CMJ metrics to monitor fatigue: height, RSI or contact time

Updated on 26 de June de 2026 by Adrián Escobar Morales

To monitor neuromuscular fatigue with the CMJ, jump height is the most practical and reliable variable on a day-to-day basis, while the RSI is the most sensitive for detecting acute changes in the function of the stretch-shortening cycle. Contact time and flight time provide complementary information that allows us to distinguish whether the origin of fatigue is neuromuscular or mechanical. Using just one variable is insufficient — combining two or three metrics offers a much more complete picture of the athlete’s true state.

What science says — executive summary

• The jump height It has an intraday CV of 5.3-5.6% and is the most used variable for routine monitoring.
• He Modified RSI is more sensitive to acute changes in fatigue than height alone — detects changes that height does not
• He contact time increases with fatigue before altitude drops — an early indicator
• He flight time is directly related to height and is more stable than contact time
• A fall of 3-5% in jump height with respect to the individual reference is the practical threshold to detect significant fatigue

The CMJ (countermovement jump) is the most used neuromuscular monitoring tool in high-performance sports. Fast, non-invasive and highly reproducible, it allows the athlete’s condition to be assessed before or after a session without adding additional load. But the CMJ doesn’t give a single number — it gives several simultaneous metrics, each with different information.

The question that every preparer ends up asking is this: Which one do I focus on? Is the jump height enough? Do I need the RSI? What does contact time tell me that height doesn’t? Scientific evidence has concrete answers to each of these questions.

Why not all CMJ metrics are the same for detecting fatigue

Each CMJ variable reflects a different aspect of neuromuscular function. Understanding what each one measures is the first step to correctly interpreting the athlete’s data:

• jump height: reflects the total capacity to produce power in the lower body. It is the end result of movement — the integration of everything that occurs in the eccentric and concentric phases.
• Flight time: mathematically related to jump height. It measures the time the athlete remains in the air, which directly reflects the force applied at takeoff.
• Contact time: In reactive jumps, measure how long the foot is in contact with the ground between repetitions. It reflects the rigidity of the muscle-tendon system and the efficiency of the eccentric-concentric transition.
• RSI (Reactive Strength Index): quotient between jump height and contact time. It integrates the two previous variables into a single index that reflects the efficiency of the stretch-shortening cycle (SSC).

Fatigue does not affect all of these variables in the same way or at the same time. That is precisely what makes them complementary.

Jump height: the most practical variable for routine monitoring

CMJ jump height has an intraday coefficient of variation (CV) of 5.3% according to Gathercole et al., and 5.6% according to more recent studies — which makes it a sufficiently reliable variable to detect real changes with a standardized protocol of 3-5 jumps.

Its main advantage is the ease of interpretation: Any coach immediately understands that if an athlete jumps 5 cm less than his reference, something has changed. That makes it the ideal metric for routine monitoring, especially when time is limited or the trainer is working with large groups.

Its limitation is that It is not very sensitive to small changes or in early phases of fatigue. Jump height can remain stable even when the athlete is already accumulating fatigue — simply because it compensates with a different movement strategy: more countermovement time, more knee flexion, different timing between phases. That’s where the other metrics come in.

Practical fatigue threshold with jump height

The most used reference in practice is a drop in 3-5% compared to the individual reference average of the athlete. Below that threshold, the variation may be measurement noise. Above, it is likely to reflect a real change in neuromuscular status.

Flight time: the mirror of height

Flight time and jump height are mathematically related (h = g × t²/8), so they provide very similar information. Its advantage is that some devices — such as ADR Jumping — calculate it directly from the infrared cells, without the need for a force platform.

For fatigue monitoring, flight time follows the same trend as height: it drops when fatigue is significant, is stable under normal conditions and responds proportionally to the accumulated load. It is a useful variable to confirm what the height indicates and to have a second reference without additional measurement cost.

Contact time: the early indicator of fatigue

This is the most undervalued variable of the CMJ and possibly the most interesting for early detection of fatigue. The reason is a well-documented physiological mechanism: When the neuromuscular system fatigues, muscle-tendon stiffness drops and contact time increases — before the jump height begins to decrease.

In other words: contact time is a leading indicator. If you see an athlete’s contact time consistently rising relative to their baseline, fatigue is likely already present even if the height is still maintained. Acting at this time — reducing load or adding recovery — can prevent a subsequent drop in performance.

Its limitation is that it is more variable than height and more sensitive to changes in technique. That is why it is always advisable to interpret it in context, especially if the athlete has recently changed the testing protocol or has injuries that affect the movement pattern.

RSI: the most complete metric to evaluate CEA

The RSI (height / contact time) integrates the two previous variables into a single index that reflects the efficiency of the stretch-shortening cycle (SSC) — the ability to absorb energy in the eccentric phase and release it quickly in the concentric phase.

