Flexor Digitorum Superficialis Tendon Research Articles

Fractional Lengthening of Forearm Flexor Tendons: A Cadaveric Biomechanical Analysis

Multiple procedures have been described for wrist and finger flexion contractures and spasticity. Fractional lengthening of forearm flexor tendons involves making parallel transverse tenotomies at the musculotendinous junction to elongate the muscle. Currently, there is limited literature to define the biomechanical consequences of this lengthening technique. Forty-eight flexor tendons were harvested from eight paired upper limbs including flexor carpi radialis, flexor carpi ulnaris, flexor pollicis longus, and flexor digitorum superficialis tendons. Each tendon that was lengthened was paired with the contralateral tendon as a control. A pair of transverse tenotomies were completed for the fractional lengthening. The first tenotomy was performed at the musculotendinous junction where the tendon narrowed to 75% of its maximal width. The second tenotomy was made 1 cm distal to the first. Tendon length was measured before and after fractional lengthening at a constant resting tension of 1 N. The maximum load at failure of each tendon and the mechanism of failure were each measured and compared with the contralateral side. After fractional lengthening, the mean increase in resting tendon length was 4 mm. When loaded to failure, the mean maximum load of fractionally lengthened tendons was 42% of the mean maximum load of intact tendons. All lengthened tendons failed at the distal tenotomy site. Fractional lengthening resulted in an increase of 3-6 mm (mean: 4 mm) in tendon length at resting tension. There was a significant loss in tensile strength and load to failure following fractional lengthening compared with an intact musculotendinous unit. The reduction in tensile strength following fractional lengthening results in loads at failure that are, in some cases, lower than the estimated forces required to perform basic tasks. Caution during the healing and rehabilitation period is warranted.

Treatment for complete extensor tendon rupture: A case report on extensor pollicis longus tendon transfer and tenodesis procedure to radius for a patient with rheumatoid arthritis.

The complete loss of finger extension leads to significant inconvenience in daily life and often requires surgical treatment. Despite some disadvantages, the Boyes method, which uses the flexor digitorum superficialis tendon, is commonly performed for complete extensor rupture. We report the case of a 73-year-old woman living alone diagnosed with a subcutaneous rupture of all extensor tendons from the index to the little finger. The favourable range of motion of her wrist allowed us to perform extensor tenodesis. Additionally, the patient had a dislocated thumb interphalangeal (IP) joint, enabling us to use the extensor pollicis longus (EPL) tendon for tendon transfer in combination with thumb IP joint fusion. The patient demonstrated favourable finger range-of-motion outcomes at the 6-month postoperative assessment. The case shows that EPL tendon transfer and tenodesis may be a viable treatment option for patients with complete extensor rupture accompanied by thumb IP joint deformity and normal wrist range of motion.

A Comparison of Patient Characteristics and Outcomes Between Patients Receiving Flexor Digitorum Superficialis Slip Excision or Isolated A1 Pulley Release for Trigger Finger

Resection of the radial or ulnar slip of the flexor digitorum superficialis (FDS) tendon is a known treatment option for persistent trigger finger. Risk factors for undergoing FDS slip excision are unclear. We hypothesized that patients who underwent A1 pulley release with FDS slip excision secondary to persistent triggering would have a higher comorbidity burden compared to those receiving A1 pulley release alone. We identified all adult patients who underwent A1 pulley release with FDS slip excision because of persistent triggering either intraoperatively or postoperatively from 2018 to 2023. We selected a 3:1 age- and sex-matched control group who underwent isolated A1 pulley release. Charts were retrospectively reviewed for demographics, selected comorbidities, trigger finger history, and postoperative course. We performed multivariable logistic regression to assess the probability of FDS slip excision after adjusting for several variables that were significant in bivariate comparisons. We identified 48 patients who underwent A1 pulley release with FDS slip excision and 144 controls. Our multivariable model showed that patients with additional trigger fingers and a preoperative proximal interphalangeal (PIP) joint contracture were significantly more likely to undergo FDS slip excision. Patients who underwent A1 pulley release with FDS slip excision were significantly more likely to have multiple trigger fingers or a preoperative PIP joint contracture. Clinicians should counsel patients with these risk factors regarding the potential for FDS slip excision in addition to A1 pulley release to alleviate triggering of the affected digit. Therapeutic III.

