Sunday, May 25, 2008

A Crafty Bugger

When Schooner Mark II arrives (due sometime in the next week or so), then I not only have the challenge of learning to walk on the new leg (which shouldn't take too long) but of also rigging up and adjusting a new cycling leg.

As mentioned before, the new prosthetic leg has been designed to enable a fluid pedalling motion by allowing sufficient freedom of movement around the knee joint (especially behind the knee). It is that very freedom which means I will take a little time to re-adapt to walking on it - my knees, hips and all the associated muscles, tendons etc need to have the ability to cope with the new level of freedom. That will mostly be sorted out simply through using it and maybe a few supplemental exercises.

The prosthetic is also designed so that my walking foot extension can be readily removed from the leg socket and a cycling leg extension attached. Undo/redo a couple of allen bolts to drop one out and put the other in.

Getting it right requires a few planets to align, with one of them just being put into the right orbit today.

At the moment the problem is being tackled from each end of the prosthetic - firstly the end where the remainder of my left leg attaches (fits into) to the prosthetic socket. That's been the job of my prosthetics specialist, George. I'm going to write more about that when I get the new leg.

The other end is the interface with the bicycle pedal. I still want to be able to use the standard clipless pedal system for all the normal advantages that provides (for the non-cyclists amongst you - that's a pedal system whereby a cleat on the bottom of your cycling shoe enables you to "click" into a pedal, which then holds your foot securely onto the pedal, similar to how a ski boot is secured onto a ski).

I also did some research which indicated that riders using prosthetic legs have had two main approaches to using pedals. One has been to use their normal prothetic foot (or a proxy prosthetic foot) and wear a cycling shoe on that. This has some inherent problems, as there no longer exists much of the lower leg required to stablise the foot section and the lever it creates puts non-radial forces through your leg to socket interface. The other approach is simply to place a cleat directly under the end of the pipe section that extends down from the leg socket.

This latter arrangement is effectively like pedalling with a cleat attached under your heel. That means that forces are applied in a line that goes directly down through your prosthetic socket, straight down the pipe section and into the pedal. This is radial alignment is also the strongest arrangement, and since I have no foot on that leg, there are no toes to overlap the front wheel (which is why you could not position a cleat as such with a normal foot).

But hang on - isn't that going to do some weird things with my pedalling action and balance considering my other foot will use the standard shoe/cleat and pedal position?

Well of course it will, but not as bad as one might at first think. The one advantage of a prosthetic leg is that I can make it as long or short as I want. It can be made to extend out at any angle I desire. The cleat can be moved fore/aft, side to side and rotated in its own plane, so there are significant types of adjustment should it be required. Indeed there are also shims that would enable the cleat to be slightly rotated perpendicular to its plane if that were necessary.

OK, so how about shifting that planet's orbit to make it happen?

In steps my good buddy Peter Barnard and all his crafty machining and cycling nouse on how to design and manufacture a solution for me.

The challenge was to fashion an adapter so that a cycling cleat with a standard 3-hole fit like this slightly worn Campag cleat:

could be attached to a standard attachment block used for prosthetic legs:

Here are some pics of the solution. This is the adapter plate Pete fashioned up:

Note the overall shape and how it is designed to be flat on one side for the prosthetic attachment block and curved on the other to match the curve of the cycling cleat. Also the attachment of the prosthetic block requires four screws in a square arrangement and the cycling cleat requires three screws in a triangular arrangement.

The way the design is made is so that two of the prosthetic block screw holes were able to be lined up with two of the cleat attachment holes (which was simply fortuitous), so that enables all three to be securely fitted together with two nuts and bolts and reduces the overall number of screws needed by two.

So, attaching the prosthetic block on one side:

and attaching the cleat to the other side:

and it all looks like this when together. From side on:

and head on:

So that's either end of the planetary orbits being sorted out. Now I need to work on some of the middle planets. There are quite a few, not least of which is the limited flexion of my knee joint at the moment!

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Thursday, May 15, 2008


Today was the day I was going in to to get my new leg. Or so I thought. As it turns out, I hadn't anticipated or realised (or asked or was told) that there was another intermediate step to the process.

This involved testing out a prototype of the leg with a socket made from a transparent plastic of some kind. Clearly it makes a lot of sense to test out the fitting with a prototype and not the finished product, so I feel like a bit of a dill to have not realised this.

It meant that the socket could be fine tuned to suit all of the individual characteristics of my stump/knee/tendons etc. It is very interesting to note how many elements go into getting the socket to be sized and shaped precisely to meet your individual needs, and then for the pipe section and foot attached below to be adjusted so that your gait is strong and as fluid and natural as possible.

