The Tactical Athlete's Guide to Balancing Strength and Endurance

The ongoing debate about the optimal sequencing of strength and endurance training has roots in the “interference effect,” where concurrent training seems to compromise muscle-building when endurance and resistance exercise are combined. Research suggests that doing endurance work after strength training is generally favored for those aiming for weight loss or body recomposition. However, a closer examination of the science behind the interference effect and how energy systems are utilized reveals more nuances, and many people wonder why endurance training is not encouraged before strength training.

The Interference Effect and AMPK Activation

The "interference effect" refers to the conflicting molecular signals triggered by endurance and resistance training. Endurance exercise primarily activates AMP-activated protein kinase (AMPK), which is responsible for improving endurance adaptations like mitochondrial biogenesis and fat oxidation. Resistance training, on the other hand, activates mTOR (mammalian target of rapamycin), a pathway critical for muscle hypertrophy and strength gains. AMPK and mTOR are in some ways antagonistic—when AMPK is elevated, it can inhibit the muscle-building signal of mTOR, reducing the potential for muscle growth.

This molecular tug-of-war is why some researchers argue against combining high-intensity endurance training and strength training in the same session, as AMPK activation can hinder the muscle-building signals activated by strength training. It’s not that doing cardio after lifting weights is inherently counterproductive, but the sequence can affect the adaptations you seek.

Why Endurance After Strength Training Is Preferred

The primary reason for recommending endurance work after strength training, particularly for those aiming for body recomposition or weight loss, lies in energy availability and the effectiveness of the strength session itself.

1. Energy Systems and Glycogen Use: Strength training relies heavily on glycogen, the stored form of glucose, as the primary fuel source. Glycogen is depleted during high-intensity, short-duration activities like weight lifting. By starting your workout with strength training, you're ensuring that glycogen levels are higher, enabling you to lift heavier, recruit more muscle fibers, and drive the anabolic response needed for muscle growth and maintenance. If you perform endurance work first, glycogen stores are depleted early, leading to reduced performance in strength training. Studies have shown that resistance training, when performed in a glycogen-depleted state, can reduce the effectiveness of hypertrophy and strength gains due to early fatigue and reduced muscle fiber recruitment.
2. Optimizing Fat Oxidation: Once strength training has utilized a good portion of your glycogen stores, doing low-intensity endurance work afterward taps into different energy systems—primarily fat oxidation. With depleted glycogen, the body shifts toward using fat for energy. This is where endurance training can contribute significantly to fat loss and body recomposition goals.
3. Insulin Sensitivity and Metabolic Benefits: Performing cardio after strength training can also optimize insulin sensitivity, further aiding in glucose disposal and the body's ability to store nutrients more effectively. The “afterburn effect” from the combined efforts of both types of training helps sustain elevated calorie burn for hours post-exercise, particularly useful for weight loss goals.

Why Endurance Training Before Strength Training Isn't Ideal

Although AMPK activation from endurance training does interfere with the mTOR pathway, there are additional reasons why performing cardio before lifting isn’t typically encouraged.

1. Reduced Performance in Strength Training: Performing endurance exercise before resistance training induces early fatigue. Since strength work requires a significant amount of neuromuscular coordination and the ability to lift heavy loads, starting your workout in a fatigued state compromises your ability to perform high-quality resistance training. This can diminish your ability to recruit the highest-threshold motor units necessary for muscle growth and strength gains. Evidence from various studies, including those conducted on young male athletes, show that when endurance work is performed first, the power output and strength capacity during subsequent resistance exercises are significantly reduced. This can limit the hypertrophic and strength benefits that would otherwise be maximized by starting with resistance work.
2. Suboptimal Hormonal Environment: Strength training causes a spike in anabolic hormones like testosterone and growth hormone, both of which are crucial for muscle repair and growth. Endurance training, particularly long-duration aerobic exercise, can suppress testosterone and increase cortisol levels, a catabolic hormone that breaks down tissue. Performing endurance work first creates a less-than-ideal hormonal environment for muscle growth, as the cortisol increase can be prolonged, and testosterone response blunted.

AMPK-MTOR Crosstalk: Does It Really Matter?

While it’s true that AMPK activation through endurance work can inhibit mTOR, the magnitude of this interference effect depends on the intensity and type of endurance training, as well as individual factors like training experience. Low-intensity, steady-state cardio, for example, doesn’t activate AMPK to the same degree as high-intensity interval training (HIIT). This is why low-intensity cardio performed after strength training is less likely to interfere with muscle-building processes, especially if the primary goal is to increase energy expenditure and promote fat loss.

The Role of Training Goals

The sequence of endurance and strength training should also be tailored to the individual’s goals. For endurance athletes, for example, prioritizing endurance training before strength work may make sense, as their primary goal is to improve endurance performance, not necessarily maximize muscle mass. However, for those focusing on hypertrophy, strength gains, or body recomposition, the current consensus leans toward doing resistance training first, followed by low-intensity endurance work.

Concurrent Training: Striking a Balance

Studies show that combining both types of training in the same session can still lead to significant improvements in both strength and endurance, as long as the sequence is carefully managed(Influence of strength e…). Concurrent training can offer benefits for overall health and fitness, but managing the interference effect by manipulating intensity, duration, and sequencing is key.

A potential compromise involves using a periodized approach, where periods of focused strength work are alternated with periods of endurance work. This allows the body to adapt to each stimulus without causing excessive fatigue or compromising the adaptations of one system over the other.

Conclusion

The prevailing recommendation to do endurance training after strength training, especially for weight loss or body recomposition, is rooted in optimizing both strength performance and metabolic outcomes. Endurance work performed afterward taps into fat oxidation and maximizes energy expenditure, while strength training first ensures that glycogen stores are available to fuel heavier, more effective resistance work. Although AMPK activation from cardio may interfere with the muscle-building signals of mTOR, the order of exercises should primarily reflect the goals of the individual and the type of endurance work being performed. Ultimately, the sequence matters when you want to achieve optimal results in strength, hypertrophy, and body recomposition.

References

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