High stiffness of human digital flexor tendons is suited for precise finger positional control

The objective of this study was to define the biomechanical properties of the human digital flexor tendons and to compare these biomechanical properties to other muscle-tendon units in the forearm. Mechanical measurements were performed on fresh-frozen tendons under physiological load and temperature conditions. Loads were determined by first measuring the physiological cross-sectional area of each digital belly of the flexor digitorum superficialis (FDS) and flexor digitorum profundus (FDP) and estimating maximum tension (P_o) of that specific muscle head. Loading each tendon to the appropriate P_o resulted in no significant difference in tendon strain among any of the tendons within each muscle (P > 0.05; digits 2-5) or between muscle types (FDP vs. FDS). The one exception to this finding was that a significantly higher strain at Po was observed in the FDP tendon to the small finger (P < 0.05). Average absolute strains observed for the FDP and FDS tendons (1.20 ± 0.38%, mean ± SD; n = 39) were significantly lower than those observed previously in a study of the prime movers of the wrist. The measured strain of ∼1.5% was less than half of that predicted to occur in muscles of this architectural design. Modeling sarcomere shortening magnitudes during FDP or FDS contraction yielded a value of only 0.10 μm, which would have a negligible effect on the force generating capacity of these muscles. Thus the high stiffness of the digital flexor tendons suits them well for fine positional control and would render their muscle spindles quite sensitive to length perturbations at the fingertips.