episode43

043 The Details of Muscle Contraction

Leslie Samuel IBTV, Physiology, The Muscular System 267 Comments

Join Leslie as he shares this last video on muscle contraction explaining with full clarity the smallest details on how this works.

Enjoy!

Transcript of Today’s Episode

Hello and welcome to another episode of Interactive Biology TV where we’re making Biology fun! My name is Leslie Samuel and in this episode, Episode 43, I am going to go into the details of muscle contraction. This is going to be the last video in the muscle contraction series, so, enjoy! Let’s get right into it.

You can always go back to Episode 42 to refresh your memory but, we said that the functional unit of contraction is called the sarcomere and, that is what we’re looking at right now. This unit is one sarcomere. We said that we have a thick filament that is called myosin and, we have a thin filament that is called actin. We said that, when muscle contraction happens, the more the neuron releases neurotransmitter that stimulates calcium release. When that happens, the fibers slide against each other just like this. So, as the muscle fibers becomes shorter, that is the muscle contracting. And you can clearly see that in this animation.

The reason we said that this can happen is because on the myosin filaments, we have these heads and those heads extend and bind to the actin. When they bind, they kind of flex so, it moves in this direction and that pulls the actin shortening the sarcomere.

What we are going to do today is we’re going to look at the details of what is happening there. We are going to look at six steps in muscle contraction. This is another image that’s showing something similar to what we’ve looked at. We have the myosin heads. Let me do that in a different color so that you can make sure to see it because we have a lot of red there. We have the myosin heads that are binding to the actin filaments.

Here, we are going to be looking at that in more details. We have the actin. Yes, it’s a different spelling because it’s from a different language but, on the actin filaments, there are two things that are very important. We have tropomyosin as you see here so that’s this long strand here. On top of the tropomyosin, we have troponin. This is a complex that we find all along the actin filaments.

Here’s the situation. Because this is here, the myosin heads want to bind to the actin. There’s some binding sites on the actin so, let’s say this is a binding site right here. But, what’s the problem? The tropomyosin is covering that binding site so, the myosin heads cannot bind. Okay, so, we have these myosin head-binding sites all along the actin; myosin heads want to bind, we have all these myosin heads ready to do their business but, they cannot because it’s blocked by the tropomyosin.

All right so, let’s go now and look at the six steps of muscle contraction. Step number one. Calcium is released from the terminal cisternae. Remember we said that the terminal cisternae is a part of the sarcoplasmic reticulum and that is where calcium is stored. So, calcium is released. You can see here, we have this little binding site for the calcium so the calcium now comes and binds the troponin. So, here we have calcium and binding to the troponin. And then, what that does is it causes a conformational change. To put it more simply, we’re just moving the tropomyosin-troponin complex. So, that moves and, when that moves, it exposes the binding sites on the actin. That’s step number one. So, step number one: We had calcium in the terminal cisternae that is released when there’s a stimulus. The calcium ions bind to the troponin causing a conformational change in the troponin-tropomyosin complex. In other words, it’s moving out of the way. And then, the next step can happen. That step is, the myosin heads can bind to the binding sites on the actin. So, this is the one binding site. For simplification we’re just showing one myosin head but, as you know we have many myosin heads all along this actin.

In order for that to happen, we said that there’s normally ATP that’s on the myosin heads and you saw that, you saw that in the previous figure. But, that ATP has to be hydrolyzed to become ADP and an inorganic phosphate (Pi). So, we have ADP and the phosphate. All right, so, we have the myosin head that has bound to the binding site on the actin. That was step number two. Step number three. This ADP and Pi is released from the myosin head. I’m not showing that in the figure but, just imagine that being released. When that is released it causes the power stroke. In other words, it causes this guy here to flex. And when it flexes, it moves in this direction and that causes the actin to slide across the myosin.

Okay, so, calcium is released, step number one, binds to the troponin, causes this change in the troponin-tropomyosin complex so that it gets out of the way; ATP being hydrolyzed into ADP and inorganic phosphate is a state that this needs to be in for the myosin head to bind. When those are released, the myosin head flexes and we get the power stroke. That’s step number three.

Let’s go to step number four. Step number four is another one that I’m not showing but, we have ATP. So, this is an ATP molecule that comes in and binds to the myosin head. When that binds to the myosin head, the myosin head then detaches from the actin so, we no longer have that connection. That’s step number four.