Infiltration Technique for the Fourth FDS Opponensplasty Under WALANT

Background: Wide-awake Local Anesthesia No Tourniquet (WALANT) is a surgical technique that relies on anesthetic and hemostatic agents to provide conditions suitable for hand surgery without sedation and a tourniquet. The opponensplasty using the fourth flexor digitorum superficialis tendon transfer with the pisiform pulley is one of the most used in severe carpal tunnel syndrome patient who is unable to perform thumb opposition due to thenar muscle atrophy with the benefit of small incision, less soft tissue dissection, easy to perform, good esthetic result and has minor morbidity. Key advantages of the pisiform pulley provide balancing abduction and flexion strength of the thumb. To date, there is no report using the WALANT technique for the fourth flexor digitorum superficialis opponensplasty with pisiform pulley. Materials and Method: Between January 2019 and January 2022, 18 patients (4 males and 14 females) with severe carpal tunnel syndrome with thenar muscle atrophy underwent fourth flexor digitorum superficialis opponensplasty as day surgery under the WALANT technique. Operative data, visual analog scale (VAS) pain scores, and complications were recorded. All patients underwent the procedure without any complications. Results: The average age of the patients was 58.4 (43 to 62) year-old. The average operative time was 38.4 (17 to 47) minutes. The average VAS score during the anesthetic injection was 6.43 (4 to -8). The average VAS pain score during the operation was 2.1 (1 to 3). The mean intraoperative blood loss was 3 (2.6 to 6.2) mL. Conclusions: The WALANT technique is safe and can be used as alternative anesthesia for the fourth flexor digitorum superficialis opponensplasty with pisiform pulley. This technique overcomes the problem of judging the appropriate amount of transfer tension by observing awake patients actively. Levels of Evidence: Therapeutic study, level IV

Time-varying changes in median nerve deformation and position in response to quantified pinch and grip forces.

The ability of the median nerve (MN) to adapt in response to altered carpal tunnel conditions is important to mitigate compressive stress on the nerve. We assessed changes in MN deformation and position throughout the entire time course of hand force exertions. Fourteen right-handed participants ramped up force from 0% to 50% of maximal voluntary force (MVF) before ramping force back down in three different hand force exertion tasks (pulp pinch, chuck pinch, power grip). Pinch and grip forces were measured with a digital dynamometer, which were time synchronized with transverse carpal tunnel images obtained via ultrasound. Ultrasound images were extracted in 10% increments between 0% and 50% MVF while ramping force up (loading phase) and down (unloading phase). MN deformation and position relative to the flexor digitorum superficialis tendon of the long finger were assessed in concert. During loading, the nerve became more circular while displacing dorsally and ulnarly. These changes primarily occurred at the beginning of the hand force exertions while ramping force up from 0% to 20%, with very little change between 20% and 50% MVF. Interestingly, deformation and position changes during loading were not completely reversed during unloading while ramping force down. These findings indicate an initial reorganization of carpal tunnel structures. Mirrored changes in nerve deformation and position may also reflect strain-related characteristics of adjoining subsynovial connective tissue. Regardless, time-varying changes in nerve deformation and position appear to be an important accommodative mechanism in the healthy carpal tunnel in response to gripping and pinching tasks.

Paediatric trigger fingers: a 47-year experience.

Paediatric trigger finger is rare compared to adult trigger finger or paediatric trigger thumb, and the aetiology is unclear. Proposed causes include local trauma, anatomical anomalies and systemic conditions. The aim of the present study was to detail the anatomical causes of surgically treated paediatric trigger fingers and provide an operative algorithm based on the anatomical findings. A total of 76 trigger fingers in 38 patients were identified retrospectively at our institution between 1975 and 2022. In total, 41 fingers in 26 patients had anatomical variations. A nodular thickening on the tendon, similar to Notta's nodule in trigger thumbs, was the most common anatomical cause. Abnormal decussation of the flexor digitorum superficialis tendon was the second most common variation. The recurrence rate was significantly lower after resection of one slip of the flexor digitorum superficialis tendon compared to other surgical techniques in these patients. We recommend that surgeons assess for possible anatomical variation during surgery for the trigger finger.Level of evidence: IV.

