To generate movement, muscles contract to exert forces against external objects. Muscles are attached to bones by connective tissues, with specific attachment points influencing their function:
Origin (proximal attachment): Closer to the body’s center.
Insertion (distal attachment): Farther from the body’s center.
Roles of muscles:
Agonists (prime movers): Responsible for primary movement.
Antagonists: Oppose the movement of agonists to stabilize joints and prevent injury.
Synergists: Assist agonists indirectly by stabilizing surrounding joints or fine-tuning movement.
Stabilizers: Maintain posture and joint position, providing a steady base so prime movers and synergists can act efficiently.
Muscle mechanics:
Movement efficiency depends on factors such as muscle fiber alignment, tendon insertion points, and contraction type (e.g., concentric, eccentric, or isometric).
Concentric contraction: The muscle shortens as it produces force, such as during the upward phase of a biceps curl.
Eccentric contraction: The muscle lengthens under tension, controlling or resisting movement, such as lowering the weight in a biceps curl.
Isometric contraction: The muscle generates force without changing length, stabilizing joints or holding a position, such as holding a plank or pausing mid-curl.
Tendon insertion: The point where the tendon attaches to the bone greatly affects muscle function. Tendons inserted farther from the joint’s center require the muscle to contract more to produce the same amount of joint rotation but allow for greater torque generation. Conversely, tendons inserted closer to the joint produce faster joint motion but at the expense of reduced torque. For example, athletes with distal tendon insertions may excel in tasks requiring strength, while those with proximal insertions may favor speed-based activities.
Levers in the musculoskeletal system
In the human body, bones act as levers, joints serve as fulcrums, and muscles generate the forces required for movement. The effectiveness of these levers depends on the interaction between muscle force, resistive force, moment arm, and torque.
Key concepts
Muscle force (Fm):
The force generated by the contraction of muscles, transmitted to the bones through tendons.
This force is influenced by factors like muscle size, neural activation, and angle of muscle pull relative to the lever.
Resistive force (Fr):
The external force opposing muscle action, such as the weight of a barbell or body segment.
Resistive force varies throughout the range of motion due to changes in leverage and mechanical advantage.
Moment arm:
The perpendicular distance from the line of action of a force to the fulcrum (axis of rotation).
A longer moment arm for muscle force increases torque production, while a longer resistive force moment arm increases the load on the muscle.
Torque (T):
The rotational effect of a force, calculated as: Magnitude of a force x the length of its moment arm.
Muscles must generate sufficient torque to overcome the resistive torque (caused by the load).
Lever
Lever classifications
First-class levers:
Fulcrum is positioned between the applied muscle force and resistive force.
Example: The triceps during elbow extension (e.g., a triceps pushdown).
These levers may provide mechanical advantage or disadvantage depending on the moment arm lengths.
First-class lever
Second-class levers:
Resistive force lies between the fulcrum and applied muscle force.
Example: Standing calf raises, where the ball of the foot acts as the fulcrum, body weight provides the resistive force, and the calf muscles generate the applied force.
These levers typically provide a mechanical advantage, as the moment arm for muscle force is greater than that of the resistive force.
Second-class lever
Third-class levers:
Muscle force is applied between the fulcrum and resistive force.
Example: Biceps curls, where the elbow acts as the fulcrum, the biceps generate muscle force, and the dumbbell creates resistive force.
These levers generally operate at a mechanical disadvantage, requiring high muscle force to move relatively small resistive forces. However, they allow for greater speed and range of motion.
Most human joints function as third-class levers
Third-class lever
Mechanical advantage and training implications
Mechanical advantage (MA):
When , the lever system operates efficiently, requiring less muscle force to overcome the resistive force.
When , as in most human joints, the muscle must produce greater force than the resistive load, increasing the demand on the musculoskeletal system.
Practical application:
Exercises can be modified by changing body position or equipment to adjust the moment arms. For example:
A deeper squat increases the moment arm of the resistive force, requiring greater torque from the quadriceps.
Using a wider grip on a bench press increases the moment arm of resistive force at the shoulders, altering muscle recruitment patterns.
Torque and movement efficiency
Torque is the cornerstone of human movement mechanics. During exercises:
High torque demand: Exercises with long resistive moment arms (e.g., squats, deadlifts) require higher muscle force and joint stability.
Variable torque in range of motion: Torque requirements change throughout the range of motion due to shifting moment arm lengths. For instance:
In a biceps curl, the torque is highest when the forearm is parallel to the ground because the resistive moment arm is longest.
Torque and strength curves:
Ascending strength curve: Force output increases through the range of motion (e.g., squat, deadlift).
Descending strength curve: Force output decreases as the range of motion progresses (e.g., bench press, triceps extension).
Bell-shaped strength curve: Force output is greatest in the middle portion of the movement (e.g., biceps curl).
By understanding and manipulating these principles, coaches can optimize exercise selection and execution to target specific muscles and improve performance outcomes.
Anatomical planes
Human movement occurs within three primary anatomical planes, which divide the body into sections and help describe motion and exercise mechanics:
Sagittal plane: Divides the body into left and right halves. Movements in this plane include flexion and extension, such as bicep curls, squats, and running.
Frontal plane: Divides the body into front and back halves. Movements in this plane include abduction and adduction, such as lateral raises, side lunges, and jumping jacks.
Transverse plane: Divides the body into top and bottom halves. Movements in this plane include rotational actions, such as trunk twists, throwing motions, and cable woodchops.
Anatomical planes
Importance in training:
Effective resistance training incorporates movements across all planes to ensure balanced development and reduce the risk of injury.
Understanding these planes allows coaches to design programs that improve functional performance for specific sports or activities.
Movements in the transverse plane are critical in rotational sports such as tennis, baseball, and golf.
Movements in the frontal plane (e.g., lateral cutting, side shuffles) are especially important in sports like football, basketball, and soccer.
The sagittal plane dominates in forward and backward movements such as sprinting, cycling, and squatting.
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