My hips have weakened and so I need to do a little strength work on them, but apart from that I seemed to be able to use the new low-cut style socket quite well. I will likely need to use a crutch or walking stick as an aid in the first days/week as I develop the natural strength and motor control skills required.

So while I am disappointed that I don't have my new leg just yet, what I do know from the session today is that the use of the new system (distal cup and liner with the pin and socket arrangement) is significantly more secure and comfortable than what I currently use. This is good news since over the last week I was becoming a little anxious about today's session. My main concern has been whether the discomfort and pain that I have been experiencing over the last two months would go away with the new leg. That would appear to be the case.

It should be about another two weeks for George to complete the new socket. So the saga continues....

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Friday, May 02, 2008

The seven deadly sins

This will be old hat to anyone that's been around the world of training with power meters for some time. However, having monitored the cycle training forums lately, it seems the question about how to estimate a rider's Functional Threshold Power (FTP) is something that comes up quite regularly.

So I thought I'd write a post about it in the hope that it will at least help clarify one or two things for people.

Before I go into the various methods used, it's probably worthwhile quickly revisiting what FTP is and why it is important to know.

FTP is a practical and readily measurable indicator of a rider's aerobic fitness. It was introduced to the world by Dr Andrew Coggan and for all intents and purposes it removes the confusion that exists over the term "threshold" and all of the various terms associated with it.

It is important to know FTP for a number of reasons:

-- threshold power is the single most important physiological determinant of endurance cycling performance (covering events from individual pursuits of 2 km long, up to stage racing lasting several weeks). Hence improving FTP needs to be the primary focus of our training, and measuring FTP on a regular basis is an excellent means of tracking fitness changes through the course of a season.

-- it enables a rider to define and measure intensities of riding (or power levels) relative to their own current level of fitness, expressed in a manner that relates to the primary physiological adaptation that occurs at each intensity (power) level. This is very useful for guiding training and making sure that the mix of intensity and duration during a workout or training cycle is appropriate for gaining the specific fitness required for a rider's target events.

-- it is a key input into other metrics which enable a rider/coach to monitor overall training stresses, both long term training loads and recent fatigue levels.

-- it also provides an excellent guide to how a rider should most effectively pace themselves, especially in races such as time trials (or during a breakaway in a road race or criterium)

Of course you need to have an on-bike power meter or a stationary ergometer that measures power in order to measure or estimate FTP.

FTP is simply defined as follows:

"FTP is the highest power that a rider can maintain in a quasi-steady state without fatiguing for approximately 1 hour.

When power exceeds FTP, fatigue will occur much sooner, whereas power just below FTP can be maintained considerably longer".

Okay, so that's easy. If you want to know your FTP, just go out and ride your bike as hard as you can for an hour and see what the average power was. In essence this is the gold standard measure of a rider's FTP. Unfortunately it is not always possible nor practical for everyone to do a one hour time trial like test. And not all such tests are well paced. A poorly paced effort may result in a lower average power than a well paced effort.

So what are all the alternatives available to us to estimate FTP?

Well, Dr Coggan kindly made a list of these, titled "the seven deadly sins" and posted them to the wattage forum in June 2004. Here is the original post reproduced:

"the seven deadly sins...., ways of determining your
functional threshold power (roughly in order of increasing certainty):

1) from inspection of a ride file.
2) from power distribution profile from multiple rides.

3) from blood lactate measurements (better or worse, depending on how it is done).

4) based on normalized power from a hard ~1 h race.

5) using critical power testing and analysis.

6) from the power that you can routinely generate during long intervals done in training.

7) from the average power during a ~1 h TT (the best predictor of performance is performance itself).

Note the key words "hard", "routinely", and "average" in methods 4, 6 and 7..."

Okay, so #7 is obviously the "Gold Standard". What about the others?

Inspection of Ride File / Power Distribution Profile
#1 and #2 require you to inspect data using power meter data analysis software. The method is described in more detail in the book "Training and Racing with a Power Meter" by Allen and Coggan. In general these two methods are more useful as a means to check whether a rider's FTP may have changed, than for estimating FTP itself. With #2, it is important that the selection of ride files chosen contain efforts such as races or very hard training.

Blood Lactate Measurements
#3, done properly, usually requires you to visit a sports science laboratory or a well set up cycling coach's facility. Even then, interpretation of the blood lactate data may not result in practical information for the rider. If you have a power meter, there really is no need to have a blood lactate test performed.

