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Heritability of displacement speed in a 30-m sprint.

INTRODUCTION

Displacement speed is a primary required motor skill for success in several sports. Therefore, an understanding of the magnitude of the influence of genetic factors and environmental variables that act on displacement performance may partially explain the differences between athletes who perform the same type of training but achieve completely different results (1,29).

Displacement speeds in performance were previously addressed in studies of twin pairs to further the knowledge of individual variations. The speeds were measured because they are an economically viable and practical alternative for identifying the contribution of genetic and environmental influence on the physical performance of individuals (17,30).

Previous studies have reported wide variations in the genetic and environmental influences on displacement speed (14). The primary reason for this outcome is that few references have included admixed populations such as the Brazilian population. Many of the surveys were conducted in countries where climatic conditions and the racial distribution of the population are distinct from the Brazilian population, and many factors can cause wide variations in results and hinder potential comparisons, such as differences between tests, age of twin pairs, errors in the determination of zygosity, gender differences, and maturational intrapairs (7,11)

Investigating the genetic and environmental influences on displacement speeds is important from a theoretical point of view and in its practical application, which will aid exercise physiologists in their understanding of the potential of displacement speed training in selection processes for activities or high-performance sports (29). Thus, this study evaluated the relative power of genetic, environmental, and gender contributions on displacement velocity in monozygotic (MZ, identical) twins and dizygotic (DZ, not-identical) twins.

METHODS

Subjects

The subjects consisted of volunteer MZ and DZ twins from cities near Natal (Natal and Sao Goncalo do Amarante cities), Rio Grande do Norte, Brazil. The sample included 41 pairs of twins (82 individuals) who were divided by their zygosity into 25 pairs of MZ twins (50 individuals: 28 females and 22 males) and 16 pairs of DZ twins (32 individuals: 18 females and 14 males), aged 8 to 26 yrs old; all of these twin pairs resided in the same households.

Zygosity was determined through telephone interviews with the mothers of the assessed twins (20). The similarity between each pair of twins was assessed by checking hair color, eye color, facial features, height, and body mass index. Cases in which the twin pair zygosity was doubted following the performed assessments were excluded from the analyses.

The evaluated twins were interviewed after zygosity determinations using a questionnaire that was developed specifically for this study. The questionnaire included five questions related to personal data, medical history, and habits. Subjects under the age of 14 were interviewed in the presence of their mothers. All answers were "yes" or "no". If the answer was affirmative, the subject or his/her guardian was asked about the nature of the problem. The questionnaire included the following questions: Do you have a chronic illness (such as high blood pressure, diabetes, or dyslipidemia)? Do you smoke or drink alcohol frequently?

Do you engage in some physical activity? Do you have any orthopaedic problems? Have you undergone any medical treatments? In cases of disagreement in the responses of twin pairs that indicated differences in clinical history and habits, the twins were not included in the analysis.

Procedures

The subjects were interviewed using the Physical Activity Readiness Questionnaire (PAR-Q), which evaluates the readiness for physical exercise (27). Cases of disagreement in differences of readiness for physical activity in the MZ and DZ twin intrapair responses were removed from analyses. The DZ twin pairs who indicated that physical activity or any suspected health problem prevented the practice of regular physical activity were advised to seek medical attention, and their results were removed from the analyses.

Self-assessments of sexual maturation were performed in all individuals under 19 yrs of age following the interviews of the readiness for physical activity. Evaluations were conducted individually in a common room. The subjects were initially presented with photographs of different gender-specific development stages of secondary sexual characteristics, with two picture boards for each gender. This procedure was performed with each picture board under a blank sheet to avoid curiosity and facilitate understanding. The child was instructed to carefully observe each photo and mark the assessment sheet with the number of the photo that most resembled her/his development at that moment (1,19).

The literature describes great difficulties in medical evaluations that measure sexual maturation. However, self-evaluation using the projective technique is a practical, simple, and unsophisticated method that may be used in both sexes from 6 yrs of age and any socioeconomic level. This technique is an increasingly used as an alternative practice in national and international studies (9,26).

Two measures were performed in the present study to increase confidence in the self-assessment of the sexual maturation results. One measurement was performed after the preliminary interviews, and the other measurement was performed at the end of the physical assessments. This procedure permitted the calculation of a weighted kappa index among the repeated measurements of self-assessments of sexual maturation in MZ and DZ subjects. The observed values were greater than 0.890 (P<0.001). Intrapair twins who exhibited differences in sexual maturation outcomes were removed from the analysis.

