Evaluation of Flexor Digitorum Superficialis-Small Function: A Novel Dynamic Assessment Technique in Professional Music Performance

Abstract

Background:

The flexor digitorum superficialis of the little finger (FDS-small) exhibits anatomical variation, with absence reported in 2% to 30% of individuals. As FDS is the only flexor of the proximal interphalangeal joint (PIPJ), its absence may affect finger dexterity, particularly in professional musicians who depend on precise finger control. This study evaluates the impact of FDS-small presence or absence on the technical abilities of pianists and string musicians using novel dynamic assessment tools.

Methods:

Thirty-one Bachelor of Music students (21 string players, 10 pianists) were assessed for FDS-small function and classified as “independent,” “common,” or “absent.” Tests included grip strength, static gap, and stretch tests, and a novel assessment method comprising custom-composed double-stop trill etudes for piano and strings. Technical performance was evaluated by 2 professional musicians using predefined criteria. Statistical analyses were conducted using Fisher’s exact tests, t tests, analysis of variance, and nonparametric equivalents.

Results:

Ten participants had absent FDS-small function in at least 1 hand. Pianists with bilateral absent FDS-small demonstrated superior coordination and regularity compared with those with FDS-small presence, while FDS-small absence did not significantly affect string players. Female pianists excelled in dynamic tests despite male musicians’ higher grip strength and trill speed.

Conclusion:

Absence of FDS-small does not impair, and may even enhance, fine motor performance in pianists. For string musicians, FDS-small status does not influence technique. These findings may contribute to evaluating the risk versus benefits of FDS-small surgical repair and have implications for instrument selection and pedagogy.

Introduction

The anatomical variations of the flexor digitorum superficialis of the little finger (FDS-small), especially its absence, ranges from 2% to 30%.^1,2 Since FDS is the only flexor of the proximal interphalangeal joint (PIPJ), independent flexion of the little finger at the PIPJ may thus be functionally different between individuals with differing anatomy.^3,4 Consequently, individuals in whom the FDS-small is absent depend solely on the flexor digitorum profundus for concurrent flexion at both interphalangeal joints of the little finger.^5,6 As a result, if there is an absence of FDS-small, there may be a difference in dexterity and fine motor function of the little finger, which may impact individuals who rely heavily on precise finger movement. Professional musicians require a large repertoire of highly skilled finger movements, including but not limited to accurate, precise, and dexterous maneuvers. ⁷ They thus represent a relevant population for studying the FDS-small, which will provide valuable insights into the functional implications of FDS-small absence in real-world contexts.

The authors believe that evaluating the impact of absent or present FDS-small function on the technical abilities of professional musicians requires a combination of both static and dynamic testing methods. To the authors’ knowledge, this is the first study in the literature which has developed novel dynamic testing and standardized criteria to evaluate the impact of FDS-small on the technical abilities of string players and pianists.

This also has wider implications for the general public. For example, if tendon lacerations are sustained to the FDS-small, especially in zone II of the hand, which is a challenging site to operate, ⁸ a surgeon would need to weigh the advantages and disadvantages of such an operation, and thus the exact importance of FDS-small needs to be carefully ascertained.

The aim of our study is to use a novel dynamic testing methodology to assess the impact of FDS-small presence or absence on the fine motor abilities of professional musicians, which will aid in evaluating the benefits and risks associated with its surgical repair.

Methods

We conducted an anatomic study in a music conservatory of a tertiary institution, and Bachelor of Music students who are professionally trained were recruited from February 2024 to October 2024.

Flexor digitorum superficialis of the little finger function was assessed using the standard (Supplemental Figure 1) and modified tests (Supplemental Figure 2), it is classified as either “independent,” “common,” or “absent” (Table 1). Independent function requires more than 80° flexion of the little finger at the PIPJ.^9,10

Table 1.

Description of the Standard and Modified FDS-Small Function Tests, Which Are Used to Classify the FDS-Small Function as “Independent,” “Common,” or “Absent.”

Test	Description	Outcome	Interpretation
Standard Test	(a) The hand is placed on a table in full supination with the wrist placed in neutral extension. (b) The index, middle, and ring fingers are held in full extension by an observer at the metacarpal phalangeal joint and interphalangeal joints (IPJ). This extension immobilizes the FDP of the little finger, thus any active flexion of the PIPJ is due to FDS-small alone. (c) Participants were asked to flex the little finger as much as possible.	More than 80° flexion of the little finger at the PIPJ, while keeping the distal IPJ (DIPJ) fully extended.	A positive standard test that indicates “independent” FDS-small function.
		Inability to flex little finger PIPJ more than 80°, or if PIPJ flexion is only seen in conjunction with DIPJ flexion.	FDS-small function is not independent; the modified test is then applied.
Modified Test	(a) The ring finger is now released, but the middle and index fingers are still held in extension by an observer.	If PIPJ flexion of the little finger improves together with the ring finger, this demonstrates that the FDS-small tendon is connected to the FDS-ring tendon.	This is classified as “common” FDS-small function.
		If there is an inability to improve the little finger PIPJ flexion with release of the middle finger, or if PIPJ flexion is only seen in conjunction with DIPJ flexion.	This will be considered as “absent” FDS-small function.