Normalised Power (from a Hard ~1hr Race)
#4 is pretty handy, particularly as an indicator of when a rider's FTP may have changed. Frequently riders who do not do time trials, but do other races such as shorter road races or criteriums of approximately 1 hour duration, can use this as a crosscheck of their current FTP. Assuming the race was hard (that is, you were pretty much on the limit for most of the race), and you were not overly fatigued beforehand, then the 60 minute maximal Normalised Power should be at least at your FTP if not a little higher (up to about 5% higher). If your 60 minute Normalised Power is reported as more than 5% above your FTP, then that is a strong sign that your FTP needs re-setting (upwards).

Critical Power
#5 is also a very useful means of estimating FTP. It explores the relationship between work performed (kJ) and duration (seconds). Essentially all you need is at least two (or more) maximal efforts of at least three minutes and less than 30 minutes duration, say one of five minutes and another of 20 minutes, although the choice is arbitrary and up to the individual. You then enter the average power and durations ridden into the Critical Power model. The model will calculate what is called "Critical Power", which is essentially equivalent to FTP (or at least a very good estimation of FTP).

A couple of notes: the "test" rides chosen should have been performed within a reasonably close timeframe (say within the same week), and should not be cherry picked from other rides. They need to be stand-alone maximal efforts. It is also preferable to have two very good data points rather than three or more unreliable data points. I recommend reading about it here (this links to a pdf document by Eddie Monnier) and downloading the spreadsheet as well. It also helps to use the same (or very similar) durations for all future Critical Power test inputs.

Interval Training
#6 is great for riders that regularly do hard aerobic interval work, especially indoors. The intervals need to be of sufficient duration, I would say at least two efforts of 20 minutes (with a short break between) at time trial power/pace. When done on an indoor trainer, then it is common for longer maximal effort intervals of 30 to 40 minutes be nearly equivalent to FTP. As training progresses over the weeks and months, then changes in sustainable power during these intervals is a great guide to changes in FTP.

I'd suggest the Seven Deadly Sins also include the following methods:

MAP Testing
5a) by conducting a Maximal Aerobic Power (MAP) test, using the test protocol on Ric Stern's website . FTP typically falls within the range of 72%-77% of MAP.
An example of a MAP test can be viewed here.

Shorter Time Trials
5b) by conducting a time trial effort of sufficient duration (say at least 20-min), with FTP typically falling into a range of percentages for TTs of this duration e.g.:
- FTP = 93% +/- 3% of 20-minute maximal average power
- FTP = 94% +/- 3% of 16km (10-mile) TT avg power
Of course everyone is different and some may fall outside of these ranges.

There really is no reason to nail it down to the nearest watt. Setting FTP to the nearest 5 watts is sufficient. I only change the FTP setting if there is hard evidence of a change of at least 5-10W.

Of course, getting the number right does depend on ensuring that a rider's power meter is correctly calibrated and any zero offsets needed are done. Strange numbers are usually strange for a good reason.

Remember, these are all just ways of estimating FTP and some are better than others at nailing down the number (and for many, some are more practical to perform than others). The final two methods for example, would typically get you to within a few percent either side and can then be cross referenced with another method.

It all depends on a rider's circumstances. Not everyone is in the position to do a ~1 hour time trial with sufficient regularity.

What do I use?
For the purposes of tracking aerobic fitness changes, and the setting of training levels, then performing a Maximal Aerobic Power test, combined with one of the other tests for FTP (usually a 16km or 40km time trial), is the method that I typically use with my coaching clients. Having this combination is particularly useful when assessing the training priorities for an athlete.

Of course, you can always track fitness and base training levels on a mean maximal power for a duration of less than 1 hour (e.g. a 20-minute test, or as has been suggested, 2 x 8-minute test efforts). However, by doing so you start to introduce the influence of anaerobic energy production into the test result, which means you may not be entirely sure which component of your fitness is changing, and hence be uncertain as to what type of training is needed in order to progress further.

So which sin will you choose?

This isn't the end of it of course. There are still a multitude of factors to consider, such as the impact of the following on FTP:
- Environmental effects
- Point of training cycle
- Chronic Training Loads
- Training Stress Balance
- Altitude
- Hills vs Flat terrain
- Different trainer types
- Different bikes and rider positions
- Motivation

I'll save that for another post.....

1. Coggan, A. Ph.D, Allen, H. Training & Racing with a Power Meter, Velopress 2006.
2. Monnier, E.
Using the Critical Power Model to Predict Various Points Along the Power-duration Curve., 2004
Stern, R. What is MAP?,, 1999

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