Skeletal Muscle

All the anatomically labeled muscles (quadriceps, biceps, etc.) are composed of thousands of muscle cells. These individual muscle cells contain two proteins - actin and myosin - which chemically interact to shorten the muscle cell (and along with it the entire anatomic muscle). The stimulus for this shortening is the release of calcium initiated by a nerve impulse. The shortening of the actin-myosin complex requires energy. This energy comes from ATP. A recent study found that caffeine, enhances this process by increasing the release of calcium from the sarcoplasmic reticulum of the muscle cell. This resulted in a 7% increase in power output over a 6 second cycle exercise test.

Most of us think of muscle activity in terms of daily activities and athletic performance. But muscle cells also serve an endocrine function by moderating blood glucose levels, thus reducing the risk for metabolic diseases associated with abnormal glucose metabolism (type 2 diabetes and the spectrum of fatty liver diseases are just two). And along with its role in glucose metabolism, skeletal muscle has a role in the regulation of inflammation. During exercise, muscle cells release proteins with anti-inflammatory effects which has a role in modulating chronic, low-grade inflammation (thought to play a role in many age-related diseases including cardiovascular disease, arthritis, and Alzheimer's disease.)

TWO TYPES OF SKELETAL MUSCLE CELLS

Type I (slow twitch, SO fibers) - These cells shorten at a relatively slow speed with energy for the contraction being provided by both fats and carbohydrates via aerobic metabolism . They are the major muscle fiber in use when we exercise at 70-80% VO2max. and plenty of oxygen is available. Type I cell characteristics include:

• high concentration of mitochondria (for aerobic metabolism)
• increased intracellular myoglobin (which gives the muscle its characteristic red color) to store and transport O2
• low concentration of glycolytic enzymes (used for anaerobic metabolism)
• relatively fatigue resistant

• low concentration of mitochondria
• high concentration of ATP and glycolytic (ATPase) enzymes
• a rate of shortening 3 to 5 times that of a type I muscle cell

Successful endurance athletes have a preponderance of slow twitch muscle fibers (they make up as much as 90% of the fibers in the calf in cross country skiers) while sprinters have more fast twitch fibers. Short term studies in bicyclists (5 months) failed to show a change in the percentage of slow vs fast twitch fibers in leg muscles, but a longer multi-year study has suggested that this ratio can change with time, and continue to change for at least 5 years with regular training.

Both slow and fast twitch fibers markedly improve their metabolic capacity with training. (see (see also Principles of Training). But too much much training can also hinder muscle cell adaptation. This recent article described muscle cell biopsy changes in athletes who:

• had a history of high volume exercise training for years (5 of 9 had performed at the national or international level)
• presented with chronic fatigue
• had excessive late onset muscle soreness and stiffness

RATE OF FORCE DEVELOPMENT (RFD)

RFD and power are closely related but describe different aspects of muscle performance. RFD measures how fast force increases while power measures how much work is done per unit of time, calculated as force x velocity (of a limb, pedal, or crank).

A higher RFD helps you reach effective force levels sooner, which can increase movement velocity earlier in an effort and can contribute to higher instantaneous power. But after the initial contraction phase, RFD no longer matters as much and sustained power depends on how much force you can continue to apply.

In cycling, higher RFD helps you "snap" up to speed faster (quicker acceleration or sprint launch), but maximal or sustained watts depend on both your peak force and how fast you can keep the pedals turning under that load. RFD enhances explosive responsiveness - how quickly you can hit maximum power - but sustained or peak power output depends on strength and your metabolic conditioning. A higher RFD helps in accelerations, sprints, and attacks, not in raising average watts over long rides.

MUSCLE TEMPERATURE VERSUS CYCLING PERFORMANCE

One measure of this improved performance is an increase in Rate of Force Development (RFD) a measure of how quickly a muscle or group of muscles can generate force which reflects explosive strength. This graph shows the relationship (chat gpt generated based on this article.

The shaded area is the normal range of human core temperatures - an average of 37.0 C (98.6 F). There is a normal daily circadian rhythm in our core temperature with 36.0 C (96.8 F) the average "normal" in the early morning (4-6 a.m.) which rises to 37.5 C (99.5 F) in the late afternoon or early evening. Physical activity or exercise can temporarily increase the core temperature by 0.5-1.0 C.

Key points:

• The RFD rises steadily from 35 C to about 38.5 C
• It peaks near 38.5 C
• Above 39 C, the curve begins to fall off, reflecting a decline due to increasing enzyme and calcium-handling inefficiencies.

The graph demonstrates the benefits of warming the muscles to maximize physical performance. As the optimal performance (reflected by RFD) is slightly above our normal core temperature an exercise warm up moves us closer to that optimum.

THE BENEFITS OF WARMING UP

Although the body is quite effective at keeping our core (heart, lungs, brain) temperature within a narrow range of 97F to 99F, the temperature in our peripheral muscles can vary significantly based on factors such as ambient termperature, distance from the core, and cardiac output.

A warmup increases blood flow to the peripheral muscles, transferring heat generated by any muscle activity to all peripheral tissues. This graph (from The Journal of Applied Physiology ) shows the effect. Core temperature is represented by diamonds and is well maintained within a narrow range. The other three measurements are all from the thigh at increasing distance from the femoral artery, the triangles being the greatest distance away.

