Speed
As you extend your stride or quicken the rate of your leg and foot turnover when you’re trying to pick up your pace, the lower abs-including the transversus and rectus abdominis-and lower back are called into action. The stronger and more stable these muscles are, the more force and speed you can generate as you push off the ground.
Uphills
The glutes and lower abs support the pelvis, which connects to the leg muscles needed to get uphill. If the core is strong, the legs will have a stable plane to push from, for a more powerful ascent. When you swing your leg forward, the hip-flexor muscles, such as the rectus femoris, pull on the pelvis. As you push off the ground, the glutes and hamstrings are engaged.
Downhills
When you’re flying down a slope, you need strong gluteal muscles to help absorb the impact and counter the momentum of the forward motion. As fun as it may be to zoom down, without the core strength to control your movement, your quads and knee joints bear the extra pounding of your body weight, which can lead to fatigue, pain, and even injury.
Endurance
As you’re nearing the end of a race, a solid core helps you maintain proper form and run efficiently, even through fatigue. With strong lower abs and lower-back muscles, such as the erector spinae, it’s easier to stay upright. If your core is weak, you may end up shuffling, slouching, and putting too much stress on your hips, knees, and shins.
Lateral Movement
Whenever you have to suddenly move to the side-to turn the corner on a track, dodge a pothole, or navigate undulating terrain-the obliques provide stability and help keep you upright. If your core is weak, then you may end up leaning into the movement, which can put excess weight and strain on the joints in your legs and feet.
Charlyne Bourion 