  Purpose: We study the relationship between surgical gestures and motion
primitives in dry-lab surgical exercises towards a deeper understanding of
surgical activity at fine-grained levels and interpretable feedback in skill
assessment.
  Methods: We analyze the motion primitive sequences of gestures in the JIGSAWS
dataset and identify inverse motion primitives in those sequences. Inverse
motion primitives are defined as sequential actions on the same object by the
same tool that effectively negate each other. We also examine the correlation
between surgical skills (measured by GRS scores) and the number and total
durations of inverse motion primitives in the dry-lab trials of Suturing,
Needle Passing, and Knot Tying tasks.
  Results: We find that the sequence of motion primitives used to perform
gestures can help detect labeling errors in surgical gestures. Inverse motion
primitives are often used as recovery actions to correct the position or
orientation of objects or may be indicative of other issues such as with depth
perception. The number and total durations of inverse motion primitives in
trials are also strongly correlated with lower GRS scores in the Suturing and
Knot Tying tasks.
  Conclusion: The sequence and pattern of motion primitives could be used to
provide interpretable feedback in surgical skill assessment. Combined with an
action recognition model, the explainability of automated skill assessment can
be improved by showing video clips of the inverse motion primitives of
inefficient or problematic movements.
