Lower Body and Core Muscle Anatomy

Compared with the intricacies of the upper body muscle anatomy, the lower body has large muscles with fewer attachments. You only need a squat to address all the major muscle groups and perhaps an interval with something athletic such as sprinting or running stairs to address the lower body stabilizers. The core works on nearly every exercise since it stabilizes the torso, but especially on an upper body pulling exercise.

We will focus on the lower body muscles without discussing the joints. They show how the body functions as a unit and not as bits and pieces, giving evidence in favor of compound over isolation exercises. We will focus on insights versus an in-depth knowledge.

Muscles get their names in several ways. These include structure, shape, function, location, bony attachments, size, and other qualities. These can help you remember where they are and how they work.

Lower Body Muscles

Hip and Pelvis

Back:

Muscle: Gluteus Maximus

Function: It performs hip extension and external rotation at the hip. Different portions focus on different actions due to their large areas, although hip extension remains the main action.

Comments: This muscle can help extend the knee since part of the muscle attaches to the femur, and another part inserts into the iliotibial tract, a thick tendon band located on the outer side of the thigh. This mirrors the same advantages addressed with the upper body synergy for pushing and pulling.

Muscle Groups: Internal Hip Rotators (semitendinosus, semimembranosus, popliteus, gracilis, sartorius) and External Hip Rotators (piriformis, quadratus femoris, gemellus superior, gemellus inferior, obturator internus, and obturator externus)

Functions: These perform unloaded actions.

 Thigh

An unbalanced hamstrings-to-quadriceps strength ratio is often suggested as the basis for injuries related to hamstrings strains. Unfortunately, this condition gets addressed with poor exercises, such as a leg curl, instead of applying a load properly that loads both hip and knee extension, such as a barbell squat with a low enough bar position.

Back:

Muscle Group: Hamstrings (Biceps Femoris (lateral with two heads), Semitendinosus (medial), Semimembranosus (distal))

Function: These muscles perform hip extension and knee flexion. The biceps femoris also rotates the hip outward while all the other hamstrings muscles serve to rotate the hip inward.

Comments: Despite many trainees performing exercises at the gym addressing these muscles through knee flexion using a machine, with the leg curl or glute-ham raise, these muscles perform best as hip extensors due to having better leverage.

The open-chain function of knee flexion exists for repositioning and not for heavy loads.

Postural tightness can occur since the hamstrings pull on the ischial tuberosity, which creates a posterior tilt of the pelvis. Tight hamstrings prevent good posture. Modern society, with plenty of sitting and less activity than our ancestors experienced can allow them to shorten and cause pain and discomfort for the lower back.

Muscle: Plantaris

Function: It has an unknown function. It likely assists in plantarflexion and weakly in knee flexion. This muscle does not exist in some people and may represent a relic from our evolution.

Muscle: Popliteus

Function: It performs medial rotation and knee flexion to unlock the knee joint and resides deeply.

Front:

Knee Extensor Muscles:

Muscle: Quadriceps (Rectus Femoris (front), Vastus Intermedius (central), Vastus Lateralis (external), Vastus Medialis (internal))

Function: These four heads of the quadriceps extend knee. The Rectus Femoris also flexes the hip. The vastus medialis and vastus lateralis work together to keep the patella centered in the patellofemoral groove.

Comments: Some early evidence suggested that the Vastus Medialis muscle had a greater activation during the degrees nearest to leg extension (20°-30°). This would seem to defend the leg extension machine. The anatomy fails to support this notion.

Other more recent evidence suggests that all four of the quadriceps muscles remain equally active throughout the range of motion with a significant load.

The rectus femoris acts strongly only when the knee moves along with the hip, as occurs during a squat.

Muscle: Articularis Genu

Function: This small, flat muscle likely pulls the joint capsule superiorly as the knee extends to prevent impingement.

Muscle: Sartorius

Function: This long muscle flexes, abducts, and externally rotates the hip. It also flexes the knee and medially rotates it.

Muscle: Tensor Fasciae Latae

Function: This flexes, abducts, and internally rotates the hip and extends the knee.

Muscle: Iliopsoas (two parts: iliacus and psoas major),

Function: These muscles work together to perform hip flexion and external rotation of the hip.

Comments: This muscle group has received more and more attention. These may link with lower back problems as their strength and tightness can lead to misalignment. This provides an example of how loading a muscle meant for unloaded actions may harm its function.

Muscle: Pectineus

Function: This performs adduction and flexion of the hip.

Lateral:

These muscles function as stabilizers during hip and knee extension movements.

Muscle: Gluteus Medius

Function: This performs abduction of the hip but usually prevents it. Since this muscle is large, different portions can focus on different actions. The anterior portion focuses on flexing and medially rotating the hip. The posterior portion extends and laterally rotates the hip, which prevents dropping of the pelvis.

