From The Ergoweb® Learning Center

Tool Force Predicts Incidence of Upper Extremity Injury

A strong association was found between tool handle force and upper extremity injury in a study at an automobile assembly plant by Ku et al.  The tool mean peak handle force at injury workstations was 29.99 N as compared to 12.97 N at the non-injury workstations.  Further, the risk of injury increased in proportion to peak tool handle force with the odds of an injury case increasing nearly 5 percent for each additional Newton of hand force.  The authors expressed caution regarding a dose-response relationship given the limited size of their research.

Although not statistically significant, the average handle displacement was nearly 2.5 times greater at the injury workstations as compared to the non-injury workstations.

Tool peak handle force and displacement for the same/similar tool showed wide variation dependent on workstation and job parameters.  The same tool with same target torque used at two workstations to different joint hardness tasks and workstation orientations produced peak handle forces of 0.87 N and 7.42 N.   A right-angle tool applied to a soft joint (plastic fastened to plastic or there was foam or an O-ring between materials) had the greatest peak handle force (mean of 45.57 N) and displacement (mean of 38.78 mm).


Risk of injury was not related to tool target torque.


Study Design

From 69 trim and chassis workstations, 15 distinct nutrunners were identified.  For this study, inclusion criteria consisted of:

1.      The job used only a single pneumatic power nutrunner

2.      The nutrunners did not have a counterbalance or reaction bar


Measurements involving the job and the tool were taken to estimate tool handle reaction force and displacement through the use of the Lin et al. (2003) tool operator mechanical model.  Tool parameters that were measured included:

·         Tool geometry (handle and tool lengths)

·         Shape (pistol or right angle)

·         Mass (kg)

·         Motor free running speed (RPM)

·         Motor stall torque (Nm)

·         Location of the center of mass (cm)

·         Threaded fastener joint hardness (soft, medium, and hard)

Job parameters that were measured included:

·         Fastener horizontal and vertical distances from the operator

·         Tool orientation (horizontal or vertical)


Over a 12 month automobile model year, upper extremity musculoskeletal disorders were identified from plant OSHA 100 and 200 logs.  An upper extremity musculoskeletal disorder was defined as a nonacute injury to the fingers, hands, wrists, forearms, upper arms, or shoulders (i.e., tendonitis, epicondylitis).


Other Findings

Peak handle force and displacement was greatly effected by type of tool and threaded fastener joint hardness.  Tool characteristics are outlined in Table 1 while Table 2 addresses joint hardness.


Average Peak Handle Force

Average Displacement

Right Angled Nutrunner

1.37 N to 57.81 N

0.32 mm to 38.4 mm

Pistol Grip Nutrunner

0.35 N to 18.14 N

0.19 mm to 9.21 mm

Table 1: Tool Characteristic Impact On Handle Peak Force and Displacement


Joint Type

Average Peak Handle Force

Average Displacement

Soft Joint

20.69 N

18.65 mm

Medium Joint

18.75 N

12.88 mm

Hard Joint

8.89 N

3.75 mm

Soft Joint = plastic fastened to plastic or there was foam or an O-ring between materials.
Medium Joint = sheet of metal fastened to a piece of plastic.
Hard Joint = both fastened materials were metals.

Table 2: Joint Characteristic Impact On Handle Peak Force and Displacement


The impact of tool design and joint type on average peak handle force and average displacement is presented in Table 3.

Joint Type

Tool/Handle Force/Displacement

Average Peak Handle Force

Average Displacement

Right Angled Nutrunner

Pistol Grip Nutrunner

Right Angled Nutrunner

Pistol Grip Nutrunner

Soft Joint

45.57 N

14.84 N

38.78 mm

13.91 mm

Hard Joint

10.16 N

7.86 N

4.80 mm

2.90 mm

Table 3: Relationship Between Joint Type and Tool On Average Peak

Handle Force and Average Displacement.


Study Limitations

Reservations effecting of this research include:

1.      Without measurement of other risk factors such as duration and repetition, the impact of force on injury may be over emphasized.

2.      Differences in operator characteristics such as age, strength, size, or experience were not evaluated.


Article Title: Power Hand Tool Kinetics Associated with Upper Limb Injuries in an Automobile Assembly Plant


Publication: Journal of Occupational and Environmental Hygiene 4: 391-399, 2007


Authors: C H Ku, R G Radwin, and B T Karsh

This article originally appeared in The Ergonomics Report™ on 2007-07-06.