0:00:06.808,0:00:08.557 Each time you take a step, 0:00:08.557,0:00:12.219 200 muscles work in unison[br]to lift your foot, 0:00:12.219,0:00:13.918 propel it forward, 0:00:13.918,0:00:15.693 and set it down. 0:00:15.693,0:00:20.703 It's just one of the many thousands[br]of tasks performed by the muscular system. 0:00:20.703,0:00:24.550 This network of over 650 muscles[br]covers the body 0:00:24.550,0:00:26.400 and is the reason we can blink, 0:00:26.400,0:00:27.650 smile, 0:00:27.650,0:00:29.010 run, 0:00:29.010,0:00:30.450 jump, 0:00:30.450,0:00:31.909 and stand upright. 0:00:31.909,0:00:35.741 It's even responsible for [br]the heart's dependable thump. 0:00:35.741,0:00:39.720 First, what exactly [br]is the muscular system? 0:00:39.720,0:00:42.218 It's made up of three main muscle types: 0:00:42.218,0:00:46.470 skeletal muscle, which attaches[br]via tendons to our bones, 0:00:46.470,0:00:50.260 cardiac muscle, which is only[br]found in the heart, 0:00:50.260,0:00:54.562 and smooth muscle, which lines[br]the blood vessels and certain organs, 0:00:54.562,0:00:57.510 like the intestine and uterus. 0:00:57.510,0:00:59.942 All three types are [br]made up of muscle cells, 0:00:59.942,0:01:03.772 also known as fibers,[br]bundled tightly together. 0:01:03.772,0:01:06.361 These bundles receive signals from[br]the nervous system 0:01:06.361,0:01:11.720 that contract the fibers, which in turn[br]generates force and motion. 0:01:11.720,0:01:14.382 This produces almost all[br]the movements we make. 0:01:14.382,0:01:16.142 Some of the only parts of the body 0:01:16.142,0:01:18.834 whose motions aren't governed [br]by the muscular system 0:01:18.834,0:01:20.992 are sperm cells, 0:01:20.992,0:01:23.344 the hair-like cilia in our airways, 0:01:23.344,0:01:25.491 and certain white blood cells. 0:01:25.491,0:01:29.044 Muscle contraction can be split[br]into three main types. 0:01:29.044,0:01:32.552 The first two, shortening muscle fibers[br]and lengthening them, 0:01:32.552,0:01:34.989 generate opposing forces. 0:01:34.989,0:01:38.974 So the biceps will shorten[br]while the triceps will lengthen or relax, 0:01:38.974,0:01:42.781 pulling up the arm and making it [br]bend at the elbow. 0:01:42.781,0:01:45.349 This allows us to, say, pick up a book, 0:01:45.349,0:01:49.449 or if the muscle relationship [br]is reversed, put it down. 0:01:49.449,0:01:53.714 This complementary partnership[br]exists throughout the muscular system. 0:01:53.714,0:01:57.945 The third type of contraction[br]creates a stabilizing force. 0:01:57.945,0:02:01.525 In these cases, the muscle fibers[br]don't change in length, 0:02:01.525,0:02:04.165 but instead keep the muscles rigid. 0:02:04.165,0:02:08.274 This allows us to grip a mug of coffee[br]or lean against a wall. 0:02:08.274,0:02:13.386 It also maintains our posture[br]by holding us upright. 0:02:13.386,0:02:16.526 Skeletal muscles form the bulk[br]of the muscular system, 0:02:16.526,0:02:19.705 make up about 30-40% of the body's weight, 0:02:19.705,0:02:22.286 and generate most of its motion. 0:02:22.286,0:02:26.242 Some muscles are familiar to us,[br]like the pectorals and the biceps. 0:02:26.242,0:02:29.362 Others may be less so,[br]like the buccinator, 0:02:29.362,0:02:32.493 a muscle that attaches your cheek[br]to your teeth, 0:02:32.493,0:02:34.532 or the body's tiniest skeletal muscle, 0:02:34.532,0:02:38.143 a one-millimeter-long tissue fragment[br]called the stapedius 0:02:38.143,0:02:41.125 that's nestled deep inside the ear. 0:02:41.125,0:02:45.733 Wherever they occur, skeletal muscles are[br]connected to the somatic nervous system, 0:02:45.733,0:02:49.504 which gives us almost complete control[br]over their movements. 0:02:49.504,0:02:52.833 This muscle group also contains[br]two types of muscle fibers 0:02:52.833,0:02:58.764 to refine our motions even further,[br]slow-twitch and fast-twitch. 0:02:58.764,0:03:01.783 Fast-twitch fibers react instantly[br]when triggered 0:03:01.783,0:03:04.805 but quickly use up their energy[br]and tire out. 0:03:04.805,0:03:07.956 Slow-twitch fibers, on the other hand,[br]are endurance cells. 0:03:07.956,0:03:12.315 They react and use energy slowly[br]so they can work for longer periods. 0:03:12.315,0:03:15.586 A sprinter will accumulate more[br]fast-twitch muscles in her legs 0:03:15.586,0:03:17.255 through continuous practice, 0:03:17.255,0:03:21.425 enabling her to quickly, if briefly,[br]pick up the pace, 0:03:21.425,0:03:24.156 whereas back muscles contain [br]more slow-twitch muscles 0:03:24.156,0:03:27.685 to maintain your posture all day. 0:03:27.685,0:03:31.116 Unlike the skeletal muscles, the body's[br]cardiac and smooth muscles 0:03:31.116,0:03:33.605 are managed by [br]the autonomic nervous system 0:03:33.605,0:03:35.616 beyond our direct control. 0:03:35.616,0:03:38.686 That makes your heart thump[br]roughly 3 billion times 0:03:38.686,0:03:40.427 over the course of your life, 0:03:40.427,0:03:43.347 which supplies the body [br]with blood and oxygen. 0:03:43.347,0:03:46.666 Autonomic control also contracts[br]and relaxes smooth muscle 0:03:46.666,0:03:48.549 in a rhythmic cycle. 0:03:48.549,0:03:52.106 That pumps blood through the smooth[br]internal walls of blood vessels, 0:03:52.106,0:03:56.367 enables the intestine to constrict and[br]push food through the digestive system, 0:03:56.367,0:04:01.708 and allows the uterus to contract[br]when a person is giving birth. 0:04:01.708,0:04:06.828 As muscles work, they also use energy[br]and produce an important byproduct, heat. 0:04:06.828,0:04:10.896 In fact, muscle provides [br]about 85% of your warmth, 0:04:10.896,0:04:14.616 which the heart and blood vessels[br]then spread evenly across the body 0:04:14.616,0:04:15.928 via the blood. 0:04:15.928,0:04:18.755 Without that, we couldn't maintain[br]the temperature necessary 0:04:18.755,0:04:20.495 for our survival. 0:04:20.495,0:04:23.670 The muscular system [br]may be largely invisible to us, 0:04:23.670,0:04:26.619 but it leaves its mark on almost[br]everything we do, 0:04:26.619,0:04:30.077 whether it's the blink of an eye[br]or a race to the finish line.