The second clinical study investigating the surgical method for the kineticomyographic control implementation of the bionic hand

In 2018, during our first clinical study on the kineticomyographic (KMG)-controlled bionic hand, we implanted three magnetic tags inside the musculotendinous junction of three paired extensor-flexor transferred tendons. However, the post-operative tissue adhesions affected the independent movements of the implanted tags and consequently the distinct patterns of the obtained signals. To overcome this issue, we modified our surgical procedure from a one-stage tendon transfer to a two-stage. During the first surgery, we created three tunnels using silicon rods for the smooth tendon gliding. In the second stage, we transferred the same three pairs of the forearm agonist–antagonist tendons through the tunnels and implanted the magnetic tags inside the musculotendinous junction. Compared to our prior clinical investigation, fluoroscopy and ultrasound evaluations revealed that the surgical modification in the current study yielded more pronounced independent movements in two specific magnetic tags associated with fingers (maximum 5.7 mm in the first trial vs. 28 mm in the recent trial with grasp and release) and thumb (maximum 3.2 mm in the first trial vs. 9 mm in the current trial with thumb flexion–extension). Furthermore, we observed that utilizing the flexor digitorum superficialis (FDS) tendons for the flexor component in finger and thumb tendon transfer resulted in more independent movements of the implanted tags, compared with the flexor digitorum profundus (FDP) in the prior research. This study can help us plan for our future five-channel bionic limb design by identifying the gestures with the most significant independent tag displacement.

Relating in-vivo Strain of the Flexor Digitorum Superficialis Tendon with Grip Force

Epidemiological studies have shown a relationship between hand exertion and risk of developing distal upper extremity musculoskeletal disorders (DUEMSDs). Recently, fatigue-failure models were proposed for estimating the risk of DUEMSD development. However, models that incorporate tendon strain are primarily based on in-vitro data and may be better informed using in-vivo data. This methodological pilot study aimed to establish an approach for correlating grip force and spatiotemporal strains of the flexor digitorum superficialis (FDS) tendon using ultrasound imaging. Three image texture correlation techniques to measure in-vivo strains were explored and compared: digital image correlation, direct deformation estimation, and StrainNet, a novel deep learning neural network for strain prediction. StrainNet resulted in more accurate strain measurements than conventional image assessment tools, enabled continuous prediction of FDS tendon strain, and allowed for comparison of median bulk tissue strain during isometric contraction to grip force. Future work will study more participants and viscoelastic behavior.

Endoscopic Resection of Ulnar Bursa of the Palm: 2 Ulnar Portals Technique

Ulnar bursa of the hand invaginates the flexor digitorum superficialis and flexor digitorum profundus tendons of the little, ring, long, and index fingers. Distension of this bursa can have an inflammatory or infective cause. It can also originate from pathology of another site (e.g., degenerative wrist joint). Because of the tough overlying palmar aponeurosis, distended ulnar bursa can have swelling at a remote site. The distended ulnar bursa can also have compressive neuropathy to the ulnar nerve and median nerve. The purpose of this Technical Note is to describe the technical details of the 2 ulnar portals technique involving endoscopic resection of the ulnar bursa. This is indicated for symptomatic distension of the ulnar bursa, especially with hypothenar extension.

MRI-Based Evaluation of the Flexor Digitorum Superficialis Anatomy: Investigating the Prevalence and Morphometry of the "Chiasma Antebrachii".

Recent dissection studies resulted in the introduction of the term "chiasma antebrachii", which represents an intersection of the flexor digitorum superficialis (FDS) tendons for digits 2 and 3 in the distal third of the forearm. This retrospective investigation aimed to provide an MRI-based morphologic analysis of the chiasma antebrachii. In 89 patients (41 women, 39.3 ± 21.3 years), MRI examinations of the forearm (2010-2021) were reviewed by two radiologists, who evaluated all studies for the presence and length of the chiasma as well as its distance from the distal radioulnar and elbow joint. The chiasma antebrachii was identified in the distal third of the forearm in 88 patients (98.9%), while one intersection was located more proximally in the middle part. The chiasma had a median length of 28 mm (interquartile range: 24-35 mm). Its distances to the distal radioulnar and elbow joint were 16 mm (8-25 mm) and 215 mm (187-227 mm), respectively. T1-weighted post-contrast sequences were found to be superior to T2- or proton-density-weighted sequences in 71 cases (79.8%). To conclude, the chiasma antebrachii is part of the standard FDS anatomy. Knowledge of its morphology is important, e.g., in targeted injections of therapeutics or reconstructive surgery.

