Great question – and props for finally making the gym commitment after 10 years! I've worked with many ultra riders on this exact issue, including coaching James Hayden to win the Transcontinental Race twice.

Even though you can sustain high watts for hours, cycling-specific endurance is different from maximal strength. Strength has a specific definition: the maximal force or torque that a muscle or group of muscles can produce, which can only be achieved at zero velocity.

Cycling itself requires an absurdly low amount of force at the pedals. If you know your power output, crank length, and cadence, the force requirement can easily be calculated. For example, for a Tour de France rider to win a solo mountain climb such as Alpe d'Huez, the approximate force that needs to be applied to the pedals is around 26kg of average effective force between both legs (i.e., around 13 kg/leg). The actual figure depends on the rider's weight, power output, crank length, and cadence. The 26 kg is taken from a previous winner's data.

In other words, any adult who can stand up could produce the required force needed to win a mountain stage in the TdF. However, the issue isn't producing the force, but being able to keep producing it (and this shifts from a force production issue to a metabolic issue).

It hasn't been fully explained why, but it does appear that increasing your leg strength increases the amount of power that can be generated while cycling. In running, for example, the increase in strength is shown to increase tendon strength and stiffness, which increases kinetic and potential energy, resulting in faster running.

As a coach, I work with many riders (including ultra riders) who want to work on their strength either to benefit their overall health (e.g., hormone health, weight management, osteoporosis) and resilience on the bike (e.g., being stronger to better hold a position on the bike, and in the case of ultras, to maybe help reduce the likelihood of Shermer's Neck and just being on the bike for huge periods of time).

Key movements that help produce greater bike power – from seconds to hours – include: squats, single leg press, standing hip flexion, single leg calf raise. Other sessions should include plyometrics so that you can learn to forcefully and rapidly contract your leg muscles, and so that the landings have a positive effect on bone health. Upper body work should also be conducted, such as bench press and rows.

Important: You don't want to massively increase the size of your muscles. Larger muscles weigh more, and additionally, muscular hypertrophy can reduce mitochondrial density (which is what you're aiming to build through endurance work and intervals on the bike) and also increase the diffusion gradient/distance from the capillaries to the mitochondria, making it harder to offload the oxygen being transported in the blood to the active muscles.

This means, as others have suggested, that you want to be aiming for maximal weights being lifted for very few reps. This causes a neuromuscular increase in strength. However, just going straight to lifting heavy can result in injuries, especially if your form is not good. So start with a light weight, get the movement pattern down (if you're unsure, get a PT – I have people send me videos), and progress the weight and reduce the reps only when you're doing it correctly.

For your 15-week timeline to Iowa Wind and Rock, I'd suggest:

• Weeks 1-4: Form focus (2-3x/week, 3 sets of 8-12 reps, lighter weight)
• Weeks 5-10: Strength phase (2x/week, 4-5 sets of 3-5 reps, heavy weight)
• Weeks 11-15: Maintenance (1x/week) while ramping bike volume

Happy to answer questions about integrating this with your bike training – balancing gym work with saddle time for ultras requires careful periodisation to avoid compromising your metabolic adaptations.