How Muscle Typology Influences the Number of Repetitions to Failure
by Benjamin Bunting BA(Hons) PGCert
Written by Ben Bunting: BA, PGCert. (Sport & Exercise Nutrition) // British Army Physical Training Instructor // S&C Coach.
Muscle fiber type can have a dramatic impact on your workout ability and recovery from intense training sessions. It may even dictate which sports you excel at naturally and how quickly they recover after intense sessions.
However, studies that use biochemical techniques to determine muscle fiber types have some restrictions; such methods can only be applied to a limited sample of muscle.
Type I Muscles
Skeletal muscles consist of individual muscle fibers that can be divided into slow-twitch or fast-twitch categories based on the myosin heavy chain expressed. Slow-twitch fibers, also referred to as Type I fibers, tend to be fatigue resistant and specialize in slower movements such as postural control. Slow-twitch fibers contain more mitochondria and myoglobin than fast-twitch fibers, and tend to be more aerobic in nature compared with fast-twitch muscle fibers (also referred to as Type IIa or IIx fibers).
They are commonly found among elite endurance athletes such as long distance runners or cyclists as well as power athletes such as sprinters or weightlifters who rely heavily on them as they generate force per unit of time than slow twitch muscle fibers (also referred to as Type IIa or Type IIx fibers), producing force per unit of time with faster contractile velocity generating greater contractile velocity producing greater force per unit of time producing greater force per unit of time than slow twitch fibers).
Both Type I and II muscle fibers may coexist within one muscle, with certain muscle groups tending toward being richer in either one than the other.
Most human skeletal muscles contain both types of fibers; their ratio of slow to fast fibers tends to be quite evenly spread among individual muscles. Some, however, such as quadriceps and hamstrings of the lower body tending toward slower-twitch fibers; whereas, fast twitch fibers make up most of what comprises upper-body muscle composition
There is no practical test available to accurately identify whether your muscles are predominantly fast-twitch or slow-twitch; nor is there any clear indication of why this matters for training purposes. There have also been no longitudinal studies conducted to study how exercise training affects fiber type shifts over time and thus current data on this subject remains limited.
Researchers have observed in numerous experiments that exercise induces changes to fiber type distribution. While the exact mechanisms responsible remain unknown, intra-myocellular signaling pathways like calcium-activated protein kinases and nuclear factor of activated T cells (NFAT) appear to play a part. Resistance training increases expression of PPARGC1A both slow fibers and fast fibers which is believed to account for some of these observed fiber type shifts during training sessions.
Type II Muscles
No matter if it be running sprints on the treadmill, dribbling across a basketball court or lifting weights with your arms overhead - fast-twitch muscle fibers from type II muscles are at work! Also referred to as intermediate fast-twitch fibers or fast oxidative fibers, these fast oxidative fibers utilize both aerobic and anaerobic metabolisms to create energy for energy production and are typically larger, paler and can produce greater force faster than slow twitch muscles.
Most individuals possess about 50% type I and fast-twitch muscle fibers in their skeletal muscles. Athletes participating in high-speed sports like sprinting or football tend to possess more type II fibers; endurance athletes usually contain more type I muscle fibers.
Training to Failure
Note that no single solution exists when it comes to training. All muscles contain both types of muscle fibers, and therefore respond differently to various resistance exercises. Some require longer rest periods between sets while others demand that participants move through their full range of motion in a single repetition or as many as they can before failing.
When it comes to predicting maximal isometric or dynamic strength, muscle typology makes only a minor contribution, with repetitions to failure being much more dependent on exercise selection and intensity than muscle typology alone. Giordano referenced a study which found very weak correlation between fiber type proportions and reps to failure on bicep curls and leg extensions at 70-86% of one repetition maximum (1RM), using knee extension peak torque measurements instead of taking muscle biopsies which would have provided more accurate information but would not been practical for most individuals.
Researchers did only observe a weak correlation between muscle type proportions and reps to failure; nevertheless their results are worthy of closer examination. Their subjects were divided into two separate groups and underwent six weeks of lower-body resistance training with low or high loads. While low load saw no increase in type I muscle fiber CSA increases for low load groups; on the other hand, their high load counterparts saw substantial gains across all major fiber types, especially large, slow twitch fibers.
Type III Muscles
Muscles work with our brain and nerves to bring movement. There are three primary types of muscles found within a human body: skeletal, smooth and cardiac. Each myofibril contains actin and myosin proteins which interact to generate force through creating ATP energy that generates force for movement.
Myofibrils are organized into motor units - parts of muscles which contract in response to electrical signals sent from our nervous systems - each innervating thousands of muscle fibers at once; individual fibers may come in flat shape (triangular or circular shape); each type responds differently with training regimens.
