High knees recruit a wide chain of muscles from your feet to your shoulders, but the real workhorses are your hip flexors, quadriceps, and core. Because the movement combines rapid knee drives with an upright posture and quick ground contacts, it demands both powerful leg muscles and strong stabilizers working together at speed.
Hip Flexors: The Primary Drivers
The upward knee drive that defines high knees is powered almost entirely by your hip flexors. The most important of these is the psoas (pronounced “SO-ez”), a deep muscle that runs from your lower spine through the pelvis to the top of your thighbone. As the Cleveland Clinic describes it, the psoas allows you to move your knees up toward your body, exactly the motion you perform with every rep. The iliacus, which lines the inside of your pelvis and works alongside the psoas, fires in tandem to pull your thigh upward.
Because high knees demand that you lift each knee above waist height, your hip flexors work through a large range of motion at high speed. This makes the exercise particularly effective at building both strength and endurance in these muscles, which is one reason coaches use it as a warm-up drill for sprinters.
Quadriceps and Rectus Femoris
Your quadriceps, the four muscles on the front of each thigh, play a dual role during high knees. The three single-joint quads (vastus lateralis, medialis, and intermedius) help control your knee position as your leg swings up and extends back down. But the rectus femoris, the quad muscle that crosses both the hip and the knee, does something extra: it assists the hip flexors in driving the knee upward while also helping straighten the knee on the way down.
This double duty creates an interesting mechanical challenge. The rectus femoris becomes less effective at one job when it’s fully engaged in the other. When the hip is deeply flexed (knee high), the muscle has less capacity to extend the knee, and vice versa. In practical terms, this means the exercise forces the rectus femoris to coordinate rapidly between two opposing tasks, which builds the kind of dynamic quad strength useful in running and jumping.
Core Muscles as Stabilizers
High knees work your abs, but not in the way a crunch does. Your core muscles engage isometrically, meaning they contract and hold rather than moving through a range of motion. Their job is to keep your torso upright and your pelvis stable while your legs cycle rapidly beneath you.
The transverse abdominis, your deepest abdominal muscle, wraps around your midsection like a corset and is responsible for stabilizing your spine and pelvis against the rotational forces each knee drive creates. Your obliques, both internal and external, fire to prevent your torso from twisting or tilting side to side as you alternate legs. The rectus abdominis (your “six-pack” muscle) contracts to prevent excessive arching in your lower back, especially as fatigue sets in and form starts to break down.
This stabilizing work is constant. Every time a foot leaves the ground and a knee drives upward, your abdominals must contract to maintain posture and protect your spine. Over a 30- or 60-second set, that adds up to significant core endurance training, even though the abs aren’t the primary target.
Glutes and Hamstrings
While your hip flexors power the upswing, your glutes and hamstrings control what happens on the other side. Each time a lifted leg returns to the ground, the gluteus maximus and hamstrings of that leg work eccentrically to decelerate hip flexion and prepare for ground contact. On the stance leg (the one still on the ground), the glutes stabilize your pelvis so it doesn’t drop or shift laterally.
The hamstrings also play a role in bending the knee slightly during the recovery phase, helping the foot clear the ground quickly before the next drive upward. At higher speeds, this contribution becomes more important because you need faster leg turnover.
Calves and Lower Leg
Each rep of high knees starts with a push off the ball of your foot, which means your calf muscles generate the initial upward force. The gastrocnemius, the larger calf muscle that crosses the knee joint, contributes most of this push-off power when the knee is relatively straight at the start of each drive. The soleus, a deeper calf muscle that only crosses the ankle, provides steady force regardless of knee angle.
Research on calf activation shows that gastrocnemius activity drops by about 35% when the knee is bent compared to when it’s straight, while soleus activity actually increases by 15 to 28% in a flexed-knee position. During high knees, this means the gastrocnemius dominates the initial push-off (when the stance leg is nearly straight), while the soleus picks up more work as the knee bends during the transition. Your anterior tibialis, the muscle on the front of your shin, also fires briefly to pull your toes up (dorsiflex) so your foot clears the ground between reps.
Arms and Upper Body
The arm swing in high knees isn’t just for balance. Pumping your arms in opposition to your legs (left arm forward with right knee, and vice versa) involves your biceps and triceps, your anterior deltoids (front of the shoulders), and your upper back muscles. The arm action counterbalances the rotational forces your legs create, making it easier for your core to keep your torso stable. Faster, more aggressive arm pumps also help drive your knees higher and increase the overall intensity of the exercise.
How the Muscles Work Together
High knees happen in two rapid, overlapping phases for each leg. During the drive phase, the hip flexors and rectus femoris contract concentrically to pull the knee upward while the core braces isometrically and the opposite arm swings forward. During the return phase, the glutes and hamstrings decelerate the leg, the calf muscles of the landing foot absorb impact and immediately reload for the next push-off, and the core adjusts to the shift in weight from one leg to the other.
Because both legs cycle continuously, all of these muscle groups are working at all times, just alternating between concentric (shortening), eccentric (lengthening), and isometric (holding) contractions. This is what makes high knees such an efficient full-body drill: you’re not isolating any single muscle, but rather training the entire chain of muscles involved in running mechanics at an exaggerated range of motion and controlled pace.