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  "events": [ {
    "tStartMs": 6808,
    "dDurationMs": 1749,
    "segs": [ {
      "utf8": "Each time you take a step,"
    } ]
  }, {
    "tStartMs": 8557,
    "dDurationMs": 3662,
    "segs": [ {
      "utf8": "200 muscles work in unison\nto lift your foot,"
    } ]
  }, {
    "tStartMs": 12219,
    "dDurationMs": 1699,
    "segs": [ {
      "utf8": "propel it forward,"
    } ]
  }, {
    "tStartMs": 13918,
    "dDurationMs": 1775,
    "segs": [ {
      "utf8": "and set it down."
    } ]
  }, {
    "tStartMs": 15693,
    "dDurationMs": 5010,
    "segs": [ {
      "utf8": "It's just one of the many thousands\nof tasks performed by the muscular system."
    } ]
  }, {
    "tStartMs": 20703,
    "dDurationMs": 3847,
    "segs": [ {
      "utf8": "This network of over 650 muscles\ncovers the body"
    } ]
  }, {
    "tStartMs": 24550,
    "dDurationMs": 1850,
    "segs": [ {
      "utf8": "and is the reason we can blink,"
    } ]
  }, {
    "tStartMs": 26400,
    "dDurationMs": 1250,
    "segs": [ {
      "utf8": "smile,"
    } ]
  }, {
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    "dDurationMs": 1360,
    "segs": [ {
      "utf8": "run,"
    } ]
  }, {
    "tStartMs": 29010,
    "dDurationMs": 1440,
    "segs": [ {
      "utf8": "jump,"
    } ]
  }, {
    "tStartMs": 30450,
    "dDurationMs": 1459,
    "segs": [ {
      "utf8": "and stand upright."
    } ]
  }, {
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    "dDurationMs": 3832,
    "segs": [ {
      "utf8": "It's even responsible for \nthe heart's dependable thump."
    } ]
  }, {
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    "segs": [ {
      "utf8": "First, what exactly \nis the muscular system?"
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    "segs": [ {
      "utf8": "It's made up of three main muscle types:"
    } ]
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    "segs": [ {
      "utf8": "skeletal muscle, which attaches\nvia tendons to our bones,"
    } ]
  }, {
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    "segs": [ {
      "utf8": "cardiac muscle, which is only\nfound in the heart,"
    } ]
  }, {
    "tStartMs": 50260,
    "dDurationMs": 4302,
    "segs": [ {
      "utf8": "and smooth muscle, which lines\nthe blood vessels and certain organs,"
    } ]
  }, {
    "tStartMs": 54562,
    "dDurationMs": 2948,
    "segs": [ {
      "utf8": "like the intestine and uterus."
    } ]
  }, {
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    "dDurationMs": 2432,
    "segs": [ {
      "utf8": "All three types are \nmade up of muscle cells,"
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  }, {
    "tStartMs": 59942,
    "dDurationMs": 3830,
    "segs": [ {
      "utf8": "also known as fibers,\nbundled tightly together."
    } ]
  }, {
    "tStartMs": 63772,
    "dDurationMs": 2589,
    "segs": [ {
      "utf8": "These bundles receive signals from\nthe nervous system"
    } ]
  }, {
    "tStartMs": 66361,
    "dDurationMs": 5359,
    "segs": [ {
      "utf8": "that contract the fibers, which in turn\ngenerates force and motion."
    } ]
  }, {
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    "segs": [ {
      "utf8": "This produces almost all\nthe movements we make."
    } ]
  }, {
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    "segs": [ {
      "utf8": "Some of the only parts of the body"
    } ]
  }, {
    "tStartMs": 76142,
    "dDurationMs": 2692,
    "segs": [ {
      "utf8": "whose motions aren't governed \nby the muscular system"
    } ]
  }, {
    "tStartMs": 78834,
    "dDurationMs": 2158,
    "segs": [ {
      "utf8": "are sperm cells,"
    } ]
  }, {
    "tStartMs": 80992,
    "dDurationMs": 2352,
    "segs": [ {
      "utf8": "the hair-like cilia in our airways,"
    } ]
  }, {
    "tStartMs": 83344,
    "dDurationMs": 2147,
    "segs": [ {
      "utf8": "and certain white blood cells."
