Biomechanical Analysis of the Index Finger Motion for Optimal Trigger Design Using Cadaver Experiments.

Restricted (Penn State Only)
Author:
Chang, Joonho
Graduate Program:
Industrial Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
May 14, 2014
Committee Members:
  • Andris Freivalds, Dissertation Advisor
  • Neil Sharkey, Dissertation Advisor
  • Andris Freivalds, Committee Chair
  • Neil Sharkey, Committee Chair
  • David J Cannon, Committee Member
  • Ling Rothrock, Committee Member
  • Hyun Min Mike Kim, Special Member
Keywords:
  • Cadaver study
  • Trigger grip design
  • Triggering force
  • Trigger grip span
  • Handle grip span
  • and FDP to FDS tendon force ratio
Abstract:
A trigger grip has been usually used to operate powered hand tools and helps operators to control powered hand tools quickly and simply by reducing manual force requirements. However, operating the trigger excessively and repeatedly may let workers be exposed to work related musculoskeletal disorders (WRMSDs) such as trigger finger or carpal tunnel syndrome. Thus, the proper design of a trigger to reduce excessive and repetitive finger motions is required to prevent these WRMSDs. Although there have been a variety of related biomechanical analyses on the index finger or a trigger design, little research focusing on biomechanical interaction between index finger motions and trigger mechanism exists. However, they even have limits to show detailed biomechanical interaction between the index finger and triggers. Therefore, more specific and detailed biomechanical analyses focusing on the index finger mechanism during triggering are required. The present study aims to conduct a biomechanical analysis on the index finger mechanism during triggering action using cadaver experiments. The specific objectives of the study include 1) developing a two-dimensional biomechanical static model for the index finger which accounts for index finger force mechanism while operating a trigger, 2) building an index finger motion simulator to support and control a cadaver hand, 3) conducting cadaver experiments to observe the relationship of external trigger forces to the internal tendon forces depending on different external force contact locations, trigger grip spans, FDP to FDS tendon force ratios, and handle grip spans, 4) validating the proposed biomechanical model, and 5) recommending optimal trigger grip span, optimal contact location for finger triggering, and optimal handle grip span for trigger and pistol designs in terms of force efficiency between externally generated trigger forces and internal tendon forces. The index finger motion simulator was developed for cadaver experiments. The primary purpose of this simulator was to support and control a cadaver hand for observing relationship between internal tendon forces and external trigger forces, during triggering. The simulator was comprised of five essential parts: 1) the support frame to secure the specimens, 2) the motion delivery unit to control FDP and FDS tendons in the specimens 3) the data acquisition system to measure internal tendon forces and external triggering forces, 4) the vision system to obtain joint flexion angles on the index finger, and 5) the operation program to control the simulator and save the results of the experiments. The cadaver experiments for index finger’s triggering motion was conducted with the index finger motion simulator. Five fresh-frozen right human cadaveric hand specimens (average age = 46; SD = 5.7) without apparent musculoskeletal disorders and anatomical abnormalities were employed in the study. And the experiments consisting of two phases investigated triggering forces by the index finger as function of 1) total internal tendon forces (FDP + FDS; 40, 70, and 100 N), 2) FDP to FDS tendon force ratios (1 to 1, 1.5 to 1, 2 to 1, and 3 to 1), 3) trigger grip spans (40, 50, and 60 mm), 4) three different contact locations (F1 = the middle of the distal phalange; F2 = the distal edge of the medial phalange; F3 = the middle of the medial phalange) between a trigger and the index finger, and 5) handle grip spans (40, 50, and 60 mm) for the middle, ring, and little fingers; Phase I observed effects of 1), 2), 3), and 4), while Phase II investigated effect of 5). Also, force efficiencies were computed based on their internal tendon force to external triggering force ratios. As a result, the following key findings were found from the experiments: 1) FDP to FDS tendon force ratios didn't affect triggering forces by the index finger, 2) triggering forces increased significantly while total internal tendon force increased from 40 to 100 N, 3) The maximum triggering forces were found at 50 mm trigger grip span and the triggering forces at 60 and 40 mm trigger grip spans followed it in turn, 4) triggering forces increased significantly while external force contact location moved proximally from F1 to F3, and 5) 50 mm handle grip span showed the maximum triggering forces and 40 and 60 mm handle grip spans followed it in turn. Also, force efficiencies found the following important features: 1) approximately 10 to 30% of internal tendon forces could be converted into external triggering forces while triggering and 2) force efficiencies declined gradually while total internal tendon force increased from 40 to 100 N. A biomechanical index finger model for triggering was developed to observe relationships between internal tendon forces and externally generated triggering forces, based upon the index finger anatomy; three mathematical models on the three external force contact locations, F1, F2, and F3, were developed based on biomechanical assumptions and conditions of static equilibrium. Also, the models were simulated mathematically based on the unknown variables determined by the experimental conditions, in order to evaluate the validity of the models. Consequently, the simulation results showed high similarities to the results of the experiments: 1) the estimated triggering forces increased significantly while total internal tendon force increased from 40 to 100 N, 2) The maximum estimated triggering forces were found at 50 mm trigger grip span and the triggering forces at 60 and 40 mm trigger grip spans followed it in turn, and 3) the estimated triggering forces increased gradually while external force contact location moved proximally from F1 to F3. In sum, the models predicted similar triggering force patterns to the triggering forces measured in the experiments. However, overall, triggering forces were over-estimated by the models, except for at F1 contact location; relatively accurate triggering forces were estimated at F1 contact location, but the models predicted triggering forces two and three times higher than the measured triggering forces at F2 and F3 contact locations, respectively. Finally, the present study provided design recommendations for an optimal one-finger trigger design. The following four design guidelines were defined based on the results of the cadaver experiments: 1) a force requirement to activate a trigger should be set as low as possible, 2) a trigger grip may afford users to use the medial phalange, 3) 50 mm trigger grip span is generally recommended as the optimal trigger grip span to accommodate most people, and 4) 50 mm is recommended as the trigger handle grip span.