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Evaluation of flexor digitorum superficialis function in adolescent baseball players

Open AccessPublished:October 08, 2022DOI:https://doi.org/10.1016/j.jseint.2022.09.009

      Abstract

      Hypothesis and/or Background

      Increased flexor digitorum superficialis (FDS) tendon activity can be a therapeutic target for elbow disorders in adolescent baseball players. The proportion of adolescent baseball players who can use FDS independently is unknown, and which finger is most often used remains unclear. This study investigated whether adolescent baseball players intentionally used FDS on each finger.

      Methods

      Adolescent baseball players were recruited and assessed for FDS function for each finger using the standard technique.

      Results

      Sixty-nine participants (mean age: 10.4 years) were recruited. Participants numbered 33, 56, 59, and 25 on the throwing side and those numbered 28, 46, 54, and 33 on the non-throwing side could independently flex the proximal interphalangeal joint (PIP) while holding their palms in the index, middle, ring, and small fingers, respectively. When assessing both throwing and non-throwing participants, a significant number of participants could independently flex the PIPs of the ring and middle fingers but had difficulty with the index and small fingers (P<0.001). No significant difference was noted between the throwing and non-throwing participants in any finger (P>0.05).

      Discussion and/or Conclusion

      One study reported that participating baseball players with elbow pain have more medial elbow joint space than those without pain symptoms. In another study on finger movements during pitching motion, the force of the thumb, index, middle, and ring fingers was greatest immediately before maximum external rotation. According to both reports, FDS function, especially in the index finger, can be a therapeutic target for medial-sided elbow injuries in adolescent baseball players.

      Keywords

      Approximately 30% of adolescent baseball players experience elbow pain.
      • Matsuura T.
      • Iwame T.
      • Suzue N.
      • Arisawa K.
      • Sairyo K.
      Risk factors for shoulder and elbow pain in adolescent baseball players.
      Although elbow disorders can occur at various sites, medial elbow disorders, which account for more than 50% of cases, are most common in baseball players.
      • Lyman S.
      • Fleisig G.S.
      • Waterbor J.W.
      • Funkhouser E.M.
      • Pulley L.
      • Andrews J.R.
      • et al.
      Longitudinal study of elbow and shoulder pain in youth baseball pitchers.
      ,
      • Matsuura T.
      • Suzue N.
      • Kashiwaguchi S.
      • Arisawa K.
      • Yasui N.
      Elbow injuries in youth baseball players without prior elbow pain: a 1-year prospective study.
      When medial elbow disorders progress to a severe stage in adults, surgery such as medial collateral ligament (MCL) reconstruction is performed in top players, for example, major league baseball players.
      • Conte S.A.
      • Fleisig G.S.
      • Dines J.S.
      • Wilk K.E.
      • Aune K.T.
      • Patterson-Flynn N.
      • et al.
      Prevalence of ulnar collateral ligament surgery in professional baseball players.
      The frequency of MCL ligament reconstruction in professional baseball players has increased significantly from 2012 to 2018.
      • Leland D.P.
      • Conte S.
      • Flynn N.
      • Conte N.
      • Crenshaw K.
      • Wilk K.E.
      • et al.
      Prevalence of medial ulnar collateral ligament surgery in 6135 current professional baseball players. A 2018 update.
      Furthermore, MCL reconstruction has become more frequent in adolescent athletes.
      • Hodgins J.L.
      • Vitale M.
      • Arons R.R.
      • Ahmead C.S.
      Epidemiology of medial ulnar collateral ligament reconstruction. A 10-year study in New York state.
      ,
      • Petty D.H.
      • Andrews J.R.
      • Fleisig G.S.
      • Cain E.L.
      Ulnar collateral ligament reconstruction in high school baseball players: clinical results and injury risk factors.
      Therefore, preventing medial elbow disorders and performing post-event rehabilitation are important for decreasing the number of MCL reconstruction cases in adolescent baseball players.
      The valgus stress applied during pitching induces medial elbow disorders.
      • Fleisig G.S.
      • Andrews J.R.
      • Dillman C.J.
      • Escamilla R.F.
      Kinetics of baseball pitching with implications about injury mechanisms.
      The MCL and flexor/pronator group of the forearm are involved in controlling the elbow joint against valgus stress during pitching.
      • Morrey B.F.
      • An K.N.
      Articular and ligamentous contributions to the stability of the elbow joint.
      ,
      • Udall J.H.
      • Fitzpatrick M.J.
      • McGarry M.H.
      • Leba T.B.
      • Lee T.Q.
      Effects of flexor-pronater muscle loading on valgus stability of the elbow with an intact, stretched, and resected medial ulnar collateral ligament.
      The control mechanisms of the MCL and flexor are usually disrupted in patients with medial elbow disorders.
      • Conway J.E.
      • Jobe F.W.
      • Glousman R.E.
      • Pink M.
      Medial instability of the elbow in the throwing athletes. Treatment by repair or reconstruction of the ulnar collateral ligament.
      Contraction of the flexor digitorum superficialis (FDS) can decrease medial elbow joint space.
      • Hoshika S.
      • Nimura A.
      • Takahashi N.
      • Sugaya H.
      • Akita K.
      Valgus stability is enhanced by flexor digitorum superficialis muscle contraction of the index and middle fingers.
      ,
      • Otoshi K.
      • Kikuchi S.
      • Shishido H.
      • Konno S.
      Ultrasonographic assessment of the flexor pronator muscles as a dynamic stabilizer of the elbow against valgus force.
      In addition, some studies have reported that increasing the activity of the FDS improves medial elbow disorders.
      • Sakata J.
      • Miyazaki T.
      • Akeda M.
      • Yamazaki T.
      Return-to-play outcomes in high school baseball players after ulnar collateral ligament injuries: dynamic contributions of flexor digitorum superficialis function.
      Therefore, increasing FDS activity can be a therapeutic target for elbow disorders in baseball players.
      However, the proportion of adolescent baseball players who can use FDS independently is unknown, and which finger is used is unclear. Finger flexion is possible with flexor digitorum profundus (FDP) activity alone. We hypothesized that most adolescent baseball players would show a lack of independent movement of the proximal interphalangeal (PIP) joint in every finger. The purpose of this study was to investigate whether adolescent baseball players intentionally used FDS on each finger.

