Editor’s note: Dana Santas, known as the “Mobility Maker,” is a certified strength and conditioning specialist and mind-body coach in professional sports, and is the author of the book “Practical ...

Muscle contraction is the tightening, shortening, or lengthening of muscles when you do some activity. It can happen when you hold or pick up something, or when you stretch or exercise with weights.

Panel A shows the microscopic myoarchitecture of the circular and longitudinal muscle layers of the lower esophageal sphincter and stomach. The circular muscle fibers of the esophagus cross each other ...