Note. FDS-small = flexor digitorum superficialis of the little finger; FDS-ring = flexor digitorum superficialis of the ring finger; IPJ = interphalangeal joints; PIPJ = proximal interphalangeal joint; DIPJ = distal interphalangeal joints.

All participants were asked to perform a grip strength test, static tests and a dynamic test—this comprises the Liau Dexterity Hand Assessment which is a self-developed assessment by the authors that incorporates existing literature (Table 2).^11,12 Bilateral hand grip strength was measured for all participants using a handheld dynamometer. Static tests include a gap (Supplemental Figure 3) and stretch test (Supplemental Figure 4), while the dynamic test required the participants to play a double-stop trill etude, which consist of a strings musical excerpt by Yang-Liau and a piano musical excerpt by Liau-Yang (Supplemental Figure 5). ¹³ Both musical excerpts are original compositions by our authors, which are designed to technically challenge the presence or absence of FDS-small. Two professional musicians then assessed the performances independently, using a predevised set of criteria that can holistically judge the technical ability of participants (Table 3).

Table 2.

Description of the Tests Assessed and Measurements Taken From All Participants.

Test	Instructions	Measurements taken
Static Gap Test (Violin)	(a) Place index, middle, ring, and little finger on the E string. (b) Attempt to raise the ring finger off the E string while leaving other fingers down.	Measure the distance between the E string and the tip of the ring finger (mm).
Static Stretch Test (Violin)	(a) Place index, middle, ring, and little fingers on the E string. (b) Attempt to reach over for the G string with the ring finger without raising the other fingers from the E string.	Observe if there is loss of DIPJ flexion in the ring and little finger, if the ring finger pulp touches the D string and if the little finger involuntarily moves with the ring finger.
Dynamic Test (Violin)	(a) Sight read the double-stop trill etude. (b) Play the double-stop trill etude after 30 seconds of preparation.	Video of the performance is taken, which is then judged by 2 professional musicians.
Static Gap Test (Cello/Viola)	(a) Place index, middle, ring, and little finger on the A string. (b) Attempt to raise the ring finger off the A string while leaving other fingers down.	Measure the distance between the A string and the tip of the ring finger (mm).
Static Stretch Test (Cello/Viola)	(a) Place index, middle, ring, and little fingers on the A string. (b) Attempt to reach over for the C string with the ring finger without raising the other fingers from the A string.	Observe if there is loss of DIPJ flexion in the ring and little finger, if the ring finger pulp touches the G string and if the little finger involuntarily moves with the ring finger.
Dynamic Test (Cello/Viola)	(a) Sight read the double-stop trill etude. (b) Play the double-stop trill etude after 30 seconds of preparation.	Video of the performance is taken, which is then judged by 2 professional musicians.
Static Gap Test (Piano)	(a) Right hand: While placing the thumb, index and little finger on the C, D, and G keys, raise up the middle and ring fingers as high as possible above the E and F keys. (b) Left hand: While placing the thumb, index and little finger on the C, F, and G keys, raise up the middle and ring fingers as high as possible above the D and E keys.	Measure the distance between the tip of the ring finger and the F key (right hand), D key (left hand).
Dynamic Test (Piano)	(a) Sight read the double-stop trill etude. (b) Play the double-stop trill etude after 30 seconds of preparation. (c) Repeat for both right and left hand.	Video of the performance is taken, which is then judged by 2 professional musicians.
Grip Strength	(a) Ensure participant’s shoulders are in a neutral position, elbow flexed at 90° with forearm in a neutral position. (b) Get a comfortable grip around the handheld dynamometer, and squeeze as hard as possible for 3 seconds. (c) Repeat the test 3 times for each hand, repeat for both right and left hands.	Measure the single maximal grip force value during each test.
Musicians Reported Difficulty Level	(a) Participants scores from 1 to 10 the perceived extent to which lack of independent movement of the ring and little fingers poses a problem (1 = minimal problem, 10 = significant problem) (b) Participant scores from 1 to 10 the difficulty involved to execute the stretch test (1 = with ease, 10 = most difficult). (c) Participant scores from 1 to 10 the difficulty involved to play the etude (1 = with ease, 10 = most difficult).
Static Gap Test (Double Bass)	(a) Place index, middle, ring, and little finger on the G string. (b) Attempt to raise the ring finger off the G string while leaving other fingers down.	Measure the distance between the G string and the tip of the ring finger (mm).
Static Stretch Test (Double Bass)	(a) Place index, middle, ring, and little fingers on the G string. (b) Attempt to reach over for the E string with the ring finger without raising the other fingers from the G string.	Observe if there is loss of DIPJ flexion in the ring and little finger, if the ring finger pulp touches the A string and if the little finger involuntarily moves with the ring finger.
Dynamic Test (Double Bass)	(a) Sight read the double-stop trill etude. (b) Play the double-stop trill etude after 30 seconds of preparation.	Video of the performance is taken, which is then judged by 2 professional musicians.

