been paid to patellar tendon (PT) injury, reported as an overuse syndrome. (3) It is noteworthy that normal function of this tendon is essential for breakdancers. Moreover, studies have shown that various forms of athletic training can affect the tendon's physiology and morphology. (4),(5) Therefore, studying PT function may help to understand the underlying mechanisms of tendon adaptation and its contribution to breakdancers' performance.

Tendons are responsible for transmitting the force produced by muscles and for storing and releasing elastic energy during movements. (6) This double function allows for efficient use of the muscle-tendon complex (MTC) during human movement. (7),(8) Tendons respond to mechanical stimuli by altering their structure, composition, and mechanical properties, a process known as tissue mechanical adaptation. (9) Collagen synthesis and turnover, increase in water binding proteins (e.g., proteoglycans) of the extra-cellular matrix, and enlarged collagen fibril diameter have been proposed as the underlying mechanisms that change tendons' structure and composition following the application of different mechanical-loading stimuli. (10),(11) These structural changes lead to adaptations of tendons' mechanical properties. (12) Stiffness, the relationship between the force applied to a tissue and the displacement it exerts, is one of the mechanical properties most studied in relation to the tendon adaptation process. (13),(14) Increased stiffness has been observed in both short-term and long-term training regimens, demonstrating that tendons adapt to mechanical stimuli independent of the training length. (15) Furthermore, changes in stiffness have been reported in different pathologies, such as tendinopathy. (16) It has been shown that structural changes associated with tendinopathy result in reduced tendon stiffness, altering the normal function of the tendon. (17),(18) Thus, assessment of tendon stiffness is needed to fully understand tendon function during training adaptation and resulting pathology.

Dynamometry combined with ultrasonography is the gold standard method for measuring passive tendon stiffness. (14) However, it is an expensive and operator-dependent technique that can only be used in laboratory settings. Recently another technique has been used to evaluate tendon mechanical properties.(19),(20) This technique uses myotonometric measurements, a noninvasive and painless method base on the free oscillation theory. (21) A brief mechanical impulse is released over the skin by a probe, and parameters, such as stiffness, can be obtained. (22) Studies that have used myotonometric measurements have mainly measured muscle properties,(23-25) so studies assessing tendon stiffness with this technology are needed. The aim of the current study was to describe mean PT stiffness in elite breakdancers using myotonmetric measurements and compare those results with control subjects. Data about PT stiffness may contribute to an understanding of the underlying mechanism of tendon adaptation in elite breakdancers.

Material and Methods

Subjects

Twenty-five elite male breakdancers (29.3 [+ or -] 2.6 years; 69.1 [+ or -] 8.0 kg; 169.9 [+ or -] 6.2 cm) were evaluated during the "El Boske City" tournament, a high-performance tournament for breakdancers. Inclusion and exclusion criteria for the breakdancers were defined as follows: 1. danced for at least 10 years, practiced three times a week or more, and worked out for at least 3 hours per training session; 2. participated once a week in advance-category tournaments and qualified for the two most important championships ("Sudaka Breakdance" and "El Boske City"); 3. no history or diagnosis of patellar tendinopathy; and 4. no history of knee or ankle surgery in the last year. A group of 25 healthy male controls (27.3 [+ or -] 2.9 years; 71.2 [+ or -] 6.7 kg; 171.6 [+ or -] 6.9 cm) was also recruited, and their inclusion and exclusion criteria were: 1. no history or symptoms of patellar tendinopathy; 2. no history of knee or ankle surgery in the last year, and 3. performed less than 150 minutes per week of moderate and vigorous physical activity. (26) Anthropometric characteristics for each group are shown in Table 1. All subjects provided informed consent before participating in the study, which was approved by the Bioethics Committee of the authors' university to confirm that the study was conducted in accordance with the Declaration of Helsinki.

Measures

Patellar tendon stiffness was assessed using myotonometric measurements obtained with the Myoton Pro device (Myoton AS, Tallin, Estonia). The measuring method of the device is based on the free oscillation technique. (22) A brief mechanical impulse was applied to cause tissue damped oscillation, the signal of which was recorded by an accelerometer. From the obtained signal stiffness can be mathematically inferred as Stiffness = a0 * mProbe / At, where ao is the maximal acceleration of the curve (first negative peak), mProbe is the mass of the measurement mechanism (probe), and [DELTA]l is the maximal displacement of the tissue (Fig. 1) obtained by double integration of the acceleration curve.

