Graduation Year


Document Type




Degree Granting Department

Public Health

Major Professor

Hamisu Salihu


dynamic forces, maximal achievable effort, measurement of forces, musculoskeletal injuries, needle stick prevention, repetitive movements


Background: Jet injectors are advantageous over needle injectors by eliminating sharps hazards. The Government Accountability Office estimates 29% preventable sharp injuries with an estimated direct cost of more than $500 million out of the CDC's reported incidence of 385,000 needle stick injuries per year among US hospital healthcare workers. Yet the forces required to set and trigger devices using spring mechanisms for medication delivery have not been explored. This laboratory experiment measured forces exerted by healthcare workers (HCWs) using a particular jet injector approved by FDA in 2011.

Objectives: In order to quantify the ergonomic impact on HCWs using a needle-free injector, the first objective was to evaluate the dynamic forces required to activate the trigger injector button and the reset station for the injector, with their respective means, for each of the parameters studied. The second objective was to compare these forces to those required to use four previously analyzed retractable intramuscular syringes with needles. Finally, the third objective was to assess potential psychophysics ergonomic impact on HCWs with use of these devices to formulate future design changes and recommendations for manufacturers and HCWs, respectively.

Methods: This laboratory experiment was conducted through a multi-disciplinary team approach. It included a total of 136 trials (10 validation trials, 116 experimental trials and 10 padded trials for soft tissue simulation), which were conducted using the PharmaJetTM Injector. A force gauge and a load cell were integrated into the triggering setup and reset station, correspondingly, enabling force measurements to be obtained directly from the human-machine interfaces. These force data allowed for observations of force profiles in time by the healthcare worker as researcher while preparing for and administering injections. Data collection used three software applications for force conversions and data manipulation. Data were analyzed using descriptive statistics and analytical results by using ANOVA for the trigger injector & reset station with multiple comparison tests for parametric and non-parametric distributions, respectively.

Results: The descriptive results indicated an average force for triggering the injector in the 116 trials was 15.92 lbs. (70.8 N) with a range of 9.77-26.46 lbs. (43.46-117.69 N). The measured forces for the reset station ranged from 5.35-82.78 lbs. (5.35-368.22 N) with an average of 25.32 lbs. (112.62 N) (SD 12.36). Spurious findings presented with tensile forces to fill the syringes resulting in hand strain in the first metacarpal joint after repetitive pinprick motion. The analytical results showed an ANOVA for trigger injector with a parametric-normal distribution with an F (2,133) Ratio 10.0472, p- value (F) 0.0001<0.05, showing statistical significance and with a Tukey's comparison test showing a significant difference in between the means of the padded trials vs. the validation & experimental trial groups. The ANOVA for the reset station showed a Kruskal Wallis H-statistic of 0.2568, p-value (H) 0.8795>0.05 presenting NO statistical significance with a Dunn's comparison test confirming NO difference in between the medians or mean ranks of all three groups.

Conclusions: Triggering the injector and resetting the station required considerable effort in comparison to activating 4 retractable intramuscular syringes with needles from our previous studies, the range of mean forces were 3.63-17 lbs (16.19-77.53 N) for those syringes with the trigger injector maximum voluntary force of 71 N being above the recommend 56.6 N.The jet injector required more force per effort than 2 (4.4x) syringes & similar to other 2 syringes (0.9x) previously tested when considering the compression forces related with the trigger injector. Additional vector forces (displacement & gripping of reset station) could increase the cumulative effort affecting different musculoskeletal components when the whole components of the procedure are taken into account.

Suggestions for the manufacturer regarding design changes to facilitate HCWs' use of this device are warranted, since some of the summation forces during the 12 mini-steps could be avoided to achieve a higher efficiency. This information may be useful for health care facilities when choosing devices to protect their workers from ergonomic injuries.