The most important findings of this study were that FDS activity alone contributes to medial elbow stability. In addition, the FDS muscles of the index and middle fingers provide more dynamic stability against valgus stress than that of the ring finger.
Grip strength is an important indicator of hand function, including the functions of the FDS and FDP muscles. A hand dynamometer is used to evaluate the maximum power of the total grip strength of all fingers . However, the FDP is considered a primary finger flexor, generating greater gripping strength than the FDS . A recent biomechanical study demonstrated that the contribution of the FDS and FDP to the grip strength depends on the hand dynamometer angulation and contact area of each finger . In addition, it is difficult to accurately evaluate grip strength in each finger by using the dynamometer. Finally, grip strength of all fingers could not be associated with the FDS function in each finger. In our study, to obtain isolated FDS activation, the subject was asked to flex the PIP joint while the DIP joint remained quiescent and not flexed in each finger . Clearly, our study showed the contribution of isolated FDS function in each finger to the dynamics of the MJD.
Some anatomical studies reported that the FDS was the individual muscle best suited to provide medial elbow support because of its proximity and relatively large bulk among the FPMs [6,8]. Current in vitro research recognized that FDS activity had significant effects on MJD using stress US [14,17]. However, these studies overlooked the importance of assessing FDS activity of individual fingers, as opposed to the activity in all of them collectively, through grip motion; thus it remained unclear which finger’s FDS contraction contributed most to elbow stability. This study demonstrates that FDS contraction of the index and middle fingers has a larger effect on MJD than that of the ring finger. This explains how FDS activation of the index and middle fingers has a primary role for stabilization of the medial elbow against valgus stress.
The stress US study showed that healthy volunteers displayed 0.7 mm of change in the MJD as measured with non-stress and stress . Another study demonstrated that the change in the MJD under loaded and loaded-contracted conditions of the FDS was 0.2 mm . Based on the previous studies, a minimum of 0.2 mm could be interpreted as a clinically significant difference in the loaded and loaded-contracted conditions compared to the unloaded condition. In the present study, the change in the MJD under loaded and loaded-contracted conditions of the isolated FDS compared to the unloaded condition was 1.3 and 0.8 mm, respectively. Finally, the isolated FDS contraction could have clinically significant effects on the MJD to protect from valgus stress.
Previous studies have reported that the UCL of the dominant elbow in professional baseball pitchers was thicker than that of the non-dominant elbow and its thickness was associated with increased laxity against valgus stress [20,25]. Recently, Hoshika et al  identified that the anterior bundle of the UCL could be interpreted as part of the tendinous complex, which consisted of the tendinous septum (TS) and the FDS muscle. In other words, UCL thickness could be interpreted as the FDS muscle thickness. Given the anatomical fact, we could hypothesize that UCL thickness will change with medial elbow stabilization by the FDS contraction of each finger. Future research is needed to assess the changes in muscular thickness of the FDS with valgus stability by FDS contraction of each finger using the MJD through the stress US.
Based on these findings, an interpretation of the dynamic stabilizing effect of FDS function in these specific fingers can be described anatomically as follows: At the level of the mid forearm, the FDS muscles of the index and middle fingers are located on the radial side, and the ring and little fingers on the ulnar side. In addition, the FDS muscle of the index finger is located in the superficial layer and that of the middle finger in the deep layer . Hoshika et al.  also reported that the tendinous complex might be a pathway of the muscular power of the FPMs to the humeroulnar joint. Taking these anatomical concepts into consideration, the FDS muscles of the index and middle fingers may connect with the TS, thus these fingers could provide more dynamic stability against valgus stress than that of the ring finger via the TS.
Our study provides clinically relevant information that can be used in the development of a program to prevent UCL injury. Recently, there has been a focus on hand muscle training to prevent throwing injuries as well as to improve performance in generating ball velocity [14,26,28]. However, the Thrower’s Ten, which has a long history of use by overhead throwing athletes, incorporates multiple strengthening exercises for the shoulder, elbow, and wrist, but fails to address the finger flexors . Taking our current results into consideration, the preventive care program could be improved by incorporating finger exercises that focus particularly on the FDS muscles of the index and middle fingers. This study might aid development of such programs.
The study has the following limitations: First, FDS activity of the little finger was not examined because it has no impact on throwing a ball. Second, we enrolled participants from the general population, and the medial elbow of baseball players may have adapted to chronic medial elbow stress and react differently. Future research in these athletes will allow the assessment of how these characteristics change in a sport-specific population. Third, we did not check any electromyography of the FDS and FDP muscles for the precise assessment of muscle activity in each finger, when cued to perform isolated FDS contraction without the effect of the FDP muscle. Given these limitations, this is the first study to assess MJD during FDS contraction in each finger.