[Script Info]
Title: 
[Events]
Format: Layer, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text
Dialogue: 0,0:00:06.81,0:00:08.56,Default,,0000,0000,0000,,Each time you take a step,
Dialogue: 0,0:00:08.56,0:00:12.22,Default,,0000,0000,0000,,200 muscles work in unison\Nto lift your foot,
Dialogue: 0,0:00:12.22,0:00:13.92,Default,,0000,0000,0000,,propel it forward,
Dialogue: 0,0:00:13.92,0:00:15.69,Default,,0000,0000,0000,,and set it down.
Dialogue: 0,0:00:15.69,0:00:20.70,Default,,0000,0000,0000,,It's just one of the many thousands\Nof tasks performed by the muscular system.
Dialogue: 0,0:00:20.70,0:00:24.55,Default,,0000,0000,0000,,This network of over 650 muscles\Ncovers the body
Dialogue: 0,0:00:24.55,0:00:26.40,Default,,0000,0000,0000,,and is the reason we can blink,
Dialogue: 0,0:00:26.40,0:00:27.65,Default,,0000,0000,0000,,smile,
Dialogue: 0,0:00:27.65,0:00:29.01,Default,,0000,0000,0000,,run,
Dialogue: 0,0:00:29.01,0:00:30.45,Default,,0000,0000,0000,,jump,
Dialogue: 0,0:00:30.45,0:00:31.91,Default,,0000,0000,0000,,and stand upright.
Dialogue: 0,0:00:31.91,0:00:35.74,Default,,0000,0000,0000,,It's even responsible for \Nthe heart's dependable thump.
Dialogue: 0,0:00:35.74,0:00:39.72,Default,,0000,0000,0000,,First, what exactly \Nis the muscular system?
Dialogue: 0,0:00:39.72,0:00:42.22,Default,,0000,0000,0000,,It's made up of three main muscle types:
Dialogue: 0,0:00:42.22,0:00:46.47,Default,,0000,0000,0000,,skeletal muscle, which attaches\Nvia tendons to our bones,
Dialogue: 0,0:00:46.47,0:00:50.26,Default,,0000,0000,0000,,cardiac muscle, which is only\Nfound in the heart,
Dialogue: 0,0:00:50.26,0:00:54.56,Default,,0000,0000,0000,,and smooth muscle, which lines\Nthe blood vessels and certain organs,
Dialogue: 0,0:00:54.56,0:00:57.51,Default,,0000,0000,0000,,like the intestine and uterus.
Dialogue: 0,0:00:57.51,0:00:59.94,Default,,0000,0000,0000,,All three types are \Nmade up of muscle cells,
Dialogue: 0,0:00:59.94,0:01:03.77,Default,,0000,0000,0000,,also known as fibers,\Nbundled tightly together.
Dialogue: 0,0:01:03.77,0:01:06.36,Default,,0000,0000,0000,,These bundles receive signals from\Nthe nervous system
Dialogue: 0,0:01:06.36,0:01:11.72,Default,,0000,0000,0000,,that contract the fibers, which in turn\Ngenerates force and motion.
Dialogue: 0,0:01:11.72,0:01:14.38,Default,,0000,0000,0000,,This produces almost all\Nthe movements we make.
Dialogue: 0,0:01:14.38,0:01:16.14,Default,,0000,0000,0000,,Some of the only parts of the body
Dialogue: 0,0:01:16.14,0:01:18.83,Default,,0000,0000,0000,,whose motions aren't governed \Nby the muscular system
Dialogue: 0,0:01:18.83,0:01:20.99,Default,,0000,0000,0000,,are sperm cells,
Dialogue: 0,0:01:20.99,0:01:23.34,Default,,0000,0000,0000,,the hair-like cilia in our airways,
Dialogue: 0,0:01:23.34,0:01:25.49,Default,,0000,0000,0000,,and certain white blood cells.
Dialogue: 0,0:01:25.49,0:01:29.04,Default,,0000,0000,0000,,Muscle contraction can be split\Ninto three main types.
