Digit ratio (2D:4D) and handgrip strength in a Chinese population of Han ethnicity

Highlights

• Sexual dimorphism of 2D:4D and HGS were found, males had significantly lower 2D:4D and greater HGS than females.

• 2D:4D in both hands were significantly negative correlated with HGS in females but not in males.

• 2D:4D in both hands were significantly negative correlated with HGS in females but not in males in a Chinese population of Ningxia Han ethnicity.

Abstract

Background

In humans, the relative lengths of the index finger to the ring finger (2D:4D) is a sexually dimorphic trait which correlated with prenatal sex steroids and has been increasingly used as a promising tool to evaluate the impact of prenatal hormone exposure in some traits, such as physical performance. Handgrip strength (HGS) is one potent index of physical ability and its relationship with 2D:4D ratio has been discussed in several ethnic groups.

Aims

To investigate whether there is a correlation between 2D:4D ratio and HGS in Chinese college students of Ningxia Han ethnicity.

Methods

608 students (211 males and 397 females) of Han ethnicity were recruited from Ningxia medical university. Photocopies and HGS of both hands were collected at Yinchuan city, in the Ningxia province of China.

Results

Sexual dimorphism of 2D:4D and HGS were found, males had significantly lower 2D:4D and greater HGS than females. 2D:4D in both hands were significantly negative correlated with HGS in females and not in males.

Conclusions

2D:4D ratio is negative correlated with HGS in a Chinese population of Ningxia Han ethnicity and this association should be considered on the anthropological research within an evolutionary concept in the future.

Introduction

Digit ratio, especially second-to-fourth digit ratio (2D:4D), is sexually dimorphic: males generally have a relatively shorter 2D:4D compared to females across ethnicities and countries [1]. Sex difference on 2D:4D is generally established during the first trimester of the embryo development coincided with sex steroids changing and be relatively stable across the lifespan without affection of puberty [2]. In humans, this dimorphic trait is influenced by the level of prenatal hormone exposure of the embryo [3]. Since the levels of prenatal testosterone (PT) and prenatal estrogen (PE) are difficult to directly assess, progress has been rapid in exploring surrogate measures of hormone exposure throughout the lifespan to better illuminate hormone-related events [4].

Based on evidence that 2D:4D might provide a window into the external environmental changes in terms of PT and PE exposure, low 2D:4D indicates high PT (low PE), whereas high 2D:4D indicates low PT (high PE). Many researchers have used 2D:4D as a promising index in studying hormone-related behavior, such as physical ability, to evaluate the impact of PT [5], [6], [7], [8], [9], [10], [11]. A meta-analysis by Hönekopp et al. [12] showed that athletic prowess is negatively correlated with 2D:4D of each hand for both sexes.

Handgrip strength (HGS) is considered to be one of the authentic parameters of physical ability [13]. Increasing evidence shows that HGS is also a sexually dimorphic trait, just as 2D:4D, with men being typically stronger than women [14]. In recent years, measures adopted to investigate the correlation between 2D:4D and HGS have been discussed in several ethnic groups [15], [16], [17], [18], [19], [20], [21], [22]. However, the results of the association between 2D:4D and HGS are inconsistent. Low 2D:4D has been found to be correlated with high HGS in samples of European (German) and Asian (Mizos) men [15]. Subsequently, this finding has only been verified in a sample of US men [16], in Chinese men of Han [11] and Hani [17] ethnicity, in Turkish older men [18], and in boys from the UK [19]. A recent study including students and staff of two UK universities (ethnically mixed) has also reported that 2D:4D is a negative correlate of HGS in men in the challenge conditions [20]. Unlike with the findings mentioned previously, other studies including US and European samples failed to replicate the link between 2D:4D and HGS in men and women [21], [22], [23], whereas Meltem et al. [18] found this link in both sexes in elderly patients with sarcopenia. Early experimental evidence suggested that PT improves HGS and muscle mass. Since the relationship between PT and 2D:4D is at least as well substantiated in women as it is in men, if lower 2D:4D is associated with increased adult HGS, then an association between them should be apparent in women as well as men [23]. Therefore, if this hypothesis is correct, we would guess that there should be a common link between 2D:4D and HGS in both sexes for any ethnicity in humans.

Zhao et al. have reported a negative correlation between right-hand 2D:4D and HGS in Chinese men of Han ethnicity (Shanxi province) [11]. Here, we consider an association between 2D:4D and HGS in both sexes from a different region of China (i.e. Ningxia province). Our aim was to investigate the correlation between 2D:4D and HGS, and, at the same time, to determine whether the findings of Zhao et al. could be generalized to other groups of Chinese Han ethnicity.