    } ]
  }, {
    "tStartMs": 85491,
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    "segs": [ {
      "utf8": "Muscle contraction can be split\ninto three main types."
    } ]
  }, {
    "tStartMs": 89044,
    "dDurationMs": 3508,
    "segs": [ {
      "utf8": "The first two, shortening muscle fibers\nand lengthening them,"
    } ]
  }, {
    "tStartMs": 92552,
    "dDurationMs": 2437,
    "segs": [ {
      "utf8": "generate opposing forces."
    } ]
  }, {
    "tStartMs": 94989,
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    "segs": [ {
      "utf8": "So the biceps will shorten\nwhile the triceps will lengthen or relax,"
    } ]
  }, {
    "tStartMs": 98974,
    "dDurationMs": 3807,
    "segs": [ {
      "utf8": "pulling up the arm and making it \nbend at the elbow."
    } ]
  }, {
    "tStartMs": 102781,
    "dDurationMs": 2568,
    "segs": [ {
      "utf8": "This allows us to, say, pick up a book,"
    } ]
  }, {
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    "dDurationMs": 4100,
    "segs": [ {
      "utf8": "or if the muscle relationship \nis reversed, put it down."
    } ]
  }, {
    "tStartMs": 109449,
    "dDurationMs": 4265,
    "segs": [ {
      "utf8": "This complementary partnership\nexists throughout the muscular system."
    } ]
  }, {
    "tStartMs": 113714,
    "dDurationMs": 4231,
    "segs": [ {
      "utf8": "The third type of contraction\ncreates a stabilizing force."
    } ]
  }, {
    "tStartMs": 117945,
    "dDurationMs": 3580,
    "segs": [ {
      "utf8": "In these cases, the muscle fibers\ndon't change in length,"
    } ]
  }, {
    "tStartMs": 121525,
    "dDurationMs": 2640,
    "segs": [ {
      "utf8": "but instead keep the muscles rigid."
    } ]
  }, {
    "tStartMs": 124165,
    "dDurationMs": 4109,
    "segs": [ {
      "utf8": "This allows us to grip a mug of coffee\nor lean against a wall."
    } ]
  }, {
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    "dDurationMs": 5112,
    "segs": [ {
      "utf8": "It also maintains our posture\nby holding us upright."
    } ]
  }, {
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    "segs": [ {
      "utf8": "Skeletal muscles form the bulk\nof the muscular system,"
    } ]
  }, {
    "tStartMs": 136526,
    "dDurationMs": 3179,
    "segs": [ {
      "utf8": "make up about 30-40% of the body's weight,"
    } ]
  }, {
    "tStartMs": 139705,
    "dDurationMs": 2581,
    "segs": [ {
      "utf8": "and generate most of its motion."
    } ]
  }, {
    "tStartMs": 142286,
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    "segs": [ {
      "utf8": "Some muscles are familiar to us,\nlike the pectorals and the biceps."
    } ]
  }, {
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    "dDurationMs": 3120,
    "segs": [ {
      "utf8": "Others may be less so,\nlike the buccinator,"
    } ]
  }, {
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    "segs": [ {
      "utf8": "a muscle that attaches your cheek\nto your teeth,"
    } ]
  }, {
    "tStartMs": 152493,
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    "segs": [ {
      "utf8": "or the body's tiniest skeletal muscle,"
    } ]
  }, {
    "tStartMs": 154532,
    "dDurationMs": 3611,
    "segs": [ {
      "utf8": "a one-millimeter-long tissue fragment\ncalled the stapedius"
    } ]
  }, {
    "tStartMs": 158143,
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    "segs": [ {
      "utf8": "that's nestled deep inside the ear."
    } ]
  }, {
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    "segs": [ {
      "utf8": "Wherever they occur, skeletal muscles are\nconnected to the somatic nervous system,"
    } ]
  }, {
    "tStartMs": 165733,
    "dDurationMs": 3771,
    "segs": [ {
      "utf8": "which gives us almost complete control\nover their movements."