Step number five. ATP is hydrolyzed which re-energizes the myosin head. So, once we have ATP being hydrolyzed like it is here, that re-energizes the myosin head and it’s ready to go again.

One more step, step number six. This calcium here needs to be gone. So, we have the terminal cisternae and it’s not shown here. So, I’m just going to draw it with my great artistic skills. We have calcium pumps in the terminal cisternae. What that does is it basically pumps that calcium back in. So, we have calcium being pumped back in that is going to cause the troponin-tropomyosin complex to go back to where it was and it’s going to be blocking… I should have done that in green to keep the consistency. Oh, I still can, why not. Okay, so we have the troponin-tropomyosin complex that is blocking once again the binding site for the myosin head.

All right, so let’s recap on that real quick. Step number one: Calcium is released, binds to the troponin. When it binds, it causes a conformational change or a shift, whichever one you want to call it, it causes that shift exposing the binding sites on the actin. Step number two: Myosin head binds to the actin. Remember that the ATP has to be hydrolyzed into ADP and Pi in order for that to take place, it has to be re-energized. It gets that from that hydrolysis process. Step number three: ADP and Pi release that causes power strokes that causes this guy to flex. Then, ATP comes in, binds to the myosin head, causes the myosin head to be released from the actin. The myosin head gets re-energized when the ATP is hydrolyzed back into ADP and Pi. Calcium ions are pumped back into the terminal cisternae and this process can happen again.

Well, that’s pretty much it. That is muscle contraction. If you have any questions, go ahead and leave them in the comments. That’s all for this video, and I’ll see you on the next one.

Comments 267

  1. HD Choi

    This video is “Fabulous”.
    I’m a Univ. student in korea, and my proffesor can never make me and my friends understand.
    but this 10 mins video makes me understand!!!!
    You r the best

    ——a Student who loves Biology

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  3. Tara Schlitz

    Dear Leslie, I am 44 years old and going back to school to become a Nurse. I have to upgrade my Biology. I have been struggling with studying. I stumbled onto one of your videos on You Tube when I was searching for information on Na+/K+pump. I found myself on your site watching many of your lectures. I would like to thank you for making it fun, interesting, and easy to understand. I have learned so much from just watching and listening to you.
    Deepest Thanks, Sincerely, Tara

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      Leslie

      Hi Tara,

      You are very much welcome. I’m soooo glad that you have found the videos to be helpful, and fun. That’s my goal. I know that it can be complicated studying from the textbook, so I want to do whatever I can to help. You can help me by sharing it with your classmates, and even professors.

      All the best with your program, and check back often because we’ll be doing some awesome stuff in the near future :)

      Leslie

  4. Maurizio

    Hi, I’m from Italy, I am studying physiology and your videos are helping me so much. Thanking you is just not enough! You are really skilled in teaching.

    Hope I didn’t make bad grammar mistakes!

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  5. Imran

    It would be useful if there was something describing the difference between fast twitch and slow twitch fibres in terms of structure ect.
    But very well explained!

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  6. Vickey

    I was totally confused about this after my anatomy class…then I watched your video a couple of times and can not only understand what is happening but explain it to others…hope I do well on my exam tomorrow!! Thank you so much!

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      Lrsamuel

      Vickey, that’s SO AWESOME. Glad to hear that. Every time I get a message like that it motivates me to do more. All the best on your exam tomorrow. Make sure to come back and let me know how it went.

  7. Estrellita Calaro

    Hi Leslie,

    Please bear with me. I really like to understand how neuron cells send signals to different types of muscles. Lets say, there is an excess extracellular calcium, how is this going to affect the contraction of the skeletal muscle cell? How about the smooth muscles like the intestines? Does it have the same effect like in cardiac muscle cells?

    Thanks a lot.

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      Lrsamuel

      Not a problem. Feel free to ask your questions. I assume that the effect is the same. However, I haven’t studied that in depth so I cannot guarantee that it’s definitely that way. You’d have to do more research on the subject.

  8. Djalitana

    great video. thanks. tell me during a muscle ATP converts to ADP only once or many times? I actually need to watch your vid about ATP and ADP. need to know what happens there and how they brake done and build up. Is awesome and your voice and your english is excellent and clear.

  9. Bas

    Hello Leslie, I have some questions, what happens when a muscle get tired?

    Why does i hurt when you over excersize.

    How does “lactic acid” fit in to the picture.

    You do a great job in explaning how a human works, i never learned so much in so short amount of time. Bas.

