What is Flexion?

What is Flexion?

Flexion is the movement that bends a joint and brings two bones closer together, decreasing the angle between them. When you curl a dumbbell and your forearm travels up toward your shoulder, your elbow is flexing. When you lie on a leg-curl machine and your heel travels toward your glutes, your knee is flexing. When you lift your thigh up toward your chest, as in a hanging knee raise or a standing march, your hip is flexing. Most flexion happens in the sagittal plane, the front-to-back plane that divides the body into left and right halves, and it is the direct opposite of extension, which straightens the joint and opens the angle back up. A useful way to picture it: flexion is usually the bending or curling action, and extension is the straightening action. Flexion shows up constantly in training because so many exercises are built around bending a joint against resistance. Every biceps curl variation is elbow flexion. Every leg curl is knee flexion. Hip flexion drives knee raises, leg raises, and the recovery phase of running and sprinting. There are important regional exceptions to the sagittal-plane rule, most notably the spine, where lateral flexion is bending the trunk sideways in the frontal plane, and the thumb, which flexes across the palm. Knowing which muscles produce flexion at each joint, and how strong they are through the range, lets you target the biceps, hamstrings, or hip flexors precisely, and it clarifies why a curl and a leg curl, despite looking nothing alike, are the same underlying action at two different joints.

How it works

Mechanically, flexion is rotation of one bone toward another around the joint axis, shortening the muscle that crosses the front (or, at the knee, the back) of that joint. At the elbow, flexion is produced mainly by the biceps brachii, brachialis, and brachioradialis. The brachialis is the workhorse under the biceps and contributes regardless of forearm position, while the biceps is most effective as an elbow flexor when the forearm is supinated (palm up), which is why a supinated curl loads it hard and a hammer or reverse grip shifts more work to the brachialis and brachioradialis. The biceps also crosses the shoulder, so it assists shoulder flexion, but its headline job in the gym is bending the elbow under load. At the knee, flexion is driven by the hamstring group, the biceps femoris, semitendinosus, and semimembranosus, which bend the knee and, because they also cross the hip, extend the hip at the same time. This two-joint arrangement is why hamstrings can be trained either as knee flexors (leg curls) or as hip extensors (Romanian deadlifts, hip hinges), and why leg-curl exercises are the cleanest way to load their knee-flexion role. At the hip, flexion lifts the thigh forward and up and is driven by the iliopsoas (the psoas major plus the iliacus), assisted by the rectus femoris, sartorius, and tensor fasciae latae. The iliopsoas is the primary and strongest hip flexor and is loaded in leg raises, knee raises, and marching drills, as well as being active through the swing phase of walking, running, and kicking. Range of motion is joint-specific and measured with a goniometer from the anatomical zero position, where the joint is straight. Elbow flexion typically reaches about 140 to 150 degrees, knee flexion around 130 to 140 degrees, and hip flexion roughly 120 to 125 degrees with the knee bent. In training terms, the strength curve of a flexion movement depends on the tool: a dumbbell curl is hardest at the mid-range where the forearm is horizontal and the moment arm is largest, while a cable or machine can shift peak tension toward the stretched or shortened end. Matching the exercise to the part of the range you want to challenge is a big part of programming flexion work well.

How to apply it

• Curl variations for elbow flexion: Supinated barbell and dumbbell curls bias the biceps, while hammer and reverse curls shift work to the brachialis and brachioradialis. Train them for 8 to 15 reps, control the lowering phase, and avoid swinging the torso to keep tension on the flexors.
• Leg curls for knee flexion: Lying, seated, and standing leg curls load the hamstrings in their knee-flexion role. The seated version places the hamstrings at longer muscle lengths. Use 8 to 15 reps with a controlled eccentric and a brief squeeze at peak knee flexion.
• Hanging and lying leg raises for hip flexion: Raising the thighs toward the chest trains the iliopsoas and hip flexors. Keep the movement deliberate rather than swinging, and control the descent. These also demand core bracing, so treat them as combined hip-flexor and trunk work for 8 to 15 reps.
• Choose grip and tool for the range you want: A dumbbell curl challenges the mid-range hardest, while a cable or preacher setup can bias the stretched or shortened position. Selecting the tool that loads your weak range is often more productive than simply adding weight.
• Own the lengthened position: Lower under control into the stretched position on curls and leg curls rather than letting the weight drop. Loaded stretch at long muscle lengths is a strong driver of hamstring and biceps growth and builds control where strains tend to occur.
• Balance flexors with extensors: Train the flexor and extensor of each joint in rough balance: biceps with triceps, hamstrings with quadriceps, hip flexors with glutes. Even development supports joint health and prevents the strength gaps that come from favoring one side.

Types

Worked example

A flexion-focused accessory block that trains bending at the elbow, knee, and hip. It pairs well after a pressing or lower-body session. Loads are examples only; pick weights that make the last few reps demanding while you keep the lowering phase slow and controlled.