    } ]
  }, {
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    "segs": [ {
      "utf8": "This muscle group also contains\ntwo types of muscle fibers"
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  }, {
    "tStartMs": 172833,
    "dDurationMs": 5931,
    "segs": [ {
      "utf8": "to refine our motions even further,\nslow-twitch and fast-twitch."
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  }, {
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    "dDurationMs": 3019,
    "segs": [ {
      "utf8": "Fast-twitch fibers react instantly\nwhen triggered"
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  }, {
    "tStartMs": 181783,
    "dDurationMs": 3022,
    "segs": [ {
      "utf8": "but quickly use up their energy\nand tire out."
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  }, {
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    "dDurationMs": 3151,
    "segs": [ {
      "utf8": "Slow-twitch fibers, on the other hand,\nare endurance cells."
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  }, {
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    "segs": [ {
      "utf8": "They react and use energy slowly\nso they can work for longer periods."
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  }, {
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    "segs": [ {
      "utf8": "A sprinter will accumulate more\nfast-twitch muscles in her legs"
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  }, {
    "tStartMs": 195586,
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    "segs": [ {
      "utf8": "through continuous practice,"
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  }, {
    "tStartMs": 197255,
    "dDurationMs": 4170,
    "segs": [ {
      "utf8": "enabling her to quickly, if briefly,\npick up the pace,"
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  }, {
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    "segs": [ {
      "utf8": "whereas back muscles contain \nmore slow-twitch muscles"
    } ]
  }, {
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    "segs": [ {
      "utf8": "to maintain your posture all day."
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  }, {
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    "segs": [ {
      "utf8": "Unlike the skeletal muscles, the body's\ncardiac and smooth muscles"
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  }, {
    "tStartMs": 211116,
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    "segs": [ {
      "utf8": "are managed by \nthe autonomic nervous system"
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  }, {
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    "segs": [ {
      "utf8": "beyond our direct control."
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  }, {
    "tStartMs": 215616,
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    "segs": [ {
      "utf8": "That makes your heart thump\nroughly 3 billion times"
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  }, {
    "tStartMs": 218686,
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    "segs": [ {
      "utf8": "over the course of your life,"
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  }, {
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    "segs": [ {
      "utf8": "which supplies the body \nwith blood and oxygen."
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  }, {
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    "segs": [ {
      "utf8": "Autonomic control also contracts\nand relaxes smooth muscle"
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  }, {
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    "segs": [ {
      "utf8": "in a rhythmic cycle."
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  }, {
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    "segs": [ {
      "utf8": "That pumps blood through the smooth\ninternal walls of blood vessels,"
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  }, {
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    "segs": [ {
      "utf8": "enables the intestine to constrict and\npush food through the digestive system,"
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  }, {
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    "segs": [ {
      "utf8": "and allows the uterus to contract\nwhen a person is giving birth."
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    "segs": [ {
      "utf8": "As muscles work, they also use energy\nand produce an important byproduct, heat."
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  }, {
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    "segs": [ {
      "utf8": "In fact, muscle provides \nabout 85% of your warmth,"
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  }, {
    "tStartMs": 250896,
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    "segs": [ {
      "utf8": "which the heart and blood vessels\nthen spread evenly across the body"
    } ]
  }, {
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    "segs": [ {
      "utf8": "via the blood."
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  }, {
    "tStartMs": 255928,
    "dDurationMs": 2827,
    "segs": [ {
      "utf8": "Without that, we couldn't maintain\nthe temperature necessary"
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  }, {
    "tStartMs": 258755,
    "dDurationMs": 1740,
    "segs": [ {
      "utf8": "for our survival."
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  }, {
    "tStartMs": 260495,
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    "segs": [ {
      "utf8": "The muscular system \nmay be largely invisible to us,"
    } ]
  }, {
    "tStartMs": 263670,
    "dDurationMs": 2949,
    "segs": [ {
      "utf8": "but it leaves its mark on almost\neverything we do,"
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  }, {
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    "dDurationMs": 3458,
    "segs": [ {
      "utf8": "whether it's the blink of an eye\nor a race to the finish line."
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