 When we take out a nerve muscle preparation from a frog and stimulate the nerve, the muscle contracts. And we can record this contraction and relaxation of the muscle. So when the nerve is stimulated and the muscle contracts and relaxes, the record obtained is known as simple muscle twitch. So if we see this record here, this is showing the point of stimulus. So that is the time when the nerve is stimulated. Then there is some time which the muscle takes to start contracting. So this is the time the muscle takes to start contracting and this upward phase is showing the contraction and this downward phase is showing the relaxation of the muscle. So basically we see there are three phases in simple muscle twitch. One is the latent period that is the time taken from the point of stimulus to the start of the contraction. Then there is contraction period that is from the start of the contraction to the maximum strength of contraction. So this is contraction period and then there is relaxation period that is when the muscle starts relaxing from the point of maximum contraction to the end of relaxation. So this is relaxation period. Latent period is in a frog is approximately 10 to 20 milliseconds, contraction period is approximately 40 to 50 milliseconds and relaxation period is approximately 50 to 60 milliseconds. Now the question arises that why is there a latent period? Well, you see we are stimulating the nerve and what happens when we stimulate the nerve with adequate strength of a stimulus, there is an action potential and this action potential travels along the nerve. So action potential propagation takes some time, right? Then once it reaches the neuromuscular junction, there will be steps of neuromuscular transmission, then only the skeletal muscle will be excited, right? So there is neuromuscular transmission and when the skeletal muscle is excited, there will be release of calcium from the stores and then the calcium is going to bind to troponin. So there is something known as excitation contraction coupling. So this much time is required for the start of the contraction, for the propagation of the nerve impulse, for neuromuscular transmission and excitation contraction coupling. So these are the reasons why there is some delay between the point of a stimulus and the start of contraction reasons for latent period. Now, if the device on which we are recording is a mechanical device that is there is a liver which is attached to the muscle, then this mechanical device has some inertia as well. Well, this is not the actual setup just for explaining that there will be inertia for recording I have just put to write. So this is not the way that the liver is attached. Anyways, so in a mechanical device, there will be inertia of the liver as well. If we are recording electronically, then this inertia will not be there. In mechanical device, this also becomes a reason for latent period. Fine. Now, if I ask you a question that while recording simple muscle twitch, is the whole muscle contracting or only a few muscle fibers contracting? Well, it depends that how much strength of contraction which we have used, right? If the strength of contraction is such that the whole nerve that is all the neurons in the nerve are stimulated, then the whole muscle is going to contract. You see, the muscle is made up of lot of muscle fibers, right? So these are the muscle fibers and alpha motor neurons supply few of these muscle fibers. So this alpha motor neurons with the muscle fibers which it supplies that is known as motor unit. So here, this will form one motor unit, this is going to form one motor unit. Now, the stimulus which we use may excite all the motor units. If it is too high a strength of a stimulus or if it is of lesser strength, then it may excite maybe only a single motor unit. But because that particular muscle fibers will contract and relax, yet we will record the simple muscle twitch, right? So simple muscle twitch can be obtained with any strength of a stimulus. Obviously, it should be above the threshold stimulus for this concept on strength of a stimulus and recruitment of motor units. Watch another video on recruitment of motor units. Thanks for watching the video. If you like the video, do press the like button, share the video with others. And don't forget to subscribe to the channel physiology open. And now you can also access notes of physiology, the link for which I have given in the description section below. You can click on it, go to the website and check them out. Thank you. Once again, have a nice day.