The effects of the presence or the absence of flexor digitorum superficialis tendon of the little finger on the handgrip strength of athletes

Abstract

BACKGROUND:

Many daily functions and sporting events need high activity level of the flexor muscles of the forearms and hands. Handgrip strength is measured by a handheld dynamometer which is a reliable, safe and inexpensive device.

OBJECTIVES:

The study aims to find out whether the presence or absence of flexor digitorum superficialis tendon of the fifth (little) fingers (FDS-V) affects handgrip strength of athletes from different types of sports.

METHODS:

The effect of presence or absence of the FDS-V in a total number of 788 fingers of athletes (male $n=$ 200, female $n=$ 194) was assessed using isometric handgrip strength.

RESULTS:

No statistically significant difference existed in handgrip strength of the participants with and without the FDS-V of the little finger ( $p>$ 0.025).

CONCLUSION:

The presence or absence of FDS-V of the little finger in athletes has no effect on handgrip strength.

Keywords

Handgrip strength flexor digitorum superficialis tendon little finger

1. Introduction

The assessment of handgrip strength (HGS) is assumed to be highly important in the evaluation of performance of some sports activities [1, 2]. In health-related cases, HGS is evaluated as an indicator of overall health and upper extremity force [3, 4]. Moreover, HGS is used as an indicator of overall muscular force [5, 6]. Many different factors such as age, height, body weight and BMI affect HGS [7, 8, 9]. However, anthropometric variables should be taken into account while evaluating HGS. Some other variables are important in the evaluation of HGS. For example, it was reported that HGS could depend on sports participation [10], or the level of sport: elite or sub-elite [11] or the type of occupation [12].

Highly common anatomic variations might also affect HGS and these variations can lead to significant differences in HGS. Some muscles in forearm that actively take part in HGS activities have different anatomic structures like muscle belly mass or abnormal origins and insertions. These variations are relatively widespread in general population. Another variation that might be statistically significant in HGS is the FDS-V [13, 14] which is often interconnected with FDS tendon of the fourth (ring) finger [15] or completely absent [16]. In their study on the FDS-V of a non-homogeneous native Asian population, Puhaindran et al. [17] reported that 6% of the population lacked this muscle and that this absence had no effect on HGS of the study groups. Bowman et al. [18] reported that the absence of the FDS-V was 17.6% in US adults and that this absence adversely affected the HGS values of the individuals. Guler et al. [19] stated that the overall incidence of absence of FDS of little fingers was 18.5% in Turkish population.

We have failed to find any study relating to the effect of the absence of FDS-V on HGS is athletic groups, hence the present study which aimed at assessing the effect of this anatomical variation on male and females athletes doing different sports and effects of the case on the handgrip strength of the participants.

2. Methods

2.1 Participants

The study included 394 athletes ( $n=$ 200 males, $n=$ 194 females) from different types of sports. Table 1 shows the demographic characteristics of the participants. Two men and one woman were left-side dominant. All participants verbally declared that they had no history of any hand injury or harm.

Table 1
Demographic characteristics of the male and female athletes

Groups	Characteristics	Minimum	Maximum	Mean	Std. deviation
Male ( $n=$ 200)	Age (years)	18.00	28.00	18.97	1.70
	Height (m)	1.57	1.92	1.75	0.06
	Body weight (kg)	53.00	140.00	74.25	14.60
	BMI (kg/m ${}^{2}$ )	17.84	45.71	24.09	4.40
Female ( $n=$ 94)	Age (years)	18.00	27.00	18.47	1.27
	Height (m)	1.48	1.85	1.63	0.061
	Body weight (kg)	36.00	114.00	55.80	9.83
	BMI (kg/m ${}^{2}$ )	14.42	37.65	20.85	3.33

The study complies with the Declaration of Helsinki and the ethical approval of the study was obtained from the University of Necmettin Erbakan, Meram Medical Faculty (approval number: 2019/2208). Individuals having trouble in hand force and/or hand use due to any previous operations or traumas, connective tissue diseases, rheumatoid arthritis and under 18 age were excluded.

