Lecture 10 - Biomechanics of Muscle: Strength, Length, and Velocity

Question 1

What is muscular strength, and how is it defined?

Answer 1

Muscular strength refers to the ability of a specific muscle group to generate torque at a particular joint.

It is the force that attempts to cause rotation around a fixed center, influenced by factors such as muscle tension, moment arms, and the direction of torque.

Question 2

Explain how torque is derived in the context of muscular strength.

Answer 2

Torque is derived from the force generated by the muscle and the moment arm.

The force applied by the muscle and the distance from the axis of rotation (joint center) determine the torque.

Increasing force and the distance from the axis of rotation result in greater torque

Question 3

What factors influence the amount of torque a muscle can generate?

Answer 3

The amount of torque a muscle can generate depends on factors such as muscle length (joint angle), muscle velocity (joint velocity), and the moment arms of contributing muscles with respect to the joint center.

Question 4

Define moment arm and its significance in muscular strength.

Answer 4

The moment arm is the perpendicular distance from the line of action of the muscle to the axis or center of rotation (joint).

It determines:

leverage of the muscle force

torque generated around the joint.

Question 5

How does going through a range of motion affect torque output in muscles?

Answer 5

Going through a range of motion can change both the moment arm and the muscle length, ultimately affecting the torque output of the muscle.

This implies that torque output can vary depending on the position of the joint and the speed of movement.

Question 6

What components make up muscle forces, especially concerning their intersection with bones?

Answer 6

Muscle forces are often divided into two components:

the rotary component, which is perpendicular to the bone and causes torque around the joint,

and

the stabilizing/dislocating component, which acts along the axis of the bone either pulling the bones together or apart.

Question 7

How is muscular strength quantified in terms of torque?

Answer 7

Muscular strength, or torque, is quantified as the product of the force exerted by the muscle and the moment arm: Tm = Fm × d, where Tm represents torque, Fm is the force of the muscle, and d is the moment arm.

Question 8

Lecture Practice Question on Heidi Doc or lecture slides

Question 9

What does the muscle force-length relationship describe?

Answer 9

The muscle force-length relationship describes how the force a muscle can produce is dependent on its length at a given point in time.

Question 10

What are the two sources of force production that contribute to the overall force output of a muscle?

Answer 10

The two sources of force production are

the contractile component, representing the force output of the cross-bridges controlled by the brain,

and

the passive elastic component, which includes structures such as parallel and series elastic elements that stretch as the muscle lengthens.

Question 11

How does the passive elastic component contribute to muscle force production?

Answer 11

The passive elastic component includes structures like parallel and series elastic elements that get stretched as the muscle increases in length.

These elements contribute to muscle force production by adding passive tension to the muscle.

Question 12

What is the role of parallel elastic structures in the force-length relationship of muscles?

Answer 12

Parallel elastic structures contribute to the force-length relationship when muscles are not activated. They affect the force-length curve of the muscle.

Question 13

How does the series elastic component influence the muscle force-length relationship during the stretch-shortening cycle?

Answer 13

During the stretch-shortening cycle, the series elastic component comes into play when muscles are activated.

It contributes to the overall force output of the muscle during dynamic movements where the muscle undergoes both stretch and shortening phases.

Question 14

What factor determines the ability of cross-bridges to develop force within a muscle?

Answer 14

The ability of cross-bridges to develop force depends on the overlap between actin and myosin filaments within the muscle. When perfectly aligned, there is maximum contractile force generation capability.

Question 15

How does muscle length affect the overlap between actin and myosin filaments, and consequently, force generation?

Answer 15

When the muscle is lengthened, there is less overlap between actin and myosin filaments, resulting in fewer cross-bridges forming and reduced force generation capability. Conversely, when the muscle is shortened, the alignment is suboptimal, leading to fewer cross-bridges hitting binding sites, thus reducing maximum force.

If the muscle is too short or too long, maximum force is reduced; this is associated with reduction in available cross-bridges

Question 16

What happens to muscle tension and passive tissues beyond the resting length of a muscle?

Answer 16

Beyond the resting length, tension builds up in passive tissues such as cell membranes and tendons. This leads to a buildup of elastic energy, which contributes to muscle tension.

Question 17

How do eccentric contractions contribute to force production in muscles?

Answer 17

Eccentric contractions cause more force production due to the passive tension generated in the muscle. This passive tension increases the total tension the muscle can produce, allowing it to generate greater force. Eccentric contractions require the muscle to be stretched beyond its resting length to fully exploit this mechanism

Question 18

What happens to the maximum force capability of a muscle if it is either too short or too long?

Answer 18

If a muscle is either too short or too long, its maximum force capability is reduced. This reduction is associated with a decrease in the availability of cross-bridges for force generation.

Question 19

What is active insufficiency, and when does it occur?

Answer 19

Active insufficiency occurs when a muscle is so short that it fails to produce force when it becomes slack. This typically happens when the muscle crosses two joints.

Question 20

Can you provide an example of active insufficiency?

Answer 20

An example of active insufficiency is the decreased ability to form a fist when the wrist is in flexion. In this scenario, the muscle responsible for flexing the wrist and fingers may become too short to generate sufficient force for fist formation.

Question 21

What is passive insufficiency, and under what conditions does it occur?

Answer 21

Passive insufficiency refers to the restriction of joint range of motion when muscles are fully stretched. It occurs when muscles that cross multiple joints reach their maximal length.