The evidence shows that the modified RSI is more sensitive to acute changes in fatigue than height alone. This occurs because when the athlete fatigues, he or she tends to increase contact time to compensate for the loss of strength — causing RSI to drop even when height remains relatively stable. The RSI captures that deterioration that height overlooks.

A study by Gathercole et al. (2015) in the International Journal of Sports Physiology and Performance showed that CMJ variables related to the eccentric phase — including RSI — were more sensitive to fatigue induced by intense training than jump height alone, with larger effect sizes and earlier detection of changes.

For fatigue monitoring with RSI, the recommended protocol is the repeated drop jump (5-10 consecutive jumps) instead of the standard CMJ, because it maximizes the demand on the CEA and makes the RSI more informative. With the standard CMJ the RSI is also useful but less discriminating.

How to combine metrics in practice

The evidence does not point to a single winning metric — it points to using combinations depending on the monitoring objective:

Aim	Primary Metric	Supplementary Metric
Daily routine monitoring	jump height	Flight time
Early detection of fatigue	Contact time	RSI
CEA evaluation and plyometrics	RSI	Contact time
Comparison between athletes or sessions	jump height	RSI
Post-match or post-competition evaluation	Jump height + RSI	Contact time

The most efficient protocol: 3 hops, 2 metrics

For most practical contexts — especially time-limited team sports — the most efficient protocol is:

1. 3-5 standard CMJ with hands on hips, maximum intention in each jump
2. Register the average jump height of the 3 best attempts
3. Register the average contact time if the device provides it
4. Compare both with the athlete’s individual reference (average of the last 4-6 weeks)
5. If the height drops more than 3-5% OR the contact time increases more than 5-8%: sign of fatigue

With this protocol you have enough information to make loading decisions in less than 3 minutes per athlete — without the need for a force platform or complex analysis.

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Conclusion

There is no single CMJ metric that is sufficient to comprehensively monitor fatigue. Jump height is the most practical and reliable for routine use, but contact time detects fatigue earlier and RSI is the most sensitive indicator of CEA function.

The smartest strategy is to start with jump height as the primary reference — for its simplicity and ease of interpretation — and incorporate contact time and RSI when the goal is to detect fatigue early or evaluate the quality of CEA in plyometric or high-impact training contexts.

Frequently asked questions

Which CMJ variable is best for detecting fatigue?

It depends on the type of fatigue and the moment of detection. For routine monitoring, the jump height is sufficient with a drop threshold of 3-5% with respect to the individual reference. For early fatigue detection, contact time is more sensitive because it increases before height drops. The RSI is the most complete indicator because it combines height and contact time into a single value that reflects the efficiency of the stretch-shortening cycle.

How much should the height of the CMJ fall to be considered fatigue?

The most used reference in practice is a drop of 3-5% with respect to the athlete’s individual reference average. Below that threshold the variation may be measurement noise. Above, it is likely to reflect a real change in neuromuscular status that warrants adjusting training load.

What is RSI and how is it used to monitor fatigue?

The RSI (Reactive Strength Index) is the ratio between jump height and contact time. It reflects the efficiency of the stretch-shortening cycle — the ability to absorb energy in the eccentric phase and release it quickly in the concentric phase. For fatigue monitoring, RSI is more sensitive than height alone because it captures the increase in contact time that occurs as the athlete fatigues, even when height remains relatively stable.

How many jumps are necessary to monitor fatigue with the CMJ?

With 3-5 standardized CMJ is enough to obtain reliable data. It is recommended to use the average of the 3 best attempts to reduce variability. The entire protocol — including comparison to the individual reference — can be completed in less than 3 minutes per athlete.

What device to use to measure CMJ metrics?

ADR Jumping measures jump height, flight time, contact time and RSI in each jump in real time. It works with the free ADR System app, requires no force platform, and stores each athlete’s complete history in the cloud to compare sessions over time.

Literature

1. Gathercole, R.J. et al. (2015). Countermovement jump performance with increased training loads in elite male rugby league players. International Journal of Sports Physiology and Performance, 10(6), 703–710. See study →
2. Watkins, C.M. et al. (2017). Determination of vertical jump as a measure of neuromuscular readiness and fatigue. Journal of Strength and Conditioning Research, 31(12), 3305–3310. See in PubMed →
3. Claudino, J.G. et al. (2017). Countermovement jump as a means of monitoring neuromuscular status in elite soccer players. International Journal of Sports Physiology and Performance, 12(S2), S2–34. See study →
4. Lonergan, B. et al. (2025). Inter-day reliability of countermovement jump metrics in elite academy soccer players. International Journal of Strength and Conditioning. See study →
5. Flanagan, E.P. & Comyns, T.M. (2008). The use of contact time and the reactive strength index to optimize fast stretch-shortening cycle training. Strength and Conditioning Journal, 30(5), 32–38. See study →