2.2 Experimental protocol

2.2.1 Functional ability of the FDS

The study was conducted in two parts. In the first part, the functional ability of FDS tendon to flex the proximal inter-phalangeal (PIP) joint of little finger was assessed. The participants were asked to sit in a chair with the left arm on the table with palm facing up. The examiner sat across the table from the participant and showed the correct positioning. The examiner asked the participant to hold his/her hand in supine position (while dorsum of the forearm, hand wrist and hand were in contact with the table). Two different methods were used to observe FDS anomalies of little finger on each hand. In the first method, the examiner verbally told the participant to flex PIP joint of little finger more than 90 degrees and keep the second, third and fourth fingers on right hand in extension. For flexion measurements, a goniometer was used. The same steps were repeated for the left hand and the findings were noted on FDS anomalies form. If the participant could not flex the fifth PIP joint beyond 90 ${}^{\circ}$ , the second method (the modified method) was used. In this modified method, the examiner instructed the participant to keep the second and third fingers of right hand in extension and to flex the fourth and fifth fingers more than 90 ${}^{\circ}$ simultaneously. The same steps were repeated for the left hand. Then, all participants were grouped depending on the results of the tests. If the participant was able to flex the PIP joint of fifth finger more than 90 ${}^{\circ}$ during the standard test, the hand was classified as independent of FDS (FDS-independent). If the participant wasn’t able to do so during the standard test, but was able to flex PIP joint of the little finger more than 90 ${}^{\circ}$ during the modified test, the hand was classified as FDS-common. If the participant was not able to flex PIP joint of little finger more than 90 ${}^{\circ}$ during both tests, the hand was classified as FDS-absent [18, 20, 17, 21].

Table 2
Correlations between right-left hand HGS values and other variables: anthropometric characteristics, age of groups

Male $\rightarrow$ Female $\downarrow$	Right hand HGS		Left hand HGS		Age (years)		Height (m)		Body weight (kg)		BMI (kg/m ${}^{2}$ )
Right hand HGS (kg)			0	.778 ${}^{\ast\ast\ast}$	0	.253 ${}^{\ast\ast\ast}$	0	.055	0	.396 ${}^{\ast\ast\ast}$	0	.400 ${}^{\ast\ast\ast}$
Left hand HGS (kg)	0	.861 ${}^{\ast\ast\ast}$			0	.276 ${}^{\ast\ast\ast}$	0	.028	0	.428 ${}^{\ast\ast\ast}$	0	.446 ${}^{\ast\ast\ast}$
Age (years)	0	.169 ${}^{\ast}$	0	.173 ${}^{\ast}$			$-$ 0	.017	0	.103	0	.127
Height (m)	0	.169 ${}^{\ast}$	0	.135	$-$ 0	.117			0	.359 ${}^{\ast\ast}$	$-$ 0	.037
Body weight (kg)	0	.420 ${}^{\ast\ast\ast}$	0	.347 ${}^{\ast\ast\ast}$	0	.119	0	.357 ${}^{\ast\ast\ast}$			0	.917 ${}^{\ast\ast\ast}$
BMI (kg/m ${}^{2}$ )	0	.349 ${}^{\ast\ast\ast}$	0	.298 ${}^{\ast\ast\ast}$	0	.177 ${}^{\ast}$	$-$ 0	.102	0	.890 ${}^{\ast\ast\ast}$

${}^{*}p<$ 0.05, ${}^{**}p<$ 0 .01, ${}^{***}p<$ 0.001.

2.2.2 Measurement of HGS

The second part of the study included measurement of HGS using a calibrated Jamar dynamometer (Asirnow Engineering Co, Los Angeles, CA, USA) at level 2 in a standardised position, as described by the American Association of Hand Therapists [22]. The participant sat with his/her shoulder adducted and neutrally rotated, elbow flexed at 90 ${}^{\circ}$ and the forearm and wrist in neutral position. The participants were instructed to take two to three seconds to reach the maximum effort and they were consistently encouraged verbally throughout all measurements. For each strength test, the scores of three successive trials were recorded for each hand. The highest HGS value of each hand was used for analysis. The trials for each measurement were separated by a rest of at least one minute to minimize fatigue. Observed HGS values were recorded in kilos.