Design and Procedures

Breakdancers who participated in the tournament were asked to be part of this study. A questionnaire developed by the authors and a clinical interview were administered to assess inclusion and exclusion criteria. For the breakdancers group, the dominant limb was identified as that used for initial push-off during execution of breakdancing skills (i.e., splits, spins, and handstands); for the control group it was determined as the one preferred when kicking a ball. Measurements were taken in the early morning, before competition. Subjects were instructed not to warm up until the test was completed. Hydration for each participant was the same as they used before every competition. No studies have evaluated the effects of hydration level or body temperature on tendon stiffness as assessed by myotonometric measurements. The control group was recruited from the same city as the breakdancers, and their evaluation was performed under similar environmental and physiological conditions.

The subjects were evaluated without shoes, with their knees in 90[degrees] of flexion, and they were asked to be completely relaxed (Fig. 2). The anatomical landmark for the PT stiffness assessment was marked by one of the evaluators at 3 cm above the tibial tuberosity. This landmark approximates the mid-portion of the PT and was used to avoid potential bias caused by bone tissue contact during the myotonometric measurement. Immediately after the skin was marked, the myotonometric assessment was conducted by another evaluator. The myotonometric evaluation was repeated three times per extremity, with a rest period of 10 seconds between trials. Intra- and inter-rater reliability of the device has been tested, showing a high reliability between trials. (27),(28) Only measures that had a coefficient of variation lower than 3% were retained; otherwise the measurement was repeated.

Data Analysis

Statistical analyses were performed using IBM SPSS Statistics software version 20 (IBM, Armonk, New York). For all outcome measurements, the Shapiro-Wilk test was applied to determine normality of the data. Mean and standard deviation were used to describe the data obtained. An unpaired test was used for comparison between groups. The level of significance was set at a = 0.05.

Results

The mean PT stiffness of breakdancers was 1,045 [+ or -] 202 Nm and 1,084 [+ or -] 193 Nm for the dominant and nondominant limb, respectively. For the control group, the mean PT stiffness for the dominant limb was 902 [+ or -] 166 Nm, and for the non-dominant limb 862 [+ or -] 159 Nm. Thus, breakdancers showed significantly higher mean PT stiffness than controls for both the dominant (p = 0.045) and non-dominant limbs (p < 0.0001). Figure 3 displays the mean PT stiffness values of both groups.

Discussion

The aim of the study was to describe the mechanical properties of the PT in elite breakdancers using myotonmetric measurements and to compare those results with control subjects. No previous studies have evaluated PT stiffness using this technique. The results showed that trained breakdancers have higher mean PT stiffness values when compared to controls.

Breakdance is a discipline characterized by high physiological demand during the execution of specific skills. To achieve this, breakdancers train for years. Associated training movements include continuous stretch-shortening of the quadriceps MTC. (2) During such movements, the PT enables quadriceps force to be transmitted during the shortening phase of the cycle and to release elastic energy in the stretch phase. (29),(30) To facilitate this mechanism, the PT must adapt to the loads exerted during this type of movement. It has been shown that stretch-shortening training, for example plyometrics, results in PT stiffening. (31),(32) Although breakdancers do not utilize plyometric training, the nature of their required movements and skills may produce similar tendon adaptations. Hence, the higher mean PT stiffness in breakdances compared to controls may result from the stretch-shortening movements that breakdancers commonly practice.

When tendons are adapted to mechanical loads, they exhibit structural and material modifications. (6),(11-13),(15) These changes can develop after weeks of (short-term) training or several years of (long-term) training. After 12 weeks of training, it has been shown that tendon becomes stiffer through changes in its composition, whereas its structure (i.e., cross sectional area) remains the same. (12),(13) Moreover, after long-term training, it has been shown that a stiffer tendon is obtained as a result of increase in its cross-sectional area (CSA), suggesting this is the mechanism that explains why our breakdancers had stiffer PTs than the control group. (12),(13),(33) That is, the results obtained in this research showed that the elite breakdancers exhibited changes in PT stiffness, which may have been due to the long-term mechanical loads to which they had been subjected.

