Module 4 Section 3

Question 1

Describe what is meant by motor unit recruitment.

Answer 1

Greater # of fibres recruited to contract = greater muscle tension.

Motor unit: when a motor neuron (innervates multiple muscle fibres) is activated, it causes all of its muscle fibres to contract.

In order to cause a stronger contraction, multiple motor units need to be recruited.

To prevent fatigue, the body can selectively rotate the activation of motor units so that some can rest while others take over. By rotating motor unit activation, contractions can be maintained for much longer.

Question 2

Describe the length-tension relationship for skeletal muscle.

Answer 2

The amount of tension that can be generated at tetanus is greatly dependent upon the length of the muscle at the onset of contraction.

Each fibre has an ideal or optimal length at which max force can be generated.

Less than optimal length:
- As the fibre length is shortened, the thin filaments are overlapping the part of the thick filament w/o cross-bridges, so the efficiency of contraction decreases as does tension.
If shortening continues, the thin filaments from opposite sides of the sarcomere become overlapped and the thick filaments come in contact w/ the z-lines
- This occurs at about 70% of the optimal length

Question 3

Describe what is meant by twitch summation and tetanus.

Answer 3

Twitch summation
- If a muscle fibre is restimulated before it has completely relaxed, the second twitch is added on to the first twich, resulting in twitch summation.

Tetanus

• If a muscle fibre is stimulated so rapidly that the twitches overlap, tetanic contraction occurs.
• The muscle fibres don’t completely relax before the next stimulus b/c they’re being stimulated at such a high rate, or fused, where there is no relaxation of the muscle fibres b/w stimuli.
• It’s the strongest single-unit twitch in muscle contraction.

Optimal length:

• At this length, the max number of cross-bridge binding sites are available to the cross-bridges for binding.
• At rest, muscle fibres are generally at the optimal length. B/c of this, they can achieve max contraction when needed.

Greater than optimal length:

• During passive stretch the distance b/w z-lines increases the amount of overlap of the thick and thin filaments decrease.
• With less overlap, less cross-bridges are available and less tension can be developed.
• At about a 30% increase of optimal length, only 50% of optimal tension can be generated.
• Sarcomere length can be increased so much that there is no overlap b/w the filaments and contraction can’t occur at all. This happens at about 70% longer than the optimal length.

Question 4

Compare and contrast the different types of skeletal muscle contractions.

Answer 4

At the level of the motor unit: isotonic and isometric contractions

Isotonic contractions:
- The muscle fibre tension remains constant as it changes length (e.g., movement goes up or down -> doing weights)

Isometric contractions:
- The muscle fibre tension increases as it remains at the same length. This is a static muscle contraction (no movement).

At the level of the whole muscle: concentric dynamic and eccentric dynamic contractions. Both require work/effort. Both a subsect of isotonic contraction.

Concentric dynamic contractions:
- It produces tension while the muscle shortens. Ex: lifting an object by the biceps muscle (bicep curls).

Eccentric dynamic contractions:
- It produces tension while the muscle lengthens. Ex: controlled lowering pf the object using the biceps muscle (lowering a weight).

Question 5

What would occur if you held a 10lbs weight with a straight arm for 5 minutes?

Answer 5

When holding a heavy load, motor units are stimulated by motor neurons at very high frequency. Each stimulus causes a twitch. If you were to hold the 10 pound weight for 5 continuous minutes with no period of relaxation, the twitches would overlap and cause what is known as fused tetanus.

The maximal contraction would occur until fatigue. At this point, the 10lbs weight would be dropped or the arm position would change in order to recruit other muscles.

Question 6

Determine whether the sarcomere would shorten (S), lengthen (L), or not change in shape (N) during concentric muscle contraction.

• Sarcomere:
• A-band:
• I-band:
• H-zone:
• Muscle length:

Answer 6

• Sarcomere: S
• A-band: N
• I-band: S
• H-zone: S
• Muscle length: S

Question 7

Determine whether the sarcomere would shorten (S), lengthen (L), or not change in shape (N) during eccentric muscle contraction.

• Sarcomere:
• A-band:
• I-band:
• H-zone:
• Muscle length:

Answer 7

• Sarcomere: L
• A-band: N
• I-band: L
• H-zone: L
• Muscle length: L

Question 8

Determine whether the sarcomere would shorten (S), lengthen (L), or not change in shape (N) during isometric/static muscle contraction.

• Sarcomere:
• A-band:
• I-band:
• H-zone:
• Muscle length:

Answer 8

• Sarcomere: S
• A-band: N
• I-band: S
• H-zone: S
• Muscle length: N