A training regimen that includes both strength and endurance workouts (not always on the same day) is called concurrent training. Unfortunately, endurance workouts can reduce the gains (in strength, power, and muscle size) from strength workouts. This “interference effect”, first described in a landmark 1980 study, can be explained by several mechanisms:
Reductions in motor-neuron recruitment and motor-unit discharge (a motor unit being the combination of a motor neuron and all the muscle fibers it innervates)
Acute fatigue, if a strength workout is performed soon after an endurance workout
A reduction in energy status (due to fasting or exercising, for instance) leads to reductions in AMP:ATP and ADP:ATP ratios. Sensing this, AMPK restores energy balance by inhibiting ATP-consuming pathways. One such pathway is mTOR, an intracellular signaling protein that plays a critical role in stimulating muscle protein synthesis (MPS). In short, endurance exercise leads to an increase in AMPK activation, which leads to a decrease in mTOR activation, which leads to a decrease in MPS.
This was a meta-analysis of 43 controlled trials that compared strength training (ST) with concurrent training (CT; same strength training + endurance training) for ≥4 weeks.
Three outcomes were assessed:
Maximal strength (37 trials, 967 healthy adults) was assessed via squats, bilateral leg press, bilateral knee extension, or unilateral knee extension.
Explosive strength (18 trials, 478 healthy adults) was assessed via jump height, squat-jump power, leg-press power at 50% of 1RM, or isometric rate of force development.
Muscle size (15 trials, 389 healthy adults) was assessed via segmental DXA scan of the lower extremities, thickness of the vastus lateralis muscle, or cross-sectional area of the quadriceps femoris muscle.
Six potential moderators were assessed in subgroup analyses:
Age (18–40 vs. >40 years old)
Training status (active vs. untrained)
Type of endurance workout (cycling vs. running)
Concurrent-training frequency (4.1 vs. 6.1 weekly sessions)
Concurrent training: timing of the strength and endurance workouts (different days vs. same day vs. same session)
Concurrent training: order of the strength and endurance workouts (when performed in the same session)
Increases in explosive strength were smaller in the CT group.
This effect was statistically significant only when the strength and endurance workouts took place less than 3 hours apart. (There weren’t enough trials to assess the effect of workout order.)
This effect was statistically significant for cycling but not running; however, when the authors excluded a potential outlier study, there were no differences between the cycling and running trials.
This effect was statistically significant for low but not high training frequencies; however, when the authors removed the same outlier study, there were no differences based on training frequency.
Increases in maximal strength and increases in muscle size didn’t differ between the two groups.
The big picture
At least two other meta-analyses have assessed the effects of concurrent training on strength and power adaptations.
In a 2012 meta-analysis of 21 controlled trials, strength-only training led to a greater increase in lower-body power and nonsignificantly greater increases in lower-body strength and muscle size. Concurrent training and endurance-only training both increased VO2max.
A 2021 meta-analysis of 27 controlled trials compared people who were trained (i.e., who’d participated in a structured training program for ≥3 months before the intervention), moderately trained (i.e., who were physically active but hadn’t participated in a structured training program for ≥3 months before the intervention), and sedentary.
In trained individuals, strength-only training led to greater 1RM increases for squat and leg press.
In moderately trained individuals, strength-only training led to nonsignificantly greater 1RM increases for squat and leg press.
In sedentary individuals, strength-only training and concurrent training had similar benefits.
When the authors stratified the results, they found that concurrent training attenuated 1RM increases only when the strength workout and the endurance workout were performed during the same session.
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