How Quickly Do Athletes Adapt to Training?

By Jim Rutberg,
CTS Pro Coach, co-author of “Ride Inside“,
“The Time-Crunched Cyclist”, and
“Training Essentials for Ultrarunning”

When it comes to training, time can be your nemesis or your greatest ally. Goal-oriented people want to see results quickly and they spend billions of dollars on supplements and get-fit-quick or lose-weight-quick schemes. But meaningful and measurable physiological adaptations to training require diligence and patience. Total training time, in terms of monthly and yearly training hours as well as years of training history, is the most powerful factor for improving athletic performance. Along the way, however, fitness improves incrementally through your daily and weekly training behaviors, supported or hindered by your decisions on recovery, nutrition, sleep, and stress management. So, how long is this going to take and what gains can you expect to see over days, weeks, and years?

Timeline for adaptations

When athletes are new to endurance training or returning to training after a long hiatus, increased physical activity stimulates a predictable cascade of physiological adaptations. The timeline below assumes athletes consume adequate calories to support their training, and that they follow a routine that incorporates adequate rest between exercise bouts.

First Month (1-30 days): Enhanced cardiovascular efficiency and improved blood flow. Plasma volume increases, sweat response accelerates, neuromuscular pathways adapt to recruit muscle fibers more efficiently. Beginnings of capillarization to improve perfusion of oxygenated blood to working muscles.

Second Month (31-60 days): Increased mitochondrial development in terms of size, number, and efficiency. The cardiovascular system adapted to deliver more oxygenated blood to working muscles, now the muscles are adapting to liberate energy from fat and carbohydrate more efficiently. Fat oxidation rates increase dramatically, because exercise intensity to this point has pushed the body to disproportionately rely on carbohydrate. If training stress stimulated an increase in red blood cell production (e.g. altitude and/or heat exposure), athletes may see an increased hematocrit within this timeframe.

Two to Four Months (8-12 weeks): Maximum capacities start to increase. Athletes see meaningful increases in lactate threshold and VO2 max, both in terms of absolute values and how long these intensities can be sustained. Carbohydrate replenishment becomes more efficient, leading to increased glycogen storage between exercise bouts.

Long term (four months to years): Adaptations to cardiovascular and neuromuscular systems continue, along with mitochondrial development and more efficient fuel utilization. Structural changes, however, only happen over many months and years. These include adaptations to muscle fiber type (e.g, some fast twitch muscle fibers adapt to function more like slow twitch fibers) and remodeling of connective tissues (e.g., tendons and ligaments) and bones in response to stress.

Time Course of Adaptation for Experienced Athletes

How do endurance athletes who have been training for 10 or 20 years adapt to training? For experienced athletes, many of the essential adaptations to endurance training occurred long ago. Although none of these adaptations (e.g. capillarization, mitochondrial density, plasma volume) are permanent or irreversible, athletes who train consistently experience diminishing returns. In short, some aspects of physiology have adapted as much – or nearly as much – as they can.

So, what does the time course of adaptation mean for experienced athletes? What can you still change in the short term to improve particular aspects of performance?

What can change in a matter of days?

If you have built a deep foundation of aerobic conditioning over the past 20 years, it’s going to take weeks or months to meaningfully change your aerobic fitness. But there are factors for performance that you can alter within days:

• Plasma Volume: The fluid component of blood, plasma volume responds very quickly to exercise and changes in elevation and temperature. Plasma volume can increase within hours and is one of the first and fastest responses to exposure to high altitude and/or hot environments. It can also change rapidly from dehydration/fluid replenishment because plasma volume is your body’s primary reservoir for sweat production.
• Glycogen Stores: Experienced athletes have increased capacity for glycogen storage compared to untrained athletes, as well as the opportunity to take advantage of glycogen supercompensation. When you partially or fully deplete muscle glycogen stores through exercise, acute replenishment can increase muscle glycogen storage to a higher level than before exercise.
• Sleep: Sleep deprivation and/or disturbed sleep hurts endurance performance. However, sleep extension – adding an hour to your normal nightly sleep duration – over a period of a few days or a week can improve readiness to perform, alertness, and cognitive function.

What can change in a matter of weeks?

Many experienced cyclists have great aerobic foundations. Even if you are not feeling as fit as you once were, those years of training have given you the tools to rapidly improve aspects of fitness:

• Power for high intensity efforts: When you have a strong aerobic base, your body can adapt quickly to training efforts above lactate threshold or FTP. Incorporating 1-2 sprint workouts or VO2 max workouts (i.e., 4-6 intervals, 4 minutes each, RPE of 9-10/10, separated by 4 minutes easy spinning) can be enough to see meaningful improvements in power and repeatability for short, high-intensity efforts.
• Fractional Utilization of VO2 max: Fractional utilization of VO2 max is your FTP as a percentage of power at VO2 max. It’s how much of your maximum aerobic capacity you can use at your maximum sustainable intensity. FTP is more responsive to training than VO2 max, so within 3-6 weeks FTP workouts (i.e., 3-4 intervals, 10-20 minutes each, RPE of 7-8/10, power at 91-105% of FTP, separated by 5-10 minutes easy spinning) may increase fractional utilization while VO2 max is a bit slower to increase itself.
• Red Blood Cell Count: Endurance training increases production of red blood cells, and this increase can be even greater with altitude training or heat exposure. It takes weeks, however, for the cascade of signals and adaptations to measurably increase red blood cell count.