Raceproof measures fitness in two dimensions: how much power you produce at a physiological threshold, and where on the time axis that threshold fires. This is the vocabulary for understanding what that means.
A Seat is the position of a single bioenergetic threshold in two-dimensional space. One axis is power (watts). The other is offset (duration in seconds). Together, these two coordinates locate where a specific energy system transition currently lives: its power and its duration.
Every athlete has seven seats, one for each of PTA's seven thresholds. Each seat can move independently in either dimension. Power can increase without offset shifting. Offset can extend without power changing. Both can move at once. Each movement is a distinct development signal.
This is the fundamental departure from single-number fitness metrics. A seat is not a watt value. It is a position. When you train, the position moves through that two-dimensional space, and the direction it moves tells you something specific about what kind of adaptation is happening.
Offset is the duration, measured in seconds, at which a bioenergetic transition occurs. It answers the question: how far down the time axis can this energy system sustain its contribution before handing off to the next one?
This is PTA's novel contribution. Conventional power analysis measures the Y-axis: how many watts at various durations. PTA adds the X-axis: the temporal position of the energy system handoff, treated as an independent, trainable variable.
The distinction matters because offset and power can move in opposite directions. An athlete whose VO2max offset extends from 3:00 to 4:00 without the power number changing has made a real physiological adaptation. The aerobic system sustains its contribution for a full minute longer. Under conventional analysis, nothing happened. Under PTA, the seat moved.
Poole et al. (2016) formalized this in the critical power framework: the simplest adequate model of the power-duration relationship requires at minimum two independent parameters (CP and W'). Reducing that relationship to a single number discards information. PTA extends this principle across seven transitions. (PubMed 27031742)
Power is the watts the athlete produces at a given threshold position. This is the dimension the sport already measures comprehensively. FTP, CP, peak power at 5 seconds or 5 minutes: these are all power measurements at specific durations.
PTA does not discard power. It measures power at each of seven thresholds, just as existing tools do. What PTA adds is the second dimension. The power at your VO2max threshold tells you how strong that system is. The offset tells you how long it sustains that strength. Together, they describe the shape of the system, not just its peak output.
In practice, power is the metric athletes and coaches already know how to interpret. "Your PT-V is 480 watts" is immediately meaningful. The offset dimension is newer and requires a different kind of reading. But the two dimensions together produce signals that neither can produce alone.
PTA identifies seven bioenergetic transition points, each a power and a duration. Each transition corresponds to a shift in energy system dominance: from the phosphocreatine sprint system through glycolytic anaerobic metabolism and into the aerobic capacity and threshold systems. The seven thresholds are detection points, not zones. PTA finds where your physiology actually shifts rather than prescribing where it should.
The offset ranges shown above are population means. Every athlete's actual transition points are unique. That individual variation is the signal, not noise. It is what PTA detects.
Seven thresholds, each with an independent power and offset dimension, produce fourteen development signals. The sport has historically tracked one (FTP) or at most two (CP and W'). Gastin (2001) quantified the energy system contributions at each of these duration ranges, establishing the physiological basis for the transitions PTA measures. (PubMed 11547894)
Each seat has two identity peaks: the highest offset this seat has ever recorded, and the highest power this seat has ever recorded. These are not necessarily from the same ride. They represent the ceiling in each dimension independently, drawn from the athlete's full recorded history.
Identity peaks serve as the reference frame for measuring current fitness. They answer: relative to the best this energy system has ever produced in each dimension, where does the current seat sit?
The peaks are self-referenced. There is no population comparison, no external benchmark, no idealized standard. The question is always internal: how fully are you expressing what you have already demonstrated you can do?
This framing is deliberate. An athlete in their first season of racing has a narrow history and low identity peaks. Their Expression scores may be high, because they are close to their own ceiling. A veteran with years of peak performances has a deep reference frame and may score lower, because the ceiling is further away. Both readings are honest. The metric adjusts to the athlete it is measuring.
Expression is the product of two ratios. The first ratio measures how close the seat's current offset is to its all-time peak offset. The second measures how close the current power is to its all-time peak power. Multiplied together, they produce a single percentage that captures how fully the seat is being realized across both dimensions simultaneously.
An Expression of 100% would mean the seat is at its all-time peak in both dimensions at the same time. In practice, this is rare. Power and offset tend to trade off against each other: when power surges, offset often contracts, and when offset expands, power may give back some of its gains. This dynamic is consistent with the Billat finding that intensity and duration at VO2max are inversely correlated.
The value of Expression is that it resolves the ambiguity of one-dimensional progress. A power gain that narrows the offset may lower Expression. An offset expansion that comes with modest power loss may raise it. Expression tells you whether the net movement across both dimensions represents a step toward or away from the fullest version of that energy system.
When an athlete increases their power at a given threshold, the offset at that threshold often contracts. When the offset expands, power sometimes declines. This is not a flaw in the measurement. It reflects the physiology.
Billat and Koralsztein established this relationship at the VO2max transition: athletes with higher vVO2max sustain it for less time. The inverse correlation (r = -0.35) has been replicated across multiple sports and populations. The finding holds within individuals over training cycles, not just between athletes in cross-sectional comparisons.
The mechanism is intuitive. A stronger output at any given threshold depletes its supporting energy system faster. Higher watts at VO2max burn through the finite capacity above critical power more quickly, shortening the sustainable duration. Training that expands the offset typically does so by improving efficiency, buffering capacity, or substrate availability, which may not increase peak watt output at all.
For athletes and coaches, the implication is that power gains and offset gains may require different training stimuli and may not arrive at the same time. A winter of high-intensity intervals might surge power while compressing offset. A spring of sustained threshold work might expand offset while power plateaus. Both are development. Neither is visible in a power-only view.
This oscillation is what the Expression article tracks across a ten-month season: repeating cycles of power-push and offset-absorption that leave the seat wider after each cycle.
The Seat Trends page in the Raceproof app plots each seat's power and offset over time as separate trend lines. This separation is the point. Collapsing power and offset into a single line would hide the dynamics between them: the moments when power surges while offset contracts, the periods when offset expands while power holds steady, the oscillations that produce structural seat expansion over months.
Expression appears as a third line on the same chart, computed from the relationship between the power and offset trends and the seat's identity peaks. When power and offset are both near their peaks, Expression is high. When one dimension retreats while the other advances, Expression resolves the net movement into a single score.
The most revealing view is often two adjacent seats displayed together. Adjacent seats interact: when one seat's Expression rises, the neighboring seat's Expression sometimes gives ground. These lead-follow patterns can reveal adaptation sequences that no single seat's chart would show.
Billat V, Koralsztein JP. Significance of the velocity at VO2max and time to exhaustion at this velocity. Sports Medicine 22(2):90-108, 1996. PubMed
Gastin PB. Energy system interaction and relative contribution during maximal exercise. Sports Medicine 31(10):725-741, 2001. PubMed
Morin JB, Samozino P. Interpreting power-force-velocity profiles for individualized and specific training. International Journal of Sports Physiology and Performance 11:267-272, 2016. PubMed
Poole DC, Burnley M, Vanhatalo A, Rossiter HB, Jones AM. Critical power: an important fatigue threshold in exercise physiology. Medicine & Science in Sports & Exercise 48(11):2320-2334, 2016. PubMed