Question 22

Could you give an example of passive insufficiency?

Answer 22

An example of passive insufficiency is the decreased range of motion for wrist extension when the fingers are extended. In this situation, the muscles responsible for wrist extension and finger extension may reach their maximum length, limiting the overall range of motion at the wrist joint.

Question 23

In which scenario does active and passive insufficiency particularly occur?

Answer 23

Active and passive insufficiency are particularly evident in muscles that cross multiple joints. When these muscles reach extreme lengths or shortening, they may experience limitations in force production (active insufficiency) or range of motion (passive insufficiency).

Question 24

How is force generation related to length change velocity in muscle contraction?

Answer 24

Force generation in muscle contraction is inversely related to length change velocity.

Question 25

Explain the relationship between force and velocity in concentric muscle contractions.

Answer 25

In concentric muscle contractions, the faster the muscle shortens, the lower the force developed.

Question 26

What characterizes isometric muscle contractions concerning the force-velocity relationship?

Answer 26

In isometric muscle contractions, where the rate of length change is zero, it serves as a reference for muscle force production.

Question 27

How does the force generated in eccentric muscle contractions correlate with length change velocity?

Answer 27

In eccentric muscle contractions, the faster the muscle lengthens, the greater the force developed.

Question 28

Why does eccentric contraction typically produce greater force?

Answer 28

Eccentric contractions involve fighting against external forces, leading to greater force production compared to concentric contractions.

Question 29

What is the stretch-shortening cycle in the context of the musculotendinous unit?

Answer 29

The stretch-shortening cycle refers to a sequence of muscle contractions where an eccentric contraction (muscle lengthening) is immediately followed by a concentric contraction (muscle shortening).

Question 30

How does the stretch-shortening cycle utilize potential energy stored in elastic tissues?

Answer 30

During the eccentric phase of the stretch-shortening cycle, elastic tissues in the muscle-tendon unit store potential energy as they are stretched. This potential energy is then released along with the force produced by the contractile component during the subsequent concentric contraction.

Question 31

Can you provide examples of activities that involve the stretch-shortening cycle?

Answer 31

Examples of activities that utilize the stretch-shortening cycle include squat jumps and plyometric exercises.

Question 32

What factors influence the tension-generating capability/force of muscle tissue?

Answer 32

The tension-generating capability of muscle tissue is influenced by factors such as muscle cross-sectional area, the training state of the muscle, and instantaneous muscle length and velocity, which are affected by joint angle.

Question 33

How does muscle cross-sectional area affect muscular strength?

Answer 33

Muscle cross-sectional area is positively correlated with muscular strength. A larger cross-sectional area indicates a greater number of muscle fibers, which can generate more force.

Question 34

What role does the training state of the muscle play in determining muscular strength?

Answer 34

The training state of the muscle, including factors such as muscle hypertrophy and neural adaptations, directly affects muscular strength. Regular resistance training can increase muscle strength by enhancing muscle fiber size and recruitment efficiency.

Question 35

How does the moment arm influence muscular strength?

Answer 35

The moment arm of a muscle crossing a joint affects muscular strength by determining the leverage and torque produced around the joint. It is influenced by the distance between the muscle attachment to the bone and the joint center, as well as the angle of the muscle attachment to the bone, which is affected by joint angle.

Question 36

What is the significance of the angle of muscle attachment to bone in relation to muscular strength?

Answer 36

The angle of muscle attachment to bone, which varies with joint angle, affects the moment arm and thus the torque generated by the muscle. This angle determines the effectiveness of muscle contraction in producing force around the joint.

Question 37

How is muscular power defined?

Answer 37

The product of muscular force and the velocity of muscle shortening

(Force) X (velocity)

Question 38

What is the formula for calculating muscular power in terms of force and velocity?

Answer 38

(Force) × (Velocity)

Question 39

How is muscular power related to torque production and angular velocity at a joint?

Answer 39

Muscular power can also be expressed as the product of net torque and angular velocity at a joint, which is

(Net torque) × (Angular velocity of joint).

Question 40

Difference Between Step and Stride

Answer 40

Step: movement from one foot onto the other foot

Stride: movement from one foot back onto that same foot (essentially two steps; considered one full gait cycle)

Question 41

What is the stance phase of gait, and what percentage of the gait cycle does it typically comprise?

Answer 41

Stance: when your foot is on the ground (60%)

Question 42

Describe the beginning, middle, and ending stages of the stance phase in terms of pressure and foot position.

Answer 42

Beginning: The beginning of the stance phase occurs at heel contact, characterized by more pressure on the foot, typically observed as the first peak on a pressure graph.

Middle: The middle stage of the stance phase occurs when the foot is more or less flat, resulting in less pressure on the foot, often indicated as a dip in the pressure graph.

Ending: The ending stage of the stance phase involves toe-off or toe contact just before leaving the ground, characterized by increased pressure on the foot, typically observed as the second peak on a pressure graph.

Question 43

What defines the swing phase of gait, and what proportion of the gait cycle does it represent?

Answer 43

Swing: when your foot is in the air (40%)

Question 44

Types of Support (based on stance phase)

Answer 44

Single support: when only one foot is on the ground

Double support: when both feet are on the ground (doesn’t typically happen when we run)

Flight phase (in running): when no feet are on the ground