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      Lrsamuel

      Hi Bas,
      Glad to know you are learning so much. To answer your questions . . .
      Muscles get tired because they run out of the stuff they need (ATP, O2)
      It hurts because your muscle fibers actually tear. They tear and are rebuilt bigger and stronger over time.
      Lactic acid is produced during anaerobic respiration as a byproduct. That also causes some of the burning that you feel.
      Hope that helps.

  10. Bas

    Hello, I forget to ask an other question.

    What is the difference between a hart muscle (get never tired)
    and a normal muscle?

    Bas.

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    Lrsamuel

    Different structure, doesn’t work as hard as when you are exercising hard. Your questions are too detailed to answer in a simple comment. Sorry that I can’t go into too much detail in the comments. Too much to do, too little time.

  12. ruddergrl1

    I can’t tell you how much you are helping me w/ my physiology class. My professor does not teach it well at all and doesn’t realize that he is ineffective. You make me feel like my goal is attainable. Don’t ever feel like you don’t make a difference…….

  13. 369thegoose1

    Hi…may I ask a question ….. so is it true that the myosin myofilaments do not move and the actin myofilaments move by sliding on the myosin myofilaments? Thanks, I hope I hear from you real soon….
    Thanks
    :)

  14. 369thegoose1

    Hi…may I ask a question ….. so is it true that the myosin myofilaments do not move and the actin myofilaments move by sliding on the myosin myofilaments? Thanks, I hope I hear from you real soon….
    Thanks
    :)

  15. InteractiveBiology

    @369thegoose1 All questions are answered in the Interactive Biology community forums from now on. Go to the website in the description and then visit the community. This is to make it as efficient as possible as we have multiple people over there to help answer questions.

    All the best

  16. InteractiveBiology

    All questions are answered in the Interactive Biology community forums from now on. Go to the website in the description and then visit the community. This is to make it as efficient as possible as we have multiple people over there to help answer questions.

    All the best

  17. youngLeemac

    I don’t know self how to thank you, coz these sequence of events of contractile process in skeletal muscle #excitation-contraction coupling of your’s helped me alot. Thanks very much!

  18. Anne-Marie

    Leslie…you are gold!
    I have just found your website and simply could not wait to tell my lecturers/student body about you!You have made my study a pleasure and I look forward to more of your work. From the bottom of my heart THANK-YOU!
    From Australia with Gratitude!
    Anne-Marie

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  19. gorgeouslady14

    What about the Mg++ that is responsible for the Ca++ to go back the Terminal Cisternae? Otherwise, this video is very very very helpful. Thanks

  20. rriggs2

    Huge Fan!!! I’ve been using your videos to help me get through the start of my doctoral degree in Physical Therapy – MUCH THANKS!!!

  21. morboris

    when i first heard you say make biology fun i thought that this would be crap
    but you really did make it fun and understandable

  22. AznMilu

    why is this so much different than my textbook? i dont know which to study, but yours is more clear and specific. thanks and nice job!

  23. hensonc4098

    I love how you go over everything a few times to help learn it, and that it’s easy to understand. This has been great to help me learn and understand muscle contraction as in the lectures, they will only explain really once or twice and are sometimes more difficult to understand. Thanks a lot!

  24. hensonc4098

    I love how you go over everything a few times to help learn it, and that it’s easy to understand. This has been great to help me learn and understand muscle contraction as in the lectures, they will only explain really once or twice and are sometimes more difficult to understand. Thanks a lot!

  25. qngo1

    Your videos are very helpful. Thank you for contributing such great videos. I am med student and there are some great points that I am picking up from your videos. Thank you.

  26. qngo1

    Your videos are very helpful. Thank you for contributing such great videos. I am med student and there are some great points that I am picking up from your videos. Thank you.

  27. Eamyeddi

    I appreciate your videos verry much and iam really intereasted on them.Iam astudent iam studying pharmacy.ihope that you will make videos about renal physiology and endocrinology if you can .thankfuland iam soo

  28. Eamyeddi

    I appreciate your videos verry much and iam really intereasted on them.Iam astudent iam studying pharmacy.ihope that you will make videos about renal physiology and endocrinology if you can .thankfuland iam soo

  29. AcousticMadnessMusic

    What about the acetocholinase? that assists in the process of removing calcium and resituating the muscle fibrile in its relaxed state by eating up the inorganics leftover? I’m sure you know what I’m talking about. Or, by all means correct me if I am wrong. Great info though. You really can explain it in a way that fits A to B to C….etc. Thanks.