2.3 Statistical analysis

For the analysis of the data, SPPS 25 (IBM Corp. Released 2017. IBM SPSS Statistics for Windows, Version 25.0. Armonk, NY: IBM Corp.) was used. Before basic analysis, descriptive statistical values regarding demographic characteristics were studied and the relations among study variables were analyzed by Pearson correlation analysis. The differences between right and left hand HGS values of male and female groups were identified by Paired Sample $t$ -Test. To study whether differences existed between groups, ANCOVA was preferred. Following the correlational analysis, variables of age, height, body weight and BMI, which were significantly related to right and left hand HGS values, were controlled in comparison analyses. In accordance with the aim of the study, we performed two different ANCOVA analyses, hence, Bonferroni correction was supposed to be necessary [23] and p value was accepted as 0.025.

3. Results

The demographic characteristics regarding age, height, body weight and BMI of male and female athletes are shown in Table 1. Of all 788 (male $n=$ 200, female $n=$ 194) little fingers examined, 678 (86.04%) had the FDS tendon. Of these, 176 (22%) demonstrated a common function (male: $n=$ 71, female $n=$ 105) while 502 (63.7%) fingers (male: $n=$ 275, female $n=$ 227) showed independent function of FDS. FDS of little finger was absent bilaterally in 42 (10.6%) participants, [male: $n=$ 21 (10.5%), female: $n=$ 21 (10.8%) and unilaterally in 26 (6.5%) participants. The number of the participants without unilateral FDS was 12 in males and 14 in females of a total of 12 men, half of whom did not have FDS-V in right hand and half, in the left. In 14 women, 9 did not have FDS-V in the right hand and 5 in the left. An overall prevalence of absence of FDS-V (both unilateral and bilateral) was observed in 68 participants (17.2%) 33 of whom were men and 35 were women.

The correlation analyses of right-hand HGS values and age, height, body weight and BMI variables are shown in Table 2. The right and left hand HGS values in men were correlated with age, body weight and BMI. Also, we found a high correlation between HGS values of both hands in men. Body weight was correlated with height, whereas BMI was correlated with body weight (Table 2). The right hand HGS values in women athletes were correlated with age, height, body weight and BMI and that their left hand HGS values were correlated with age, body weight and BMI (Table 2). In addition, the HGS values of both hands in women were correlated and BMI, height and body weight and BMI were correlated (Table 2).

When the results and the effects of age, height, body weight and BMI variables were checked, no observable differences existed between the groups ( $p>$ 0.025). Whereas height, body weight and BMI control variables had significant effect on right hand HGS values in women, the same variables had no significant effects on left hand HGS values. For men, only age had a significant effect on both right and left hand HGS values (Table 3).

Table 3
Comparisons of groups for HGS

Gender	Source	df	Right hand HGS (kg)				Left hand HGS (kg)
			Mean square	F	$p$	Partial eta squared	Mean square	F	$p$	Partial eta squared
Male	Corrected model	6	596.32	8.15	0.000	0.202	662.51	10.29	0.000	0.242
	Intercept	1	40.45	0.55	0.458	0.003	20.53	0.31	0.57	0.002
	Age	1	621.21	8.49	0.004	0.042	654.10	10.16	0.002	0.050
	Height (m)	1	29.29	0.40	0.528	0.002	14.59	0.22	0.63	0.001
	B.weight (kg)	1	44.63	0.61	0.436	0.003	23.77	0.36	0.54	0.002
	BMI (kg/m ${}^{2}$ )	1	11.47	0.15	0.693	0.001	1.65	0.02	0.87	0.000
	FDS	2	44.24	0.60	0.547	0.006	6.32	0.09	0.90	0.001
Female	Corrected model	6	293.16	10.48	0.00	0.253	221.52	6.09	0.000	0.16
	Intercept	1	176.42	6.30	0.013	0.033	36.27	0.99	0.31	0.005
	Age	1	76.19	2.72	0.101	0.014	97.67	2.68	0.10	0.014
	Height (m)	1	158.90	5.68	0.018	0.030	28.78	0.79	0.37	0.004
	B.weight (kg)	1	251.06	8.97	0.003	0.046	61.65	1.69	0.19	0.009
	BMI (kg/m ${}^{2}$ )	1	181.41	6.48	0.012	0.034	32.94	0.90	0.34	0.005
	FDS	2	92.67	3.31	0.039	0.034	78.68	2.16	0.11	0.023

