The force produced by a skeletal muscle depends on three main factors: (a) the number of active fibres, (b) the length of the fibres, and (c) the level of activation of the fibres.
In this lab practical you will do a set of virtual experiments on a simulated skeletal muscle and study its behaviour. The parameters that you can control are: (a) muscle length, and (b) stimulation rate to adjust activation level.
Aim: (1) To study twitch, summation and tetanus in skeletal muscle.
(2) To study the length-tension behaviour of skeletal muscle.
(3) To observe fatigue and recovery in different types of skeletal muscle.
(4) To understand the force-velocity experiment.
Procedure:
In the first experiment you will see a screen similar to that shown in the following figure. The main portion of the window is a grid where the muscle force for 4 seconds at the current time is displayed.
On the top of the window are a set of controls to adjust muscle length, and stimulation. There is a checkbox on the top right that can be used to freeze the display – this is useful if the force changes and you want to note the changing values.
On the left side is a cartoon picture of a muscle as it would appear in a real physical experiment. One of the muscle is fixed and the other end is moved to adjust length. A force transducer is attached at one of the ends to record the muscle force.
Passive length-tension property:
If you adjust the muscle length, the stretched muscle exhibits some tension and this is reflected in the display. Change the muscle length in small increments and record the tension in Newtons.
Twitch, Summation and Tetanus:
Keep the muscle length at 35mm and set the stimulation rate at 0.5 Hz. Now if you press the “stimuate” button, one stimulus will be delivered and you will see a single twitch. Pressing the stimulate button once delivers stimuli at the selected rate for 2 seconds. Repeat this with stimulation rates of 1, 2, 4, 8, 16, and 32Hz. Note the peak force in each case. The peak active force must be measured from the baseline force – i.e., subtract the passive tension from the total force. At what frequency does summation of twitches occur?
Active length-tension property:
Set the stimulation frequency at 16Hz and for different muscle lengths, measure the total force and passive tension – using these values calculate the active force of the muscle at each length. Plot tension/force against length – passive, total and active.
Repeat the length-tension measurements at two different stimulation frequencies, 4Hz and 32 Hz.
The next virtual experiment will be on muscle fatigue. Muscle fatigue may be defined as a decrease in the capacity of a muscle to produce force. Therefore, if under identical conditions of stimulation, the muscle produces less force after exercise, then the muscle may be said to be fatigued. Recovery from fatigue will depend on the duration of rest. Different types of muscle have different susceptibility for fatigue.
Select Type I muscle, and a stimulation rate of 32Hz. When you turn ON stimulation it will continue until you turn it OFF. Turn on stimulation. Note the initial maximum tetanic force. Use your watch to note the decreasing force against time. When the force has dropped to 10% of the initial value, turn OFF the stimulator. Now you must observe recovery. In order to note the recovery of force, you should periodically apply a stimulus to see what level the force is at. If you use a high rate of stimulation, the test stimuli will themselves cause fatigue. Therefore, use a twitch to monitor recovery. To measure the twitch force accurately you will need to magnify the display and set it to Gain=5 (or some other suitable value). Now with the stimulus rate set to 0.5Hz, turn on the stimulus at regular intervals (say 5s) and note the amplitude of the twitch.
Plot the fatigue and recovery as a function of time. Repeat with stimulation rates of 8Hz and 16Hz. Repeat these for Type II muscle. Here the fatigue and recovery takes place more quickly so you’ll have to take your readings more quickly.
Note that the time scales of fatigue and recovery have been speeded up in this virtual experiment to make the lab shorter.
The practical effect of the length dependence of muscle force is best understood by looking at the interaction between muscle activation and load. Use the 3rd virtual experiment tostudy thi.
The force-velocity experiment was very important in the understanding of muscle contraction. Use the accompanying ssimulation to understand how the experiment was performed.