The defining experience of a first triathlon run is the "jelly legs" effect — a strange heaviness in the legs that feels nothing like standalone running. Your pace is slower than it should be, your cadence feels laboured, and the legs that were fine on the bike have forgotten how to run. This is not a fitness problem. It is a neuromuscular transition problem, and it is almost entirely solvable with structured brick training.
Why the Run Leg Feels Different
During cycling, the quadriceps are the primary power producers, with the glutes and hamstrings playing a secondary role. During running, the movement pattern, muscle recruitment order, and force profile are fundamentally different: the glutes, hamstrings, and calves become primary movers, cadence is higher (170–180 strides/min vs 85–95 rpm on the bike), and the body must now absorb ground impact forces that don't exist in cycling.
Studies using electromyography (EMG) show altered muscle recruitment patterns lasting 5–15 minutes after transitioning from cycling to running — explaining why the first kilometres of a triathlon run feel distinct from a standalone running effort at the same pace. With regular brick training, this transition window shortens and eventually the jelly legs effect disappears.
Brick Session Protocols by Event Distance
- Sprint triathlon (20km bike / 5km run): 40-minute bike + 15-minute run at race effort. Focus on high cadence on the bike (90–95 rpm) and immediate pace pickup on the run. 1 session per week in the 6 weeks before the event.
- Olympic triathlon (40km bike / 10km run): 75-minute bike + 25-minute run including race-pace sections on both. 1–2 brick sessions per week in the final 8 weeks.
- 70.3 Half Ironman (90km bike / 21km run): 3–3.5 hour bike + 30–45 minute run. The final long brick should simulate your race nutrition strategy exactly. 1 long brick every 2 weeks from 12 weeks out.
- Full Ironman (180km bike / 42km run): You cannot replicate full race duration in training. Programme 4–5 hour bikes followed by 30–60 minute runs at marathon goal pace. 1 brick every 2–3 weeks from 16 weeks out.
Cycling Cadence as a Transition Strategy
Experienced triathletes elevate cycling cadence to 95–100 rpm in the final 10–15 minutes of the bike leg rather than grinding a lower cadence. Higher cadence is more aerobically (vs muscularly) driven — meaning the legs arrive at T2 less glycogen-depleted and more prepared for running. On the run side, targeting 175–180 strides per minute reduces ground contact time and partially offsets the neuromuscular transition delay.
Nutrition for Brick Sessions
Brick sessions over 90 minutes require fueling. The bike leg should be fully fueled — 60–90g of carbohydrate per hour with electrolytes. A common mistake is stopping fueling at the bike-run transition; the run leg requires continued carbohydrate intake at 30–60g per hour, even if the gut is less receptive in those early kilometres. Use brick sessions to practise your race nutrition including run-leg gel timing — never trial a new product on race day. Use the NorthLine Race Day Nutrition Planner to build a complete bike-and-run nutrition schedule for race day.
T2 Transition Practice
The physical transition from bike shoes to running shoes is its own trainable skill. Practise racking your bike, removing your helmet, and putting on running shoes within a consistent protocol. Elite age groupers complete T2 in 60–90 seconds. Every 10 seconds saved in T2 is 10 seconds you do not need to run faster to gain. Incorporate at least 3 full T2 transitions in your final 8 weeks of race preparation — the sequence becomes automatic with repetition.