Source: Muscle recruitment pattern in cycling: a review
The gluteus medius sits above and to the front of the glute max, on the outside of the hip. Beneath it is the glute minimus, the smallest of the three glutes. The primary role of these two smaller glute muscles is to stabilise the pelvis and abduct the leg, lifting it outward from the body's midline.
On the bike, when properly activated, these muscles help keep your pelvis steady as you push power through the pedals, keeping your leg tracking in a consistent line, which in turn improves pedalling efficiency and reduces the likelihood of knee injury. As they don't play a significant role in force production, the two muscles won't undergo notable changes in size or strength through cycling.
How Glutes Engage During a Pedal Stroke
As the authors of "Muscle recruitment pattern in cycling: a review" highlight, the pedal stroke is broken into two halves: the power phase (0–180 degrees) and the recovery phase (180–360 degrees).
By measuring muscle electrical activity with electromyography (EMG), researchers can determine the precise activation of muscle groups. In one study of 10 road cyclists, researchers found that the glute max engages from the end of the recovery phase through halfway into the power phase (from 340–130 degrees), with peak activity around 80 degrees, just before the halfway point of the power phase.
This is a wide active range compared to the other main muscles involved in the pedal stroke, including the quadriceps. However, since a greater portion of total force production comes from the quadriceps, these muscles experience more growth than the glutes.
The angle of your body is another important factor in glute engagement. A study showed that reducing the hip angle by riding in drops and on aerobars leads to higher glute max activity, but it activates later in the pedal stroke.
Cycling for Glute Strength vs. Glute Size
When a muscle grows in size in response to exercise like weightlifting, this process is called hypertrophy. Hypertrophy also occurs in response to hormones, such as during puberty.
Muscle growth is typically associated with strength gains, but the two don't always correlate. As this review of the literature shows, you can actually gain muscle strength, which is the maximum force the muscle can produce, while maintaining or even reducing the size of the associated muscle, depending on the type of exercise and the individual. Many pro cyclists tailor their strength training to achieve strength gains without muscle growth.
Strength is built through repeated movements against high resistance, whether that's carrying heavy loads at work or doing weighted squats or pull-ups in the gym.