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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.

Blood vessels are adaptable to stress. If there is hypoxic stress (low oxygen content present) that results in tissue hypoxemia (low oxygen in the tissue), a cascade of local hormonal and anabolic events occurs that produces new capillaries and new arterioles. This process is called angiogenesis and is considered to be an endurance-friendly anatomical adaptation, improving the body’s capacity to deliver oxygen to working skeletal muscle.

Read MoreThe Heart, Part 6: Blood Vessel Basics

Inside metabolically active cells, you will find little droplets of fat (more properly called lipid) that can be used as an energy source. Fat carries out many crucial roles in the body apart from being a pretty effective energy storage medium. Adipose cell accumulations, or adipose tissues, are located subcutaneously (under the skin), viscerally (surrounding internal organs), in bone marrow, in streaks along fascicular spaces in muscle, and in breast tissue.

Read MoreFat, Part 1: Anatomical Distribution

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