Tension, Compression, Shear and Torsion

Strength coaches and physical therapy types are always talking about the types of stresses our bodies undergo. But they usually sprinkle around words such as stress, strain, load, tension, shear, compression, torsion, etc. more like they are decorating a cake than trying to teach us something. I sometimes wonder why so many like to impress us with their vocabulary but so few ever want to take the time to clue us in to the fundamental meaning of their jargon. So, here I'll take the time to explain what all the words mentioned in the title mean.

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Acromioclavicular Joint (AC Joint) Overview and Injuries

The abbreviation AC or AC joint stands for the acromioclavicular joint. The acromioclavicular is one of the three articulations of the shoulder girdle. See the shoulder complex for a general overview of the shoulder girdle and its joints.

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Fundamentals of Biomechanics by Duane Knudson: GUS Recommended for Basic Biomechanics

Whether you're a personal trainer, strength coach, or just a strength training enthusiast, you really should have one good textbook on the basics of biomechanics. Although Basic Biomechanics by Susan J. Hall is often recommended for a primer, I think there are better choices today for the beginner to biomechanics concepts. The basic textbook that I recommend is Fundamentals of Biomechanics by Duane V. Knudson. Written in an accessible style and laid out so that key and related concepts are presented together, the book just makes sense, even to someone with only a basic science background. Of course, the better your background, the more in-depth your understanding will be. As with any college level textbook, not every page of the book will be useful to every person, depending on your particular needs and practice, but Fundamentals Of Biomechanics covers the basic and more advanced concepts very well. Knudson weaves simple real-world examples throughout the book to help explain the application of the different concepts discussed. The format and integration of the various concepts, I find to be superior to the Hall book.

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Inertia: The property of an object that causes it to resist changes to its state of motion by an application of force. A stationary object will tend to resist being moved, and a moving object will resist change in its speed or direction of movement. Comparing two objects of different masses, the object with the most mass will resist changes in its state of motion the most, and so is said to have the most inertia. So, for example, a train has much more inertia than a car, and a football linebacker has more inertia than a gymnast. Although the heavier object has the most inertia, as above, it is the mass that determines inertia, as measured in kilograms.

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Relative Strength

Relative Strength: Strength relative to body weight. Literally, relative strength is a person's strength per kilogram or pound of body weight. Since absolute strength is the total strength, as defined by the total force that can be exerted regardless of body weight, relative strength is found by dividing the absolute strength by body weight.

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Condyoid Joint

Condyloid Joint: Also called an ellipsoidal joint, ovoid, or condylar joint, a bi-axial diarthrodial or synovial joint were one oval-shaped articulating surface (a condyle), fits into a corresponding ovoid depression in the other articulating surface. Movement is possible in two planes and includes flexion, extension, abduction, adduction and circumduction, but not rotation. See the diarthrodial article for examples and other types of joints.

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Close-Packed Position (joints)

A close-packed position in a synovial joint is the position in which the joint surfaces become fully congruent and their area of contact is at a maximum. This position has been described as a "screwed in" or "screwed home" position, where the joint is tightly compressed and the ligaments and joint capsule are tense, allowing no more movement. This type of position results in the bones being "locked together" is essence, as if not joint existed between them, allowing them to transmit static forces most efficiently because the joint is extremely stable.

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What is the Frontal Plane?

The frontal plane is one of several anatomical planes which are used as positional references in biomechanics, kinesiology, anatomy, and related fields. They are especially useful for describing movements. The frontal plane, also called the coronal or lateral plane, is an imaginary plane (a flat, two-dimensional surface) that passes through one side of the body to the other and divides the body into front and back halfs (anterior and posterior). It is perpendicular to any sagittal plane. Many different frontal planes can be imagined to pass through the body, but we usually refer to the frontal plane intersecting the midpoint or center of gravity of the body, to divide it into equal front and back halves. This is the cardinal frontal plane. While there can be many frontal planes, there is only one cardinal frontal plane.

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Bench Press: Flaring your elbows out versus tucking them to your sides. Plus, why you lift less on incline press.

I recently got a bench press question form a member. You know it's funny, I used to get more bench press questions than anything and after a while, I started getting more deadlift questions than anything. Which I liked until I almost have grown sick of talking about the deadlift so it's sort of a treat to get a bench press question again. The question was basically this:

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Force Velocity Relationship

Force-velocity Relationship: A propery of skeletal muscle contraction in which the force capability of a given muscle contraction is dependent on the velocity of shortening of the muscle.

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Sequential and Simultaneous Lifts? What is the Difference Between Them?

Every once in a while you will hear someone calling the squat a simultaneous lift. You'll even hear people calling the deadlift a sequential lift. What does this mean, and is it correct?

Well, these terms come from the description and measurement of the coordination of human movement, a branch of biomechanics called kinematics. Movements, in this context, are looked at in terms of the movement of body segments, and this means also the action of the body's joints. You may have never given it a second thought, but during some movements the joints act "all at once" or simultaneously and in others they act one after the other in a sequence. Most movements, however, are not really so black and white and fall in a continuum between the two. Sometimes, for instance, a movement may look to be simultaneous, but upon close observation be sequential.

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Biaxial Joint

Biaxial Joints: Joints such as the ankle and wrist. They allow movement about two perpendicular axes, which are axes at right angles to each other. This movement is usually abduction/adduction about the anteroposterior axes or flexion/extension about the mediolateral axes.

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What Is Force?

Training for maximal strength is essentially training to exert maximum muscular force. So what is force? The easiest way to think of a force is as a simple push or pull. When you push or pull on a barbell or other implement you are exerting a force. The pull of the Earth's gravity on an object is a force. Friction is a force. To be more precise, then, a force is something that causes or tends to cause a change in the motion or the shape of an object.

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Is the Hamstring to Quadriceps Strength Ratio Really Important?

Many strength trainees, bodybuilders, and exercisers are told that there should be a certain ratio between the strength of their hamstring and quadriceps muscles. Called the H/Q ratio and reported to be anywhere from .50 to .75 with a normative value of .60, the strength ratio of this important agonist/antagonist pairing is considered essential to the stability of the knee joint and to prevent ACL and other injuries. It is also sometimes thought to be predictive of those at risk for hamstring strain.

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Anatomical Direction Terms: A Glossary and Reference

Directional terms are widespread in all references concerning human performance, including anatomy, kinesiology, sports medicine, athletic training; and strength and bodybuilding coaching. At first, these terms can be confusing to the student of strength training but they are easy to understand once the fundamentals are studied.

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