Dialogue: 0,0:01:29.04,0:01:32.55,Default,,0000,0000,0000,,The first two, shortening muscle fibers\Nand lengthening them,
Dialogue: 0,0:01:32.55,0:01:34.99,Default,,0000,0000,0000,,generate opposing forces.
Dialogue: 0,0:01:34.99,0:01:38.97,Default,,0000,0000,0000,,So the biceps will shorten\Nwhile the triceps will lengthen or relax,
Dialogue: 0,0:01:38.97,0:01:42.78,Default,,0000,0000,0000,,pulling up the arm and making it \Nbend at the elbow.
Dialogue: 0,0:01:42.78,0:01:45.35,Default,,0000,0000,0000,,This allows us to, say, pick up a book,
Dialogue: 0,0:01:45.35,0:01:49.45,Default,,0000,0000,0000,,or if the muscle relationship \Nis reversed, put it down.
Dialogue: 0,0:01:49.45,0:01:53.71,Default,,0000,0000,0000,,This complementary partnership\Nexists throughout the muscular system.
Dialogue: 0,0:01:53.71,0:01:57.94,Default,,0000,0000,0000,,The third type of contraction\Ncreates a stabilizing force.
Dialogue: 0,0:01:57.94,0:02:01.52,Default,,0000,0000,0000,,In these cases, the muscle fibers\Ndon't change in length,
Dialogue: 0,0:02:01.52,0:02:04.16,Default,,0000,0000,0000,,but instead keep the muscles rigid.
Dialogue: 0,0:02:04.16,0:02:08.27,Default,,0000,0000,0000,,This allows us to grip a mug of coffee\Nor lean against a wall.
Dialogue: 0,0:02:08.27,0:02:13.39,Default,,0000,0000,0000,,It also maintains our posture\Nby holding us upright.
Dialogue: 0,0:02:13.39,0:02:16.53,Default,,0000,0000,0000,,Skeletal muscles form the bulk\Nof the muscular system,
Dialogue: 0,0:02:16.53,0:02:19.70,Default,,0000,0000,0000,,make up about 30-40% of the body's weight,
Dialogue: 0,0:02:19.70,0:02:22.29,Default,,0000,0000,0000,,and generate most of its motion.
Dialogue: 0,0:02:22.29,0:02:26.24,Default,,0000,0000,0000,,Some muscles are familiar to us,\Nlike the pectorals and the biceps.
Dialogue: 0,0:02:26.24,0:02:29.36,Default,,0000,0000,0000,,Others may be less so,\Nlike the buccinator,
Dialogue: 0,0:02:29.36,0:02:32.49,Default,,0000,0000,0000,,a muscle that attaches your cheek\Nto your teeth,
Dialogue: 0,0:02:32.49,0:02:34.53,Default,,0000,0000,0000,,or the body's tiniest skeletal muscle,
Dialogue: 0,0:02:34.53,0:02:38.14,Default,,0000,0000,0000,,a one-millimeter-long tissue fragment\Ncalled the stapedius
Dialogue: 0,0:02:38.14,0:02:41.12,Default,,0000,0000,0000,,that's nestled deep inside the ear.
Dialogue: 0,0:02:41.12,0:02:45.73,Default,,0000,0000,0000,,Wherever they occur, skeletal muscles are\Nconnected to the somatic nervous system,
Dialogue: 0,0:02:45.73,0:02:49.50,Default,,0000,0000,0000,,which gives us almost complete control\Nover their movements.
Dialogue: 0,0:02:49.50,0:02:52.83,Default,,0000,0000,0000,,This muscle group also contains\Ntwo types of muscle fibers
Dialogue: 0,0:02:52.83,0:02:58.76,Default,,0000,0000,0000,,to refine our motions even further,\Nslow-twitch and fast-twitch.
Dialogue: 0,0:02:58.76,0:03:01.78,Default,,0000,0000,0000,,Fast-twitch fibers react instantly\Nwhen triggered
Dialogue: 0,0:03:01.78,0:03:04.80,Default,,0000,0000,0000,,but quickly use up their energy\Nand tire out.