Tendon mechanical properties have been studied for years. Decades ago, the common method used to measure tendon mechanical properties was in vitro assessment, in which force exerted and resulting displacement (i.e., stiffness) were obtained by testing cadaveric specimens. (34) Later, Komi et al. (35) developed an invasive in vivo technique, whereby a force transducer was inserted into the tendon and information about tendon tension (i.e., tendon force) was extracted during normal human locomotion. However, the invasive nature of the method rendered its future use impossible. Finni et al. (36),(37) then started using an optic fiber force transducer that was passed through the patellar tendon with a hollow needle to measure tendon tension during different jump tasks, such as hopping and squat jumps. Again, the invasive nature of the method prohibited its use. Currently, the most accepted method for evaluating tendon mechanical properties is ultrasonography and load cells. The present study introduces a simple and validated method to assess the described properties, and for the first time it has been used with elite breakdancers in this study.

Limitations and Future Directions

This study describes PT stiffness in trained breakdancers, which is shown to be higher than that in controls. However, the study has certain limitations. First, a greater sample size is needed to allow for more generalized interpretation of these results. Second, the myotonometric methodology implemented in this study is rather new, and more scientific studies in different populations are needed to fully validate its use. Future studies might employ an experimental design and prospectively evaluate PT stiffness after an intervention. Other studies might assess pathological tendons in breakdancers. Understanding the training-induced adaptations of tendons may help to improve training program design and prevention strategies for overuse tendon injuries as well as the optimization of tendon pathology rehabilitation.

Conclusion

Breakdancers presented significantly greater mean PT stiffness in comparison to the control group in both limbs. Patellar tendon stiffening seems to be an adaptation process associated with the continuous movements and skills performed during training and competition, which may enhance breakdancers' performance. Previous studies have shown that PT stiffening is a normal process after years of training, supporting the results found here. This information could be used as reference for future studies evaluating PT stiffness.

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Fiorella Celsi Young, MD, Iver Cristi-Sanchez, PT, Claudia Danes-Daetz, PT, Juan E. Monckeberg, MD, PhD, and Rony Silvestre Aguirre, PT, MSc

Fiorella Celsi Young, MD, Escuela de Medicina, Facultad de Medicina, Universidad de los Andes, Santiago, Chile. Iver Cristi-Sanchez, PT, Escuela de Kinesiologia, Facultad de Ciencias, Universidad Mayor, Chile, and Claudia Danes-Daetz, PT, Laboratorio Integrativo de Biomecanica y Fisiologia del Esfuerzo, Escuela de Kinesiologia, Facultad de Medicina, Universidad de los Andes, Santiago, Chile. Juan E. Monckeberg, MD, PhD, and Rony Silvestre Aguirre, PT, MSc, Clinica MEDS, Santiago, Chile.

Correspondence: Fiorella Celsi Young, MD, Universidad de los Andes, Monsenor Alvaro del Portillo 12455, Las Condes, Santiago, 7620001, Chile; ffcelsi@miuandes.cl.
Table 1 Demographic Characteristics by Group

                    Break Dance Group  Control Group
                    (N = 25)           (N = 25)       P-value

Weight (kg)          69.12 (8.01)       71.20 (6.71)  0.324
Height (cm)         169.96 (6.29)      171.60 (6.93)  0.385
BMI (kg/[m.sup.2])   23.90 (2.15)       24.16 (1.55)  0.630
Age (years)          29.32 (2.64)       27.36 (2.94)  0.016

Demographic characteristics by group shown as mean and standard
deviation. Significance difference between groups (p = 0.016) for age
can be observed.


Caption: Figure 1 Filtered acceleration curve after using MytonPRO. Displacement and velocity curves are obtained by double-integrating the acceleration curve. Stiffness = a0 * mprobe/Al, where a0 is the maximal acceleration of the curve, mprobe is the mass of the measurement mechanism (probe), and [DELTA]l is the maximal displacement of the tissue.

Caption: Figure 2 A, Position of the subject during the mechanical properties assessment. Subject was sitting with his knee in 90[degrees] of flexion. B, Mid-portion of the tendon was marked and later assessed.

Caption: Figure 3 Results for both groups for mean patellar tendon stiffness shown as mean and standard deviation. Significant differences between groups (*p < 0.05) can be observed.

Please Note: Illustration(s) are not available due to copyright restrictions.
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Author:Young, Fiorella Celsi; Cristi-Sanchez, Iver; Danes-Daetz, Claudia; Monckeberg, Juan E.; Aguirre, Ron
Publication:Journal of Dance Medicine & Science
Article Type:Report
Date:Oct 1, 2018
Words:3355
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