      Materials and Methods

      Ethical issues

      The procedures followed were in accordance with the ethical standards of the responsible committee on human experimentation (institutional and national) and the Helsinki Declaration of 1975, as revised in 2000. This experimental protocol adhered to the institutional guidelines and was approved by the Institutional Review Board (reference number: 2015-032). Oral informed consent was obtained from all participants, their parents, and the little league to which they belonged.

      Participants

      Adolescent baseball players were recruited and assessed for FDS function for each finger using the standard technique. This investigation was performed with the cooperation of local Little League Federation. The inclusion criterion was a player who had registered in this federation. The exclusion criterion was a player and/or parents who were not allowed to participate in our investigation.

      Assessment of FDS function

      Our investigation assessed the FDS function according to Daniels' manual strength and the standard technique previously reported.

      Avers D, Brown M. Daniels and Worthingham’s muscle testing: techniques of manual examination and performance testing, 10th ed. Philadelphia: Saunders. p. 183-184; 2018.

      ,
      • Watanabe Y.
      • Shirato R.
      • Wada T.
      • Iba K.
      • Sonoda T.
      • Yamashita T.
      Quantitative examination of isolated finger flexion associated with function of the flexor digitorum superficialis.
      The test was performed in the sitting position. Participants placed their hands on a desk with elbow flexion and forearm supination. The participants flexed their fingers individually while an observer held the hand (Figure 1). Each participant watched a test video on a tablet in advance, and after confirming the test method, the index, middle, ring, and small fingers were evaluated on both sides. When the participant could independently flex the PIP joint in each finger, we considered this as a positive result for FDS function. Each participant watched a test video on a tablet in advance, and after confirming the test method, the index, middle, ring, and small fingers were evaluated on both sides.
      Figure thumbnail gr1
      Figure 1Assessing of flexor digitorum superficialis (FDS) function by flexing proximal interphalangeal (PIP) joint independently

      Statistical analysis

      Statistical analyses were performed using commercial software (SPSS, version 28.0; IBM, Armonk, NY, USA). The chi-square test was used to compare the rate of positive FDS function among the four fingers. Effect size (ES) was assessed using Cramer’s V. We also assessed the prevalence of positive FDS function between the throwing and non-throwing hands. For all statistical analyses, significance was defined as P<0.05. On the strength of Cramer’s V, ES<0.3 was defined as weak, 0.3<ES<0.5 was defined as moderate, and ES>0.5 was defined as strong.