Carpal tunnel tendon and sub-synovial connective tissue mechanics are affected by reduced venous return

BackgroundThe effects of blood flow occlusion and sex-specific differences in tendon-subsynovial connective tissue relative strain are not well understood. Thus, the purpose of this study was to investigate the influence of blood flow, biological sex, and finger movement speed on carpal tunnel tendon mechanics to further develop our understanding of carpal tunnel syndrome. MethodsColour Doppler ultrasound imaging quantified relative motion between flexor digitorum superficialis tendon and subsynovial connective tissue in 20 healthy male and female participants during repetitive finger flexion-extension under brachial occlusion of blood flow and two movement speeds (0.75 & 1.25 Hz). FindingsFlexor digitorum superficialis and subsynovial connective tissue displacement decreased with occlusion (small effect) and fast speed (large effect). Speed × Condition interactions were found for mean FDS displacement and peak FDS velocity, where slow speed with occlusion reduced both outcomes. Movement speed had a small but significant effect on tendon-subsynovial connective tissue shear outcomes, where MVR decreased with fast finger motion. InterpretationThese results suggest the influence of localized edema through venous occlusion on tendon-subsynovial connective tissue gliding within the carpal tunnel. This insight further develops our understanding of carpal tunnel syndrome pathophysiology and suggests ramifications on carpal tunnel tissue motion when the local fluid environment of the carpal tunnel is disturbed.

Flexor Digitorum Superficialis Excision for Trigger Finger - A Systematic Literature Review.

Background: Division of one or more slips of the flexor digitorum superficialis (FDS) tendon has been posited as an effective surgical modality for advanced or recurrent trigger finger. This may be an effective approach among patients with diabetes or rheumatoid arthritis, or in those with fixed flexion deformities who have poor outcomes from A1 pulley release alone. However, there is limited evidence regarding the effectiveness of this procedure. The role of this study was to systematically review the evidence on functional outcomes and safety of partial or complete FDS resection in the management of trigger finger. Methods: A systematic review was performed according to PRISMA guidelines. PubMed, Cochrane CENTRAL and Ovid Medline databases were electronically queried from their inception until February 2022. English language papers were included if they reported original data on postoperative outcomes and complications following resection of one or more slips of FDS for adult trigger finger. Results: Seven articles were eligible for inclusion, encompassing 420 fingers in 290 patients. All included studies were retrospective. Isolated ulnar slip FDS resection was the most described surgery. Mean postoperative fixed flexion deformity at the proximal interphalangeal joint was 6.0° compared to 31.5° preoperatively, and the proportion of patients with fixed flexion deformity reduced by 58%. Mean postoperative total active motion was 228.7°. Recurrence was seen in 4.7% of digits, and complications occurred in 11.2% of cases. No post-surgical ulnar drift or swan neck deformities were observed. Conclusions: FDS resection for long-standing trigger finger, or in diabetic or rheumatoid populations, is an effective and safe technique with low rates of recurrence. Prospective and comparative studies of this technique would be beneficial. Level of Evidence: Level III (Therapeutic).

Use of a relative motion flexion orthosis after epitendinous zone II flexor tendon repair: A case report