Dialogue: 0,0:03:04.80,0:03:07.96,Default,,0000,0000,0000,,Slow-twitch fibers, on the other hand,\Nare endurance cells.
Dialogue: 0,0:03:07.96,0:03:12.32,Default,,0000,0000,0000,,They react and use energy slowly\Nso they can work for longer periods.
Dialogue: 0,0:03:12.32,0:03:15.59,Default,,0000,0000,0000,,A sprinter will accumulate more\Nfast-twitch muscles in her legs
Dialogue: 0,0:03:15.59,0:03:17.26,Default,,0000,0000,0000,,through continuous practice,
Dialogue: 0,0:03:17.26,0:03:21.42,Default,,0000,0000,0000,,enabling her to quickly, if briefly,\Npick up the pace,
Dialogue: 0,0:03:21.42,0:03:24.16,Default,,0000,0000,0000,,whereas back muscles contain \Nmore slow-twitch muscles
Dialogue: 0,0:03:24.16,0:03:27.68,Default,,0000,0000,0000,,to maintain your posture all day.
Dialogue: 0,0:03:27.68,0:03:31.12,Default,,0000,0000,0000,,Unlike the skeletal muscles, the body's\Ncardiac and smooth muscles
Dialogue: 0,0:03:31.12,0:03:33.60,Default,,0000,0000,0000,,are managed by \Nthe autonomic nervous system
Dialogue: 0,0:03:33.60,0:03:35.62,Default,,0000,0000,0000,,beyond our direct control.
Dialogue: 0,0:03:35.62,0:03:38.69,Default,,0000,0000,0000,,That makes your heart thump\Nroughly 3 billion times
Dialogue: 0,0:03:38.69,0:03:40.43,Default,,0000,0000,0000,,over the course of your life,
Dialogue: 0,0:03:40.43,0:03:43.35,Default,,0000,0000,0000,,which supplies the body \Nwith blood and oxygen.
Dialogue: 0,0:03:43.35,0:03:46.67,Default,,0000,0000,0000,,Autonomic control also contracts\Nand relaxes smooth muscle
Dialogue: 0,0:03:46.67,0:03:48.55,Default,,0000,0000,0000,,in a rhythmic cycle.
Dialogue: 0,0:03:48.55,0:03:52.11,Default,,0000,0000,0000,,That pumps blood through the smooth\Ninternal walls of blood vessels,
Dialogue: 0,0:03:52.11,0:03:56.37,Default,,0000,0000,0000,,enables the intestine to constrict and\Npush food through the digestive system,
Dialogue: 0,0:03:56.37,0:04:01.71,Default,,0000,0000,0000,,and allows the uterus to contract\Nwhen a person is giving birth.
Dialogue: 0,0:04:01.71,0:04:06.83,Default,,0000,0000,0000,,As muscles work, they also use energy\Nand produce an important byproduct, heat.
Dialogue: 0,0:04:06.83,0:04:10.90,Default,,0000,0000,0000,,In fact, muscle provides \Nabout 85% of your warmth,
Dialogue: 0,0:04:10.90,0:04:14.62,Default,,0000,0000,0000,,which the heart and blood vessels\Nthen spread evenly across the body
Dialogue: 0,0:04:14.62,0:04:15.93,Default,,0000,0000,0000,,via the blood.
Dialogue: 0,0:04:15.93,0:04:18.76,Default,,0000,0000,0000,,Without that, we couldn't maintain\Nthe temperature necessary
Dialogue: 0,0:04:18.76,0:04:20.50,Default,,0000,0000,0000,,for our survival.
Dialogue: 0,0:04:20.50,0:04:23.67,Default,,0000,0000,0000,,The muscular system \Nmay be largely invisible to us,
Dialogue: 0,0:04:23.67,0:04:26.62,Default,,0000,0000,0000,,but it leaves its mark on almost\Neverything we do,
Dialogue: 0,0:04:26.62,0:04:30.08,Default,,0000,0000,0000,,whether it's the blink of an eye\Nor a race to the finish line.