      Cohen J. Statistical power analysis for the behavioral sciences (2nd ed.). New York, NY: Routledge; 1988.10.4324/9780203771587

      Results

      Profile of participants

      The participants were 69 adolescent baseball players who participated in a baseball elbow examination. The average age of the participants was 10.4 years, and the sex ratio was 63 boys and 6 girls. The participants had an average height of 141.5 cm, average weight of 35.8 kg, and average BMI of 17.8 kg/m2. The average palm size was 15.3 cm for both the throwing and non-throwing sides. Four of the players had a history of medial elbow pain.
      The differences between each finger on the throwing and non-throwing sides are shown in Table 1. On the throwing side, participants who were numbered 33, 56, 59, and 25 could independently flex the PIP while holding their palms in the index, middle, ring, and small fingers, respectively. On the non-throwing side, participants who were numbered 28, 46, 54, and 33 could independently flex the PIP while holding their palms in the index, middle, ring, and small fingers, respectively. On both sides, most participants could independently flex the PIP of the ring and middle fingers but had difficulty with the index and small fingers (P<0.001). Cramer’s V values of the throwing and non-throwing sides were 0.436 and 0.303, respectively.
      Table 1Number and percentage of adolescent participants who could perform proximal interphalangeal (PIP) joint flexion independently in each finger
      SideFinger
      IndexMiddleRingSmallP-value
      Throwing, n (%)33 (47.8)56 (81.1)59 (85.5)25 (36.2)<0.001
      Non-throwing, n (%)28 (40.5)46 (66.6)54 (78.2)33 (47.8)<0.001
      P-value0.250.390.270.09
      n, number of participants who could perform PIP joint flexion independently
      No significant differences were noted between the throwing and non-throwing sides in any of the fingers with independent flexion of the PIP (P=0.25, 0.39, 0.27, and 0.09 for the index, middle, ring, and small fingers, respectively) (Table 1).