Study designCase report IntroductionRelative motion flexion (RMF) orthoses are emerging as an option for early active motion (EAM) postoperatively. Purpose of the studyTo describe the rationale and implementation of an RMF orthosis to manage a patient after partial zone II epitendinous flexor tendon repairs. MethodsThis case involves a female who sustained partial flexor tendon lacerations to her middle finger in zone II, 60% flexor digitorum superficialis (FDS) and 90% flexor digitorum profundus (FDP). After epitendinous repair she was referred to therapy for EAM with a no orthosis request. The unusual circumstances prompted the therapist, concerned about the risk of tendon rupture to engage in discussion with the surgeon. Following discussion, a decision was made to use an RMF orthosis for controlled EAM to protect the epitendinous zone II FDS and FDP repairs. Outcomes of range of motion (ROM), total active motion (TAM), %TAM, grip, and quickDASH are reported. ResultsNeither the FDP or FDS tendons ruptured, nor were there any joint contractures. “Good” %TAM outcomes were achieved at 12-week postoperatively. Quick DASH scores improved 61 points indicating a clinically meaningful difference of improved function. DiscussionThe lack of a multi-strand core suture repair is unusual in combination with EAM. The positive outcomes reported in this single patient have raised questions about the protective benefit of the RMF orthosis when used with a zone II epitendinous repair of a 90% FDP laceration. Epitendinous repair of a partial (60%) FDS injury, however, is not uncommon and often not repaired at all. ConclusionsIn this single case report the epitendinous repairs of zone II 90% FDP and 60% FDS with digital nerve involvement were successfully managed with an RMF only orthosis. The use of EAM with an epitendinous repair is in conflict to the current surgical and therapy literature.

Isolated Closed Rupture of the Middle Finger Flexor Digitorum Superficialis Tendon in a Climber.

Isolated Closed Rupture of the Middle Finger Flexor Digitorum Superficialis Tendon in a Climber.

Ulnar Superficial Slip Resection for Resistant Trigger Finger: A Minimally Invasive Technique.

Primary trigger finger is a common hand disorder for which nonoperative treatment or release of A1 pulley is usually effective. For resistant or recurrent cases, there are different surgical techniques including partial or complete opening of A2 pulley, reduction tenoplasty, and resection of the ulnar slip of the flexor digitorum superficialis tendon. Here, we present our minimally invasive technique for ulnar superficial slip resection surgery. Our indications are the patients with persistent residual proximal interphalangeal joint contracture after A1 pulley release and also the recurrent cases. Then, we report the clinical outcomes of our patients operated using this technique.

Relative motion between the flexor digitorum superficialis tendon and subsynovial connective tissue is time dependent.

The subsynovial connective tissue is an integral component of flexor tendon gliding in the carpal tunnel, which is strained during longitudinal tendon displacement. We tested the effects of repetition frequency and finger load on flexor tendon function throughout active finger movement. Eleven participants performed metacarpophalangeal joint flexion/extension of the long finger cyclically at three repetition frequencies (0.75, 1.00, 1.25 Hz) and two finger loads (3.5, 7 N). Relative displacement between the flexor digitorum superficialis tendon and subsynovial connective tissue was assessed as the shear-strain index with color ultrasound throughout the entire time history of finger flexion and extension. In addition, long finger joint angles were measured with electrogoniometry while flexor digitorum superficialis and extensor digitorum muscle activities were measured with fine-wire electromyography to characterize the finger movements. The shear-strain index increased with greater finger flexion (p = 0.001), representing higher relative displacement between tendon and subsynovial connective tissue; however, no changes were observed throughout finger extension. The shear-strain index also increased with higher repetition frequencies (p = 0.013) and finger loads (p = 0.029), further modulating time-dependent effects during finger flexion versus extension. Using ultrasound, we characterized the time-dependent response of the shear-strain index, in vivo, providing valuable data on flexor tendon function during active finger movement. Our results infer greater subsynovial connective tissue strain and shear during repetitive and forceful finger movements. Future research characterizing time-dependent effects in carpal tunnel syndrome patients may further elucidate the relations between subsynovial connective tissue function, damage, and carpal tunnel syndrome.