Note. DIPJ = distal interphalangeal joints; MRDL = Musicians Reported Difficulty Level.

Table 3.

Criteria Assessed by Two Independent Professional Musicians.

Criteria judged from the performance videos	Description	Components assessed
Number of trills performed in 10 seconds	- One trill cycle is defined as the completion of both pairs of double-stop notes	- Speed - Dexterity
Regularity	- Similarity of frequency throughout the trill and within each cycle - Scored from 1 to 10 (1 = unacceptable performance, 10 = excellent)	- Consistency - Precision
Coordination	- The coordination between alternate lifting and dropping of the ring and little fingers - Scored from 1 to 10 (1 = unacceptable performance, 10 = excellent)	- Synchrony - Control
Intonation	- Whether intonation remains stable throughout 10 seconds - Not applicable for piano trills - Scored from 1 to 10 (1 = unacceptable performance, 10 = excellent)	- Pitch accuracy
Any trick movements by the participants	Irregularities in technique to compensate when playing the trill, for example, wrist vibrato, forearm prono-supination, extended phalanges	- Compensation maneuvers

This study and research protocol was approved by the National Healthcare Group Ethics and Compliance Online System (Institutional Review Board approval number: 2023/0043). Informed consent was obtained from all participants before their enrolment into the study.

Fisher’s exact tests were used to test differences between categorical data. The Shapiro-Wilk test was used to assess the normality of the continuous data recorded. All continuous data had a normal distribution except for the regularity and intonation scores. Mean values were compared using parametric tests including Student’s t test or one-way analysis of variance (ANOVA), while medians were compared using nonparametric tests including the Mann-Whitney U test or Kruskal-Wallis H test. Values are reported with a 95% confidence interval (95% CI). A P value of <.05 was considered statistically significant.

Results

Our participants consisted of 21 string musicians and 10 pianists. String musicians were further comprised of 16 violinists and 5 cellists. Table 4 shows the demographics of the musicians and illustrates the prevalence of the different FDS-small functions in the participants. Overall, 10 out of 31 participants had absent FDS-small function in at least 1 hand. Five of them had unilateral absence while 5 had bilateral absence. Results obtained from the tests conducted are given in Table 5, and further subgroup analysis by bilateral FDS-small function and by gender are given in Table 6.

Table 4.

Demographics of Participants.

Demographics		Strings	Piano
N		21 (16 violinists and 5 cellists)	10
Age (years) (95% CI)		22.3 (18.3-26.4)	24.5 (20.7-28.3)
Male		9/21	6/10
Right-hand dominance		17/21	9/10
Years of musical experience (years) (95% CI)		16.1 (11.8-20.4)	15.8 (9.66-21.9)
Right-hand grip strength (kg) (95% CI)		25.8 (22.3-29.3)	32.0 (26.5-37.6)
Left-hand grip strength (kg) (95% CI)		24.2 (20.5-27.9)	29.2 (23.5-34.9)
Independent FDS-small function	Right hand	10/21	6/10
	Left hand	4/21	6/10
Common FDS-small function	Right hand	5/21	1/10
	Left hand	13/21	2/10
Absent FDS-small function	Right hand	6/21	3/10
	Left hand	4/21	2/10
Unilateral prevalence of absent FDS-small function		4/21	1/10
Bilateral prevalence of absent FDS-small function		3/21	2/10

Note. FDS-small = flexor digitorum superficialis of the little finger; 95% CI = 95% confidence interval.

Table 5.

Recorded Measurements of the Tests Performed on Participants.

