Digit ratio (2D:4D) and muscular strength in adolescent boys (original) (raw)
Highlights
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This was the first study to describe the relationship between digit ratio (2D:4D) and handgrip strength in adolescent boys. - •
2D:4D was very likely a moderate negative correlate of handgrip strength, even after adjustment for age and body size. - •
This result may reflect the organizational benefits of prenatal testosterone.
Abstract
Using a cross-sectional design, this study quantified the relationship between the digit ratio (2D:4D) and muscular strength in 57 adolescent boys. 2D:4D was very likely a moderate negative correlate of handgrip strength, even after adjustment for age and body size. This result may reflect the organizational benefits of prenatal testosterone.
Introduction
Digit ratio (2D:4D) is the ratio of the length of the second digit (2D) to the length of the fourth digit (4D). Males typically display lower 2D:4Ds than females, the likely result of the balance between prenatal testosterone and estrogen as the fetal 4D has a higher number of receptors for androgen [1], [2]. 2D:4D is essentially fixed in utero [3] and remains reasonably stable across the lifespan [4].
2D:4D is considered a proxy of prenatal testosterone [2]. Prenatal testosterone has numerous long-term organizational effects on the body, including growth and development of the cardiovascular, musculoskeletal, and urogenital systems [2]. 2D:4D is a negative correlate of performance in sports (e.g., basketball, fencing, rowing, soccer [football]), athletics (e.g., running), and on fitness tests (e.g., handgrip strength), although considerable variability exists across different activities [5], [6], [7]. Muscular strength (operationalized as handgrip strength) has been linked with 2D:4D, albeit with inconsistent results. Fink and colleagues [8] found that men with lower 2D:4Ds had substantially better handgrip strength. Correlational research indicates that the 2D:4D-handgrip relationship is: (a) weak to moderate and negative in men [9], [10]; (b) negligible to weak and typically positive in women [9], [10], [11]; (c) negligible to weak and typically negative in male and female college students [12]; and (d) negligible in boys and girls aged 8–12
years [13]. Negligible relationships between 2D:4D and other strength measures (e.g., static strength [upper and lower body] and explosive strength [lower body]) have also been reported for adolescent girls aged 13–18
years [14]. Unfortunately, the relationship between 2D:4D and handgrip strength in adolescent boys is unknown. The aim of this study was to quantify the relationship between 2D:4D and handgrip strength in adolescent boys.
Section snippets
Methods
This study used a cross-sectional design. Boys aged 13–18
years from Sacred Heart School in East Grand Forks, MN, USA, were invited to participate. Written informed consent was obtained from parents or legal guardians, and participants provided assent. Only participants of Caucasian ethnicity were included because of known ethnic differences in 2D:4Ds [15], with those self-reporting a major injury (e.g., a break) to either the second digit (2D) or fourth digit (4D) excluded. The Institutional
Results
Fifty-seven adolescent boys volunteered for this study. Means (SDs) for the sample were: age, 15 (2) years; height, 173 (10) cm; mass, 70 (18) kg; BMI, 23 (5) kg/m2; 2D:4D, 1.00 (0.05); and handgrip strength, 35 (10) kg.
The age- and BMI-adjusted partial correlation between 2D:4D and handgrip strength was very likely moderate and negative (r [95%CI]: −0.32 [−0.57, −0.04]) (Fig. 1), indicating that boys with lower 2D:4Ds had better handgrip strength irrespective of their age and body size. Each
Discussion
This study was the first to quantify the relationship between 2D:4D and muscular strength in adolescent boys. It showed that 2D:4D was very likely a moderate negative correlate of handgrip strength, even after adjustment for age and body size. This relationship is similar in magnitude and direction to that observed in men [8], [9], [10], and is similar in direction yet substantially larger in magnitude to that observed in younger boys (aged 8–12
years) [13]. While the observed relationship
Conclusion
This study found a moderate age- and BMI-adjusted negative (and theory-consistent) relationship between 2D:4D and handgrip strength in adolescent boys. This result is likely due to the long-term organizational benefits of prenatal testosterone. This study adds to a limited body of research examining the 2D:4D-fitness relationship in adolescents, and encourages additional 2D:4D research in girls and other ethnicities before drawing confident conclusions as to the true relationship.
Funding
Nil.
Contributors
JMT and GRT developed the research question, designed the study, had full access to the data, and take responsibility for the integrity of the data. JMT collected the data. GRT conducted the statistical analysis and wrote the report. Both authors contributed to the interpretation of results, editing and critical reviewing of the final report, and approved the final report.
Conflicts of interest statement
None declared.
Acknowledgements
We would like to thank the students at Sacred Heart School who participated in this study.
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Cited by (18)
2018, Early Human Development
The authors found negative associations of lower 2D:4D with symptoms of hyperactivity and poor social cognitive function in girls. Subsequent studies showed that the 2D:4D ratio correlates negatively with physical aggression and fitness in school children and adolescents [18–20]. Increased aggression and attention problems were also associated with lower 2D:4D ratios in Chinese school children [21].
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