  30. AcousticMadnessMusic

    What about the acetocholinase? that assists in the process of removing calcium and resituating the muscle fibrile in its relaxed state by eating up the inorganics leftover? I’m sure you know what I’m talking about. Or, by all means correct me if I am wrong. Great info though. You really can explain it in a way that fits A to B to C….etc. Thanks.

  31. avasak123

    Hi, have you wondered this method called the Max Muscle Method? (look for it on google). My sister says it gets people stronger.

  32. tmpEngine

    In 9 minutes you just did what my teacher was trying to do for over 3 months ! You explained the whole thing, and.. wouah !! I finally got it !! Thank youuuuuu

  33. langseth1410

    do you have any videos on smooth muscle contraction? This one is on skeletal muscle but I need the one with MLCK and MLC and calmodulin

  34. detox96

    I enjoyed your video. My instructor provided our class with 12 steps and our textbook has different steps as well. Though, I am starting to understand it. I would like to email you the 12 steps instead of posting them on youtube. to confirm they are correct. Could you please provide an email. Thanks

  35. kvietimas

    The greatest video ever when it comes to mind numbing muscle contraction. It just proves the power of good teaching. There are no dumb students, there are teachers who unable to explain. Cheerrrrs! I am so happy now :)

  36. Samar A.T

    These videos are life-saving. I’m a pharmacy intern but my mind is all blank when it comes to physiology, biology and anatomy. So, THANK YOU SO MUCH!

  37. Leah Olson

    This is such a helpful video! Very clear, concise explanation which brought these concepts to life for me. Thank you! Looking forward to watching more of your Biology videos. Feeling much more confident for my anatomy midterm coming up. :)

  38. aloap0907

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  39. deannwia

    I was hoping to hear more about the muscle contraction beginning from the axon terminal. With acetylcolene and sodium being released

  40. Yvonne

    Thank you for this video and for all of your videos! You always break it down in a way that makes it clear to me. I did extremely well on my test!

  41. mrsramos777

    Best YOUTUBE video explaining the function and structural components of a sarcomere in a myofibril. WHOOP – WHOOP, I’M GOING TO PASS MY A&P TEST WITH AN A+!!!! LOL

  42. Crisk68

    I have read this section again, and again in my text. Until I saw your video, it didn’t make a much sense. Now it’s much more understandable. Thanks!

  43. mattmank7

    Just make sure you are not confusing the action of the power stroke occurring in real time or if he is referring to when the action of power stroking is over

  44. drpink86

    Greetings,

    My name is Fransuas and I am a medical student about to take USMLE step 1. First I would like to congratulate you on your great videos. You teach in an excellent way I wish my professors in collage would have teach like that and now I wouldn’t have doubts on the more complicated mechanisms. I use your videos when I have doubt or to refresh my memory. I wanted to ask you if you have a smooth muscle contraction video?(con. on next post)

  45. drpink86

    I just saw the skeletal muscle contraction video and is excellent. As you know the smooth muscle is different with the Ca- calmodulin and other things that I get mixed all the time. If you have it can you send me the link? Because I can’t find it. Thank you very much I really appreciate it.
    Fransuas
    PS.
    Just in case I wrote to the email on the website also.

  46. an9593

    i seriously love you… thank you so much for this amazing video…. our teacher didn’t even teach us this and our exam is gonna cover this as well. so i thought i was pretty much screwed. but you literally saved my life….!!! again, thank you so much for uploading such an awesome video :)

  47. modernhippielife

    What role does Mg ion play in the relaxing of muscles??

    I saw the notation for it in one of the pictures, but it wasn’t mentioned. I love your videos! I didn’t understand this by just reading my anatomy book- very helpful!!!

  48. Kristoff Foster

    our country needs to invest in these kinds of videos that help students learn and understand concepts
    if I had money i’d donate.
    Brilliant.

  49. franco casal

    @unkiazs sure is right. before I started eating right it didn’t matter how much I exercised my abs were still jello. Listen In a best female model 2012 exclusive interview she had mentioned about her strong abs with this 7 food items. have a look here -> bit.ly/XE2Be0?=txhlbb

  50. Wal Sid

    Wassup. Exceptional clip. My school friend’s muscles grew extremely fast as though he was injecting anabolic steroids. Found out this man wasn’t. I’ve consistently fought to put on muscle tissue. My friend tried the Muscle Building Bible (google it) I’m going to take a crack at it.