Muscle: Gluteus Minimus

Function: This performs abduction, medial rotation, and flexion of the hip.

Medial:

These muscles function as stabilizers during hip and knee extension movements and can contribute to hip extension. The Obturator Externus, Quadratus Femoris, and lower portion of Gluteus Maximus can also assist in adducting the hip or preventing it from doing this action.

Muscles: Adductors (Adductor Magnus, Adductor Longus, Adductor Brevis, Gracilis, Pectineus).

Function: These muscles adduct the hip, with the large Adductor Magnus also rotating the hip inward, and the Gracilis flexing the knee and also rotating the hip inward. Some can act as strong hip extensors when the hip is flexed and as hip flexors when the hip is extended.

Comments: The large muscle mass exists not due to the great strength needed for adduction, but because these muscles also play valuable roles as flexors, extensors, rotators, and co-contractors of the hip. This concept shows that larger muscles tend to contribute to major movements and not focus on stabilizing alone.

 Leg and Foot

These parts help to elevate the body, but mainly serve to absorb the shock from landing after a jump or when walking and running. They provide control and work as stabilizers when the foot stays planted. They store energy to contribute more force on the push-off phase when walking, running, and jumping.

Back:

Muscle Group: Triceps Surae (gastrocnemius (two heads), soleus)

Function: The gastrocnemius can weakly flex the knee but mainly extends the ankle in an action called plantarflexion. The soleus performs only plantarflexion.

Comments: The seated hell raise exercise performed often at gyms places the gastrocnemius into active insufficiency, which bunches up the muscle fibers to invite cramping and injuries. These muscles contribute to a squat by performing plantarflexion and stabilizing the motions of the knee.

Muscle: Tibialis Posterior

Function: This deep muscle performs plantarflexion and inversion for the foot.

Peroneus Tertius

Front:

These muscles exist for foot positioning and stabilization through open-chain movement.

Muscle: Tibialis Anterior

Function: It performs dorsiflexion or ankle flexion necessary to prevent the foot from dropping forward.

Ankle and Foot:

The ankle foot flexors and extensors assist the other larger muscles and function best with a planted foot, which does not occur when isolating the muscles unnaturally leg extensions, leg curls, and hip adduction and abductor machines. They exist to support closed-chain motion and maintain the support of the foot arches during these activities.

Core Muscles

Abdomen (Core)

These muscles exist to transmit and prevent motion, especially flexion and extension of the spine. The criss-cross pattern of these muscles allow them to work effectively to prevent rotation too. Due to the unilateral motion, this shows their importance during sprinting.

Lower Back:

Muscle: Erector Spinae and various smaller, deep muscles

Function: Bilateral contraction allows backward flexing at the atlanto-occipital joint and the cervical spine. Certain muscles emphasize rotation but all muscles work as a unit especially to prevent flexing. Resistance to shoulder motion causes muscles throughout the trunk to contract as well.

Abdominal:

Muscle: Rectus Abdominis

Function: This flat, superficial muscle can perform trunk flexion, but exists to resist motion at the core.

Muscle: Obliquus Externus Abdominis

Function: It serves to unilaterally produce trunk rotation to the opposite side and side-flexing to the same side.

Muscle: Obliquus Internus Abdominis

Function: It blends with other core muscles and serves to unilaterally cause side flexing and trunk rotation leading with the opposite shoulder.

Muscle: Transversus Abdominis

Function: This innermost layer forms a corset that performs abdominal compression and vigorous exhalation and expulsion.

Muscle Group: External and Internal Intercostals

Function: These muscles exist between the ribs to elevate and depress them.

Muscle: Quadratus Lumborum

Function: This muscle is located on the posterior abdominal wall between the psoas major and erector spinae. This small muscle depresses the 12th rib and performs lateral flexion of the trunk.

Muscle: Diaphragm

Function: This serves as a dome that separates the thoracic from abdominal cavity and performs inspiration.

Lower Body Muscle Anatomy Conclusions

Muscles adapted for loaded versus unloaded actions. Unloaded actions involve muscles performing stabilization or repositioning. Loading these actions with weight may harm the joints and work the muscles poorly.

Correcting a balance should involve doing good exercises correctly instead of adding bad exercises.

Machines can work muscles unnaturally.

Tight muscles can harm the joints.

Some stabilizers of the lower body can only work when the foot stays on the floor.

Some muscles, such as stabilizers, consistently perform the same muscle role.

Most muscles have several anatomic actions. They come together to assist in one major movement though. They develop synergy for these multi-joint motions, making the whole greater than the sum of its parts.

Muscles may perform more than several functions. They usually function weakly beyond their main roles though. Some may increase their activity to compensate only for other muscles getting placed in awkward and weak positions, which can harm the joints as well.

Muscles have an ideal position to exert the most force.