Exposure Differences Between Children and Adults When Using Small and Standard Size Mice and Keyboards

Study Overview

Researchers Blackstone, Karr, Campo and Johnson investigated the physical exposure differences between children and adults when using standard sized input devices and smaller input devices.

In their introduction, the researchers review pertinent usage statistics:

• In 1984, 12% of children aged 5-17 yrs used a computer at home, 27% used them at school;
• By 2003, the numbers had increased to 83% and 92%, respectively;
• By 2000, children with home computer access averaged 1.5 hrs per day of use;
• By 2001, 75% of USA children were using computers by the age of 5 yrs; and
• By 2003, computer use was more prevalent among children than adults (90% of children vs. 60% of adults aged 18 yrs and over).

They also note that there is a large body of evidence that links computer usage to musculoskeletal disorders (MSDs) in adults, including:

• In a 2002 study, more than 50% of newly hired adults working jobs that required computer use for 15 hrs or more developed some sort of symptom within the first yr; and
• In a 1995 study, adult computer users self reporting 4 or more hrs of use per day showed a greater risk of developing MSDs.

There is less known about children and computer use, but:

• A 2002 study assessed 212 school-age children and showed 30% reported wrist pain attributed to computer use;
• A similar 2002 study reported more than 50% of children identifying pain that they thought was exacerbated by computer use; and
• A 2000 study found that 60% of a group of 314 students aged 10-17 yrs who used laptop computers for an average of 3.2 hrs per day reported increased discomfort while doing so.

The researchers also note that women have a higher prevalence for many types of MSDs, and studies have shown that female computer operators, when compared to men, apply higher relative forces; exhibit higher muscular activity; and work at greater postural extremes with greater range of motion in the wrist.

For this study, the researchers postulate that the greater apparent risk to woman may also be present with children, and that the underlying cause may be related to the relative size of the input devices.

Methods

A total of 28 subjects were recruited from a Microsoft Corporation database, including healthy schoolchildren (7 male, 7 female) between 5-7 yrs and their same-gender biological parent. Demographic and anthropometry data were gathered for each subject, and the researchers used electromyography (EMG) and electrogoniometry methods to measure posture and estimate force exertions while participants performed trials of standardized pointing and typing tasks. A representative keyboard with built-in numeric keypad on the right was used as the standard sized keyboard, and a smaller keyboard with no numeric keypad was used as the small keyboard. Likewise, representative small and standard sized mice were used. All subjects sat in a representative adult-sized office chair, and attempts were made to control gross postural differences (e.g., workstations were adjusted, where possible, such that elbows were at 90 degrees; foot rest platforms and additional back support were used for children). However, due to workstation adjustability constraints, 5 adults and 10 children worked with their elbows 2.5 cm or less below worksurface height.

This brief review does not cover all of the data collection and analysis performed in this study, but instead highlights what this reviewer finds most 
pertinent. As with all scientific studies, the authors provide a great deal of information regarding their methods and findings, and interested readers are encouraged to read the complete research article, cited below, for complete details.

Once data collection was complete, the researchers performed statistical analysis to characterize the physical exposures and task performance under the various conditions.

Results and Conclusions

Mouse Performance

• Overall, children took nearly twice as long to move between targets than adults;
• Overall, children were disproportionately slower than adults when performing the smallest target-pointing task;
• Overall, adults were significantly more accurate, and made fewer overall errors, than did children;
• When comparing the small mouse to the standard size mouse:
  • Both children and adults moved between targets significantly faster with the small mouse;
  • Both children and adults made significantly fewer errors with the small mouse;
  • Children were more accurate with the small mouse than with the standard size.

Keyboard Performance

• Adults typed significantly faster than children (29.4 WPM vs. 2.6 WPM);
• Adults typed faster on the standard sized keyboard than on the small (29.4 WPM vs. 26 WPM);
• Adults were more accurate than children (83% vs. 60.4%);
• Accuracy did not change between the keyboard sizes.

Physical Exposures

• Differences between children and adults:
  • When mousing, children exhibited less wrist extension, but greater ulnar deviation than adults;
  • When mousing, children exhibited nearly twice the range of motion than adults for both flexion/extension and radial/ulnar deviation;
  • When typing, children exhibited less wrist extension, but greater ulnar deviation than adults;
  • When typing, adults exhibited roughly twice the amount of flexor and extensor force than children, which the researchers postulate is related to the faster adult typing speeds.
• Differences between small and standard sized devices:
  • Children worked with significantly less ulnar deviation and had significantly lower muscle activity levels when using the small mouse;
  • No other significant differences were observed between the devices for either children or adults.

Standard Size Device Force Requirements

• At baseline maximal exertion measurements, children were significantly weaker than adults (about "45% as strong" as adults);
• Children worked at a higher percentage of their MVC, roughly twice that of adults, while operating the mice and keyboards (i.e., adults operated at roughly 1.5% of their MVC while children operated at roughly 3% of their MVC).

In summary, the primary findings in this study are:

• Children have greater postural exposures than adults when using an adult-sized mouse;
• Children operate at a greater percentage of their MVC than do adults when activating the keys/buttons on mice and keyboards, regardless of their size; and
• Both adults and children perform pointing tasks faster and with fewer errors when using a smaller mouse than a standard sized mouse, and children are also more accurate with a smaller pointing device.

What This Means to Ergonomists

We’ve said it many times before, and we’ll say it again: "One size does not fit all." Product designers and product purchasers — parents and schools in this case — need to understand the implications of their decisions on the health and performance of their target user population.

However, even though this study demonstrated some important differences between children and adults, it does not answer the question of whether children are at greater risk of injury or discomfort while using adult sized input devices.

Also, the finding that both parents and children performed better with the smaller size mouse deserves further research to both verify and better characterize this potential performance effect. Could it be that we might all benefit from a smaller mouse?

Reference

Blackstone, Janet M., Karra, Catherine, Campa, Janice, and Johnson, Peter W., Physical exposure differences between children and adults when using standard and small computer input devices, Ergonomics, Vol. 51, No. 6, June 2008, pp. 872–889.

This article originally appeared in The Ergonomics Report™ on 2011-07-05.