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Why Do Muscles Grow...

Metadata

• Author: @AvrahamCooperMD on Twitter
• Full Title: 1/14
  Why Do Muscles Grow...
• Category: #tweets
• URL: https://twitter.com/AvrahamCooperMD/status/1345753985467224064

Highlights

• 1/14
  Why do muscles grow in size after weightlifting or other types of resistance training?
  The answer is both more straightforward and more complicated than I realized.
  Let’s get “swol”...
  #Tweetorial #MedTwitter
  (View Tweet)
• 2/
  First, a review of skeletal muscle physiology:
  The fundamental unit of muscular contraction is the sarcomere, made up of actin and myosin proteins.
  Myosin slides along actin in an ATP-dependent fashion, shortening the sarcomere, inducing contraction.
  https://t.co/azqjOY77HH
  (View Tweet)
• 3/
  Sarcomeres line up in parallel and are bunched into myofibrils.
  Myofibrils pack together to make muscle fibers, which comprise skeletal muscle.
  https://t.co/descHaZOK4
  (View Tweet)
• 4/
  I assumed the answer to this question was simple - hypertrophy.
  But there are 3 phases to muscle growth w/ exercise:
  ✅ Muscle pump (immediate swelling)
  ✅ Inflammatory (delayed swelling, hours-days)
  ✅ Hypertrophy (days)
  Let’s break each one down.
  https://t.co/IGuanjx3nf (View Tweet)
• 5/
  ✅ First, there is immediate swelling (within minutes of exercise).
  💡Not surprisingly, blood flow to exercising muscle increases dramatically to meet metabolic demands.
  https://t.co/pSQ0YLi05t
  (View Tweet)
• 6/
  Blood flow ⬆️ to muscle during exercise b/c of the so-called muscle pump.
  Contracted muscle squeezes valved veins, pushing blood through w/ contraction and pulling in more blood w/ relaxation. 
  🔑This "pumped" blood flow leads to rapid swelling.
  https://t.co/w81b38TFdI
  (View Tweet)
• 7/
  ✅ Next let's look at delayed swelling (hours-days).
  The load from weightlifting causes microdamage to sarcomeres, as actin and myosin are pulled apart w/ forced contraction.
  💡This is known as myotrauma and is a normal part of resistance training.
  https://t.co/JTNeciXlIq
  (View Tweet)
• 8/
  Myotrauma releases Damage Associated Molecular Patterns (DAMPs).
  DAMPs recruit macrophages, neutrophils, and lymphocytes into the recently-exercised muscle.
  🔑Associated prostaglandin release and vasodilation causes delayed muscular swelling.
  https://t.co/Ttxs8n1lf8
  (View Tweet)
• 9/
  ✅ The final phase, and source of longterm growth of muscle after weightlifting, is hypertrophy.
  💡Studies after weight training show an increase in muscle fiber surface area without an increase in the number of fibers, consistent with hypertrophy.
  https://t.co/naMYlzmnKZ
  (View Tweet)
• 10/
  Why do muscles hypertrophy after resistance training? It has to do w/ repair of damaged fibers.
  We already saw that exercise ➡️ myotrauma and macrophage recruitment.
  Macrophages also activate a type of muscle stem cell called satellite cells (SC).
  https://t.co/qYDGFCpWgw
  (View Tweet)
• 11/
  Satellite cells normally reside in a quiescent state in muscle.
  ⚡️Macrophages secrete IL-6, which recruits and activates them (as do circulating growth factors).
  SCs then proliferate and form tube-like structures within the damaged myofibrils.
  https://t.co/Ttxs8n1lf8
  (View Tweet)
• 12/
  Satellite cell aggregates secrete actin and myosin, which get incorporated into the damaged myofibrils, repairing and expanding them.
  🔑 This leads to hypertrophy and growth of muscle in the days after exercise.
  https://t.co/Ttxs8n1lf8
  (View Tweet)
• 13/
  In essence, we can distill down the increased size of skeletal muscle after weight training to two factors:
  1⃣Increased blood flow (short term)
  2⃣Repair of myotrauma (longterm) (View Tweet)
• 14/
  The next time you workout and your muscles grow, the following events will have occurred:
  💪 Immediate swelling (increased blood flow from the muscle pump)
  💪 Delayed swelling (inflammatory response to myotrauma)
  💪 Hypertrophy (new protein deposition from tissue repair) (View Tweet)