Distribution of the forces produced by motor unit activity in the human flexor digitorum profundus.
Level 4 - case-series / case-control
Cross-sectional human neurophysiological study (graded by design analogy)
PubMed 12181299 · doi:10.1113/jphysiol.2002.023861
What was done
Discharge of 69 single low-threshold motor units was recorded across the human flexor digitorum profundus (FDP) muscle during weak voluntary grasping involving all fingers. Spike-triggered averaging of forces under each fingertip pad was used to determine whether force associated with a single motor unit was confined to its primary test finger or distributed across non-test fingers.
What was found
For index-, middle-, and ring-finger motor units, force changes under non-test fingers were typically less than 20% of the force under the primary test finger. For little-finger units, mean force changes under the adjacent ring finger were greater than 50% of the force under the test finger, a distribution pattern that differed significantly from the other three fingers. Occasional unloading of adjacent fingers occurred in 22 of 267 finger-unit combinations, predominantly affecting the index finger. Increases in test-finger force correlated significantly with background force for middle-, ring-, and little-finger units.
Why it matters
The findings show that human FDP motor units allow relatively selective mechanical control at the tips of the index, middle, and ring fingers during grasping, while force production by the little finger is mechanically and neurally linked to the ring finger.
Limits
The abstract does not report the number, age, or sex of human participants. The findings are restricted to low-threshold motor units during weak grasping and may not generalize to high-force contractions or non-grasping finger movements. Direct anatomical and mechanical mechanisms underlying non-test finger loading were not isolated.
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- supports The deep forearm flexors running into the fourth and fifth fingers are the weakest and least resilient to heavy gripping load.