A "brick" workout—a bike session followed immediately by a run—is the most triathlon-specific session an athlete can perform. It directly prepares your body and mind for the unique challenge of the bike-to-run transition, a critical juncture in any race. This guide covers the science behind the infamous "jelly legs" and provides practical workout templates to help you master this transition.
4.1 The Science of the "Jelly Legs"
That wobbly, uncoordinated feeling you get when you start running off the bike is a real physiological phenomenon caused by several factors:
- Hemodynamic Shift: When cycling, a large volume of blood is directed to your quadriceps and glutes. When you start running, your body must rapidly redistribute that blood flow to your calves, hamstrings, and hip flexors. This shift can cause a temporary feeling of weakness and disorientation.
- Increased Energy Cost: Running immediately after cycling is metabolically more demanding than running fresh. Research shows that the energy cost of running is elevated by 1.6% to 11.6% in the moments after dismounting the bike.
- Biomechanical Adjustments: Your running form changes after cycling. Athletes typically exhibit a more forward-leaning trunk and altered muscle activation patterns. Brick workouts train your nervous system to make these biomechanical adjustments more quickly and efficiently.
4.2 Building Your Bricks: Workouts for Every Race Distance
The structure of your brick workout should be tailored to the specific demands of your target race distance, each with a clear physiological intent.
- For Sprint/Olympic - The "Neural Primer"
- Intent: Acid tolerance. This workout simulates the lactate accumulation present in short-course racing, training the body to run fast on lactate-filled legs and clear metabolic byproducts on the fly.
- Structure: A 45-minute bike session including several efforts above threshold (Zone 5), followed immediately by a 15-minute run at or near your 5k race pace.
- For Half-Ironman (70.3) - The "Endurance Handoff"
- Intent: Glycogen management and pace maintenance. This workout trains you to run efficiently with partially depleted glycogen stores, dialing in your nutrition and hydration strategy under race-specific fatigue.
- Structure: A 2-hour bike ride at a strong endurance or Sweet Spot intensity (Zone 3), followed immediately by a 30-45 minute run at your goal Half-Ironman pace.
- For Full-Ironman (140.6) - The "Durability Test"
- Intent: Fatigue resistance. This workout focuses on maintaining form and efficiency under extreme duration. The run speed is low, but the accumulated musculoskeletal and metabolic stress is high, directly preparing you to avoid the "Ironman Shuffle."
- Structure: A 4-5 hour aerobic bike ride (Zone 2), followed immediately by a 45-60 minute easy run, focusing entirely on smooth, efficient form.
4.3 Practical Tips and Frequency
- Schedule Wisely: Brick workouts induce significant stress. It's best to schedule them once a week, typically on a weekend or a day when you have more time.
- Manage Injury Risk: Running on fatigued legs increases injury risk. Pay close attention to your form and avoid pushing through sharp pain.
- Avoid in Recovery: Due to their high stress, brick workouts should generally be avoided during designated recovery weeks to allow for full adaptation.
By consistently incorporating race-specific brick workouts into your training, you can turn the bike-to-run transition from a moment of weakness into a competitive advantage.
