Kinematic Chains and Muscle Force Generation

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Questions and Answers

What characterizes an open kinematic chain (OKC)?

The distal segment is restricted by external resistance.

The motion requires the activation of all muscle groups.

The internal load torque always exceeds the external load torque.

The distal segment moves freely without external resistance. (correct)

Which statement is true regarding closed kinematic chains (CKC)?

CKC requires the distal segment to meet considerable external resistance. (correct)

CKC is characterized by isotonic muscle contractions only.

CKC primarily focuses on muscle lengthening during movement.

CKC allows for greater freedom of motion than OKC.

What is the relationship between internal and external torque during isometric muscle contraction?

Internal torque exceeds external torque significantly.

Internal torque is unrelated to external torque.

Internal torque is equal to external torque. (correct)

Internal torque is less than external torque.

Which type of muscle contraction is characterized by muscle lengthening?

Eccentric contraction. Signup and view all the answers

In a concentric contraction, which condition must be met?

Internal torque must exceed external torque. Signup and view all the answers

Which of the following distinguishes open kinematic chains from closed kinematic chains?

The freedom or restriction of motion at the distal segment. Signup and view all the answers

Which statement regarding muscle force generation is accurate?

Concentric contractions generate force while muscles shorten. Signup and view all the answers

How does external resistance affect internal muscle torque in closed kinematic chains?

External resistance can create imbalance between internal and external torques. Signup and view all the answers

What does the elastic modulus indicate about a material?

The likelihood of an object to deform under force Signup and view all the answers

During which phase of the stress-strain curve does the elastic modulus remain constant?

Initial linear elastic region Signup and view all the answers

Which property describes a material's ability to return to its original shape after deformation?

Elasticity Signup and view all the answers

What characterizes the intermediate region of the stress-strain curve?

Nonlinear elastic plastic behavior with yielding Signup and view all the answers

Which of the following is a characteristic of viscoelastic materials?

They show both viscous and elastic responses Signup and view all the answers

What occurs to the work done on a material when it is stretched?

It is equal to the product of force and distance Signup and view all the answers

What can lead to mechanical failure in biological tissues?

Exceeding the elastic limit of the tissue Signup and view all the answers

How is viscosity affected as temperature increases?

Viscosity decreases Signup and view all the answers

What happens to energy when force is removed from a loaded structure?

Some energy is lost, typically as heat. Signup and view all the answers

How does rapid loading affect the stiffness of connective tissue?

It increases the stiffness and requires more energy to deform. Signup and view all the answers

What is a consequence of applying a load slowly to tissue?

Lower peak force can be applied compared to rapid loading. Signup and view all the answers

What is the relationship between strain-rate sensitivity and different loading rates?

Tissues behave differently under varying loading rates, affecting stiffness. Signup and view all the answers

What happens to stress-relaxation under conditions of rapid loading?

Stress-relaxation becomes larger compared to slower loading. Signup and view all the answers

Hysteresis in the load-deformation curve indicates what about energy usage?

Some energy is lost in the process, usually as heat. Signup and view all the answers

During creep, what is observed when a tissue is subjected to a continuous load?

Creep takes longer under rapid loading conditions. Signup and view all the answers

What characterizes the load-deformation curve of dense connective tissue during unloading?

It forms a different path indicating energy loss. Signup and view all the answers

Study Notes

Open vs Closed Kinematic Chain

Closed Kinematic Chain (CKC) occurs when the distal segment of the chain is restricted by external resistance, meaning that the end of the limb is fixed in space and cannot move freely. This type of movement typically engages multiple joints and muscle groups simultaneously, enhancing stability and functional strength. CKC exercises are also beneficial for rehabilitative purposes, as they promote joint integrity and balance. For instance, during a squat, the feet remain in contact with the ground, creating resistance through the lower extremity that allows for effective engagement of the hips, knees, and ankles.
Open Kinematic Chain (OKC) occurs when the distal segment of the chain moves freely without external resistance, allowing for more isolated movements primarily targeting specific muscles. This type of exercise is often performed using machines or free weights, where the movement is accustomed to the direct action of the intended muscle group. The bicep curl, for example, allows the individual to focus solely on the contraction of the biceps brachii while minimizing engagement of other muscle groups, promoting strength and hypertrophy development in that specific area.
Examples of OKC: bicep curl, leg extension, and bench press, all of which effectively target individual muscle groups while the distal ends of the limbs are not fixed. These exercises can increase muscle mass and joint flexibility.
Examples of CKC: push-ups, squats, and lunges, which require the stabilization of the body as a whole and engage several muscles in a coordinated fashion. This promotes functional fitness and is essential for activities of daily living.

Muscle Force Generation

Isometric
- Isometric contractions occur when a muscle generates tension without changing its length, which can be important for stabilizing joints and maintaining posture without moving the bone structure.
- In this scenario, the internal torque produced by the muscle is equal to the external torque exerted by the load or resistance. This balance allows the muscle to hold a position, providing the necessary strength to counteract external forces.
Concentric
- Concentric contractions involve the shortening of muscle fibers as they generate force, which is crucial during movements such as lifting weights or climbing stairs, where the muscle acts to overcome resistance.
- During concentric contraction, the internal torque exerted by the muscle exceeds the external torque from the load, allowing for upward movement against gravity. This is indicative of a successful muscular effort to perform work on an object or perform a physical activity.
Eccentric
- Eccentric contractions occur when a muscle lengthens while generating force, often during controlled movements like lowering weights or running downhill. These contractions can be particularly effective for muscle growth and strength improvements.
- In the case of eccentric contractions, the internal torque produced is less than the external torque acting on the muscle, which can lead to the lengthening of the muscle under tension. This process is significant for muscle control and the absorption of impact forces.

Basic Mechanics

Hysteresis
- When force is applied and removed from a structure this creates a load–deformation curve
- Not all energy gained during the lengthening work is recovered during shortening work, some is lost as heat
Strain-Rate Sensitivity
- Tissues behave differently when loaded at different rates
  - Stiffness increases and larger peak forces can be applied with rapid loading
  - Creep happens more slowly with rapid loading

Basic Mechanics Concepts

Stress
- The force per unit area applied to a material
Strain
- A measure of how much the object is stretched
Stress divided by Strain
- This defines the modulus of elasticity, an indicator of how likely the object is to deform under force

Stress-Strain Curves

Three Distinct Regions:
- Initial linearly elastic region is where slope = elastic modulus (E)
- Intermediate region exhibits yielding and nonlinear elasto-plastic material behavior
- Final region exhibits linear plasticity where slope = strain hardening modulus
Therapeutic Range
- The elastic region of biological tissue is most important clinically

Viscoelasticity

All connective tissues are viscoelastic materials:
- Viscosity (fluid property): material’s resistance to flow
  - High viscosity fluids flow slowly (honey)
  - Low viscosity fluids flow quickly (water)
- Elasticity: material’s ability to return to its original length or shape after a load is removed
  - Length changes are proportional to applied loads
  - Depends on collagen and elastin content
- Work and Energy
  - When stretched, work is done (force x distance) and energy in the stretched material increases
Creep: gradual deformation of a solid material under constant load, over time

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Description

Explore the principles of open and closed kinematic chains and how they influence muscle force generation. This quiz covers examples of each type of chain and different modes of muscle contraction including isometric, concentric, and eccentric actions. Test your understanding of basic mechanics related to force application and deformation.