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The CrossFit stimulus—constantly varied high-intensity functional movement coupled with meat and vegetables, nuts and seeds, some fruit, little starch, and no sugar—prepares you for the demands of a healthy, functional, independent life and provides a hedge against chronic disease and incapacity. This stimulus is elegant in the mathematical sense of being marked by simplicity and efficacy. The proven elements of this broad, general, and inclusive fitness, in terms of both movement and nutrition, are what we term our CrossFit Essentials.

The meaning of the anatomical core has become confused over time. Originally used as a word added to modify the phrase “body temperature," in the 1990s and 2000s, it became associated with lumbar and abdominal exercises designed to address lower back pain. In reality, a far greater number of muscles are part of the core's structure, and these contribute to what we refer to in CrossFit as midline stability.

Read MoreMidline Stability, Part 1: More Than the Core

Ligaments connect bone to bone, and as there are 26 bones in the foot, there are numerous ligaments connecting and crisscrossing the foot’s interior. The foot receives quite a bit of support from the simple interlocking of the tarsal bones via their shape and via the intertarsal ligaments.

Read MoreThe Foot, Part 2: Ligaments

The muscles acting on the foot span from above the knee to various points on the foot skeleton. The muscles with proximal attachments at points outside the foot are referred to as extrinsic muscles of the foot. Another set of muscles, the intrinsic muscles of the foot, have both proximal and distal attachments within the foot’s bony architecture, from calcaneus to distal phalanx.

Read MoreThe Foot, Part 1: Muscles

A virus is structurally and functionally different from a cell. Viruses are most frequently categorized according to morphology. The most pressing epidemiological virus we currently face is SARS-CoV-2, a variant of the envelope virus that includes spike proteins scientists believe exploit complementary proteins on the membranes of target cells.

Read MoreSimplified Viral Anatomy

The musculature around the hip includes some of the largest and most powerful muscles in the human body. The extension of the hip can move thousands of pounds, propel the body up and over obstacles equal to or greater in height than the body in motion, and move the body at speeds greater than 25 miles per hour. But running, jumping, and lifting are just a few of the hip’s unique abilities.

Read MoreHip Musculature, Part 1: Anterior Muscles

Movements of the hand are, by nature, complex. Even muscles attaching at the shoulder affect the orientation of the hand in space. If we look specifically at the intrinsic muscles of the hand — i.e., those that have both attachments within the carpals, metacarpals, and phalanges — we find a number of small muscles that control thumb and finger positions.

Read MoreWrist Musculature, Part 3: The Hand

The wrist includes a complex aggregation of muscles that drive movement about its joints. The posterior muscles of the wrist are generally used for extension but in a few exceptions carry out an additional action. The wrist extensors are found on the posterior arm, opposite the flexors on the anterior side. These muscles can be attached proximally to the humerus, radius, or ulna and distally to the carpals, metacarpals, or phalanges.

Read MoreWrist Musculature, Part 2: Posterior Muscles

"Trainers … apply exercises to their clients in order to induce adaptations in structure and function that lead to improved fitness. In order to do this effectively, reliably and safely, the working trainer must understand the structures they are stressing with exercise to produce the functional change that is fitness. This is the primary reason we learn anatomy and physiology."

Read the article Anatomy and Physiology Primer for CrossFit Trainers

The elbow musculature is an integral part of an upper axillary system and is capable of both extremely refined and very powerful movement. The anterior muscles of the elbow are considered elbow flexors, reducing the angle of the humerus and the two bones of the forearm, the radius and ulna. If the upper and lower arm are aligned in extension at 180 degrees (straight), flexion will reduce that angle to about 30 degrees. As with other regions, the muscles are arranged in layers: superficial, intermediate, and deep.

Read MoreElbow Musculature, Part 1: Anterior Flexors

Impingement refers to the impeding interaction of hard bony surfaces with softer muscular, connective, and neural tissues, causing pain. The conventional wisdom for the genesis of this syndrome is that the acromion process comes into contact with and impinges upon the tendinous underlying tissues. This is suggested to occur when the arm is lifted, bringing the humeral head and shaft into close proximity with the acromion. The idea is that this movement, when repeated over time, compresses and damages the softer tissues.

Read MoreShoulder Musculature, Part 4: Impingement