The plank is an isometric

Isometric exercise

Isometric exercise or isometrics are a type of strength training in which the joint angle and muscle length do not change during contraction...

 core

Core (anatomy)

In anatomy, the core refers, in its most general of definitions, to the body minus the legs and arms. Functional movements are highly dependent on the core, and lack of core development can result in a predisposition to injury...

 strength exercise that involves maintaining a difficult position for extended periods of time. The most common plank is the front plank which is held in a push-up position with the body's weight borne on forearms, elbows, and toes.

Many variations exist such as the side plank and the reverse plank. The plank is commonly practiced in pilates

Pilates

Pilates is a physical fitness system developed in the early 20th century by Joseph Pilates in Germany, the UK and the USA. As of 2005, there were 11 million people practicing the discipline regularly and 14,000 instructors in the United States....

 and yoga

Yoga

Yoga is a physical, mental, and spiritual discipline, originating in ancient India. The goal of yoga, or of the person practicing yoga, is the attainment of a state of perfect spiritual insight and tranquility while meditating on Supersoul...

, and by those training for boxing

Boxing

Boxing, also called pugilism, is a combat sport in which two people fight each other using their fists. Boxing is supervised by a referee over a series of between one to three minute intervals called rounds...

 and other sports.

The plank strengthens the abdominals, back, and shoulders. Muscles involved in the front plank include:

• Primary muscles: erector spinae, rectus abdominus
  Rectus abdominis muscle
  The rectus abdominis muscle, also known as the "six pack", is a paired muscle running vertically on each side of the anterior wall of the human abdomen . There are two parallel muscles, separated by a midline band of connective tissue called the linea alba...
  
   (abs), and transverse abdominus
  Transversus abdominis muscle
  The transversus abdominis muscle, also known as the transverse abdominus, transversalis muscle and transverse abdominal muscle, is a muscle layer of the anterior and lateral abdominal wall which is deep to the internal oblique muscle...
  
  .
• Secondary muscles (synergist
  Synergist
  A Synergist is a kind of muscle that performs, or helps perform, the same set of joint motion as the agonists. Synergists muscles act on movable joints. Synergists are sometimes called as "neutralizers" because they help cancel out, or neutralize, extra motion from the agonists to make sure that...
  
  s/segmental stabilizers
  Segmental stabilizers
  In anatomy and physiology, segmental stabilizers are the muscles which provide support across joints, as in the multifidus across spinal vertebrae.-In fitness:...
  
  ): trapezius
  Trapezius muscle
  In human anatomy, the trapezius is a large superficial muscle that extends longitudinally from the occipital bone to the lower thoracic vertebrae and laterally to the spine of the scapula...
  
   (traps), rhomboids, rotator cuff
  Rotator cuff
  In anatomy, the rotator cuff is the group of muscles and their tendons that act to stabilize the shoulder. The four muscles of the rotator cuff, along with the teres major muscle, the coracobrachialis muscle and the deltoid, make up the seven scapulohumeral muscles of the human body.-Function:The...
  
  , the anterior, medial, and posterior deltoid muscle
  Deltoid muscle
  In human anatomy, the deltoid muscle is the muscle forming the rounded contour of the shoulder. Anatomically, it appears to be made up of three distinct sets of fibers though electromyography suggests that it consists of at least seven groups that can be independently coordinated by the central...
  
  s (delts), pectorals
  Pectoralis major muscle
  The pectoralis major is a thick, fan-shaped muscle, situated at the chest of the body. It makes up the bulk of the chest muscles in the male and lies under the breast in the female...
  
   (pecs), serratus anterior
  Serratus anterior muscle
  The serratus anterior is a muscle that originates on the surface of the upper eight or nine ribs at the side of the chest and inserts along the entire anterior length of the medial border of the scapula.-Origin:...
  
  , gluteus maximus
  Gluteus maximus muscle
  The gluteus maximus is the largest and most superficial of the three gluteal muscles...
  
   (glutes), quadriceps (quads), and gastrocnemius
  Gastrocnemius muscle
  In humans, the gastrocnemius muscle is a very powerful superficial pennate muscle that is in the back part of the lower leg. It runs from its two heads just above the knee to the heel, and is involved in standing, walking, running and jumping. Along with the soleus muscle it forms the calf muscle...
  
  .

Muscles involved in the side plank include:

• Primary: transverse abdominus, gluteus medius
  Gluteus medius muscle
  The gluteus medius , one of the three gluteal muscles, is a broad, thick, radiating muscle, situated on the outer surface of the pelvis....
  
   and gluteus minimus
  Gluteus minimus muscle
  The gluteus minimus , the smallest of the three gluteal muscles, is situated immediately beneath the gluteus medius.-Origin and insertion:...
  
   muscles (abductors), the adductor muscles of the hip
  Adductor muscles of the hip
  In human anatomy, the adductor muscles of the hip is a group of muscles of the thigh.-Muscles:The adductor group is made up of:*Adductor brevis*Adductor longus*Adductor magnus*Adductor minimus This is often considered to be a part of adductor magnus....
  
  , and the external
  Abdominal external oblique muscle
  The external oblique muscle is the largest and the most superficial of the three flat muscles of the lateral anterior abdomen.-Structure:...
  
   and internal obliques
  Abdominal internal oblique muscle
  The internal oblique muscle is the intermediate muscle of the abdomen, lying just underneath the external oblique and just above the transverse abdominal muscle.-Structure:...
  
  .
• Secondary: gluteus maximus (glutes), quadriceps (quads), and hamstring
  Hamstring
  In human anatomy, the hamstring refers to any one of the three posterior thigh muscles, or to the tendons that make up the borders of the space behind the knee. In modern anatomical contexts, however, they usually refer to the posterior thigh muscles, or the tendons of the semitendinosus, the...
  
  s.

The current world record (according to Guiness World Records) for the plank position is 36 minutes and 58 seconds, set in [ Stevens Point, Wisconsin ] on 28 Sept 2011 by 71 year old Betty Lou Sweeney.