Slow-twitch fibers use oxygen to generate ATP, which is then used to initiate and sustain contractions. They can produce short bursts of power but fatigue more gradually than fast-twitch fibers.
Fast-twitch fibers use anaerobic metabolism to generate energy, making them common among athletes such as mixed martial artists, sprinters and Olympic weightlifters. Fast-twitch fibers have more ATP producing potential than slow-twitch fibers but fatigue quickly due to this.
Athletic performance can depend on their muscle composition. A marathon runner will typically possess more slow-twitch fibers than average individuals, while mixed martial artists and sprinters could contain as many as 95% fast-twitch muscle fibers in their muscles.
A muscle workout increases ATP production through its percentage of fast-twitch fibers and aerobic energy production; aerobic energy production yields 36 times more ATP molecules from one mole of glucose than anaerobic glycolysis does, yielding 36 ATP molecules for every mole of glucose consumed as opposed to two produced by anaerobic glycolysis.
Researching muscle fiber typology typically compares total reps to failure with percentages of each fiber type in muscle tissues. Unfortunately, however, this correlation can be flawed due to several factors. First of all, maximum reps at any percentage of 1RM tend to be determined by initial repetitions when maximum force production peaks; secondly, someone's fiber type breakdown can change with training (resistance included); thirdly intraset fatigue can impact total reps per set load at which one performs exercises.
Type IV Muscles
Type IV muscles consist of muscle fibers with diagonal (diagonal) or circular structures connecting their origin to their insertion, and connect from origin to insertion of each muscle. They produce movement via interaction of actin and myosin proteins released by an electrical impulse from the nervous system that initiates calcium release; force created from this interaction can then be utilized to generate movement by converting ATP (adenosine triphosphate) into tension, providing energy source for muscle contractions.
These muscles contain both fast- and slow-twitch fibers, and they tend to be more susceptible to fatigue than Type I and II muscle fibers. Furthermore, their contractile speeds tend to be slower, and their capacity for glycolytic enzymes greater; research shows that they are also less responsive to resistance training compared with them.
Cross-sectional studies have uncovered differences in the types of muscle fibers that predominate, while longitudinal data haven't seen consistent patterns emerge over time. This could be attributable to differences between methods used for identifying fiber types as well as individual responses to training modalities, but further investigation would likely reveal patterns over time.
Traditional muscle fiber categorization identifies muscle fibers by their color, depending on whether myoglobin is present. Type I fibers typically appear red, signaling high levels of myoglobin that enable them to use oxidative metabolism for producing ATP while type II fibers tend to rely more heavily on glycolytic enzymes than Type I ones.
Recent research has identified several genes associated with the predominance of either Type I or II muscle fibers; however, their influence over response to resistance training modalities remains elusive.
Studies published in 2011 discovered a weak correlation between Type II fiber density and number of repetitions performed to fatigue during exercise, specifically leg extensions. Subjects who had more Type 2 muscle fibers were not able to complete as many repetitions than those with more Type 1 muscle fibers.
Frustrating though it may be, this result shouldn't come as a total surprise. Remember that resistance training encompasses more than repetitions completed to failure - peak torque on particular exercises depends on their underlying muscle architecture as well. Furthermore, isometric strength is often better at predicting muscular adaptation than dynamic measures like bench press 1RMs.
Your number of repetitions to failure will determine the type of muscles you build; however, counting them alone won't do. Genetics, age, nutrition and training technique all play a part. Olympic sprinters tend to possess more fast-twitch fibers while marathon runners typically possess a greater percentage of slow-twitch ones.
Most humans possess a mix of slow and fast muscle fibers, roughly 50/50. These muscle types can further be classified as type I and II fibers; with slow-twitch fibers producing greater levels of force at slower shortening velocities (phasic muscles) while fast-twitch fibers creating power at greater shortening velocities generating either type IIa or type IIx fibers (fast shortening velocity fibers).
Studies examining the effect of exercise and training on muscle fiber type distribution have produced varied and sometimes conflicting findings, which may be ascribed to various factors like methods used for identifying fiber types (muscle biopsy) as well as variations between deep to superficial and proximal to distal muscles in terms of distribution of fiber types.
Ghent University Professor Wim Derave's lab (HIIT Science contributor) has discovered a way to estimate muscle fiber type using noninvasive measures of carnosine concentration (1H-MRS). They discovered that lower carnosine z-scores indicated predominately Type I fibers while high ones suggested predominantly Type II ones within the vastus lateralis muscle, suggesting athletes with more Type II fibers might benefit more from high volume overload training sessions; more research will need to be completed before this conclusion can be confirmed.