The average recreational runner usually clocks between 150 and 170 steps a minute. This means that in a light, half-hour run, your feet will strike the ground around 5000 times. "Every time your foot hits the ground, your body absorbs the impact," says John Mercer at the University of Nevada, Las Vegas.
Previous studies have shown that joggers have a clear improvement in prefrontal function.
While exercise is no doubt beneficial for overall health, running and walking put stresses on your joints.
To study how these vibrations affect our bodies, Delphine Chadefaux at Aix-Marseille University in France and her colleagues used high-speed cameras, leg sensors and force-sensing plates to follow motion, acceleration, muscle activity and the force of hitting the ground in 10 recreational runners. Each participant ran both barefoot and in shoes, and at two different speeds.
Together, these techniques enabled the researchers to home in on where the leg absorbs the impact energy from running. They found that the foot unsurprisingly absorbed the largest part, and the energy then decreased upwards through the tibia (shinbone) and knee. This was particularly pronounced in barefoot running, which produced almost four times the amount of shock energy in the foot as running in shoes.
However, in both barefoot and shoe-wearing runners, the team found that the impact shocks had almost entirely dispersed by the time they reached the hip.
Chadefaux, who presented her findings today at a conference of the Acoustical Society of America in Boston, Massachusetts, thinks this suggests that the way we use our muscles when we run is geared towards protecting the upper body from impact vibrations.
She and her team think understanding how the body does this might lead to ways to reduce shock in the lower part of the leg, and prevent common injuries such as stress fractures and joint problems.
Runners get injured because the constant, repetitive shocks can wear on bones and joints. "If you have too many impacts and the bone does not have a chance to recover, that can lead to a stress fracture or some other issue," says Mercer.
He says the team's methods are a lot more specialised and focused than previous attempts to understand the impact of running, and could improve understanding of the effects of jogging-induced shock vibrations.
Chadefaux now plans to look at how different muscle activity might control the impact of foot strikes. Using a different type of shoe or changing how a person runs might help prevent injuries, she says.