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Different Loads on Bones Can Cause them to Break

August 16, 2016

Using the injury of French gymnast Samir Ait Said during the Olympics, MIE associate professor Sandra Shefelbine explains how bones can break when different loads are placed on them.

Source: News @ Northeastern

Ear­lier this month, French gym­nast Samir Ait Said broke his leg at the Rio Olympics while vaulting during the men’s pre­lim­i­naries. When North­eastern pro­fessor Sandra She­fel­bine viewed the grue­some injury online after­ward, her reac­tion mir­rored that of most viewers. “Ouch!” she exclaimed.

Yet She­fel­bine views this injury through a dif­ferent lens than most Olympics viewers. She is an expert in bone bio­me­chanics and studies how the struc­ture and com­po­si­tion of bones influ­ence their mechan­ical properties.

It looked like his foot was planted, and his knee was twisting,” explains She­fel­bine, asso­ciate pro­fessor in the Depart­ment of Mechan­ical and Indus­trial Engi­neering. “That is called tor­sion, and bones—and most mate­rials, for that matter—aren’t as strong in tor­sion. Lig­a­ment tears usu­ally happen this way. The way his leg went down, there was prob­ably a high degree of tort, which is a rotating force.”

Bones break when the load on the bone is too much for it to with­stand, She­fel­bine says. “But one inter­esting thing about bones is that they are opti­mized for our daily loading activ­i­ties. If you put unique loads on them, they’re not as strong as when you put the normal loads on them.”

Loads, She­fel­bine explains, essen­tially refer to force. Bones also break when the force is going in a dif­ferent direc­tion than a person nor­mally experiences—in the French gymnast’s case, his leg, upon landing, was bent at a dif­ferent angle than usual.

The way he landed, it was a very dif­ferent loading sit­u­a­tion than from normal activity,” she says.

One possible cause of the gymnast's leg break is that his leg was twisting as his foot hit the mat, and the force was too great on the leg and caused the break. Graphic by Greg Grinnell/Northeastern University

One pos­sible cause of the gymnast’s leg break is that his leg was twisting as his foot hit the mat, and the force was too great on the leg and caused the break. Graphic by Greg Grinnell/​Northeastern University

Shefelbine’s research focuses pri­marily on injuries that can be pre­vented and under­standing the appro­priate levels and types of loads we should be putting on our bones. She uses in vivo exper­i­men­ta­tion and com­pu­ta­tional mod­eling to study topics ranging from how bones adapt to increased mechan­ical load to what makes the bones of patients with brittle bone dis­ease more sus­cep­tible to frac­tures. She also looks at how bones develop over the course of a single individual’s lifetime.

One track of her research focuses on working with para­plegics to help them main­tain their bone health; she explains that their bones get weaker because they’re not putting the same loads on them as they once did. Another track explores chil­dren who play youth sports at an elite level, with a par­tic­ular emphasis on whether that changes how their bones grow and whether it can lead to hip defor­mity and arthritis later in life.

That’s not to say phys­ical activity is bad for your bones. “Our bones are fan­tastic for walking and run­ning,” She­fel­bine says, noting that putting higher loads on your bones actu­ally builds more bone and makes them stronger and tougher. She says studies have shown that adult tennis players, for example, have higher bone den­sity in their upper bodies than non-​​athletes.

Freak and unpre­ventable acci­dents like the French gymnast’s will happen, she says. But, speaking gen­er­ally of the average person and injury pre­ven­tion, “By having healthy bones, you have a wider safety margin.”