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  52. branubabygirl3x3

    OMG! Thank you soooo much for explaining this! My instructor made this process hard to comprehend. Learning it from a book on your own is so one-dimensional. Thank you for explaining this so I could understand it! I will be visiting the interactive site as well.

  53. branubabygirl3x3

    OMG! Thank you soooo much for explaining this! My instructor made this process hard to comprehend. Learning it from a book on your own is so one-dimensional. Thank you for explaining this so I could understand it! I will be visiting the interactive site as well.

  54. rajan sigdel

    Have you tried Morsch Muscle Madness? (look for it on google) It is a quick and easy way to build muscle fast.

  55. Lupa Day

    Elated I found this film. Youtube is perfect for this kind of content.

    My step brother was formerly bullied. He said he was going to get bigger. I laughed… Then in just weeks he gained 40lbs of genuine muscle mass. He made use of the Muscle Building Bible (look in Google). No-one dares to intimidate the man anymore. :-) I subscribed the other day. See exactly what goes on. Not to mention this mans emails are fucking excellent…

  56. Nicole Schinzder

    whew. What a relieve. Thanks so much. I love the way you go over those steps before you start a brand new step. Love it. Thanks for posting

  57. Michael Green

    My class average on my A&P exam was a 65, I watched these videos and got a 93, i love you good sir, thank you!

  58. Kate Ricciardi

    Read 25 pages of my A&P book had no idea what they were flapping on about… thank you so much for making this so simple!!
    great teaching!!

  59. pinkfloyd1589

    does the myosin have to move the whole way across the actin? or is it some kind of graded response where the myosin can also move halfway if it likes?

  60. 29erteamAUT1536

    sure is right. even i was very sad since i workout well for abs but nothing was coming. Listen to this I saw an interview with body building champion where he talks about 7 odd foods he eats to keep his abs hard. get to know here -> bit.ly/10OySTl?=ubgor

  61. 29erteamAUT1536

    sure is right. even i was very sad since i workout well for abs but nothing was coming. Listen to this I saw an interview with body building champion where he talks about 7 odd foods he eats to keep his abs hard. get to know here -> bit.ly/10OySTl?=ubgor

  62. 627funny

    The sliding of the actin over the myosin is incremental; in order for the two actin molecules to become so close, the actin-myosin reaction has to happen multiple times. The muscle can pull the actin as close to each other as it wants, as long as there is calcium and ATP to power each slide.

    TL;DR it can be a graded response

  63. 627funny

    If you’re asking immediately where it comes from at that instant, it is brought from mitochondria which are plentiful in muscle cells.

    More specifically, within the mitochondria, the Citric Acid Cycle is used to convert the energy in glucose into smaller, more usable units of energy stored in ATP. The Citric Acid Cycle produces NADH and FADH2 which carry electrons to the Electron Transport Chain, which in turn produce ATP using ATPase in the mitochondrion. Search Google for more details.

  64. pinkfloyd1589

    thanks for the reply, sorry to be annoying but to clarify, so this sliding filament mechanism as well as the motor unit concept are both used in conjunction to give a graded response? would it be right to say that since increased action potentials stimulates more motor units?

  65. TehPwninator1

    I think videos like this are the future of teaching, it’s just so much more effective than a textbook or teaching on a chalk board. You’re also a very talented teacher, thank you for making this.

  66. TehPwninator1

    I think videos like this are the future of teaching, it’s just so much more effective than a textbook or teaching on a chalk board. You’re also a very talented teacher, thank you for making this.

  67. roshan chandima

    If you desire to have an outstanding body, you should definitely Google search for “Aston Muscle Ripper”. You are going to end up getting the appearance you want.

  68. RoseRed12393

    Sir, you are truly amazing. Your videos have been incredibly helpful for me seeing the processes and applying the concepts and this video is no exception. Keep up the great work!

  69. PineGremlins

    your accent makes learning so much more bearable, great stuff! and very selfless to put it up on-line free of charge!
    thanks a bundle

  70. zulhamakbar

    If you really want to get a six pack, you should take a look on Google for “Oak Muscle Method”. You are guaranteed to achieve the muscles you deserve.

  71. noclassmail

    Thank you so much for this video! I have a final tomorrow and I finally understand what my professor attempted to explain!

