Research Article| Volume 35, ISSUE 2, P261-266, April 2022

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In vivo evidence of lumbricals incursion into the carpal tunnel in healthy hands: An ultrasonographic cross sectional study

Published:April 30, 2022DOI:


      • The percentage of lumbrical muscles that migrated into the Carpal Tunnel during full finger flexion was 79.4%.
      • The magnitude of migration of lumbrical muscles was 2.99 cm.
      • After the lumbrical incursion, the cross-sectional area of the Carpal Tunnel increased by .13 cm2.
      • The traditional splinting properties for the treatment of Carpal Tunnel Syndrome should be revised to account for lumbrical incursion into the Carpal Tunnel.



      During finger flexion, the tendons of flexor digitorum profundus migrate proximally, along with their attached lumbrical muscles. This incursion was suggested to extend into the Carpal Tunnel. Ultrasonographic imaging can be used to assess in vivo soft tissue behavior and incursion.

      Purpose of the study

      To clinically quantify the lumbrical muscles incursion in different finger positions.

      Study Design

      Cross sectional, observational study.


      The lumbricals of 20 healthy adults with no history of hand injuries were evaluated with neuromuscular ultrasound imaging (n = 160 lumbricals). The lumbrical muscles migration was measured as the participants actively moved their fingers from full extension to 50% flexion, and 100% flexion.


      Of the 160 lumbricals measures, the incursion occurred at 18.1% of fingers at 50% finger flexion, and increased to 79.4% during full finger flexion. The lumbricals migrated a total of 2.99 cm after full finger flexion, and ended up 0.76 cm (SD = 0.86 cm) inside the Carpal Tunnel. The metacarpophalangeal joint range of motion of the index finger at the point where the lumbricals entered the distal border of the Transverse Carpal Ligament was 84.4° (SD = 6.8°). The Carpal Tunnel cross-sectional area during finger extension was 1.68 (0.35) cm2, and increased to 1.81 (0.33) cm2 after full finger flexion.


      This study showed direct evidence of lumbrical incursion into the Carpal Tunnel during finger flexion. The cross-sectional area of the Carpal Tunnel increased during full finger flexion in comparison to full finger extension, supplementing the evidence of increase content within the Carpal Tunnel. The findings of this study have significant clinical implications for the conservative treatment of the Capral Tunnel Syndrome.


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      JHT Read for Credit

      Quiz: # 855

      Record your answers on the Return Answer Form found on the tear-out coupon at the back of this issue or to complete online and use a credit card, go to There is only one best answer for each question.
      • # 1.
        The lumbricals
        • a.
          migrate distally owing to their attachment to the tendon of the FDS
        • b.
          migrate distally owing to their attachment to the tendon of the FDP
        • c.
          migrate distally independent of the movement of the flexor tendons
        • d.
          maintain a fixed position during finger flexion
      • # 2.
        Reproduction of carpal tunnel symptoms after maintaining a full fist in neutral for 60 seconds constitutes a
        • a.
          + Mills Test
        • b.
          + Phalen's Sign
        • c.
          + Watson Sign
        • d.
          + Berger's Closed Fist Test
      • # 3.
        MCP flexion was approximately _____ when the lumbricals entered the distal border of the transverse carpal ligament
        • a.
        • b.
        • c.
        • d.
      • # 4.
        With full finger flexion the
        • a.
          lumbricals flattened out
        • b.
          cross sectional area of the carpal tunnel increased
        • c.
          cross sectional area of the carpal tunnel decreased
        • d.
          cross sectional area of the carpal tunnel was unchanged
      • # 5.
        The evidence supports the phenomenon of lumbrical incursion into the carpal tunnel during finger flexion
        • a.
        • b.
      When submitting to the HTCC for re-certification, please batch your JHT RFC certificates in groups of 3 or more to get full credit.