The following groups were excluded from the study: (a) disabled subjects; (b) pregnant women; (c) individuals in drug treatment; (d) patients with endogenous or secondary obesity; and (e) patients with endocrine and genetic disorders. The following twin pairs were also excluded: (a) twin pairs who contained individuals in different genres (intrapair); (b) pairs who did not share the same physical activity habits; and (c) same-sex twin pairs who presented with different stages of sexual maturation.

All subjects were healthy individuals. The subjects or their parents or guardians of the children signed an informed consent form to participate in the research. The Research Ethics Committee of the Onofre Lopes Hospital approved this study, which was duly recognised by the National Research Ethics Committee under protocol--HUOL: 484/10--CAAE: 0042.0.2 .294.000-10 in 18/02/2011 day.

A trained evaluator assessed the body composition of the twins using anthropometric measurements. Body mass index was measured using a high-precision Filizola-110 electronic scale with a 150-kg capacity and a unit of measurement of 0.1 kg; body mass was measured only once. Height was measured twice using a Sanny stadiometer with a 0.1-cm unit of measurement, and a 0.5-cm gap between the measurements was permitted. The final results were averaged and utilized for analyses. The intraclass correlation coefficient (ICC) between repeated measurements of height was calculated separately for MZ and DZ subjects to increase confidence in the results. Values were greater than 0.996 (P<0.001), which indicates that less than 1% of the variance can be explained by variations in the measuring instrument or standardization. These evaluations were performed in subjects with minimal clothing and no shoes. The location of the evaluation was a quiet room with a temperature between 22 to 24[degrees]C. The anthropometric assessments were standardized in accordance with the procedures described by Marfell-Jones et al. (15).

The subjects' displacement speed (30 m) was conducted in a closed and covered gymnasium to minimize the influence of wind on the results. The floor was rubberized. Each subject wore light clothing and rubber-soled shoes. The room temperature was measured using a digital thermometer. The temperature ranged between 24[degrees]C-26[degrees]C. The twins were tested at a maximum interval of 60 min to avoid the possible effects of time of day on the test results. The subjects were instructed to avoid participation in any vigorous activity and the drinking of alcohol and caffeine for 24 hrs prior to testing. All subjects were informed about the importance of obtaining at least 8 hrs of sleep on the night before the procedure. All subjects were familiar with the research.

Two CEFISE photocells with millisecond precision recorded the travel times. These photoelectric cells timed the route of the 30 m. Each photocell was positioned to focus the light beam on the iliac crest. The output for the race was a standing position just behind the first photocell. The subjects were instructed to cover the 30-m distance and not slow down prior to reaching the 30-m mark. A false arrival was placed 5 m after the final mark, and all subjects were instructed to run as fast as possible until they reached the 35-m false mark. These instructions ensured that all subjects passed the photocell at the 30-m mark at maximum displacement speed (4). The test was performed three times in each subject with a 1-min interval between attempts. The average displacement speed was used in the statistical analyses. The ICC was calculated between repeated measurements of the separate displacement speeds of MZ and DZ subjects. Values above 0.995 (P<0.001) were observed, which indicates that less than 1% of the variance was explained by variations in the measuring instruments in both groups. These photoelectric cells have been used previously because these cells provide greater precision and reliability in time measurements of displacement speeds (5,8).

Statistical Analyses

Analyses were based on the results of individual MZ and DZ twins and the variance of intrapair twins. Potential confounders (such as sex, age, and biological maturation) were controlled for. Statistical analyses were performed following classical criteria for the initial investigation of sample normality: the behaviour of asymmetry (two times less than the standard error of asymmetry), kurtosis (two times less than the standard error of kurtosis), and the minimum value and maximum average (must be within three times the value of the mean). Overall, the results of this study were characterized as non-parametric. Therefore, the medians of central tendency and their respective confidence intervals (percentiles 25-75 (p25-p75)) were used. Partial correlations (r) with age control were observed between the medians of the MZ and DZ twin groups separately to investigate the degree of relationship between MZ and DZ brothers. Furthermore, the ICC of repeated measurements of heights and displacement speeds and weighted kappa values were calculated between the repeated measures of sexual maturation in MZ and DZ twin groups separately to increase the confidence in our results.

The differences between the median age, weight, height, and forward speed were calculated using a Wilcoxon test to examine significant differences between MZ and DZ twins. A linear regression was performed to observe the behaviour of the mean displacement speeds (30 m) between pairs of MZ and DZ twins for males, females, and both sexes. The following equation estimated the heritability ([h.sup.2]): [h.sup.2] = ([S.sup.2] DZ - [S.sup.2] MZ) / [S.sup.2] DZ) x 100, where [S.sup.2] represents the median of intrapair variances in each different series (18,25).

