=design =biomechanics =mechanical engineering =clothing
There's something intuitively intriguing about the concept of shoes with spring elements, something that made many kids excited about getting "moon shoes", but they found the actual item rather disappointing. Using springs somehow with legged movement also makes some logical sense: walking and running involve cyclic energy changes, and the Achilles tendon stores some elastic energy. Is that perspective missing something?
big springs
In a sense, spring elements in
shoes are standard: sneakers have elastic foam in the soles, and maximizing
the sole bounciness
does slightly improve running performance.
Jumping stilts are
the modern version of spring shoes. They actually work, which also means
they're quite dangerous - if what the kids who wanted moon shoes imagined
was accurate, their parents wouldn't have bought that for them. The concept
is obvious, but they only appeared recently; they weren't being made in
1900, and that's because high-performance materials are necessary for a net
increase in performance. Those jumping stilts typically use fiberglass
springs and modern aluminum alloys, and keeping weight low is still a
problem. As that linked video notes, even with modern materials, the
increase in jump height is only moderate.
This guy made a
different type of spring boots for increasing running speed, and 16 million
views implies that some people find the concept interesting, but it would be
better to start from a proper theoretical analysis and then properly
optimize materials and structure.
This paper
argues for a different geometry, where a spring is attached to the foot and
hip instead of the foot and shin. It notes:
To reach the theoretical top speed of 20.9 m/s in Fig. 2, the spring should (i) store 930 J energy and (ii) weigh no more than 1.5 kg
and
state-of-the-art fixed stiffness running springs made from carbon fiber offer only about 150 J/kg
It might be possible to use gas springs to get that kind of performance, though matching the desired force curves is an issue. Another obvious issue is transferring vertical forces to the hip or torso without interfering with movement too much or adding too much weight. Of course, 20.9 m/s is very fast and not very realistic in practice, but some sort of setup with a thick waist belt and gas springs + carbon fiber springs could plausibly make people run significantly faster.
heels
A lot of women wear high heels,
despite them causing higher rates of injury and foot pain than other shoes.
That popularity has something to do with the effect on apparent body
proportions and gait changes making women seem slightly more attractive. As
for why certain walks would be more attractive, my understanding is, that's
largely an association with pelvis width. (I remember being told that pelvis
width of human women had an evolutionary tradeoff between childbirth
problems and walking/running efficiency, but
apparently that was incorrect. (Learning about biomechanics of walking
hasn't made me any better at walking, and wheeled vehicles on roads are
obviously more efficient, but I guess if a Japanese billionaire ever needs
me to build an 18m bipedal running robot, I'll be ready. (Yes, that's not a
practical thing to do even if it's become possible, but neither is
this.)))
One of the main reasons that high heels are less comfortable is that there's
a greater impact on hitting the ground. Padded insoles help with that
somewhat, but the theme of this post is shoes with springs, so
here's a high heel
prototype with a spring heel. Apparently that design worked OK but was
kind of heavy; using fiberglass instead of steel would reduce the weight. I
haven't seen much interest in that sort of concept, but maybe it's actually
a good idea.
We can also ask: why would high heels
have more impact when hitting the ground? I think it's related to ankle
position relative to the impact point. Normally, the heel strikes the ground
slightly below and behind the ankle; with a high heel, the ankle is higher,
and it's also difficult to rotate the heel so it's behind the ankle while
stepping forwards. Should people who wear high heels do the sort of foot
stretches that ballerinas do so they can rotate their feet downward more?
Would that help? I'm not sure.
If the problem is the impact point
relative to the ankle position, then it might also help to have a curved
extension backwards from the bottom of the heel, so that the impact point
would be on that extension and then the shoe would roll forwards. I'm not
sure about the aesthetics or effectiveness of that, but sometimes fashion
involves gratuitous
novelty, so maybe someone should try it. And perhaps that rearward heel
extension could be...slightly springy?
Another problem with high
heels is the foot sliding forwards, causing the toes to get crushed. Perhaps
that problem could also be mitigated by...adding springs? Suppose an ankle
cuff was connected to the main body of the shoe with a fiberglass spring
that pushes the shoe forwards; when weight is on the foot, it would shift
down slightly, but when the foot gets lifted the spring would push it
forwards again. Of course, all that sliding forwards and back makes some
friction, which isn't ideal. That
happens already because the squashed toes push the shoe back when weight
is removed, but it would probably happen even more with a spring.
The
usual solution to friction between feet and shoes is wearing socks so that
sliding happens between the sock and shoe instead. This is quite effective
and socks have become fairly popular. (In fact, I'm wearing socks right
now.) In extreme cases, like soldiers or hikers carrying heavy backpacks, it
can be a good idea to wear 2 layers of thinner socks; I actually think
armies should probably issue double-layer socks as standard.
However,
socks with sandals are uncool, and high heels are often sandals or at least
sandal-like. Why is that, anyway? My theory is, socks with sandals stay
uncool because they're practical and uncool, which means that only people
who care more about practicality than fashion wear them, and those people
are considered uncool, so socks with sandals stay unfashionable by
association. An obvious possible solution is for an outfit to be fashionable
or impractical enough to avoid association with uncool german dads. High
enough heels are impractical enough to potentially make socks and sandals
OK. Combined with a girly outfit, it would be countersignalling: "nobody
could confuse me with the uncool sort of person who wears socks with
sandals, so I can".