There are nearly 7,500 emergency room visits each year due to injuries related to backpacks or book bags, according to estimages from the The Consumer Product Safety Commission (CPSC), notes principal investigator Gita Murthy, PhD, who was a fellow in UCSD’s Department of Anesthesiology during the time of the research.

"The concern of heavy backpacks and back and shoulder pain to parents is not new. However, the objective data that we have published is new and important. The more objective data that the public has, the more educated they become, and perhaps more inclined to change the way children carry backpacks," says Murthy.

"Furthermore, manufacturers and designers of backpacks often try to optimize design based upon the data available in the literature," Murthy adds. "Our shoulder-loading data may help designers and engineers design a wider shoulder strap, for example, that will help spread the load of the backpack."

Heavy Loads, Asymmetrical Distribution

Murthy and principal investigator Brandon Macias, BA, of UCSD's Department of Orthopedic Surgery decided to study backpack loads because there previously were no studies connecting physiological pain and backpack loading.

"Previous studies have measured general pain and heavy backpacks," Macias says. "But, they have not correlated backpack load distribution and pain. This correlation is important to establish how much load in a backpack is too much."

Participants in the research included five boys and five girls, aged 13 years old. Each child's backpack was fitted with pressure sensors -- the same type as those that measure standard blood pressure -- on the shoulder straps.

The children wore standard identical backpacks first carrying 10 percent of their body weight, then 20 percent and finally 30 percent. Children commonly carry backpack loads of 22 percent of their body weight, earlier studies have found.

Before the backpacks were loaded, the shoulder straps were positioned with sensors to obtain contact pressure measurements over a 30-second recording period. Contact pressures measured significantly higher on the right side than the left side at all bodyweight levels, the researchers noted, which may have been due to posture, a factor they suggest warrants further study.

Other studies have indicated that posture changed when shoulders were asymmetrically loaded, they pointed out, adding that long-term asymmetrical loading on the shoulders may alter the spine curvature and produce back pain.

Perceived Pain, Reduced Blood Flow

The children reported increased perceived pain with each weight level. Surface pressures measured by the investigators were higher than the pressure threshold to obstruct normal skin and muscle blood flow, measured at 30mmHg.

Pressures at 20 percent of body weight measured 70 mmHg on the left shoulder and 110 mmHg on the right shoulder -- skin surface pressures that are more than double and triple the threshold for reduced blood flow -- the UCSD team observed.

The research team hopes that parents will help their children avoid using heavy backpacks for prolonged periods and prevent backpack pain and related injuries.

Concentrated backpack loads should be minimized, and the way children carry backpacks should be optimized to promote safety and comfort, the researchers recommend, adding that the reported backpack average of 22 percent of body weight is too high.

It is difficult to give an overall recommendation, they say, because of different body types and life styles. However, the researchers emphasize that the average load that children are carrying now needs to be reduced, because the high pressure on the shoulder may be causing shoulder pain, and asymmetric loads may contribute to low back pain.