RESULTS

This study analyzed the heritability of displacement speeds based on intrapair variances in MZ and DZ twins and the possible influence of gender on these speeds. Table 1(a) presents the medians and confidence intervals (p25-p75) in both sexes. The intrapair r, controlled by sex and age, revealed results greater than 0.81 (P<0.001) for all study variables in MZ and DZ pairs. This high intrapair correlation demonstrated the similarities between these subjects and the possible influences of sex and age, which did not affect the behaviour of the results. Furthermore, no significant differences between the medians (Wilcoxon test) of MZ and DZ twin pairs for height, age, body mass, and displacement speeds were observed. The estimation of heritability ([h.sup.2]) in the group with both sexes was 73%.

Table 1(b) presents the medians and confidence intervals (p25-p75) in the female subjects. The intrapair r controlled for age revealed similar results as Table I(a), and values greater than 0.82 (P<0.001) were observed for MZ and DZ twins. Furthermore, no significant differences between the medians (Wilcoxon test) of MZ and DZ twin pairs were observed for height, age, body mass, and displacement speeds. The only observed difference was the low and non-significant r (r = 0.36, P>0.05) for the displacement speed of the DZ group. The estimation of heritability ([h.sup.2]) in the group with females was 85%.

Table 1(c) presents the results in males as medians and confidence intervals (p25-p75). The intrapair r, controlled by sex and age, revealed similar results as Table I(a), and values greater than 0.71 (P<0.05) were observed for all MZ and DZ pairs. No significant differences between the medians (Wilcoxon test) of MZ and DZ twin pairs were observed for height, age, body mass, and displacement speeds. The estimation of heritability ([h.sup.2]) in the group with males was 67%.

The average result of the three attempts of the displacement speed test was used for each MZ and DZ subject in both sexes to investigate the detachment of the individual results of each twin pair (Figure 1). An adjusted [R.sup.2] of 0.78 was observed, which indicates that the explained variance of the results was 78%. The average results of the three displacement speed tests for each female MZ and DZ subject were used to investigate the detachment of the individual results of each twin pairs (Figure 2). An adjusted [R.sup.2] of 0.54 was observed, which indicates that the explained variance of the results was 54%.

The results of three displacement speed tests for each of the male MZ and DZ subjects were used to investigate the detachment of the individual results of each twin pair (Figure 3). An adjusted [R.sup.2] of 0.899 was observed, which indicates that the explained variance of the results was 89.9%. The graphs demonstrate a clear decrease in the variance that was explained by gender. The group of female MZ and DZ twins exhibited the lowest variation (54%). The greatest variations were observed between DZ twins, which supports our hypothesis that the trait of "displacement speed" possesses a high heritability in admixed populations in accordance with the fact that MZ twins have a rate of 100% similarity in the DNA sequence while the DZ twins present approximately 50% similarity in their DNA sequences.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

DISCUSSION

This study provides evidence of individual differences in displacement speeds between twins that may be largely attributed to genetic differences in both genres. Little data on the influence of heritability on displacement speeds exist in the Brazilian population, but several relevant international studies have reported this influence in same-sex samples of similar age. While international studies have often contained results based on populations which are highly homogenized such as Caucasians, Africans, and Asians, studies in populations highly admixed represent a great challenge when trying to genetically understand between ethnicities. This is because a genetic characteristic is present in one ethnic group, making it difficult to discover if investigating using comparisons with only homogenous groups. This characteristic can however be found in admixed populations (24).

Despite the high estimates of heritability, previous homogenous studies (3,6) have demonstrated a wide range of 48% and 92% variation. Our findings are consistent with these results and confirmed the high hereditary dependence of displacement speeds with an estimated heritability of 73% in both sexes, 67% in the males, and 85% in the females. Interestingly, recent research by Rodrigues de Moura and colleagues (23) indicates that European (62%) ancestry is the major contributor to the genetic background of Brazilians, followed by African (21%) and Amerindian (17%) ancestries. The North East region where the present study was conducted had a larger presence of African ancestry with a general population 58% European, 27% African, and 15% Amerindian (23).

Thus, the present study confirms the results of the international studies despite the differences in racial, environmental, and climate influences on individuals who were born in northeastern Brazil (18,25). We observed a high heritability of displacement speed in both sexes and a greater influence in women. These results demonstrate that estimates of heritability vary considerably, which may be attributed to differences between tests, measuring equipment and genre. Several factors, such as mental concentration, proprioception, rhythm, motor learning, accuracy, training time, and economy interact with these types of tests and these factors are integrated with displacement speed development (28,29).