      Discussion

      This study investigated a representative percentage of the population who can perform independent FDS function in each digit in adolescent baseball players. The results of this study showed that the ring finger had the highest FDS function for both the throwing and non-throwing sides. Next, the middle finger functioned well, while the index and small fingers had difficulty using FDS. In a quantitative evaluation of FDS function in adults, the ring and middle fingers were the largest and capable of PIP flexion, and the small finger showed the highest percentage of lack of FDS function.
      • Mugalur A.
      • Shahane S.M.
      • Samant A.
      • Pathak A.C.
      • Patil A.
      • Reddy R.
      Anatomic variation of palmaris longus and flexor digitorum superficialis of small finger in Indian population.
      ,
      • Olieveria B.M.
      • Fernandes C.H.
      • Nakachima L.R.
      • Dos Santos J.B.G.
      • Hirakawa C.K.
      • Faloppa F.
      Prevalence of absence of function of the flexor digitorum superficialis muscle tendons in the fourth and fifth fingers of the hand in the Brazilian population.
      ,
      • Tan J.S.
      • Oh L.
      • Louis D.S.
      Variations of the flexor digitorum superficialis as determined by an expanded clinical examination.
      ,
      • Watanabe Y.
      • Shirato R.
      • Wada T.
      • Iba K.
      • Sonoda T.
      • Yamashita T.
      Quantitative examination of isolated finger flexion associated with function of the flexor digitorum superficialis.
      In our investigation of adolescent baseball players, the independent FDS function of the middle and ring fingers was significantly higher than that of the small finger, a similar finding to those in previous studies. Thus, concomitant with previous reports and our findings, FDS function in each finger is not related to aging. Cadaveric studies have revealed variations ranging from anomalous muscle bellies to connections between musculotendinous units to complete absence of the small finger FDS, with the latter having a reported incidence ranging from 0% to 20%.
      • Kaplan E.B.
      Muscular and tendinous variations of the flexor superficialis of the fifth finger of the hand.
      ,
      • Shrewsbury M.M.
      • Kuczynski K.
      Flexor digitorum superficialis tendon in the fingers of the human hand.
      Therefore, FDS of the small finger can be a lower function of FDS, compared with that of other fingers.
      Adolescent and college-aged baseball players with elbow pain have more medial elbow joint space than players without pain symptoms.
      • Sasaki J.
      • Takahara M.
      • Ogino T.
      • Kashiwa H.
      • Isigaki D.
      • Kanauchi Y.
      Ultrasonographic assessment of the ulnar collateral ligament and medial elbow laxity in college baseball players.
      ,
      • Watanabe H.
      • Masuma H.
      • Kenmoku T.
      • Kudo H.
      • Saito K.
      • Nagami T.
      • et al.
      Increased medial laxity of the elbow in preadolescent baseball players with or without medial elbow apophysitis.
      Recently, studies have reported that increasing FDS function increases the medial side stability in valgus stress and helps to improve baseball players with ulnar collateral ligament (UCL) injury.
      • Hoshika S.
      • Nimura A.
      • Takahashi N.
      • Sugaya H.
      • Akita K.
      Valgus stability is enhanced by flexor digitorum superficialis muscle contraction of the index and middle fingers.
      ,
      • Matsuzawa K.
      • Edama M.
      • Ikezu M.
      • Otsuki T.
      • Maruyama S.
      • Sato N.
      Contributions of second and fifth digits of the flexor digitorum superficialis muscle to elbow valgus instability.
      ,
      • Otoshi K.
      • Kikuchi S.
      • Shishido H.
      • Konno S.
      Ultrasonographic assessment of the flexor pronator muscles as a dynamic stabilizer of the elbow against valgus force.
      ,
      • Sakata J.
      • Miyazaki T.
      • Akeda M.
      • Yamazaki T.
      Return-to-play outcomes in high school baseball players after ulnar collateral ligament injuries: dynamic contributions of flexor digitorum superficialis function.
      Regarding finger movements during pitching motion, the force of the thumb and ring fingers is greatest just before the maximum shoulder external rotation (MER), while the force produced from the index and middle fingers reaches the peak just before the MER and ball release.
      • Kinoshita H.
      • Obata S.
      • Nasu D.
      • Kadota K.
      • Matsuo T.
      • Fleisig G.S.
      Finger forces in fastball baseball pitching.
      Previous studies using cadavers have reported that the index, small, and independent small fingers adhere to the anterior oblique ligament (AOL) of the MCL.
      • Agee J.
      • McCarroll H.R.
      • Hollister A.
      The anatomy of the flexor digitorum superficialis relevant to tendon transfers.
      In addition, it has been reported that the ring and middle fingers may originate from the AOL.
      • Matsuzawa K.
      • Edama M.
      • Ikezu M.
      • Kaneko F.
      • Hirabayashi R.
      • Kageyama I.
      The origin structure of each finger in the flexor digitorum superficialis muscle.
      Regarding the anatomical features of the FDS, the automatic flexion angle of the PIP joint is related to the tension of the FDS. This is a factor in the difference in the FDS function among fingers.
      • Watanabe Y.
      • Shirato R.
      • Wada T.
      • Iba K.
      • Sonoda T.
      • Yamashita T.
      Quantitative examination of isolated finger flexion associated with function of the flexor digitorum superficialis.
      FDS tension was greatest in the middle and ring fingers, followed by the index finger and small finger.
      • Brand P.W.
      • Beach R.B.
      • Thompson D.E.
      Relative tension and potential excursion of muscles in the forearm and hand.
      ,
      • Lieber R.L.
      • Jacobson M.D.
      • Fazeli B.M.
      • Abrams R.A.
      • Botte M.J.
      Architecture of selected muscles of the arm and forearm: anatomy and implications for tendon transfer.
      In addition, the individual FDS muscle contraction, particularly in the index and middle fingers, decreased the medial elbow joint space under valgus load conditions.
      • Hoshika S.
      • Nimura A.
      • Takahashi N.
      • Sugaya H.
      • Akita K.
      Valgus stability is enhanced by flexor digitorum superficialis muscle contraction of the index and middle fingers.
      The sonographic elbow medial joint space under valgus stress with FDS contraction was the most significant predictive factor for the successful treatment of UCL elbow injury.
      • Sakata J.
      • Miyazaki T.
      • Akeda M.
      • Yamazaki T.
      Return-to-play outcomes in high school baseball players after ulnar collateral ligament injuries: dynamic contributions of flexor digitorum superficialis function.
      In this study, the rate of adolescent baseball players who could use the index FDS independently was only 48% on the throwing side, although the rates for the middle and ring finger FDS were 81% and 86%, respectively. According to previous reports, FDS function can be a therapeutic target for medial side elbow injuries in adolescent baseball players, especially for the index finger. Further investigations are needed to confirm the relationship between FDS function and medial elbow pain.

      Limitations

      First, this was a cross-sectional study and could not confirm the direction of the association. In this study, we did not mention whether the participants had medial elbow pain or not. Further investigations are needed to confirm the relationship between FDS function and medial elbow pain.

      Conclusions

      The FDS function was evaluated in adolescent baseball players. The FDS function of the middle and ring fingers was high for both the pitching and non-throwing sides. However, fewer than half of the players could not use the FDS of the index and small fingers independently.

      Acknowledgments

      The authors thank Ayase Little League Federation for participating in this investigation. They also thank Editage (www.editage.com) for assistance in English language editing.

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