169. TRAINING RESPONSIVENESS CHANGE OVER TIME EVEN WITHIN THE SAME ATHLETE!
Predicting performance a priori based on a training plan could be the holy grail of endurance sports. In theory, you would simply simulate a training program based on volume, intensity, and training load variables, and then know exactly how to reach peak performance at the desired time point. This is essentially what many fitness-fatigue models (FFMs), available in several commercial training software platforms, attempt to do. Unfortunately, despite marketing claims suggesting otherwise, none of these models has been convincingly shown to accurately predict future performance. (1)
Why?
One major reason is that training responsiveness differs substantially between individuals (interindividual variability). This variability is influenced by factors such as genetics and initial fitness level. In this scenario:
the seminal HERITAGE study showed that genetics can explain around 50% of the variability in VO₂max adaptations to the same training program. (2)
A recent large review including 50 years of research (~500 studies and ~6,000 participants) reported that the higher the initial fitness level, the smaller the gains in VO₂max, skeletal muscle mitochondrial content, and muscle capillarization following training. (3)
However, if training responsiveness could be fully characterized by genetics and baseline fitness alone, fitness-fatigue models could still work by constructing individualized mathematical models. Yet, even individual fitness-fatigue models are unable to accurately predict future performance.
Why?
Another aspect that has received increasing attention is the possible existence of intraindividual variability in training response. In other words, the same individual may respond differently to the exact same training plan at different points in time.
Is responsiveness to a given endurance training plan within the same individual repeatable—and so predictable?
A hot-off-the-press study published in June 2026 by Odden and Colleagues (University of Inland Norway, Lillehammer) in the Journal of Applied Physiology tried to answer this question. (4)
WHAT DID THEY DO?
42 middle-aged, untrained men and women were recruited (Age 54, VO2max 32)
They completed two similar eight-week endurance training period (TP1 and TP2), separated by an eight-week detraining period.
The training plan consisted of 3 times per week interval sessions alternating the following 3 different sessions:
4x5 min at maximal sustainable power with 2.5 min recovery in between
6x6 min at low zone 3 in a 5-7 zones model with 1 recovery in between
4x8 doing the 30-15 short high intensity intermittent intervals with 2min recovery between series
Importantly, all training sessions were supervised in the laboratory and adjusted weekly based on both changes in power output achieved during the 4×5-min maximal sessions and the target RPE (rating of perceived exertion).
This study design permitted to assess the true variability in an individual's response to the same training stimulus when applied at different time points, while starting from a comparable baseline level.
Similar initial fitness level: achieved through the 8-week period of rest between the two training interventions.
Same training load and stimulus across both training periods: achieved through strict laboratory supervision, with weekly adjustments of training power output and using target RPE (rating of perceived exertion).