In fact, estimates of the heritability of a given variable have yielded different results in the same sample in a two-year longitudinal study (2,22). Similarly, studies of these same variables in different cross-sectional ages have demonstrated variations in the values of heritability estimates (16). These findings suggest that age, gender, and developmental factors, such as sexual maturation, affect estimations of heritability. Therefore, genetic influences are not equally expressed in samples of different ages and genres (16,19).

This study was particularly concerned with controlling the bias of age, gender, and sexual maturation. That is why the medians of the intrapair variance results of MZ and DZ twins were used to calculate heritability. Twin pairs who exhibited different intrapair maturational stages and lifestyles were excluded from the present study. These actions enforced rigorous evaluation criteria on the tests that were used to measure displacement speeds and the use of twins to minimize the impact of factors that may decrease the robustness of the results (12,13,30).

A recent review of the research literature on the genetic influence and performance refuted the assertion for the need of a minimum of 10,000 hrs of training time to achieve effective results in displacement speeds. This assertion was based on the premise that elite athletes achieve very similar results, but they rarely undergo the same training times. This observation suggests the fundamental importance of the integration of genetic and environmental factors in the development of high performance levels. Some sports may require more complex training times compared to simpler procedures (28). The need to observe training quality and the possible differences between twins should be emphasized because the women demonstrated a greater genetic influence than the men.

Another important factor that may underlie the possible differences in results and trainability between men and women is the distribution of type-II muscle fibers, especially in the lower limbs. This difference may not be related to the type of exercise, but rather an innate difference between the sexes (29). However, no conclusive answer on possible differences in the numbers of type-II fibers in men and women in the lower limbs has been identified (10,21). Several studies have suggested a better use of ATP resynthesis in the greater amounts of muscle mass in the lower limbs and the lower amount of body fat in men as the primary justification for the performance differences in displacement speed between men and women (10).

Given that the present study generated evidence on the heritability of displacement speed using the method of twins, special attention should be paid to the selection process of athletes who need to produce high rates of speed in their sports practice (given that the influence of the environment is small especially in women). This study reinforces the importance of reflection on the development of possible objectives to be achieved and the correct sporting orientation that enables the actual development of young athletes.

CONCLUSIONS

The results indicate that displacement speeds were a highly heritable trait in both sexes in a northeastern Brazilian population in age groups of 8 to 26 yrs. The findings should be of interest to a broad readership, including coaches, physical educators, athletes, and sports physicians. Exercise physiologists should pay increased attention to the need for high quality training programs, especially in females, to improve the displacement speed because of the low potential impact on training. This may be done by correcting gender gaps and leveraging the possible pre-disposition in displacement speed by considering the large influence of heritability within admixed populations.

ACKNOWLEDGMENTS

This article was supported by the National Counsel of Technological and Scientific Development (CNPq), Brazil. The authors would like to thank the Thomas Anthony Huggins for his help in the review of the manuscript.

Address for correspondence: Michelle Vasconcelos de Oliveira Borges, Alameda dos Eucaliptos Street No 12, Parnamirim, Rio Grande do Norte--Brazil--Zip Code: 59151-770 Telephone: 011 55 84 98809-4057, Email: vasmichelle@gmail.com

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Luciano Alonso [1], Michelle Vasconcelos de Oliveira Borges [5], Elys Costa de Sousa [5], Marina Morena de Souza [5], Dailson Paulucio [1,2,3], Pedro Ribeiro [2,3], Bruna Velasques [2,3], Fernando A.M.S. Pompeu [1,2], Caleb Guedes Santos [1,4], Paulo Moreira Silva Dantas [5]

[1] Affiliation Lab / Program/Company, City, Country, [2] Affiliation Lab / Program / Company, City, Country, [3] Affiliation Lab / Program / Company, City, Country [1]--Federal University of Rio de Janeiro School of Physical Education and Sports--Biometrics Lab, Brazil, [2] Federal University of Rio de Janeiro--School of Physical Education and Sports--Postgraduate in Physical Education, Brazil, [3] Federal University of Rio de Janeiro--Institute of Psychiatry--Brain Mapping and Sensory Motor Integration, Brazil, [4] Army Biology Institute, Brazil, [5] Federal University of Rio Grande do Norte--School of Physical Education and Sports, Brazil
Table 1. Median, Confidence Interval (CI), Intrapair Variance of
Displacement Speed (VDS) and Partial Correlations Controlling for
Age (r), Height, Body Mass, Displacement Speed (DS) and the
Heritability of: (a) Monozygotic and Dizygotic Twins of Both Sexes;
(b) Female Monozygotic and Dizygotic Twins; and (c) and Male
Monozygotic and Dizygotic Twins.

