Have there been done any scientific studies that proves that clipless pedals actually improve pedaling efficiency etc. over flat pedals? Clipless pedal enthusiasts are quick to say that you must use clipless pedals to improve efficiency when cycling, but I have never seen this being backed up by any scientific sources actually proving this claimed efficiency boost.
The Pedaling Technique of Elite Endurance Cyclists: Changes With Increasing Workload at Constant Cadence was published in the International Journal of Sport Biometrics 7:29-53, 1991. However, it seems to come to the conclusion that they don't really make any difference as far as pedaling efficiency goes.
"...while torque during the upstroke did reduce the total positive work required during the downstroke, it did not contribute significantly to the external work done because 98.6% and 96.3% of the total work done at the low and high workloads, respectively, was done during the downstroke."
This is echoed in Physiological and biochemical determinants of elite endurance cycling performance published in the Medicine and Science in Sports and Exercise 23:93-107, 1991. There are numerous graphs showing that pedal force is only exerted between the top and bottom of the downstroke, represented by a very sharp parabola spiking at 90 degrees from vertical.
That said, I think it's obvious to anyone who has ever done any particularly technical riding that with and without clipless pedals that clipless pedals significantly improve the handling of a bicycle. A fact which is probably more difficult to verify through scientific studies.
The case for/against clipless, or even straps, is sort of summed up in this piece from the Rivendell Bicycles website. They mention studies, albeit without citing the exact source, that actually pulling up on the pedal is extremely unlikely, except maybe on short uphill or sprint bursts, and so being attached to the pedal is far from being a must. And they suggest, without supporting evidence, that it may actually make you a better cyclist, because without attachment your legs have to learn to go in circles, and not simply be taken along for the ride.
There is also the issue with positioning your foot on the pedal: all that careful messing around with the cleats so that the ball of the foot is exactly over the pedal axle. But then there is that other blog from Joe Friel, who is an evidence nut, suggesting that there is no proper support for that being best, and that it may be more efficient to put your cleat under the arch.
Joe's blog, together with the (again, inadequately referenced) story of the Japanese competitor in Ironman New Zealand that forgot to put his shoes in the transition bag, and went to a bike personal best riding barefoot on clipless pedals, have really changed my outlook on the need for attachment to the pedals. I ride with clips on my fixie, but am more and more considering changing to plain pedals and see where that takes me.
Surprised there are not more studies on efficiency of clipless. The original question was to seek a scientific answer, not anecdotal, although many feel clipless gives more, this is subjective if not backed by science.
This is the only article I found: http://www.radlabor.de/fileadmin/PDF/PowerForce/Mornieux___Stapelfeldt_Artikel_Feedback_Pedalkraefte_2008.pdf
Will be interesting if more research is done, given the fast increasing popularity of cycling here in the UK.
The aim of this study was to determine the influence of different shoe-pedal interfaces and of an active pulling-up action during the upstroke phase on the pedalling technique. Eight elite cyclists (C) and seven non-cyclists (NC) performed three different bouts at 90 rev•min–1 and 60% of their maximal aerobic power. They pedalled with single pedals (PED), with clipless pedals (CLIP) and with a pedal force feedback (CLIPFBACK) where subjects were asked to pull up on the pedal during the upstroke. There was no significant difference for pedalling effectiveness, net mechanical efficiency (NE) and muscular activity between PED and CLIP. When compared to CLIP, CLIPFBACK resulted in a significant increase in pedalling effectiveness during upstroke (86% for C and 57% NC, respectively), as well as higher biceps femoris and tibialis anterior muscle activity (p < 0.001). However, NE was significantly reduced (p < 0.008) with 9% and 3.3% reduction for C and NC, respectively. Consequently, shoe-pedal interface (PED vs. CLIP) did not significantly influence cycling technique during submaximal exercise. However, an active pulling-up action on the pedal during upstroke increased the pedalling effectiveness, while reducing net mechanical efficiency.
It's not a study per se, but the Wattbike gym machine has a useful power output meter which shows the power being applied by each leg (explained in more detail on their website).
The basic premise is that by smoothing out the push and the pull you can see a visible effect on the power curve on the display. The real key (from personal anecdotal experience) is that clipless pedals assist in these actions because the pull isn't just employing a different set of muscles in the leg and adding more power (although it is doing that), you're also reducing the total down time in each cycle. If you're just pushing, there's going to be periods where either neither is pushing and thus the bike/revolving weight is decelerating or even where both are pushing and counteracting each other.
Clipless pedals aren't going to assist in the latter case, but they will in the former. So by pulling you're helping to compensate for the lag in the cycle until the other leg starts pushing and we all know that maintaining a speed is generally easier than accelerating to that speed. So if you've got even 5-10 degrees (or more) of arc in every cycle where neither foot is pushing (see the wattbike examples of a bad transition, the curve that looks like a figure-of-eight) then you're having to work to regain the previous power. While if you're managing the leg-to-leg transition, the effort doesn't drop off so much (resulting in a 'peanut' curve) and you're not losing as much power.
Additionally, as suggested here, the clip is allowing you to attach at a better point on the base of foot, rather than on the ball of the foot which is demanded by your not being attached.
It seems like scientists tend to test athletic performance. The actual benefits of clipless pedals are:
If you are cycling fast, you get a lot of unanticipated jolts and vibration which can make you slip off of the pedals, particularly if it is wet, muddy or bumpy. Clipless pedals keep you locked in.
