Lecture 6 - Understanding Structural Adaptations in Muscle Growth PDF

Understanding the Structural Adaptations Associated with Muscle Growth Based on reading: Identifying the Structural Adaptations that Drive the Mechanical Load‐Induced Growth of Skeletal Muscle: A Scoping Review Jorgenson, KW, Phillips, SM & Hornberger, TA (2020) Lea...

Understanding the Structural Adaptations Associated with Muscle Growth Based on reading: Identifying the Structural Adaptations that Drive the Mechanical Load‐Induced Growth of Skeletal Muscle: A Scoping Review Jorgenson, KW, Phillips, SM & Hornberger, TA (2020) Learning Objectives 1. Can you explain how mechanical loading influences the size of muscles? 2. Can you explain the difference between the mechanisms of hypertrophy? 3. Can you explain how we would identify whether sarcoplasmic hypotrophy occurs following mechanical loading? 2 Overview of Skeletal Muscle Structure Blue line – anatomical CSA (perpendicular to longitudinal axis of the muscles) Green line – physiological CSA (perpendicular to longitudinal axis of the fascicles/myofibers 3 - Mechanical Load-Induced Growth of Skeletal Muscle at the Macroscopic Level 3.1 Whole Muscle Longitudinal Growth Resulting from an increase in length of the muscle Occurs during development When adults place muscles in chronically stretched state Potentially could occur with high load resistance training suggested to be minor Radial Growth Resulting from an increase in muscle cross sectional area Occurs in response to resistance training 5-30% increase in CSA following 8-16 weeks training in humans Not consistent across the whole muscle Roman et al (1993) report 14% increase in muscle volume but 23% increase in CSA across muscle belly following 12 weeks of training 3.2 Muscle Fascicles If a 30% CSA increase was due to: Longitudinal growth Fascicle length increase by 11%, pennation angle would be unchanged Number of fascicles visible in a cross-sectional analysis of the mid-belly would increase by 30% and could lead to a misinterpretation that there are more fascicles after training if longitudinal growth occurs Radial growth Fascicle diameter increased by 14% along with a 15% increase in pennation angle (from 16 to 18.4 degrees) Does this actually occur? Ema et al. (2016) provides strong evidence to support longitudinal growth following mechanical loading Ema et al. (2016) provides strong evidence to support radial growth following mechanical loading Therefore, it is likely that both occur, but the proportion and what stimulates each is yet to be fully determined 4 - Mechanical Load-Induced Growth of Skeletal Muscle at the Microscopic Level 4.1 Longitudinal Growth of Fascicles Fascicles are composed of bundles of myofibers Myofibers can either run the entire length of the fascicle, or terminate part way through the fascicle demonstrating intrafascicular termination For fascicles with myofibers that run the full length, longitudinal growth is 100% dependent upon the growth of the individual fiber For fascicles where intrafascicular termination occurs, longitudinal growth could result from either growth of the myofibers and/or the addition of new myofibers in-series What actually happens? Evidence suggests that eccentric contractions (downhill walking and running) can lead to an increase in the number of sarcomeres per myofiber, however myofriber length was not measured in these studies  4.2 Radial Growth of Fascicles Radial growth theoretically could occur as a result of: Myofiber hypertrophy Increased diameter of the myofiber Myofiber hyperplasia Increased number of myofibers 4.2.1 Myofiber Hypertrophy Myofiber hypertrophy is well established to occur Physiologically relevant models show that resistance training can lead to 10-15% increases in myofiber CSA following 8-16 weeks of training 4.2.2 Myofiber Splitting Although there is evidence that myofiber splitting is a normal occurrence in muscle, data assessing the proportion of split fibers following mechanical loading indicate that the rate does not increase It is thought that myofiber splitting does not meaningfully add to the overall growth of the muscle 4.2.3 Hyperplasia Hyperplasia refers to the generation of new myofibers It is well established that hyperplasia occurs during developmental growth, however it is highly debated whether hyperplasia occurs in adult skeletal muscles. Remember that alterations in longitudinal growth can lead to an increase number of fascicles pre cross- sectional area From prior example – 11% increase in fascicle length = 30% increase in number of fascicles per cross-section and thus myofibers per cross-section At this point, it remains unknown whether resistance exercise can lead to hyperplasia in adult skeletal 5 - Mechanical Load-Induced Growth of Skeletal Muscle at the Ultrastructural Level 5.1 Longitudinal Growth of Myofibers Since it is established that both extreme and relevant forms of mechanical loading can lead to myofiber elongation, it makes sense that this change needs to occur at the sarcomere level Sarcomeres are maintained at approximately 2.5 μm (optimal length) Thus, the elongation is based on the addition of sarcomeres in series length Where are the sarcomeres added? Some evidence to suggest the addition at the distal end of the muscle Others have presented evidence to suggest transverse splitting or sarcomeres throughout the myofiber 1. Splitting of the myosin at the H-Zone 2. Elongation of the two halves of the myosin along with formation of new actin 3. Formation of a new Z-disc in the center of the new actin Currently still up for debate 5.2 Radial Growth of Myofibers Reminder: Specific tension = the maximal isometric force produced per CSA Primary concept – if CSA is based on the addition of new contractile units, then specific tension should not change. Myofiberal hypertrophy - If more contractile units than other cell structure, specific tension should go up Sarcoplasmic hypertrophy - if more accessory structure than contractile units, specific tension should go down 5.2.1 Sarcoplasmic Hypertrophy What happens with hypertrophy exercise Dankel et al. (2019) Overwhelming majority of studies show that specific tension either does not change or slightly increases. 5.2.2 Expansion of the Force- Generating Elements Based on specific tension being maintained as radial growth occurs, it can be assumed that radial growth is accomplished by a proportionate increase in all cellular structures Based on the current work, myofiber hypertrophy is at best 2-4% out of proportion with CSA increase 5.2.2 Expansion of the Force- Generating Elements We know the myofiber will expand radially, we know it will be a consistent expansion between contractile and non-contractile tissues, but how does this expansion occur Hypertrophy Addition of more contractile elements leading to the CSA of the myofibril Hyperplasia Division of the myofibril to lead to the development of a greater number of myofibrils in the myofiber 5.2.3 Myofibril Hypertrophy How are new myofilaments added to lead to hypertrophy? Must consider that ribosomes are typically located in the intermyofibrillar space and are active in protein synthesis Unfortunately, technology is just getting the point that we can answer this question. As of now it remains unanswered 5.5.4 Myofibril Hyperplasia Actin does not run perpendicular to the Z-disc but at a slight 6-10 degree oblique angle This angle is hypothesed to lead to increase strain on the Z-disk and can lead to splitting of the sarcomere into two daughter sarcomeres 5.2.5 Radial Growth of Myofibers – Closing Remarks It is likely that radial expansion is a combination of myofibril hypertrophy and hyperplasia Step 1 – Myofibril hypertrophies Step 2 – Myofibril splits Sept 3 – Daughter myofibrils hypertrophy 6. Take Home Messages The process of muscular hypertrophy is a complex action that involves many components radial and longitudinal growth at different levels This understanding is critical to ensure you are interpreting CSA data appropriately. Lack of appreciation of these variables can lead to misinterpretations of muscle hyperplasia, where in fact the increase number of fascicles is based on longitudinal growth It appears that hypertrophy of the myofiber is a consistent process. The idea of sarcoplasmic hypertrophy is perhaps outdated