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STRENGTH TRAINING – BIOMECHANICS


Muscle growth is predominantly caused by activated, single muscle fibers increasing in size after they have experienced mechanical loading during strength training. Before we get into the biomechanics, lets quickly look into the utility of a 'muscle'. What is its purpose, why exactly do we need muscles. Its clearly not just to flaunt a sic pack or for any other vanity purposes!!



Range of Motion

Range of motion describes the amount of mobility that can be demonstrated in a given joint. Active range of motion is the amount of movement that can be accomplished by contracting the muscles that normally act across a joint. Passive range of motion is the amount of movement that can be accomplished when the structures that meet at the joint are moved by an outside force, as when a therapist holds on to a patient’s forearm and moves it toward the arm, flexing the elbow joint.



A joint’s structure dictates the movements that occur at that joint. Some common angular movements are:


Flexion

A bending movement that decreases the angle of the joint to bring the articulating bones closer together.


Extension

A straightening movement that increases the angle of the joint to extend the articulating bones.


Abduction

Movement that takes limbs (can be anything, even fingers) away from the midline.


Adduction

Movement that brings limbs towards the midline.


Circumduction

A combination of flexion, extension, abduction, and adduction.


Internal/External Rotation

Rotate limb towards/away from the midline.



If the nerve supply to a muscle is damaged so that the muscle is weakened, the active range of motion for the joint acted on by that muscle may decrease, but the joint’s passive range of motion should remain unchanged. This can be one way of determining the underlying reason for inadequate muscle performance.


Fluid buildup and/or pain in or around a joint (as occurs when the soft tissues around the joint develop edema following an injury) can severely limit both the active and passive ranges of motion for that joint. With disuse, both the active and passive ranges of motion for a given joint decrease.


The range of motion for a given joint is influenced by a number of factors:

  1. Shape of the joint surfaces of the bones forming the joint

  2. Amount and shape of cartilage covering those surfaces

  3. Strength and location of ligaments and tendons surrounding the joint

  4. Strength and location of the muscles associated with the joint

  5. Amount of fluid in and around the joint

  6. Amount of pain in and around the joint

  7. Amount of use or disuse the joint has received over time


Right Knee Joint (a) Anterior superficial view. (b) Anterior deep view (knee flexed). (c) Posterior superficial view. (d) Posterior deep view.


Injuries Associated with Joint Movements

Movement of joints beyond the normal range of motion can cause dislocations and sprains. A dislocation (luxation), of a joint occurs when the joint surfaces of the bones are moved out of proper alignment. A subluxation is a partial dislocation. Dislocations are often accompanied by painful damage to the supporting ligaments and articular cartilage. A sprain occurs when ligaments are damaged. The degree of damage can range from stretched to completely torn ligaments. Sprains often result in inflammation, swelling, and pain.



Muscle Movements

Skeletal muscles do not work by themselves. Muscles are arranged in pairs based on their functions. For muscles attached to the bones of the skeleton, the connection determines the force, speed, and range of movement. These characteristics depend on each other and can explain the general organization of the muscular and skeletal systems.


Muscles and their bones work together through levers that can pivot, or move, at a stationary hinge. In the body, the joints function as fulcrums, the bones function as levers, and muscles provide the force. When muscles contract, the pull (P), or force, of muscle contraction is applied to the levers (bones), causing them to move a body part (the weight).



Types of Muscle Contractions

A muscle fiber generates tension through actin and myosin cross-bridge cycling (more on this in a later blog about muscle physiology). While under tension, the muscle may lengthen, shorten, or remain the same. Although the term contraction implies shortening, when referring to the muscular system, it means the generation of tension within a muscle fiber. Several types of muscle contractions occur and are defined by the changes in the length of the muscle during contraction.


Isotonic Contractions

Isotonic contractions maintain constant tension in the muscle as the muscle changes length. Isotonic muscle contractions can be either concentric or eccentric.


Tension > Load


Concentric Contractions

A concentric contraction is a type of muscle contraction in which the muscles shorten while generating force, overcoming resistance. For example, when lifting a heavy weight, a concentric contraction of the biceps would cause the arm to bend at the elbow, lifting the weight towards the shoulder.


Cross-bridge cycling occurs, shortening the sarcomere, muscle fiber, and muscle.


Eccentric Contractions

An eccentric contraction results in the elongation of a muscle while the muscle is still generating force; in effect, resistance is greater than force generated.


Cross-bridge cycling occurs even though the sarcomere, muscle fiber, and muscle are lengthening, controlling the extension of the muscle.




Isometric Contractions

In contrast to isotonic contractions, isometric contractions generate force without changing the length of the muscle, common in the muscles of the hand and forearm responsible for grip. Using the above example, the muscle contraction required to grip/hold onto but not move a heavy load would be isometric. Isometric contractions are frequently used to maintain posture.


Tension = Load


Cross-bridge cycling is maintaining tension in the muscle; the sarcomere, muscle fibers, and muscle are not changing length.



Muscle Attachments and Interactions

Most skeletal muscles are attached to bones. They extend from bone to bone (mostly! some facial muscles are attached to skin) across the linking joint. Everyday movements involve skeletal muscles contracting to pull one bone of a joint toward another bone of the same joint.


