Build-up is how well force keeps building after the first burst. Its technical name is RTD Late (RFD Late when the trial reads in force), the rate of force development across the 100 to 200 ms window after onset, and the dashboard shows that name beneath the plain one. The platform computes it the same way it computes RTD Early: a two-point slope with both endpoints linearly interpolated to land exactly on the target times, no curve fit, no goodness-of-fit statistic. The math is straightforward. The clinical reason RTD Late exists alongside RTD Early is that the two windows reflect different physiology, and reading them together unpacks a contraction in ways neither one can do alone.
If you haven't already, what Explosiveness (RTD Early) actually means covers the algorithm and the sample-rate sensitivity that applies equally here. This article focuses on what makes the late window distinct.
The definition
RTD Late is the slope between the interpolated force at onset + 100 ms and the interpolated force at onset + 200 ms. Units are Nm/s when torque conversion is available, N/s otherwise.
RTD Late = (force at 200 ms − force at 100 ms) / 0.100 s
On a dedicated RTD or ballistic trial the same tile holds a different slope, labelled RTD 20–80%: the rise between 20% and 80% of the peak, timed where the rising curve crosses each level. A fast effort can be at peak before 100 ms, so a fixed 100 to 200 ms window would measure the top of the curve rather than the rise. The 20 to 80% slope measures the rise where it actually happens, and the tile still reads as Build-up.
The 100 ms window is a third longer than the 75 ms early window, so it holds more samples: 4 at 40 Hz where the early window holds 3, and 8 at 80 Hz where the early window holds 6. The reliability badge does not reward that. Both windows are measured from the same onset, and onset can only be placed to within one sample interval, so the late tile carries the same tier as the early one: high from 100 Hz, moderate from 60 Hz, low below.
Why the late window reflects different physiology
The neuromuscular system that produces a maximum-effort contraction is layered. In the first 50 to 75 ms, motor unit recruitment speed and the maximal discharge rate of the motor neuron pool dominate the force trajectory; the muscle's own contractile machinery has barely begun to catch up. By 100 to 200 ms, the picture has inverted. Motor unit discharge rates have plateaued at similar levels across individuals, and what differentiates patients' force production is the muscle's intrinsic contractile properties.
The published evidence on this is concrete. Decomposing burst-like explosive contractions of the knee extensors:
- Force at 50 ms is explained almost entirely by EMG amplitude in the first 50 ms (R² = 0.36, neural drive).
- Force at 100 ms is explained primarily by the electrically evoked octet response (R² = 0.65, pure contractile).
- Force at 150 ms is explained primarily by maximum voluntary force itself (R² = 0.71), with a small additional octet contribution (R² = 0.06).
Source: D'Emanuele et al. 2023. The early-late distinction in RFD windows is not a stylistic convention; it's a clean dissociation of underlying mechanisms, and the platform surfaces both because the published evidence dissociates both.
Reading early and late together
Three patterns recur enough to be worth naming:
Concordant high. Both RTD Early and RTD Late are at or near the patient's healthy baseline (or the uninvolved side). The neural and contractile components are tracking together. This is what recovery looks like.
Concordant low. Both windows are suppressed relative to baseline or the uninvolved side. The deficit is global; the patient is producing force slowly in both phases. Often early in rehab, often when peak strength is also down. Treatment focuses on building capacity across the board.
Discordant. One window is recovered, the other is not. The two flavors:
- RTD Early recovered, RTD Late suppressed. The patient can fire fast, but the contractile machinery is not keeping pace. For context, after ACL reconstruction both windows are suppressed against the uninvolved limb on average, the early window slightly more: standardized mean differences of −1.07 for early-phase knee extensor RFD and −0.85 for late-phase (Turpeinen et al. 2020). That meta-analysis compares limbs at one point in time, so it does not say which window recovers first; that has to come from following your own patient.
- RTD Early suppressed, RTD Late recovered. Less common. Suggests the neural drive phase is the limiting factor while the muscle's intrinsic contractile properties are intact. Sometimes seen with arthrogenic inhibition or persistent quadriceps activation deficits.
The six-spoke radar in the expanded view of the LSI panel makes these patterns visible at a glance: the Explosiveness and Build-up spokes sit next to each other, and discordance between them shows up as a step in the radar's outline. The simplified view combines the two windows into one Speed reading and names the lower of the two beneath it, so switch the patient to expanded when the split between them is the question.
The corticomotor connection
There's also a neural-imaging thread that aligns with the early-late mechanistic split. In individuals after ACL surgery, the active motor threshold (a transcranial-magnetic-stimulation measure of corticospinal excitability) correlates with RTD across 0 to 50 ms (r = −0.51), while short-interval intracortical inhibition correlates with RTD across 100 to 200 ms (r = 0.50) (Scheurer et al. 2020). The two windows have distinct corticomotor correlates: corticospinal pathways predict the early window, intracortical inhibition predicts the late window. The platform doesn't measure either, but it's a useful frame for understanding why the early-late dissociation reads as more than a measurement convention.
When the relationship matters most
Three clinical situations where the early-late pair is more informative than either alone:
- Return-to-sport gating. A patient at 95% Strength LSI and 70% Explosiveness LSI is a different clinical picture than a patient at 95% Strength LSI and 70% Build-up LSI. The first has a neural drive deficit; the second has a contractile deficit. Both need RFD work, but the dosing and modality should differ. The LSI article covers how the platform surfaces the discordance.
- Tracking adaptations to a training block. Early adaptations to explosive strength training often show up first in RTD Late: in the training data Del Vecchio reviews, force in the first 50 ms did not change while later force did, because the training had not changed how quickly motor units were recruited (Del Vecchio 2022). Watching both windows over a block of training lets you see whether the patient is adapting in the right place.
- Disuse recovery. Periods of unloading or disuse suppress RTD disproportionately faster than maximum strength (Ruggiero & Gruber 2024), and the late window often suppresses more than the early window over short unloading periods. Tracking both during the return-to-load phase makes the recovery trajectory legible.
What to do next
- Read Build-up alongside Explosiveness on every session, not in isolation. The relationship is the diagnostic.
- For low-rate devices, watch the reliability badge on both tiles. The 100 ms window holds a sample more than the 75 ms window at 40 Hz (four against three), but both windows start from the same onset, so the two tiles always carry the same tier.
- A green Strength reading with discordant Explosiveness and Build-up is a real pattern. Surface it in the chart note; don't let the headline strength number obscure it.
References
- D'Emanuele S, Tarperi C, Rainoldi A, et al. Neural and contractile determinants of burst-like explosive isometric contractions of the knee extensors. Scand J Med Sci Sports. 2023;33(2):127-135. doi:10.1111/sms.14244
- Del Vecchio A. Neuromechanics of the rate of force development. Exerc Sport Sci Rev. 2022. doi:10.1249/JES.0000000000000306
- Turpeinen JT, Freitas TT, Rubio-Arias JÁ, et al. Contractile rate of force development after anterior cruciate ligament reconstruction, a comprehensive review and meta-analysis. Scand J Med Sci Sports. 2020;30(9):1572-1585. doi:10.1111/sms.13733
- Scheurer SA, Sherman DA, Glaviano NR, et al. Corticomotor function is associated with quadriceps rate of torque development in individuals with ACL surgery. Exp Brain Res. 2020;238(2):283-294. doi:10.1007/s00221-019-05713-w
- Ruggiero L, Gruber M. Neuromuscular mechanisms for the fast decline in rate of force development with muscle disuse, a narrative review. J Physiol. 2024. doi:10.1113/JP285667