Concurrent training
/ kənˈkʌrənt ˈtreɪnɪŋ / · noun
Also: combined training, mixed-mode training, hybrid training
The simultaneous training of two or more physical capacities (typically endurance and strength) within the same training program. The term entered exercise science literature with Hickson's 1980 study showing that adding endurance work to a strength program reduced strength gains. The interaction came to be called the "interference effect."
The interference effect
Hickson's original finding: 10 weeks of strength training alone produced a 44% increase in 1RM squat. The same protocol with added endurance training produced only a 22% increase. Endurance training appeared to "interfere" with strength adaptation.
Subsequent research identified mechanisms:
- AMPK vs mTOR signaling: endurance training activates AMPK (an energy-sensor that promotes mitochondrial biogenesis); strength training activates mTOR (which promotes muscle protein synthesis). The two pathways inhibit each other when activated simultaneously.
- Glycogen depletion: aerobic work depletes muscle glycogen, blunting subsequent strength training quality.
- Acute fatigue: doing endurance immediately before strength reduces force output and stimulus quality.
- Hormonal: chronic high-volume endurance work elevates cortisol and reduces testosterone, biasing the recovery environment toward catabolism.
Modern view: interference is real but small
The 2022 Schumann meta-analysis (43 studies, 1,090 participants) found:
- Strength gains were 8-12% lower under concurrent training vs strength-only
- Hypertrophy was reduced by ~5%: a smaller effect than originally thought
- Power output (e.g., jump squat) suffered most: particularly when endurance was high-volume running
- Aerobic gains were unaffected
The interference is real but bounded. For most hybrid athletes, the practical question is not "do I lose 10% strength" but "is the trade-off worth the dual capacity gain?" For HYROX, CrossFit, and military athletes: the answer is consistently yes.
Mitigating interference
- Separate sessions by 6+ hours: doing strength in the morning and endurance in the evening reduces simultaneous AMPK/mTOR activation
- Bias modality: cycling and rowing for endurance interfere less with leg strength than running
- Block periodization: alternate strength-emphasis and endurance-emphasis blocks rather than mixing every session
- Order matters: if same-session, strength first then endurance
- Recovery focus: 1.6-2.0g protein/kg, sleep 7-9hrs, cap weekly volume
For hybrid athletes
Concurrent training is the methodological substrate of hybrid athletes. The literature confirms what the population already knows experientially: you cannot maximize both strength and endurance simultaneously, but you can develop both to a high level: provided you accept a 10-15% ceiling reduction on each compared to specialist peers. For most hybrid athletes, that ceiling is acceptable; the dual capacity is the point.
Sources
- Interference of strength development by simultaneously training for strength and endurance
- Concurrent training: a meta-analysis examining interference of aerobic and resistance exercises
- Compatibility of concurrent aerobic and strength training for skeletal muscle size and function: an updated systematic review and meta-analysis