Final answer:
The gluteal muscles in bipeds prevent us from falling backward relative to the unsupported foot by maintaining balance and stabilizing the hip joint during the stance phase of walking.
Step-by-step explanation:
In bipeds, gluteal muscles play a critical role in preventing us from falling backward relative to the unsupported foot. The feet of bipeds, such as humans, are not very long, which makes stability in the front-to-back direction less inherent. Therefore, the muscles work extensively to maintain balance. Specifically, when one foot is off the ground during the walking cycle, the gluteal muscles on the side of the stance leg, which includes the gluteus maximus, medius, and minimus, contract to prevent the pelvis from tilting and to keep the center of gravity over the base of support. This contraction stops us from falling backward by stabilizing the hip and maintaining upright posture.