Each exercise is the same action, bending a joint, at a different joint. The higher rep ranges and slow eccentrics suit these single-joint flexion movements, where control and loaded stretch matter more than maximal weight. Keep momentum out so the flexor muscle does the work.

Flexion vs extension

Flexion and extension are the two halves of sagittal-plane motion at most joints. They are produced by opposing muscle groups, so a balanced program trains both sides of each joint rather than only the bending or only the straightening action.

By goal

• Strength and performance: Knee and hip flexor strength support sprinting, jumping, and change of direction, where the hamstrings decelerate the leg and the hip flexors drive the swing phase. Direct leg-curl and controlled leg-raise work builds resilience for these high-speed actions.
• Hypertrophy: For bigger arms, train elbow flexion with a mix of supinated and neutral-grip curls to hit the biceps and brachialis. For fuller hamstrings, include leg curls to load their knee-flexion function, which hip hinges alone do not fully cover.
• Rehab and injury prevention: Eccentric-focused knee flexion, such as Nordic curls and controlled leg-curl lowering, is well supported for reducing hamstring strain risk. Restoring balanced flexion strength after injury protects the elbow, knee, and hip during return to training.

Common misconceptions

• "The biceps is the only muscle that flexes the elbow." The brachialis lies underneath the biceps and is a major, often underappreciated elbow flexor that works in any grip. The brachioradialis also contributes, especially in neutral and pronated grips, which is why hammer and reverse curls build arm thickness the standard curl misses.
• "Hamstrings only extend the hip, so leg curls are pointless." The hamstrings cross two joints: they flex the knee and extend the hip. Hip hinges train the hip-extension role, but leg curls are the cleanest way to load knee flexion. Training both roles produces more complete hamstring development.
• "Flexion always means bending forward." Flexion means decreasing a joint angle, which looks different at each joint. Elbow flexion curls the forearm up, knee flexion bends the leg back, and hip flexion lifts the thigh forward. The common thread is a smaller joint angle, not one fixed direction.
• "Tight hip flexors are always weak and just need stretching." A feeling of tightness does not reliably mean a muscle is short or weak. Hip flexors often need strengthening through their range, not only stretching. Loaded, controlled hip flexion can improve both strength and how the muscle tolerates length.

Related terms

Flexion FAQ

What is flexion in simple terms?

Flexion is bending a joint so the angle between the bones gets smaller. Curling a weight to bend your elbow, bending your knee, or lifting your thigh toward your chest are all flexion. The opposite movement, straightening the joint, is called extension.

What muscles perform elbow flexion?

The biceps brachii, brachialis, and brachioradialis flex the elbow. The brachialis works in any grip, while the biceps is strongest with the palm up. A neutral or overhand grip shifts more work to the brachioradialis, which is why hammer and reverse curls feel different.

Which exercises train flexion?

Every curl variation trains elbow flexion for the biceps and brachialis. Lying, seated, and standing leg curls train knee flexion for the hamstrings. Hanging and lying leg raises and marching drills train hip flexion for the iliopsoas and other hip flexors.

What is the difference between flexion and extension?

Flexion decreases a joint's angle by bending it, while extension increases the angle by straightening it. They are opposite movements driven by opposing muscles, such as the biceps versus triceps at the elbow, or the hamstrings versus quadriceps at the knee.

Do leg curls and Romanian deadlifts train the same thing?

Not exactly. The hamstrings both flex the knee and extend the hip. Romanian deadlifts train the hip-extension role, while leg curls train the knee-flexion role. Because the hamstrings do both jobs, training both movements produces more complete hamstring development.

How much can the elbow, knee, and hip flex?

Elbow flexion typically reaches about 140 to 150 degrees, knee flexion around 130 to 140 degrees, and hip flexion roughly 120 to 125 degrees with the knee bent. Range is joint-specific and measured with a goniometer from the straight, anatomical zero position.

Why do hammer curls feel different from regular curls?

A hammer curl uses a neutral (thumbs-up) grip, which reduces the biceps' leverage and shifts more of the flexion work onto the brachialis and brachioradialis. That is why hammer curls build arm thickness and forearm involvement that a standard palm-up curl trains less directly.

Is spinal flexion always bad for the back?

No. Flexion is a normal, healthy movement of the spine. Problems come mainly from loading heavy flexion carelessly or with no tolerance built up. Gradually strengthening the trunk through controlled flexion generally improves resilience rather than harming the back.

References

1. Anatomy, Shoulder and Upper Limb, Biceps Muscle. StatPearls, NCBI Bookshelf
2. Anatomy, Bony Pelvis and Lower Limb, Iliopsoas Muscle. StatPearls, NCBI Bookshelf
3. Anatomy, Bony Pelvis and Lower Limb, Hamstring Muscle. StatPearls, NCBI Bookshelf
4. Veeck F, et al. Hamstring-to-quadriceps activation ratio during lower-limb strengthening exercises. Res Sports Med, 2024. PubMed 38006325
5. Goniometer. StatPearls, NCBI Bookshelf