Old name, new face: A systematic analysis of flexor digitorum superficialis muscle with “chiasma antebrachii”

The gross anatomy of the forearm flexors, particularly that of the flexor digitorum superficialis (FDS) muscle, has been described and graphically illustrated in several anatomical books and atlases starting in the middle of the century before last. However, in anatomical dissection studies as well as in clinical-anatomical courses training muscle-specific targeted injections due to movement disorders such as dystonia or spasticity, it has become apparent that there is a need for a closer investigation of the complex construction of the FDS muscle. To this end, we studied the structure of the muscle bellies and tendons of FDS on 46 human body donates that have been used either in our dissection or clinical-anatomical training courses. With this, we demonstrate here the topographical configuration of the individual muscle belly for each of digits 2 through 5 and the exact paths of their tendons until their passing through the carpal tunnel. Furthermore, we demonstrate the presence of a chiasm of the FDS tendons for the digits 2 and 3, approximately 3–4 cm proximal of the carpal tunnel. Thus, we introduce herewith the terminology “chiasma antebrachii”. These findings were confirmed in situ by imaging of fixed human body donates via MRI and corroborated by MRI and ultrasound imaging in two volunteers. Taken together, the present findings enable an updated understanding of the complex organization of the heads, bellies, and tendons of FDS that is relevant not only for anatomical teaching but also clinical interventions.

Evaluation of flexor digitorum superficialis function in adolescent baseball players.

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. Adolescent baseball players were recruited and assessed for FDS function for each finger using the standard technique. 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 nonthrowing side could independently flex the proximal interphalangeal joint while holding their palms in the index, middle, ring, and small fingers, respectively. When assessing both throwing and nonthrowing participants, a significant number of participants could independently flex the proximal interphalangeals of the ring and middle fingers but had difficulty with the index and small fingers (P<.001). No significant difference was noted between the throwing and nonthrowing participants in any finger (P>.05). 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.

Anti-Claw Procedure with Modified Dynamic Flexor Digitorum Superficialis Transfer for Ulnar Nerve Palsy: Our Experience.

The ulnar nerve palsy is a distressing injury, resulting in clawing of hand. The cause of clawing is due to paralysis of the interosseous muscles in the presence of functioning long extensor and long flexors of fingers. Various methods have been proposed to correct this deformity which include both static and dynamic procedures. In this study, we share our experience with flexor digitorum superficialis tendon transfer using Zancolli's modification for anti-claw correction. It was a retrospective case series study. A record of 53 patients was included in the study, during a period between June 2013 to July 2017 with ulnar nerve palsy. The procedure done was flexor digitorum superficialis tendon transfer as dynamic anti-claw procedure. The follow-up period was three months. The outcomes assessed were grip strength by using sphygmomanometer and active range of motion of fingers assessed by fingers tips touching the palm. Fifty-three patients were included out of them, there were fifty males and three females. The mean age was 28±10 years. All patients underwent flexor digitorum superficialis transfer for ulnar claw hand. A total of 84.9% patients have good grip strength and 83% showed good active range of motion. Flexor digitorum superficialis tendon transfer is found to be effective, reliable and reproducible technique in ulnar nerve palsy where patient need grip strength, good range of motion and acceptable hand function for daily routine work.

Distally-extending muscle fibers across involved joints: study of long muscles and tendons of wrist and ankle in late-term fetuses and adult cadavers.

It is unclear whether forearm and crural muscle fibers extend distally across the wrist and ankle joints, respectively. We hypothesized, in late-term fetuses, an over-production of muscle bellies extending over the joint. Muscle fibers in histological sections from unilateral wrists and ankles of 16 late-term fetuses (30-40 weeks) were examined and compared with 15 adult cadavers. Muscle fibers of the flexor digitorum profundus (FDP) and flexor digitorum superficialis (FDS) in fetuses, especially muscle bellies to the third and fourth fingers, were found to extend far distally beyond the radiocarpal joint. The extensor digitorum and extensor pollicis longus on the extensor side of the wrist were found to carry distally-extending muscle fibers, but these fibers did not extend beyond the distal end of the radius. In the ankle, most muscle bundles in the flexor hallucis longus (FHL), fibularis brevis (FB) and extensor digitorum longus extended distally beyond the talocrural joint, with most FB muscle fibers reaching the level of the talocalcaneal joint. In adult cadavers, muscle fibers of the FDP and FHL did not reach the levels of the radiocarpal and talocrural joints, respectively, whereas the FB muscle belly always reached the talocalcaneal joint. Similarly, some of the FDS reached the level of the radiocarpal joint. Generally, infants' movements at the wrist and ankle could result in friction injury to over-extended muscle. However, the calcaneal and FDP tendons might protect the FB and FDS tendons, respectively, from friction stress.