Once an isolated muscle has been attached to a 300C series dual mode lever system or a separate force transducer and length controller, the optimal resting length (Lo) and resting tension of the muscle should be obtained. The optimal resting length is the length at which the muscle produces the maximal isometric twitch force. The muscle is stimulated with a single electrical pulse to produce a twitch response. Muscle length is increased or shortened in small increments with rest periods of at least 5 seconds between twitch stimulations to avoid fatigue of the muscle. Optimal resting length is the length at which the isometric twitch force produced is maximal.
ASI has developed an easy-to-use and time saving approach for obtaining optimal resting length of a muscle. Our DMC control software contains a feature called InstantStim. InstantStim allows the user to save the parameters of an electrical twitch stimulus which can be activated through our live data monitor with real-time feedback of the resulting force production. To setup InstantStim, go to the Setup tab on the main window and select InstantStim (Figure 1). The Setup InstantStim applet will appear (Figure 2). Enter the parameters of your twitch and press OK.
Figure 1: Dynamic Muscle Control Main Window
Figure 2: InstantStim window
Typical parameters for InstantStim may look something like this:
Pulse Frequency: 100Hz (This parameter is not used for twitch stimulations)
Pulse Width: 0.2mS – 0.5mS
Number of Pulses: 1
Train Frequency: 0.2-0.033Hz
Run Time: approximately 60 seconds
With the muscle attached the live data monitor should also be turned on to monitor the force production of the muscle (Figure 3). The resting baseline and twitch tension of the muscle will be shown in real-time. Note that the orange InstantStim button will be highlighted only when the parameters of the InstantStim are entered in the InstantStim setup window.
Figure 3: Live data monitor window
By pressing the InstantStim button on the live data monitor window a single electrical stimulus, defined by your pre-prescribed parameters, will stimulate the muscle. The resulting isometric twitch force will be seen on the live data monitor. InstantStim can be used to control the timing between each successive stimulation pulse. For instance if you would like to use 10 seconds of rest between each stimulation, enter a train frequency of 0.1Hz. This will ensure that you have the same amount of rest between each stimulation pulse while you manually increase or decrease the length of the muscle.
If incrementally increasing the length of the muscle causes the envelope of the twitch force to increase (the difference between baseline and peak) then continue incrementally increasing. If the force is less than the first twitch then shorten the length of the muscle. Continue this until the maximal isometric twitch force is obtained. Once obtained measure the length of the muscle with calipers and enter it into the DMC software. This is the Lo value of the muscle. There is a spot to enter Lo on the main DMC window which will be recorded into your data file for later analysis.
InstantStim can also be used to optimize the voltage or current used to stimulate the muscle in your experiment. Check back for our blog “Optimizing voltage/current for stimulation of muscle.”
The advantage of using InstantStim is that you do not have to run, save and analyze a protocol at each length, thereby saving valuable time that can be spent on gathering experimental data.
You are now ready to start your experimental protocol!
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