Movement Science Week 3 - Biomechanics Review

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Questions and Answers

What is the resultant force in a situation where a 25-pound traction force acts upwards while a 10-pound weight acts downwards?

15 pounds (correct)

25 pounds

35 pounds

10 pounds

Which component of a muscle force is responsible for causing torque or rotation at a joint?

Resultant force

Y component force (correct)

X component force

Joint reaction force

What does the term 'joint reaction force' refer to?

The opposite force to the resultant force at a joint (correct)

The gravitational pull on a joint

The total weight supported by a joint

The force applied by a muscle at a joint

What two elements are essential for quantifying a vector?

Magnitude and direction Signup and view all the answers

In the context of vector resolution, what happens when the line of pull runs through the axis of rotation?

It produces no torque Signup and view all the answers

Which statement best describes spatial orientation in relation to a vector?

It refers to the position of the shaft of the arrow Signup and view all the answers

How does changing the angle of a joint affect muscle forces during exercise?

It can alter the amount of torque produced Signup and view all the answers

How can vectors be classified in terms of their direction?

As positive or negative Signup and view all the answers

What does the point of application of a vector refer to?

Where the vector arrow contacts the body Signup and view all the answers

In a clinical setting, gravity acts on which part of the body segment?

The center of mass of the body segment Signup and view all the answers

What happens to the torque when the knee is flexed and the foot is brought up?

The torque decreases significantly. Signup and view all the answers

In a first class lever, where is the axis of rotation typically located?

In the middle of two opposing forces. Signup and view all the answers

What is the primary mechanical advantage of flexing the knee during a lift?

To reduce the size of the external moment arm. Signup and view all the answers

Which class of lever is characterized by the internal moment arm being greater than the external moment arm?

Second class lever. Signup and view all the answers

What is the torque calculated when the leg is fully extended at 24 pounds and 15 inches?

360 inch pounds. Signup and view all the answers

Which scenario exemplifies a second class lever in the human body?

Standing on tiptoes. Signup and view all the answers

Which factor does NOT significantly influence muscle force production in manual muscle testing?

Muscle temperature. Signup and view all the answers

What is the effect of reducing the external moment arm during lifting exercises?

It decreases the torque required. Signup and view all the answers

Study Notes

Biomechanics: Vectors

A vector is a force represented by an arrow, defined by magnitude (size, indicated by arrow length) and direction (arrowhead).
Four factors describe a vector completely: magnitude, direction, spatial orientation (shaft position; positive vectors point up/right, negative down/left), and point of application (where the arrow base contacts the body; gravity acts on the center of mass).
Vectors can be combined; the sum of forces is the resultant force. Joint reaction force is the equal and opposite force to the resultant force (Newton's Third Law).
Vector resolution breaks down a force into its components (e.g., X and Y components). The Y component creates torque (rotation), while the X component causes compression or distraction (depending on joint angle). Torque requires a force and a moment arm (perpendicular distance between axis of rotation and line of pull). The X component produces no torque if its line of pull passes through the axis of rotation.
Joint angle alters the balance between torque and compression/distraction, impacting muscle force requirements.

Biomechanics: Levers

A lever is a rigid bar pivoting around a fixed point (axis of rotation) to transmit force; bones function as levers to convert linear force to rotary torque.
Three lever classes:
- First-class lever: Axis of rotation between opposing forces (e.g., seesaw, head); mechanical advantage can be <1, >1, or =1.
- Second-class lever: Axis of rotation at one end; internal force arm > external force arm, resulting in mechanical advantage >1 (e.g., standing on tiptoes, wheelbarrow).
- Third-class lever: Axis of rotation at one end; external force arm > internal force arm, mechanical advantage <1 (most common in the musculoskeletal system; e.g., bicep curl, chopsticks).
Mechanical advantage (MA) = internal moment arm length / external moment arm length (or force arm length / resistance arm length). MA > 1 requires less muscle force; MA < 1 requires more.

Lever Systems in the Human Body

Three classes of levers exist in the human body: first, second, and third class.
First-class levers have the axis of rotation between the muscle force and the resistance force. An example is the head balanced on the neck.
The mechanical advantage (MA) in a first-class lever can be greater than or less than 1 depending on moment arm lengths. A MA > 1 means less muscle force is needed.
- Example: Head weighing 10.5 pounds, with a MA of 1.25, requires only 8.4 pounds of muscle force.
Second-class levers always have a MA greater than 1, meaning the muscle requires less force than the resistance. The resistance is between the axis and the force.
- Example: Gastrocnemius/soleus muscles lifting the body weight (150 pounds). MA of 4, meaning only 37.5 pounds of muscle force needed.
Third-class levers have a MA always less than 1, requiring significantly more muscle force than the resistance. The muscle force is between the axis and resistance. This is the most common type in the human body.
- Example: Lifting a 15-pound weight may require 105 pounds of muscle force due to a MA of 0.143.
- Examples of third-class levers in the body include the biceps brachii at the elbow, quadriceps at the knee, and supraspinatus at the shoulder.

Altering Mechanical Advantage

Therapists can alter the external moment arm, for example during manual muscle testing by changing hand placement on a limb.
Surgeons can alter the internal moment arm length by repositioning a muscle's tendon. Increasing this length increases torque.
Increasing the internal moment arm length improves torque per level of muscle force but causes:
- Reduced angular displacement of the joint.
- Reduced velocity of movement.

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Description

Test your knowledge on the fundamentals of vectors in biomechanics. This quiz covers vector definitions, factors influencing vectors, and the principles of vector resolution and torque. Perfect for students looking to solidify their understanding of biomechanical forces.