• Skip to primary navigation
  • Skip to main content
  • Skip to footer
Plastic Expert Group

Materials Testing & Plastic Consultants

  • HOME
  • ABOUT US
  • SERVICES
    • Overview
    • Plastic Failure
    • Pipe Failure Analysis
    • Injection Molding Defects
    • Materials Testing
    • CPVC Fire Sprinkler Piping
  • LEARN
  • CONTACT US

How “Burst Resistant” Exercise Balls Can Explode Causing Serious Injury

Categories

You are here: Home / Plastic Failure Analysis & Testing / How “Burst Resistant” Exercise Balls Can Explode Causing Serious Injury

January 4, 2019

Swiss balls are good for your muscles, but are they safe?

Exercise balls are used extensively by people for exercising and rehabilitation and by athletes for strength training.

“Burst-resistant” exercise balls first became popular in Switzerland about 40 years ago. Physiotherapists and sports trainers in North America began to incorporate “Swiss Balls” in their programs and the use of exercise balls in the US became widespread in the 1990s.

The primary benefit of using an exercise ball compared to exercising directly on a hard flat surface is that the body responds to the instability of the ball to remain balanced, engaging many core body muscles.

The increased demand for exercise balls caused many companies to begin marketing these products. As with most items, competition led to most exercise balls being manufactured in China.

How an exercise ball works

The most common material for use in the manufacture of exercise balls is plastisol (plasticized PVC). The ball is made by a process called rotational molding (or “rotomolding”). A sphere is heated while being rotated and loaded with plasticized PVC powder containing a chemical blowing agent dissolved in the material. Pigments are also added to the formulation to add coloration to the ball.

Regular inflated balls burst when punctured under load because of the stress concentration effect of the puncture/hole. Without a puncture, stresses are equally distributed over the entire surface of the ball. But when a ball or balloon is punctured, the stresses are suddenly locally concentrated at the edges of the puncture, causing rapid propagation at supersonic speed through the dense material under stress.

However, fractures initiated in a properly manufactured burst-resistant exercise ball don’t propagate because the ball has a cellular structure. This cellular structure helps reduce stress concentration because the thousands of cells help to distribute the stresses around a puncture and thus reduce the stress concentration effect of the puncture. Also, the thousands of tiny air pockets in the wall of the ball act to interfere and disrupt fracture propagation.

Dr. Duane Priddy

“The cellular nature of the material is what creates the burst resistance and is critical to make the balls safe for use. Without this burst resistant feature, it would be extremely dangerous to use inflated balls to support body weight during exercise routines.”

Dr. Duane Priddy, Plastic Expert Group, Founder & CEO

When and why swiss balls fail

Unfortunately, no standard exists regarding the manufacture and testing of exercise balls. Therefore, failures of burst-resistant balls are common. Our laboratory has tested exercise balls involved in over a dozen different litigations.

One of the litigations was high profile as it involved the serious injury of Sacramento Kings basketball star Francisco Garcia. Francisco was lifting hundred-pound weights while laying on a burst-resistant ball. When the ball exploded, the bones in both of his arms were seriously fractured, causing him serious injury.

The company that manufactured the balls made them available in four different colors: blue, green, black, and yellow. We purchase duplicate balls in all four colors and burst-tested each. Both the blue and the green balls exploded when punctured, but the black and yellow balls slowly deflated when punctured.

Burst Tet
Figure 1. Blue and green balls all exploded when punctured, while the black and yellow balls did not.

Analysis of the four different colored balls revealed that the blue and green balls contained copper-based pigments, while the black and yellow balls did not. We believe the copper pigment caused the PVC to degrade.

In other exercise ball explosions, we’ve found that the balls exploded because of extreme thickness variation around the circumference of the ball.

When a portion of the ball wall is thinner than the rest of the ball, stresses are concentrated in the thin region, making the ball more likely to explode under load as demonstrated using computer models.

Photograph of thinnest and thickest area of ball reveals extreme thickness variation.
Figure 2. Photograph of the thinnest and thickest area of the ball reveals extreme thickness variation.
Computer model of ball with thickness variation shown
Figure 3. Computer model of the ball with thickness variation is shown in Figure 2.

 

Plastic Expert Group: preventing exercise ball failures nationwide

In conclusion, we believe burst-resistant exercise balls should be carefully inspected before every use using a magnifying device. Balls should not be used to support body weight if they have cuts, scratches, or abrasions on the surface.

Another thing to look for is ball surface gloss or shininess. A non-reflective, dull surface is caused by ruptured cells on the surface of the ball. If a ball has a matt or dull surface, the ball is defective and should be discarded.

Exercise balls should be manufactured to be burst-resistant as rupture/explosion of a ball during use may result in serious personal injury. Unfortunately, since 2000, there have been at least 47 injuries reported due to exploding exercise balls.

Our experts have worked on several different litigations involving injuries to people caused by defectively manufactured exercise balls. We’ve used material analysis, burst testing, flaw sensitivity testing, and computer modeling to identify and confirm the defects.

Exercise Ball Surfance
Figure 4. Comparison of the appearance of the surfaces of good and bad balls. The image on the right shows a microscope image of the ruptured cells on the ball with the non-reflective/matt surface.

Contact us for a free initial consultation if you have questions about exercise ball failure.

Footer

Plastic Molding Defects
Our plastic consultants can help you discover the best process to make your part or product. If you’ve already discovered a defect, we will root out the source of the problem and recommend the best countermeasures and solutions to remedy the issue.
Find out more
Plastic Pipe Failure Analysis
Plastic isn’t perfect. CPVC, PEX and PVC pipes and components fail from time to time. Our consultants investigate pipe failure including failure analysis in our state-of-the-art testing laboratory.
Find out more
Material Testing
If a single component made from defective material goes into production, the entire product may be compromised. Using cutting edge tools and analysis, our team of expert consultants can prevent product failure, and accurately predict component lifespan based on the proposed application.
Find out more

Plastic Expert Group & Failure Labs Copyright © 2023 Toll Free USA: (877) 668-4345   •   International: +1 (989) 281-4465

www.plasticpipefailure.com   •   www.plasticfailure.com

Plastic Failure Services:  Molding Defects   •   Pipe Failure Analysis   •   Materials Testing Consultants   •   Plastic Failure   •   PEX Failure   •   Polymer Consulting