Will lifting make me bulky or slower?

The short answer

In trials where cyclists and runners added 8 to 16 weeks of heavy lifting to their training, body weight held steady while the trained muscles grew a little, about 3 to 6%. Endurance held or improved in those trials, and in 1 trial riders gained power at the end of a 3 hour ride (Rønnestad 2011). Heavy lifting with few reps builds strength well, and it takes many hard sets close to failure to build a lot of size, which endurance strength plans keep low.

What the trials found for body weight

The best direct evidence comes from small trials of trained riders and runners who kept their usual endurance training and added 2 to 3 heavy leg sessions a week.

• Cyclists. 8 weeks of heavy half squats, 4 sets of 4, 3 times a week, left body weight unchanged and raised cycling economy 4.8% (Sunde 2010).
• Runners. The same squat program in well-trained runners raised max strength 33% and running economy 5%, again with body weight unchanged (Støren 2008).
• Across trials. A review of this research found total body mass held steady over 8 to 16 weeks of added strength work in cyclists and runners, while the trained muscles grew about 3 to 6% (Rønnestad and Mujika 2014).

The muscles that grow are the ones that push the pedals and the ground. In 1 trial of female cyclists, the quadriceps grew more in the riders who lifted, and the riders whose quadriceps grew most tended to gain the most 40 minute power (Vikmoen 2016). A small gain in leg muscle is part of how the lifting helps.

In 8 weeks of heavy half squats, runners raised max strength 33% and running economy 5% with no change in body weight.

What the trials found for speed

VO2max held steady in these trials, and endurance either rose or stayed the same. After 12 weeks of 2 heavy leg sessions a week, riders raised 5 minute all-out power at the end of a 185 minute ride from 371 to 400 W (Rønnestad 2011). Riders also gained 40 minute power and economy (Vikmoen 2016), and riders and runners held their efforts longer before tiring (Sunde 2010; Støren 2008).

Not every trial in this research found a gain. In 19 female endurance athletes, 11 weeks of heavy lifting raised leg strength 40% while 40 minute running performance and economy held steady (Vikmoen 2016, PLoS One). In highly trained road riders, the 2 trials that added lifting on top of a full riding week found no improvement, and the 3 that replaced some riding with lifting did (Yamamoto 2010).

Why heavy, low-rep lifting builds strength more than size

Strength and size respond to different parts of a program. When every set was taken to failure, heavy loads gave larger gains in 1 rep max strength, while muscle growth was similar from light and heavy loads (Schoenfeld 2017, load review). Size depends more on how many hard sets a muscle does each week. More weekly sets went with more growth, about 0.37% more for each added set (Schoenfeld 2017, volume review).

How close each set goes to failure matters for size as well. Muscle grew more as sets ended closer to failure, while strength gains were similar whether sets stopped short or went close (Robinson 2024).

Strength plans for endurance athletes sit at the strength end on both counts. The trials above used 2 to 4 heavy sets of 4 to 10 reps for 2 to 3 sessions a week, and leg muscle grew a few percent. Building much more muscle takes many more hard sets per muscle each week, which is how a muscle-building plan such as Full Body differs.

What cardio does to muscle growth

The research on doing both is called concurrent training, and its findings have shifted over time.

A 2022 meta-analysis of 43 studies found that endurance training alongside lifting left muscle size and max strength intact. Gains in explosive strength were smaller, mostly when both were done in the same session (Schumann 2022). A companion analysis looked at single muscle fibers and found slightly less growth with concurrent training, with the drop in type I fibers showing up with running and not with cycling (Lundberg 2022).

An older meta-analysis of 21 studies was less reassuring. Lifting alongside running, though not cycling, reduced gains in muscle size and strength, and the reduction grew as endurance sessions became more frequent and longer (Wilson 2012). The 2 reviews used different study sets and methods, and the newer one is larger. Read together, they suggest your endurance training will, if anything, hold muscle growth back a little, especially with a lot of running.

If you want more muscle

Riders and runners who want more muscle can build it with more weekly sets per muscle, taken closer to failure, which is what the research links to more growth. The small cost of endurance work showed up in leg muscle fibers, and in 1 of the 2 older analyses only with running (Lundberg 2022; Wilson 2012). Adding size to the upper body, which endurance training loads less, is a coaching choice that follows from this rather than a tested result.

How Peak Hybrid uses this

Strength for Cyclists lifts heavy for a few reps, 2 days a week, with the main lifts at 3 sets of 4 to 6. Leg days fit around your key rides. Foundations teaches the squat, hinge, press, and pull at about 40 minutes a session, with every set stopping about 3 reps short. Full Body is built to add strength and muscle, with more weekly sets and an optional upper day. The plan details and research for each are on the plans page.