  72. kwenzie

    wow, wow, wow! you really make this simple and straight to the point….i have an exam in 3 days and i for once can say “im ready for the physiology paper”. thanks a lot you are really heaven sent to tutor me and GOD bless you

  73. Weston Stover

    Step 1 can be expanded on a bit:
    – AP traveling down T-tubule activates (Dihydropyridine) L-type Ca channels causing Ca influx
    – Ca influx acts at Ryanodine Ca dependent Ca channels on the Sarcoplasmic reticulum causing the release of sarcoplasmic reticulum Ca stores.
    – Ca then can bind to troponin C, etc, etc ,etc

    Other than great video!

  74. Weston Stover

    Step 1 can be expanded on a bit:
    – AP traveling down T-tubule activates (Dihydropyridine) L-type Ca channels causing Ca influx
    – Ca influx acts at Ryanodine Ca dependent Ca channels on the Sarcoplasmic reticulum causing the release of sarcoplasmic reticulum Ca stores.
    – Ca then can bind to troponin C, etc, etc ,etc

    Other than great video!

  75. visswaayful

    Thank you!!! I really enjoy your presentations,very thorough. I really get a clear understanding everytime I watch your tuts. Really appreciate it!!
    Tks : )

  76. Julia Khestanova

    THANK YOU!!! Instead of reading a boring 10 pages chapter (which takes me 2-3 hours to comprehend), I watch your 10 min video and understand everything! Very helpful especially if English is a second lang! ( I am Russian)

  77. rubenator6

    yes I agree cant even believe it. i know the bad fat is the reason that stopping 6 pack coming outside even we work out well. Listen The surprising part is my friend who is not doing much excercises, maintaining his six pack with this secret food items. check it out here bit.ly/10zdh63?=jrekr

  78. Sani Sanusi

    Thank you for your wonderful presentations. Please kindly make another video on action potential regarding Numbness ,when anaesthetic solution is injected at Axon terminal.

  79. Rakesh Patel

    you are correct. you can make your workout sessions much more productive if your eating the right foods to avoid fat and belly. Between In a best female model 2012 exclusive interview she had mentioned about her strong abs with this 7 food items. you can see it here bit.ly/17c5ASh?=pohpw

  80. Fred

    The lessons on skeletal muscle contraction were excellent! The lecture and the book gave me enough information to confuse me, but your presentation brought it all home!!!!
    I now have you book-marked
    Thanks!

  81. divakar singh

    yep i agree. even i was very sad since i workout well for abs but nothing was coming. And My international body building trainer also recommended this 7 food elements to kill your fat belly. you can watch out here bit.ly/12rlpky?=xedhl

  82. respeezy

    Tx for the vid, haha the accent helps a lot its so much better to listen to than some other boring vids with monotone voices.

  83. Federico Dezi

    yes i mean it. you can make your workout sessions much more productive if your eating the right foods to avoid fat and belly. Listen to this In a best female model 2012 exclusive interview she had mentioned about her strong abs with this 7 food items. have a look here bit.ly/16ZDK9R?=zchmb

  84. Lucas Cocco

    Titin is a giant protein that functions as a molecular spring which is responsible for the passive elasticity of muscle.

  85. Avik Mukherjee

    yes I agree cant even believe it. i know the bad fat is the reason that stopping 6 pack coming outside even we work out well. But I saw an interview with body building champion where he talks about 7 odd foods he eats to keep his abs hard. if you are serious go for it now bit.ly/14jXzLt?=dmkty

  86. Patricia Caulline Mauricio

    THANK YOU! Because of you, I’ll be able to pass my quiz tomorrow.
    Great explanation. I more understand it. God bless you! :)

  87. angiii

    thanku very much i:am a 10 class grader and really appeariciate,,, ur video it actually helped me a lott and to understand every single step in binding

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  89. Enrika

    Hi Leslie, thanks for great videos they are so easy to understand and informative. However I did missed one think in muscle contraction video, at school in the UK we been talk about the cross bridge, can you explain me which part of the contraction called cross bridge.
    Thank you!!

  90. TheJamesPark

    i have physiology right now and you sir saved me a whole bunch. REALLY love your videos. very simple to understand and enjoyable!!!!
    wish u were the professor…

  91. sagala40

    Wow, you are a great teacher. Your voice, your animation, your simplicity, and the steps you go over and over makes it soooo simple to learn!!!! Awesome! :)))))

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