The two points of attachment of each muscle to the bone via tendons (dense connective tissue) are called the origin and the insertion.


Muscle Attachment

Muscles are attached to bones by tendons. The biceps brachii has two heads, which originate on the scapula. The triceps brachii has three heads, which originate on the scapula and the humerus. The biceps brachii inserts onto the radial tuberosity and onto nearby connective tissue. The triceps brachii inserts onto the olecranon process of the ulna.



Origin: Also called the fixed end, is usually the most stationary, proximal (closest to the fixed end) end of the muscle. Some muscles have more than one origin. For example, the triceps brachii has three origins that join together to form one muscle.


Insertion: Or mobile end, is usually the distal (farther away from the fixed end) end of the muscle attached to the bone being pulled toward the other bone of the joint.


Belly: Is the part of the muscle between the origin and the insertion. The bulge in a bicep is the belly!


Agonist: Although a number of muscles may be involved in an action, the principal muscle involved is called the prime mover, or agonist.


Antagonist: Is the muscle with the opposite action of the prime mover. Antagonists play two important roles in muscle function: (1) they maintain body or limb position, such as holding the arm out or standing erect; and (2) they control rapid movement, as in shadow boxing without landing a punch or the ability to check the motion of a limb.


Synergist: Are muscles which assist the prime mover. They can also be a fixator that stabilizes the bone that is the attachment for the prime mover’s origin.



For example, The biceps brachii (agonist and triceps are antagonist) flex the lower arm. The brachoradialis, in the forearm, and brachialis, located deep to the biceps in the upper arm, are both synergists that aid in this motion. Triceps on the other hand are responsible for extending (triceps are agonists in this instance and biceps become antagonists) the arm by contracting, while biceps brachii relaxes.


Most joints in the body have agonist and antagonist groups or pairs.


Working along the direction of muscle fibers from insertion (mobile end) to origin (fixed end) is super important to yield the best results in terms of contraction, leading to strength gains and muscle mass. Shortening the muscle from insertion to origin, to initiate a certain movement will illicit the muscle contracting (concentric) and stretching the muscle away from point of origin (eccentric) will lead to muscle tears, which will eventually lead to additional strength gains and muscle mass we observe. So, its all about shortening and elongating the muscle fibers!!!



The main determinant of this mechanical loading is the force-velocity relationship. However, the length-tension relationship can also have an effect, and it explains why full ranges of motion and eccentric training produce the results that they do.


The force produced by any given muscle fiber during a muscular contraction (and therefore the degree of mechanical loading that it experiences) is largely determined by

  • Whether the muscle fiber is activated, and

  • The speed at which the muscle fiber shortens, due to the force-velocity relationship.


Muscle Force – Velocity Relationship

The force generated by a muscle depends on the number of actin and myosin cross-bridges formed; a larger number of cross-bridges results in a larger amount of force. At maximum velocity no cross-bridges can form, so no force is generated, resulting in the production of zero power. So, if myofilaments slide over each other at a faster rate the ability to form cross bridges and resultant force are both reduced.


Muscle Length – Tension Relationship

Due to the presence of titin, muscles are innately elastic. Skeletal muscles are attached to bones via tendons that maintain the muscle under a constant level of stretch called the resting length. Muscles exist in this state to optimize the force produced during contraction, which is modulated by the interlaced myofilaments (Actin & Myosin – Active Force) and the elastic structural (Titin & to a small extent Collagen – Passive Force) elements inside the muscle fiber of the sarcomere.


Lengthening passive elements produces tensile forces without any chemical reactions,

which is why the force is called “passive” instead of active.


If a sarcomere is stretched too far, there will be insufficient overlap of the myofilaments and the less force will be produced. If the muscle is over-contracted, the potential for further contraction is reduced, which in turn reduces the amount of force produced. So, it makes sense that the sarcomere needs to be stretched enough that there's still some room for contraction in order to exhibit some force and not overstretched so that the myosin heads can still fully contact the actin in order to contract. And so these are the states that produce the most force


Muscle Length and Tension

The length of a muscle before it is stimulated influences the muscle’s force of contraction. As the muscle changes length, the sarcomeres also change length.



It has been known for some time that eccentric contractions can produce the greatest amounts of force, recruit fast twitch fibres with little effort, and place greater strain on the muscle to induce favourable adaptations. Now we know why!! The key feature of the length-tension relationship is the extra force that can be exerted during muscular contractions when both active and passive elements are able to contribute, which occurs when the muscle is elongated to long lengths during normal strength training, and also during eccentric training.


This extra force seems to be provided largely by titin, which contributes high levels of passive tension both when the muscle is elongated to long lengths (as in strength training with full ranges of motion) and also when the muscle is lengthened after being activated (as in eccentric contractions). The muscle’s elastic components have a major impact on force production.


Seventy two percent of the elastic energy regeneration comes from tendon tissue,

whereas 28% comes from contractile elements of the muscle.


Exercise design should take a multi-joint and multi-movement approach, operating at long lengths and high/moderate intensities. Engaging in yoga and other forms of stretching helps in packing muscle and strength a lot faster and is more efficient so try to incorporate them in your fitness regiment. Next we will look into the physiology of 'Muscle Building'.

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