4. [LAB] Skeletal Muscle Contraction - Twitch, Recruitment, Summation and Tetanus

Question 1 What was the main question of the experiment?

What happens when the interval between two electrical stimuli delivered to a nerve supplying skeletal muscle is progressively decreased?

Question 2 What type of electrode was used in the experiment?

A stimulating bar electrode.

Question 3 Where was the electrical stimulus applied?

To the volunteer's hand.

Question 4 How was the stimulus intensity set?

At 5 mA above the maximal stimulus.

Question 5 What is a stimulus 5 mA above maximal called?

A supramaximal stimulus.

Question 6 Why was a supramaximal stimulus used?

To ensure the same available motor axons were activated during each trial so stimulus timing was the main variable.

Question 7 How many electrical stimuli were delivered during each trial?

Two.

Question 8 What was the longest stimulus interval tested?

1000 ms.

Question 9 What was the shortest stimulus interval tested?

50 ms.

Question 10 List the stimulus intervals used in the experiment.

1000, 500, 200, 150, 100, and 50 ms.

Question 11 What frequency corresponds to a 1000 ms interval?

1 Hz.

Question 12 What frequency corresponds to a 500 ms interval?

2 Hz.

Question 13 What frequency corresponds to a 200 ms interval?

5 Hz.

Question 14 What frequency approximately corresponds to a 150 ms interval?

6.6 Hz.

Question 15 What frequency corresponds to a 100 ms interval?

10 Hz.

Question 16 What frequency corresponds to a 50 ms interval?

20 Hz.

Question 17 What happens to stimulation frequency as the stimulus interval becomes shorter?

Frequency increases.

Question 18 What happened to the two muscle responses at long stimulus intervals?

They remained relatively separate.

Question 19 Why were the twitches separate at 1000 and 500 ms?

The muscle had enough time to contract and relax before the second stimulus.

Question 20 What happened as the stimulus interval became shorter?

The second twitch occurred before complete relaxation from the first, causing overlap.

Question 21 At which interval was twitch overlap most obvious?

50 ms.

Question 22 At approximately what interval did the twitches likely begin to summate?

About 200 ms.

Question 23 What frequency corresponded to the likely onset of summation?

About 5 Hz.

Question 24 What happened to the overlap at 150, 100, and 50 ms?

It became progressively more pronounced.

Question 25 Why did the force of the second contraction add to the first?

The second stimulus arrived while the muscle was still partially contracted.

Question 26 Which ion remained in the sarcoplasm when the second stimulus arrived early?

Calcium.

Question 27 Why was calcium still present in the sarcoplasm?

It had not yet been completely transported back into the sarcoplasmic reticulum.

Question 28 What effect did residual calcium have on cross-bridge formation?

It allowed additional actin-myosin cross-bridges to form.

Question 29 What is temporal or frequency summation?

The addition of successive twitch forces when stimuli arrive before complete muscle relaxation.

Question 30 What can repeated rapid stimulation eventually produce?

Incomplete tetanus followed by complete tetanus.

Question 31 What is an isotonic contraction?

A contraction in which muscle length changes while the load or tension remains relatively constant.

Question 32 What happens to muscle length during isotonic contraction?

It changes.

Question 33 Does an isotonic contraction perform external work?

Yes, when the muscle moves a load.

Question 34 How does load affect the speed of isotonic shortening?

A lighter load allows faster shortening, whereas a heavier load slows or may prevent shortening.

Question 35 What is a concentric contraction?

An isotonic contraction in which the muscle shortens while generating force.

Question 36 What condition allows concentric shortening?

The force generated by the muscle exceeds the external load.

Question 37 What example of concentric contraction is given in the file?

The upward lifting phase of a biceps curl.

Question 38 What is an eccentric contraction?

A contraction in which the muscle lengthens while still generating force.

Question 39 What condition produces eccentric lengthening?

The external load exceeds the force generated by the muscle.

Question 40 What example of eccentric contraction is given in the file?

Controlled lowering of a weight or walking down stairs.

Question 41 What is an isometric contraction?

A contraction in which overall muscle length remains unchanged while tension increases.

Question 42 Why can tension increase without visible movement during an isometric contraction?

The load is greater than the force generated, so contractile elements shorten while elastic elements stretch.

Question 43 Is external mechanical work performed during a purely isometric contraction?

No.

Question 44 Does an isometric contraction still consume energy?

Yes.

Question 45 What physiologic function commonly uses isometric contractions?

Maintenance of posture against gravity.

Question 46 What is a muscle twitch?

The mechanical response of a muscle to a single brief electrical stimulus.

Question 47 What electrical event appears rapidly after a single stimulus on the EMG channel?

A compound muscle action potential or CMAP.

Question 48 What does CMAP stand for?

Compound Muscle Action Potential.

Question 49 What is the latent period of a muscle twitch?

The brief delay between the electrical stimulus and measurable force development.

Question 50 What occurs during the latent period?

Action potential propagation and excitation-contraction coupling, including calcium release.

Question 51 What does the force channel look like during a single twitch?