$p<$ 0.025.

Table 4

Mean standard deviation, mean marginal rate and confidence interval range values of right and left hand HGS values

HGS	Gender	N	%	FDS	Mean	Std dev	Marj mean	95% Confidence interval		$p$
								Lower bound	Upper bound
Right hand	Male	137	68.5	Independent	48.51	9.67	48.48	47.03	49.92	0.54
HGS (kg)		36	18.0	Common	49.27	9.18	49.01	46.17	51.85
		27	13.5	Absent	46.11	8.423	46.63	43.26	50.01
	Female	114	58.7	Independent	29.76	6.50	29.90	28.91	30.89	0.03
		50	25.7	Common	27.84	4.10	27.92	26.44	29.39
		30	15.5	Absent	31.36	6.33	30.71	28.79	32.62
Left hand	Male	138	69.0	Independent	48.04	9.57	47.85	46.50	49.20	0.90
HGS (kg)		35	17.5	Common	46.69	8.00	47.18	44.49	49.87
		27	13.5	Absent	47.37	7.87	47.64	44.51	50.78
	Female	113	58.2	Independent	29.12	6.33	29.27	28.14	30.39	0.11
		55	28.3	Common	26.70	5.88	27.16	25.51	28.81
		26	13.4	Absent	30.50	7.57	28.90	26.48	31.32

$p<$ 0.025.

Mean standard deviation, mean marginal rate and confidence interval range values are shown in Table 4. Depending on FDS-V (independent, common and absent) on right hand and left hand of the groups, no statistically significant difference was observed between HGS values and right and left hand FDS tendon independent, common and absent values ( $p>$ 0.025). Moreover, according to the findings of Paired Sample t-test analysis no statistically significant difference was observed in right and left hand HGS values of male groups, however, in female groups, the right hand HGS values were higher than left hand HGS values ( $p<$ 0.001).

4. Discussion

Several previous clinical studies investigated the prevalence of absence of FDS tendon of little finger in different ethnic groups. In the study examining the FDS tendon absence in little fingers of females and males (121 women and 49 men), Bowman et al. [18] reported that FDS was absent in 30 participants, common in 62.5 participants and independent in 77.5 participants. The researchers also stated that the absence percentage of FDS tendon was the same in male (18.6%) and female groups (15.3%). Furthermore, gender had a significant effect on HGS as males have higher HGS values than women ( $p=$ $<$ 0.0001). In another study, Puhaindran et al. [17] declared that they studied 402 individuals (804 little fingers) and they found FDS tendon existed in 765 little fingers. They also stated that the tendon was common in 306 little fingers and independent in 450 little fingers. FDS-V was absent bilaterally in 18 participants, the tendon was bilateral and in 12 individuals it was unilateral. Mishra [24] studied the absence of FDS-V in Indian patients (50 patients, 100 hands) and declared that the absence rate was 11%. In another study on the correlation of the absence of FDS-V and palmaris longus tendon of Irish individuals (300 participants), Thompson et al. [25] stated that 10 participants did not have the FDS-V. Guler et al. [19] studied the prevalence of absence of FDS-V with 400 adult patients (200 males, 200 females) and declared that of all 800 little fingers, 703 had FDS (87.8%). Of these 703, 309 had FDS-common (38.6%) whereas 394 had FDS-independent (49.2%). Moreover, FDS was absent bilaterally in 23 patients (5.7%) and unilaterally in 51 patients (12.8%). Furthermore, the same researchers reported that overall prevalence of absence of FDS-V (both unilateral and bilateral) was 18% in males and 19% in females. In their study on the absence of palmaris longus muscle and the variations of FDS muscles in males and females in Turkish population (272 males, 261 females, 1066 hands in total), Irmak et al. [26] reported that the absence of active and isolated movement of proximal interphalangeal (PIP) joint of little finger was 23.7%. Moreover, the absence of the isolated and active flexion of fourth and fifth fingers’ PIP joint was 11.8%. In addition, Irmak et al. [26] stated that FDS tendon movement disorder was more frequent in females (24.6%) than males (22.7%). Godwin et al. [27] studied whether the absence of independent movements of little fingers affects musical abilities of string players and they found that the absence of FDS function in professional musicians (3.3%) was at significant lower prevalence and that the functional absence of FDS didn’t exist in left hands of the musicians. Moreover, the researchers also stated that independent FDS function is quite important for professional musicians that play at an elite level.

