A volunteer swinging their leg back to rotate the hip while the gases that they exhale are recorded. Photo credit: Negin Fallah.
Skipping about as youngsters, it can be hard to imagine that age will ever catch up, but it does, and by the age of 65, walking can consume 10–25% more energy than it did in youth. But what drives this increase? Owen Beck from the University of Texas at Austin, USA, explains that as we age, the way we walk alters; we use our ankles less and compensate by generating more propulsion from our hips while our bodies swing forward during a stride. But it was unclear whether favouring the hips over the ankles causes the increase in energy use by older walkers, or whether other factors are responsible. Intrigued, Beck teamed up with Master's student Negin Fallah (University of Texas at Austin) to put healthy young adults through their paces to find out why walking like an older person uses more energy.
The duo decided to compare how the volunteers rotated their ankles and hips while simulating aspects of a stride. They braced the youngsters’ legs in the position they assume during the middle of a stride, when standing on one leg as the body moves forward. Then they asked each individual to rotate the ankle as if extending the foot to push off with the toe or swing the leg as if extending it back from the hip during the stance portion of a step. While the volunteers repeated the manoeuvres, the duo measured the rotational force generated at the ankle and hip, in addition to measuring the activity of the muscles around the joint. They also recorded how much oxygen the volunteers consumed, and the amount of CO2 they produced, during the ankle and hip movements to calculate the amount of energy used by each joint when walking.
When the duo compared the volunteers’ ankle movements with those of the hips, however, they were surprised. The youngsters were unexpectedly producing more thrust from the ankle than the hip and exerting the ankle extensor muscles more. But when the researchers compared the amount of energy consumed by pushing off with the foot versus the hip, ‘the participants expended approximately twice as much metabolic power to produce the hip movements’, says Fallah. In short, shifting to using the hips more than the ankles makes walking more inefficient as we age, because swinging the leg back during a stride is more energetically costly than pushing off from the ankle.
But why do hip muscles use more energy while producing less force? There may be several reasons. One is that the fibres in the muscles at the hip are longer than those at the ankle, potentially causing the hip muscles to consume more energy as they contract during a step forward. In addition, hip muscles may simply be less efficient than ankle muscles, as they consume more ATP while generating force, driving up the cost of walking when we use our hips more. However, the duo suspects that older walkers’ movements might also be subtly different from those of younger walkers, so the volunteers may not have faithfully reproduced the joint movements of older walkers, and they encourage other researchers to look in further detail at older walkers.
More importantly, Fallah and Beck suggest that supporting older walkers at the hip, to reduce the effort required to swing their legs back, could help them to regain or improve mobility after injury. Either way, walking becomes more difficult as we get older, so next time you stride out, make sure you give older people a bit more time and space; they're working harder than you are.