Component	Test	Strings	Piano
Gap Test	Gap between string and ring finger (mm) (95% CI)	26.1 (17.8-34.5)	Right: 37.6 (27.8-47.4) Left: 41.1 (34.2-48.0)
Stretch Test (only applicable for string players)	Loss of ring finger DIPJ flexion	6/21	NA
	Loss of little finger DIPJ flexion	5/21	NA
	Ring finger pulp touches the adjacent D string	3/21	NA
	Little finger involuntarily moves with the ring finger	0/21	NA
Dynamic Test	Number of trills performed in 10 seconds (95% CI)	27.8 (23.9-31.7)	Right: 34.3 (30.0-38.7) Left: 34.6 (30.2-39.0)
	Regularity (95% CI)	6.11 (5.39-6.83)	Right: 5.43 (3.98-6.87) Left: 5.53 (4.09-6.96)
	Coordination (95% CI)	4.93 (4.20-5.66)	Right: 4.83 (3.30-6.35) Left: 4.88 (3.22-6.53)
	Intonation (95% CI)	7.51 (7.13-7.90)	NA
	Trick movements	2 musicians used forearm prono-supination, 1 musician used hyperextended phalanges	1 musician used forearm prono-supination
Musicians Reported Difficulty Level	Perceived extent to which lack of independent movement of the ring and little fingers poses a problem (95% CI)	5.14 (3.98-6.31)	5.65 (4.32-6.98)
	Difficulty of stretch test (95% CI)	3.45 (2.10-4.80)	NA
	Difficulty of double-stop trill etude (95% CI)	6.74 (5.67-7.81)	Right: 6.00 (4.42-7.58) Left: 3.80 (2.50-5.10)

Note. 95% CI = 95% confidence interval; DIPJ = distal interphalangeal joints.

Table 6.

Subgroup Analysis by Bilateral FDS-Small Function and by Gender.

	Strings			Piano
Component	Bilateral absent FDS-small function	Nonbilateral absent FDS-small function	P value	Bilateral absent FDS-small function	Nonbilateral absent FDS-small function	P value
No. of trills (95% CI)	25.7 (5.22-46.1)	28.1 (23.8-32.5)	.66	37.0 (30.0-44.0)	33.8 (29.8-37.8)	.50
Regularity (95% CI)	7.17 (6.22-8.11)	5.93 (5.11-6.75)	.16	7.94 (5.82-10.1)	4.86 (3.56-6.16)	.044
Coordination (95% CI)	5.50 (4.27-6.73)	4.83 (3.98-5.69)	.52	7.69 (5.57-9.81)	4.14 (2.71-5.57)	.023
Intonation (95% CI)	8.08 (7.73-8.44)	7.42 (6.98-7.85)	.088	NA	NA	NA
Grip strength (kg) (95% CI)	22.8 (7.27-38.4)	25.4 (21.4-29.5)	.59	29.5 (23.9-35.1)	30.7 (24.2-37.2)	.82
	Male	Female		Male	Female
Perceived extent to which lack of independent movement of the ring and little fingers poses a problem (95% CI)	6.72 (5.58-7.86)	3.96 (2.31-5.61)	.014	5.00 (2.70-7.30)	6.25 (3.42-9.08)	.47
Right-hand grip strength (kg) (95% CI)	31.7 (26.4-37.0)	21.5 (18.5-24.5)	<.001	33.2 (26.0-40.4)	28.5 (24.1-33.0)	.43
Left-hand grip strength (kg) (95% CI)	28.7 (22.6-34.8)	20.9 (16.6-25.2)	.025	30.8 (23.3-38.3)	24.5 (21.9-27.0)	.29
No. of trills (95% CI)	32.9 (27.4-38.5)	23.9 (19.1-28.8)	.013	32.3 (28.8-35.7)	37.8 (33.1-42.4)	.040
Regularity (95% CI)	5.39 (3.77-7.01)	6.65 (6.16-7.14)	.23	4.23 (3.40-5.06)	7.34 (6.48 8.21)	<.001
Coordination (95% CI)	4.44 (2.89-6.00)	5.29 (4.54-6.04)	.24	3.46 (2.59-4.33)	6.94 (5.93-7.95)	<.001
Intonation (95% CI)	7.22 (6.55-7.89)	7.73 (7.22-8.23)	.30	NA	NA	NA

Note. Bold indicates statistical significance. FDS-small = flexor digitorum superficialis of the little finger; 95% CI = 95% confidence interval.

