Concurrent training — the simultaneous development of strength and endurance within a single programme — creates a physiological tension. Both modalities compete for the same cellular machinery: the mTOR pathway drives muscle protein synthesis and hypertrophy (strength adaptation), while AMPK activation drives mitochondrial biogenesis and aerobic capacity (endurance adaptation). These two pathways are partly antagonistic: AMPK activation from endurance exercise suppresses mTOR signalling, potentially blunting strength adaptation. This is the "interference effect," identified in research since Hickson's landmark 1980 paper. However, 40 years of subsequent research has substantially refined our understanding of when, how much, and for whom interference actually matters.
What the Interference Effect Actually Means for Endurance Athletes
The interference effect is real, but its magnitude is often overstated. Meta-analyses of concurrent training literature (Wilson et al., 2012; Sabag et al., 2018) show:
- Maximal strength gains (1RM): Concurrent training reduces maximum strength development by approximately 30% compared to strength training alone — most relevant to athletes primarily pursuing maximal strength, less relevant to endurance athletes seeking functional strength for performance and injury prevention.
- Endurance adaptations: Concurrent training does NOT significantly impair VO2max gains. Adding strength training to endurance programmes typically produces equivalent or slightly superior VO2max and lactate threshold improvement compared to endurance training alone.
- Hypertrophy: Muscle mass gains are most significantly impaired by concurrent training — approximately 50% less hypertrophy compared to resistance training alone. For endurance athletes, this is largely irrelevant and potentially beneficial (less mass = better power-to-weight ratio).
- Running and cycling economy: Concurrent training consistently improves running economy and cycling economy (watts per VO2) — the primary mechanism is neural, not hypertrophic. This is the primary benefit of strength training for endurance athletes.
Minimising Interference: What Actually Works
The key modifiable variables that minimise the interference effect:
- Session separation: A minimum of 6 hours between strength and endurance sessions significantly reduces interference. The molecular conflict between AMPK and mTOR is most acute in the 3–6 hours post-exercise when both pathways are maximally activated. Separating sessions by 6–8 hours (morning endurance, evening strength) produces much smaller interference effects than same-session concurrent training.
- Session ordering: Endurance before strength (on the same day, when unavoidable) produces greater interference than strength before endurance. If you must do both in one day, lift first, then do the endurance session. This preserves mTOR activation from strength training before AMPK suppression from endurance exercise.
- Endurance modality: Running causes greater interference with lower body strength adaptation than cycling, due to running's higher eccentric muscle damage (which activates inflammatory pathways that suppress mTOR). Triathletes and cyclists can combine strength training with cycling more effectively than with high-volume running.
- Prioritising intensity over volume in endurance training: Low-volume, high-intensity endurance training (4–6 hours/week HIIT-focused) causes less AMPK-mediated mTOR suppression than high-volume, moderate-intensity endurance training (10–15 hours/week), making it more compatible with concurrent strength development during focused strength blocks.
Practical Concurrent Training Programme Structure
For most endurance athletes, the optimal annual structure alternates between strength-priority and endurance-priority phases:
- Off-season strength block (8–12 weeks): Reduce endurance volume by 30–40%. 3 strength sessions/week at high intensity (4–6 × 80–85% 1RM). 2–3 low-intensity endurance sessions. Allow the strength adaptation to occur with minimal interference. This is the phase where actual strength gains are made.
- Transition phase (4 weeks): Increase endurance volume progressively while reducing strength to 2 sessions/week. Maintain strength with 2–3 × 75% 1RM. The strength maintenance load preserves off-season strength gains without competing for adaptation resources.
- Race season (ongoing): 1 strength session/week at maintenance dose (2–3 sets, 75–80% 1RM). Schedule on the same day as moderate endurance sessions. The goal is preserving neural efficiency gains, not building new strength.
Nutrition Strategy for Concurrent Training Days
Fueling correctly around concurrent sessions is essential for managing the molecular competition. Protein is the critical nutrient: consuming 30–40g of high-quality protein (whey preferred for fast amino acid delivery) within 30 minutes of the strength session counteracts AMPK-mediated mTOR suppression by directly providing leucine, the mTOR activating amino acid. Carbohydrate availability before each session is critical — both strength and endurance sessions are impaired by glycogen deficit. On two-a-day concurrent training days, prioritise carbohydrate replenishment between sessions: 1.2g/kg of carbohydrates in the 2 hours between morning endurance and afternoon strength. Use the NorthLine Race Day Nutrition Planner to structure fuel intake across the training day, ensuring both sessions receive adequate glycogen support and post-session protein is timed optimally.