This post introduces a new way of plotting performance to better visualize W’ and Critical Power. I haven’t seen anyone plot performance this way, so I’m going to take the liberty to name it the Veloclinic plot. In descriptive terms, it would be accurate to call it a W’ envelope plot or a Critical Power Subtraction plot. An example of the new plot is below:
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Note that the y axis has now become Joules or capacity, and the x axis has become Watts or power. The plotted line is the capacity that can be generated from W’ at any given power, ie W’ envelope plot. The W’ capacity is isolated by calculating the capacity that would be generated CP and then subtracting it out, ie CP subtraction plot.
The motivation to develop the VC plot came out of frustration with the visually un-intuitive traditional Power Duration plot.
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In the traditional PD plot. The y axis is power and the x axis is duration. The x axis is typically converted to a log scale so that details of the curve can be better brought out. Even on a semi-log plot however, the only performance measure that is visually clear is Pmax.
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In contrast, on the VC plot, CP is visually obvious as the first x intercept. Just as clear, W’ is the y asymptote and Pmax the x intercept or asymptote.
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Of course, CP can be plotted on the traditional PD curve. The impression though is of a straight line randomly intersecting a slant. There is no obvious visual feedback whether the CP estimate is correct or a curve fitting artifact. Even more problematic is illustrating W’ as neither scale is in capacity units.
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The only way to represent W’ is to plot it with CP in the critical power model form. This representation at times is reasonably effective. However, I am often left wondering if the CP model is correctly centered over the appropriate range of the PD curve or whether it should be shifted a bit to the left or right.
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An advantage of the VC plot is that it is obvious if the CP estimate is wrong. In the plot above, I lowered CP by a small but meaningful 5 percent. Underestimating CP skews the curve left and creates an odd peak above a slanted asymptote.
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The opposite is true of Inflating CP by the same 5%. This changes creates a notable rightward shift of the asymptotic section producing the windblown appearance above.
When the CP estimate is correct, the VC plot simply looks right.
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The fit of hyperbolic shapes on the other hand can be difficult to visually assess. The same small but meaningful differences in parameter estimates make no dramatic changes in the shape. The result is the appearance of a reasonable fit simply shifting to a slightly different region of the curve:
The VC plot can be further refined by using the Ward-Smith equation to generate a smooth x axis:
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Note that the flat region of the y asymptote becomes even more distinct, and the goodness of fit of the WS model is visually confirmed by good alignment of the model and subject data.
An important detail that now also emerges is a concept that I am calling the Super Critical Power.
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Going from left to right, the VC plot starts at 0 at CP and jumps up to the y asymptote at W’. The flat region at the asymptote represents the range of power where W’ can be fully or nearly fully developed. In this region, changes in power result in no significant loss in W’ availability. Following this region, the curve falls exponentially away from the asymptote. The implication is of an upper threshold were subsequent increases in power results in an exponential cost in terms of a loss of total W’. I am terming this threshold, the Super Critical Power. As a working definition, SCP is a power threshold above which less that 95% of W’ can be generated before failure occurs.
Super critical power has important implications in terms of Skiba’s intermittent W’ balance model as well as underlying physiological implications.
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Once possible way that VC plot can be used to integrate some of the more interesting physiology publications is to think of the dominant limiters of each zone. I will start with the W’ zone. In my mind this zone may be best explained by complete peripheral fatigue of fast and intermediate twitch motor units. This zone may result from the sequential fatigue of these motor units in a power range that allows for complete depletion without other limiters becoming dominant prior to failure. Evidence for this mechanism comes from studies that demonstrate the development of the the slow component above CP and attainment of VO2max preceding effort failure. Similarly, once failure occurs in this zone, some studies show that outside stimulation of the muscle can not elicit a response greater than voluntary contraction. In order for this to occur, it follows that all easily fatigueable intermediate and fast twitch motor units must have been fully recruited and depleted. In contrast, fatigue generated by efforts above SCP have been shown to not necessarily produce complete peripheral fatigue. When fatigue is produced near maximal efforts, external stimulation can potentially generate muscle contraction greater than voluntary contraction. These findings suggest that a central fatigue occurs resulting in sub-maximal motor unit recruitment. Similarly, I anticipate that SCP should correspond to a threshold power above which failure occurs before complete development of the slow component and therefore VO2max can not be reached. Lastly, below CP studies show that the slow component does not fully develop and VO2max is not reached. VO2 kinetics are paralleled by findings of variability in sequential motor unit recruitment and lack of complete recruitment. Failure in this zone is potentially a multi-factorial systemic mechanism or may dominated by a specific factor during severe conditions such as fuel depletion, temperature dis-regulation, or as a protective central feedback mechanism.
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