Pianists with bilateral absent FDS-small function performed better than pianists with nonbilateral absent function in terms of regularity ([median 7.94, 95% CI 5.82-10.1] vs [median 4.86, 95% CI 3.56-6.16]) and coordination ([mean 7.69, 95% CI 5.57-9.81] vs [mean 4.14, 95% CI 2.71-5.57]). Pianists reported significantly greater difficulty when playing the trill with their right hand (mean 6.00, 95% CI 4.42-7.58) compared with their left hand (mean 3.80, 95% CI 2.50-5.10) (P = .026).

The absence or presence of FDS-small did not significantly affect the technique of string musicians. Both groups performed similarly for regularity, coordination, intonation and the number of trills in 10 seconds.

Female pianists performed significantly better in the dynamic tests than male pianists. They performed more trills in 10 seconds ([mean 37.8, 95% CI 33.1-42.4] vs [mean 32.3, 95% CI 28.8-35.7]), had a higher regularity score ([median 7.34, 95% CI 6.48-8.21] vs [median 4.23, 95% CI 3.40-5.06]), and higher coordination score ([mean 6.94, 95% CI 5.93-7.95] vs [mean 3.46, 95% CI 2.59-4.33]).

For string musicians, male musicians had stronger right hand grip strength ([mean 31.7, 95% CI 26.4-37.0] vs [mean 21.5, 95% CI 18.5-24.5]), stronger left hand grip strength ([mean 28.7, 95% CI 22.6-34.8] vs [mean 20.9, 95% CI 16.6-25.2]), and performed more trills in 10 seconds ([mean 32.9, 95% CI 27.4-38.5] vs [mean 23.9, 95% CI 19.1-28.8]).

Discussion

A previous study by Godwin et al only conducted the static gap and stretch tests on musicians, however we agree with Godwin et al ¹³ that their chosen static tests were arbitrary and not classical musical exercises, which have little relevance to musicians in actual performance settings. We believe that the dynamic tests that comprises double-stop trill etudes developed by Yang-Liau for strings, and Liau-Yang for piano, are more representative of the specific skills and movements musicians are required to perform, and is thus a more appropriate tool to accurately evaluate their abilities.

Interestingly, our study did not show that absent FDS-small function causes poorer performance amongst pianists. Instead, pianists with bilateral absent FDS-small function performed significantly better than those with nonbilateral absent FDS-small function, in terms of regularity and coordination. This suggests that the presence of common or independent FDS-small in either hand may hamper piano technique. Our study did not show any significant relationship between grip strength and FDS-small function, which aligns with a previous study done. ²

Our study found that the presence or absence of FDS-small function in string musicians had no significant impact on string technique. This contrasts with the findings of Godwin et al who recommended assessing FDS-small function in children before offering them the violin or viola. Their conclusion was based solely on the lower prevalence of FDS-small absence amongst professional string musicians compared with nonmusicians, without evaluating the technical abilities of musicians with or without FDS-small function. Thus, we disagree with Godwin et al’s recommendation and believe that discriminatory access to string instruments amongst young musicians who aspire to become professionals should not be based on the absence of FDS-small function.

Although male musicians have a mechanical advantage in terms of stronger right hand grip strength, stronger left hand grip strength, and can perform trills faster as corroborated by another study, they scored lower than female musicians for dynamic tests, suggesting a trade-off between strength and dexterity. ¹⁴ These findings highlight the potential value of tailoring pedagogical approaches to address gender specific tendencies in hand strength and dexterity.

In our study, our population had a higher prevalence of absent FDS-small function of 10 out of 31, which is higher than the 8.4% reported in a meta-analysis of 13 studies and is in contrast to another study which concluded that professional musicians usually have independent FDS-small function.^5,13 This could be due to our application of a more stringent criteria requiring a PIPJ flexion angle greater than 80° to define independent function, while other papers did not have a defined objective criteria for independent function.

While our findings suggest that the absence of FDS-small does not significantly impair fine motor function in professional musicians, caution should be exercised when extrapolating these results to all FDS-small injuries. Patients with acquired FDS-small function loss may experience different functional outcomes compared with those with congenital absence. Therefore, the decision to repair FDS-small in zone II injuries should be individualized, considering the potential risks of adhesions or rupture against the patient’s preinjury functional demands and expectations.

We recognize the limitations of the study. First, factors other than FDS-small function could have influenced the independent movement of the little finger, such as overall hand coordination and training . Second, definite evaluation of FDS-small presence or absence could have been evaluated with ultrasound. However, to standardize with existing literature, we instead determined the absence and presence based on the standard and modified physical examinations as described above. Third, our sample size was relatively small. However, this already represents 85% of the string musicians in the orchestra. Finally, in our study, we did not include any viola or double bass musicians. This is due to the very small numbers that preclude meaningful statistical analyses.