If your visibility is impaired by riding in the dark, or in a group, then it becomes difficult to anticipate said jolts, and in latter case the penalty for slipping off the pedals is a big pile up (which is your fault).
When tired, you don't have to expend any energy in keeping your feet planted on the pedals, you can just push down dumbly and the pedal will rotate.
When you absolutely, positively have to accelerate quickly, you need power on the upstroke and the ability to "flail" your legs without them slipping off the pedals.
(You can of course get most of these benefits from toeclips, but they are a bit harder to get it and out of, and don't fasten your foot quite as securely, without locking you in, which is what track cyclists do)
Most scientific studies point to a very small benefit in pedalling efficiency when cycling in controlled conditions, but accident avoidance is the real advantage.
Foot retention, in some form, has been around since the dawn of cycling.
There are multiple reasons for it:
Having a foot slip off the pedal during a hard effort is dangerous (especially on fixed gear bikes, but also during a race in close quarters). This isn't a risk at slow speeds, but at high cadences, it can be hard to stay on pedals without retention.
If you ride a lot, your pedal stroke will get more efficient just like a runner becomes more efficient with running. What this means on the bike is that your feet are not resisting each other on the upstrokes and wasting energy. It also means that on the upstroke your foot is applying little-to-no force on the pedal and it can easily shift position if it is not retained by clipless pedals or straps. If your foot is not in the right position, your stroke is less efficient and there is the danger of coming off the pedal during an exertion.
I think if you want scientific evidence for foot-retention, all you need to do is find evidence for having the foot placed properly on the pedal.
I think it's worth noting that if the question is about efficiency and not power, then there is probably little difference, but I find it a little dubious that so many of these references say it makes little difference whatsoever.
If the discussion is about power, I'm sure that a proper study were done on the usage of clipless pedals would show that the gains in power are rather substantial. Additionally, a study that shows there is little benefit to a rider that's unlikely to pull on the pedal doesn't mean that there aren't significant gains for one that is. I did my daily commute on platforms a couple times recently and I felt absolutely helpless at sprinting to make traffic lights, or even to get going from a stop. I had no torque available when I needed it due to being limited to a single leg and using the quads alone.
I pay very close attention to cadence and stroke while I'm riding, and the benefits in maintaining a steady cadence and using all muscle groups is quite noticeable, perhaps the existing studies are just asking the wrong questions!
in regards to the efficiency advantage of clipless pedals there are a few things to keep in mind. first off, we are converting reciprocating motion to rotational motion. secondly, that this conversion has different force vectors throughout the 360 degrees of rotation. and finally, the reciprocating component, your leg, is comprised of three constantly changing angles: hip, knee and ankle joints. let's look at the internal combustion engine as a handy example of what we're referring to: as you approach and pass the top of the stroke the reciprocating mass, your leg, is moving more sideways, applying little downward force. this is why ICE's advance the spark before the piston reaches the top of the stroke. NOW, as you continue, force will increase up to it's maximum and than taper off as you approach around 145 degrees. this is why exhaust valves open before the end of the power stroke, because there is little force to be had at the bottom of the stroke. once again, as you approach the bottom of pedal travel, the motion is more sideways, producing no downward force. also note that as the reciprocating mass, your leg, reaches the top or bottom of the stroke, it is changing direction, from up to down and back. however, since we are talking about a human leg and not an inanimate connecting rod/piston assembly, there is some pulling back and up force that can be applied to the total power input to the rotating crank. but also keep in mind that as you approach around 280 degrees of crank travel you actually pushing the pedal more forward, to top dead center, than applying any useful power to the crank. the greatest force is applied on the down stroke, so flat/clipless advantage equal. pulling the pedal back and partially up, flat/clipless advantage probably equal. as RPM's and cadence and torque and technique ( a world class cyclist compared to you and me) increase there is probably a cumulative advantage across the spectrum of factors. as power output is a combination of all the above mentioned factors, a more accurate assessment can be had by attaching sensors to the cranks and measuring force loading throughout the 360 degrees of crank rotation.
Not at all, the idea the 'clips are more efficient than flats' is based on nothing but superstition and urban myth. It's like now every rider laughs at the idea of using toe clips and wonder how cyclists could have put up with them, it was 'more efficient' back in the day. Like so many things in cycling using clips is just another superstitious practise in cycling with no scientific backup, despite even the best pros can't avoid crashing dozens of times a year using clips, they still don't give it up, and velodrome riders having to be held up at start of the race like children on stabilizers. (they may as well use stabilizers if they can't keep feet on the pedals during riding.)
GCN has done three tests on the matter, and all three show that flats are more efficient/faster than clips. I think the reason may be that as you use the hamstrings to pull up with clips, you also waste quad muscles unnecesaryly because muscles are all connected as you use the quad you also use the hamstring etc., So with clips your quads never gets a complete rest tiring you out quicker. (probably more so in climbs, which is why even pros climb so slowly) where as with flat your quads (which contribute more than 96% of the power) gets a complete rest every half rotation.
Also when you push down, you stabilize the bike and propel it forward. Upstroke is not efficient because it 'lifts' the bike up losing momentum and balance, which is why you see clip riders swing a lot from side to side (losing energy) when climbing and sprint.