In the assessment of the absence of FDS-V of our study, the FDS tendon existed in 86% of the participants. Twenty-two percent and 63.7% of the participants had FDS tendon with common function, and independent function, respectively. Furthermore, the absence of unilateral and bilateral FDS-V tendon was 17.2%. The absence of FDS tendon (unilateral and bilateral) was 18.04% in women and 16.5% in men. The general rate of absence of FDS-V tendon (unilateral and bilateral) in our study groups was in accordance with the study findings of Bowman et al. [18] and Guler et al. [19]. In addition, and in similarity to Irmak et al. [26], the rate of absence of this tendon in women was higher than in men.

Several laboratory examiners have studied the contribution of the individual fingers to total HGS using different methods [28, 29, 30, 31]. In their studies, these researchers declared that along with other fingers, little fingers positively affect total handgrip strength. Bowman et al. [18] studied the effects of presence or absence of FDS of little fingers of healthy female and male individuals on HGS. They reported that participants with no FDS had significantly lower values of HGS than participants with FDS-common or FDS-independent. Moreover, they stated that HGS values of the participants with FDS-absent were 6.9 lb lower on average than participants with FDS-common (SD $=$ 2.189; $p=$ 0.0202) and 8.14 lb lower than participants with FDS-independent (SD $=$ 1.591; $p=$ 0.0034). Furthermore, they reported that the HGS values of the participants with FDS-common and FDS-independent had no significant difference: FDS-common participants had about 1 kg of force lower ( $p=$ 0.8120) than FDS-independent participants (SD $=$ 1.366). Puhaındran et al. [17] studied the absence of FDS of little fingers and its effects on HGS values and they found that mean right hand HGS was 29.8 kg (SD: 10.1; median: 28.3; range: 6.0–62.7 kg) while the mean left hand HGS was 27.8 kg (SD: 10.0; median: 26; range: 4.7–64.3 kg). HGS values of individuals with or without FDS were not statistically significant. FDS. By using different methods, it was reported that individual digits have effects on total HGS. However, these studies [28, 29, 30, 31] didn’t focus on the effects of the presence or absence of FDS-V on total HGS. When we studied the age, body weight and BMI variables of female and male groups in our study, we found that FDS-V tendon didn’t have an effect on HGS. In this aspect, our study complies with the findings of Puhaindran et al. [17] as it clarified the presence or absence of FDS of little fingers had no effects on HGS. However, Bowman et al. [18] reported that the handgrip strength of individuals with absent or common FDS is lower than HGS of individuals with independent FDS. Our study findings vary from the findings of the study of Bowman et al. [18] because Bowmen et al. [18] didn’t study right and left hand separately and they considered all hands as independent samples.

The main limitation of the study was that it included athletes from different types of sports who joined in regional and national tournaments and none of them had participated in international tournaments.

5. Conclusions

Consequently, it might be stated that the presence or absence of the FDS-V tendon in women and men athletes has no effects on the HGS of these individuals.

Footnotes

Acknowledgments

I would like to thank all subjects and trainers for contributing to this study.

Conflict of interest

There are no conflicts of interest.

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