Muscle mechanics - muscle length, velocity and force relations (wk5)

Question 1

Describe how muscle length affects force output:
-Mainly on isometric contractions

Answer 1

-lo -> Natural resting length of the muscle
-As the muscle gets to 120% lo, the max force is at its highest (in the graph)
-A muscle’s ability to produce active force decreases when either short or long
-Proposed to occur due to extent of cross-bridge formation that can occur
-When dealing with biarticular muscles joint position can be used to manipulate muscle contribution

Question 2

Describe how muscle length affects force output
-Focus on passive insufficiency

Answer 2

-Passive insufficiency -> When an opposing muscle becomes stretched to the point where it can no longer lengthen and allow movement. Muscles have a finite length. Antagonist muscles relax to allow joint movements. The length of an antagonist muscle can limit the ROM. When dealing with biarticular muscles, the muscle may already be in a lengthened position from the first joint action. Passive insufficiency occurs when that muscle reaches its maximum length before second joint action is fully performed.

Question 3

Describe how muscle velocity affects force output:

Answer 3

-Force production is dependent on the velocity of movement
-For the same muscle length, concentric contractions generate less force than isometric or eccentric contractions
-The force output drops as the movement velocity increases
-Proposed to occur through the extent of cross-bridge formation that can occur
-Cybex chair measures extensor force. The reality is that we are weaker at concentric contractions, than of eccentric contractions

Question 4

Describe how muscle velocity affects force output:
-Focus on relationship with cross-bridge formation

Answer 4

-The contraction force of a muscle depends mainly on the number of muscle cells taking part in the contraction but it also depends upon the distance between origin and insertion when the muscle receives its contraction impulse. It’s the brain cells that send the electrical signal that initiates the contraction of a muscle in the body.
-What generates the force is the ability of the actin filaments of the muscle fibre to ‘climb’ along the myosin filaments via the heading-movement of the bridges. The ‘climbing’ can only occur for a certain distance, after which the actin from one end meet the actin from the other end. This means that the sarcomere can only be contracted to 50% of its original length.
-The total contraction force will be at it’s maximum when the distance between origin and insertion is 20% longer than in it’s resting position. In one laboratory test, it showed that when the muscle preparation is extended to an extreme, only the elasticity force remains. In this case, the ability of the actin filaments to climb over the myosin filaments is lost.

Question 5

Explain the relationship between muscles and the cross-bridge formation:

Answer 5

-Cross-bridges produce muscle force => Force production results from the interaction between myosin heads (thick filaments) and actin binding sites (thin filaments)
-Passive force originate from elastic recoil of muscle after stretch via connectin filament
-Active force is the force generated to form the cross-bridges (most Q’s directed towards active force)
-There is too much overlap. The actin is blocking the other actin filament, meaning the actin cannot go where it needs to go (start of graph)
-The majority of myosin can form onto the actin at the top of the graph due to the optimal length of the muscle. This means there can be lots of cross bridge formation, meaning a good contraction can be produced. (middle of graph)

Question 6

Describe and provide practical examples of active and passive insufficiency:

Answer 6

-Active insufficiency -> When a muscle becomes shortened to the point that it cannot generate or maintain active tension
-Passive insufficiency -> When the opposing muscle (antagonist) is stretched to a point where it can no longer lengthen and allow further movement, passive insufficiency is reached
-Soleus is long and then shortens during the lifting phase of the squat
-Gastrocnemius is long and then shortens during the lifting phase of the squat (this is because it is a bi-articular joint). It is shortened by the knee flexion and lengthened by the dorsiflexion of the ankle (active insufficiency)
-Seated position, knee flexed. When the knee flexes, it becomes shorter and so has less length to produce a force -> therefore there is a decreased force of the gastrocnemius
-Triceps brachii during shoulder extension and elbow extension (passive insufficiency)