(a) (Both               Monozygotic (n = 50)
Sexes)
                 Twin 1 (n = 25)     Twin 2 (n = 25)

                Median CI (25-75)   Median CI (25-75)      r

Age (yrs)             15.00               15.00
                  (11.50-22.00)       (11.50-22.00)
Height (cm)          154.00              154.00
                 (146.00-162.00)     (146.00-163.00)
Mass (kg)             48.80               47.60
                  (36.07-61.07)       (39.60-59.70)
DS (sec)              5.95                5.88          0.87 *
                   (5.07-6.47)         (5.11-6.66)

VDS                     0.0144 (0.0017-0.0906)
(Twin1-Twin2)

(a) (Both                Dizygotic (n = 32)
Sexes)
                 Twin 1 (n = 16)     Twin 2 (n = 16)

                Median CI (25-75)   Median CI (25-75)      r

Age (yrs)             14.50               14.50
                  (11.25-21.00)       (11.25-21.00)
Height (cm)          154.00              154.00
                 (146.00-162.00)     (146.00-163.00)
Mass (kg)             48.80               47.60
                  (36.07-61.07)       (39.60-59.70)
DS (sec)              5.95                5.88          0.84 *
                   (5.07-6.47)         (5.11-6.66)

VDS                      0.0536 (0.0101-0.1494)
(Twin1-Twin2)
                      [h.sup.2]=73% (39.00-83.00) **

(b) (Female              Monozygotic (n = 28)
Twins)
                 Twin 1 (n = 14)     Twin 2 (n = 14)

                Median CI (25-75)   Median CI (25-75)      r

Age (yrs)             15.00               15.00
                  (11.75-20.00)       (11.75-20.00)
Height (cm)          154.00              153.50
                 (146.00-162.00)     (148.00-161.00)
Mass (kg)             48.10               48.75
                  (41.35-57.16)       (39.50-54.50)
DS (sec)              6.31                6.51          0.82 *
                   (5.71-6.61)         (5.63-7.03)

VDS                     0.0162 (0.0019-0.1564)
(Twin1-Twin2)

(b) (Female               Dizygotic (n = 18)
Twins)
                 Twin 1 (n = 9)      Twin 2 (n = 9)

                Median CI (25-75)   Median CI (25-75)      r

Age (yrs)             13.00               13.00
                  (11.50-21.00)       (11.50-21.00)
Height (cm)          157.40              156.00
                 (148.20-163.00)     (146.00-164.20)
Mass (kg)             51.35               48.90
                  (44.20-57.07)       (34.40-55.15)
DS (sec)              6.29                6.54          0.36 **
                   (6.02-6.82)         (6.10-6.88)

VDS                     0.1136 (0.0206-0.2788)
(Twin1-Twin2)
                     [h.sup.2]=85% (43.90-90.70) ***

(c) (Male               Monozygotic (n = 22)
Twins)
                 Twin 1 (n = 11)     Twin 2 (n = 11)

                Median CI (25-75)   Median CI (25-75)      r

Age (yrs)             13.00               13.00
                  (9.00-24.00)        (9.00-24.00)
Height (cm)          157.00              158.70
                 (145.00-170.40)     (145.00-172.20)
Mass (kg)             50.10               46.20
                  (32.30-72.4)        (39.00-67.75)
DS (sec)              5.46                5.31          0.95 *
                   (4.91-6.16)         (4.91-6.02)

VDS                      0.0123 (0.0001-0.0800)
(Twin1-Twin2)

(c) (Male                 Dizygotic (n = 14)
Twins)
                 Twin 1 (n = 7)      Twin 2 (n = 7)

                Median CI (25-75)   Median CI (25-75)      r

Age (yrs)             16.00               16.00           --
                  (8.00-22.00)        (8.00-22.00)
Height (cm)          164.50              163.10
                 (126.50-187.00)     (131.50-187.90)
Mass (kg)             56.30               59.50
                  (31.30-84.6)        (34.60-86.80)
DS (sec)              5.22                5.19          0.71 *
                   (4.49-6.86)         (4.42-7.21)

VDS                       0.0383 (0.0009-0.0624)
(Twin1-Twin2)
                      [h.sup.2]=67%(28.20-88.80) ***

* Significant difference P<0.01, ** No significant difference, ***
[h.sup.2] = ([S.sup.2] DZ/[S.sup.2] MZ) /[S.sup.2] DZ) x 100
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Author:Alonso, Luciano; de Oliveira Borges, Michelle Vasconcelos; de Sousa, Elys Costa; de Souza, Marina Mo
Publication:Journal of Exercise Physiology Online
Article Type:Report
Date:Jun 1, 2016
Words:4982
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