To create movement, muscles contract and pull on external structures. Most skeletal muscles attach to bones through connective tissue (tendons). Where a muscle attaches affects how it moves a joint.
Origin (proximal attachment): The attachment point closer to the body’s center.
Insertion (distal attachment): The attachment point farther from the body’s center.
Roles of muscles:
Agonists (prime movers): The main muscles responsible for producing a specific movement.
Antagonists: Muscles that oppose the agonist’s action. They help control movement, stabilize joints, and reduce injury risk.
Synergists: Muscles that assist the agonist, often by stabilizing nearby joints or fine-tuning the movement.
Stabilizers: Muscles that hold posture and joint position, creating a steady base so prime movers and synergists can work 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 while producing force, such as the upward phase of a biceps curl.
Eccentric contraction: The muscle lengthens under tension to control or resist movement, such as lowering the weight in a biceps curl.
Isometric contraction: The muscle produces force without changing length, helping stabilize joints or hold a position, such as holding a plank or pausing mid-curl.
Tendon insertion: Where a tendon attaches to bone strongly influences how a muscle moves a joint.
If the tendon inserts farther from the joint’s center, the muscle must shorten more to create the same amount of joint rotation, but it can generate greater torque.
If the tendon inserts closer to the joint’s center, the joint can move faster, but torque is reduced.
For example, athletes with more distal tendon insertions may be better suited to strength-focused tasks, while those with more proximal insertions may be better suited to speed-focused tasks.
Levers in the musculoskeletal system
In the human body, bones act as levers, joints act as fulcrums, and muscles provide the forces that move the levers. How effective a lever system is depends on the relationship between muscle force, resistive force, moment arm, and torque.
Key concepts
Muscle force (Fm):
The force produced by muscle contraction and transmitted to bones through tendons.
Muscle force is influenced by factors such as muscle size, neural activation, and the angle of muscle pull relative to the lever.
Resistive force (Fr):
The external force that opposes muscle action, such as the weight of a barbell or a body segment.
Resistive force can vary through the range of motion as leverage and mechanical advantage change.
Moment arm:
The perpendicular distance from a force’s line of action to the fulcrum (axis of rotation).
A longer muscle-force moment arm increases torque production, while a longer resistive-force moment arm increases the load the muscle must overcome.
Torque (T):
The rotational effect of a force, calculated as: Magnitude of a force x the length of its moment arm.
To move a joint, muscles must generate enough torque to overcome the resistive torque created by the load.
Lever
Lever classifications
First-class levers:
The fulcrum lies between the applied muscle force and the resistive force.
Example: The triceps during elbow extension (e.g., a triceps pushdown).
These levers can create a mechanical advantage or disadvantage depending on the relative moment arm lengths.
First-class lever
Second-class levers:
The resistive force lies between the fulcrum and the applied muscle force.
Example: Standing calf raises, where the ball of the foot is the fulcrum, body weight is the resistive force, and the calf muscles provide the applied force.
These levers typically provide a mechanical advantage because the muscle-force moment arm is greater than the resistive-force moment arm.
Second-class lever
Third-class levers:
The muscle force is applied between the fulcrum and the resistive force.
Example: Biceps curls, where the elbow is the fulcrum, the biceps generate muscle force, and the dumbbell provides resistive force.
These levers usually operate at a mechanical disadvantage, meaning the muscle must produce high force to move a relatively smaller resistive force. In return, they allow 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 is more efficient and requires less muscle force to overcome the resistive force.
When , as in most human joints, the muscle must produce more force than the resistive load, increasing demand on the musculoskeletal system.
Practical application:
You can modify exercises by changing body position or equipment to change moment arms. For example:
A deeper squat increases the resistive-force moment arm, so the quadriceps must produce greater torque.
Using a wider grip on a bench press increases the resistive-force moment arm at the shoulders, which changes muscle recruitment patterns.
Torque and movement efficiency
Torque is central to human movement mechanics. During exercises:
High torque demand: Exercises with long resistive moment arms (e.g., squats, deadlifts) require higher muscle force and greater joint stability.
Variable torque in range of motion: Torque demands change through the range of motion as moment arm lengths change. For example:
In a biceps curl, 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. These planes divide the body into sections and give you a consistent way to 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 includes movements across all planes to support balanced development and reduce injury risk.
Understanding these planes helps coaches 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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