A smooth dome-shaped rise and fall.

Question 52 What happens during the contraction phase of a twitch?

Calcium binds to troponin and cross-bridge cycling increases force.

Question 53 What happens during the relaxation phase?

Calcium is pumped back into the sarcoplasmic reticulum and force declines.

Question 54 What is motor unit recruitment?

Progressive activation of additional motor units to produce greater total muscle force.

Question 55 What happens with a subthreshold stimulus?

No EMG activity and no measurable force are recorded.

Question 56 What is a threshold stimulus?

The weakest stimulus that activates the first low-threshold motor units.

Question 57 What appears at threshold on the EMG and force channels?

A small EMG response and minimal twitch force.

Question 58 What happens as stimulus intensity increases through the submaximal range?

More motor axons reach threshold and additional motor units are recruited.

Question 59 How does EMG amplitude change during recruitment?

It increases because more muscle fibers depolarize.

Question 60 How does twitch force change during recruitment?

It increases as more motor units contribute force.

Question 61 What is the maximal stimulus?

The stimulus intensity that activates all available motor units in the stimulated nerve.

Question 62 What is a supramaximal stimulus?

A stimulus above maximal intensity that produces no further increase in recruitment.

Question 63 What happens to EMG amplitude beyond the maximal stimulus?

It plateaus.

Question 64 What happens to muscle force beyond the maximal stimulus?

It plateaus.

Question 65 Why was the increased contraction in this experiment not mainly caused by recruitment?

Stimulus intensity was already supramaximal and held constant.

Question 66 What experimental variable was deliberately changed?

Stimulus interval, and therefore stimulation frequency.

Question 67 What is summation in skeletal muscle?

An increase in total contraction strength produced by adding individual twitch forces.

Question 68 What are the two types of summation discussed in the file?

Multiple fiber summation and frequency summation.

Question 69 What is another name for multiple fiber summation?

Spatial summation.

Question 70 What triggers multiple fiber summation?

An increase in the number of active motor units.

Question 71 What principle governs recruitment from small to large motor units?

The size principle.

Question 72 Which motor units are recruited first according to the size principle?

The smallest, lowest-threshold motor units.

Question 73 What happens as stimulus strength increases under the size principle?

Progressively larger motor units are recruited.

Question 74 What is the purpose of the size principle?

To provide smooth gradation of muscle force.

Question 75 What is asynchronous recruitment?

Activation of different groups of motor units at slightly different times.

Question 76 What is the benefit of asynchronous recruitment?

It smooths muscle contraction, allows some units to rest, and helps sustain long contractions.

Question 77 What is another name for frequency summation?

Temporal summation.

Question 78 What triggers frequency summation?

An increase in the firing rate of active motor units.

Question 79 What physiological factor governs frequency summation?

Accumulation of calcium in the sarcoplasm.

Question 80 What is the endpoint of multiple fiber summation?

Recruitment of all available motor units.

Question 81 What is the endpoint of frequency summation?

Tetanization or a fused sustained contraction.

Question 82 What happens at low stimulation frequency?

Separate single twitches occur with complete relaxation between them.

Question 83 What does the EMG look like at low frequency?

Separate electrical spikes with a flat baseline between them.

Question 84 What does the force trace look like at low frequency?

Separate twitches that return to baseline before the next stimulus.

Question 85 What happens at medium stimulation frequency?

Wave summation occurs.

Question 86 Why does wave summation occur?

Each new stimulus arrives before the muscle has completely relaxed.

Question 87 Do the individual action potentials fuse electrically during wave summation?

No; the EMG still shows discrete spikes because action potentials are very brief.

Question 88 What happens to the force baseline during wave summation?

It rises as each new contraction builds on residual force from the previous twitch.

Question 89 What is the main physiological cause of wave summation?

Residual sarcoplasmic calcium maintains troponin activation and additional cross-bridge cycling.

Question 90 What is incomplete or unfused tetanus?

A high sustained contraction with small oscillations because partial relaxation still occurs between stimuli.

Question 91 What does the force trace look like during incomplete tetanus?

A high plateau with small peaks and valleys.

Question 92 What does the EMG look like during incomplete tetanus?

A rapid dense train of individual spikes.

Question 93 What is complete or fused tetanus?

A maximal sustained contraction with no visible relaxation between stimuli.

Question 94 What happens to force oscillations during complete tetanus?

They disappear.

Question 95 What does the force trace look like during complete tetanus?

A smooth, sustained maximal plateau.

Question 96 What happens to sarcoplasmic calcium during complete tetanus according to the file?

It remains highly elevated or saturated.

Question 97 What happens to cross-bridge cycling during complete tetanus?

It reaches the maximum possible sustained level.

Question 98 How long can fused tetanus persist?

Until stimulation stops or the muscle fatigues.

Question 99 What is maximal force in the context of tetanization?

The highest force reached when further increases in frequency no longer increase contraction strength.

Question 100 What is the key distinction between spatial and temporal summation?

Spatial summation increases the number of active motor units; temporal summation increases the firing frequency of active units.