The study found that during exercise, core temperature initially increased rapidly and subsequently increased at a reduced rate until heat loss equaled heat production. All 3 muscle sensors, on the other hand, started from a lower baseline temperature, heated up more quickly and soon exceeded the core temperature. The warm up moved them into the known temperature range for optimal muscle power and efficiency of 96.8-100.4 F

Warm tissues are also thought to be more flexible, reducing the risk of muscle or ligament injury.

Warming up prepares the cardiovascular system to deliver more blood to the active muscles. Cardiac output increases and peripheral vessels dilate both of which work together to enhance oxygen and glucose availability.

Neurologically, a warm up improves neuromuscular coordination, leading to better reaction times and muscle control.

And finally there is the mental component. Warming up focuses the athlete, puts them into a competitive mindset, and lowers pre-performance anxiety.

IT IS HARDER TO RIDE IN THE WINTER

Reviewing the literature, it's not just my perception. It is true. For those running marathons as well as pro cyclists, cooler temperatures decrease performance. For marathoners the ideal ambient temperature plateaus at 13C (55F) with finishing times falling off above and below this temperature.

The ideal core temperature for optimal physical performance is 99.5 to 100 F. And numerous studies have shown that even a small drop in core temperature will lower oxygen uptake (VO2), aerobic power and muscle force production. This study in cross country skiers, correlating performance with ambient temperature, suggests the most likely explanation - the authors documented an increase in heat loss (reflected in decreased exposed skin temperatures) as ambient temperatures fell and assumed exercising muscle temperatures were decreasing in tandem.

When the athletes muscles are cooler, enzyme activity slows, nerve conduction velocity drops, and cross-bridging (the actin-myosin process that generates force) becomes less efficient. They literally produce less force per neural signal. Blood vessels constrict in the cold, reducing muscle oxygen delivery. Power output and coordination both suffer and the athlete generates less watts for the same perceived effort.

Cooler temperatures mean more "waste heat" from the inefficiency of muscle contraction is lost to the environment (convection from wind, conductive loss through cold clothing/air, and radiative loss) and les is available to maintain muscle temperatures. As temperatures fall this loss can soon outpace local heat production and the muscle may never reach its "optimum operating" temperature.

The sense of working harder is not the result of the discomfort of the cold wind in the face. It is a real change in the efficiency of the riding muscles.

ACTIVE VERSUS PASSIVE WARMING

Results demonstrate that increasing muscle temperature improves the RFD (Rate of Force Development) which translates into more power (watts) produced. But the absolute total force of a contraction remained unchanged. A higher RFD is crucial in explosive movements like sprinting, jumping, or weightlifting. Active warming does provide one benefit that may have been overlooked in aggregating data across numerous studies - optimizing neural activation patterns which translates into the recruitment of more individual muscle fibers per nerve impulse.

Although "warming up" has been advocated as a way to decrease injury (there is little data that this is true) warming the muscle itself, active or passive, will improve your ride.

CORE TEMPERATURE SENSORS - a tool to optimize temperature training?

These sensors provide a continuous and accurate measurement of core body temperature during training and racing. Traditional methods are often invasive (e.g., rectal thermometers, ingestible e-pills) and if not, only measure skin temperature which isn't a reliable indicator of core temperature.

While research on the specific use of core temperature sensors in biking is still evolving, here's a summary of the available data and potential benefits.

Modern sensors use a thermal energy transfer sensor placed on the skin (typically attached to a heart rate monitor strap or with adhesive patches) to estimate core temperature using algorithms which may also incorporate skin temperature and heart rate data. They typically connect via Bluetooth or ANT+ to cycling computers, smartwatches, and smartphone apps for real-time data display and recording.

Potential Benefits for Biking:

• Heat Training
  • Allows for safer and more effective heat acclimation by monitoring your core temperature in real-time and staying within specific heat training zones.
  • Heat training can lead to physiological adaptations such as increased blood plasma volume, improved VO2 max, and enhanced lactate threshold, potentially improving performance in both hot and cool conditions.
  • Helps athletes understand their individual heat response and tolerance.
• Avoiding Overheating
  • Provides awareness of when your core temperature is rising to a level that could negatively impact performance or lead to heat-related issues.
  • Allows athletes to assess the effectiveness of different cooling strategies during training to optimize race-day cooling plans.
  • Can help with pacing strategies and decisions about when to implement cooling techniques (e.g., ice vests, hydration).
• Optimizing Performance
  • Core temperature responses can vary significantly between individuals. Monitoring helps athletes understand their unique individual physiology in thermal regulation.
  • Helps understand the relationship between core temperature, effort level (power, heart rate), and performance.
  • May aid in determining optimal warm-up and cooling strategies.

Are core temperate systems accurate?

Although they provide reproducible measurements during repeated tests, the validity (accuracy compared to a gold standard like rectal temperature) has been debated. Some studies suggest potential overestimation at moderate temperatures and underestimation at higher temperatures compared to rectal measurements.

The accuracy of these non-invasive sensors may be influenced by factors such as sensor placement, skin temperature, and activity level. Ongoing research and algorithm improvements are likely to enhance the accuracy of these devices over time.

Core temperature sensors now offer a novel way to monitor physiological response to exercise and environmental conditions in cycling. While the technology is still relatively new and ongoing research is important for confirming accuracy and optimal use cases, it holds promise for elite athletes looking to optimize their training, particularly for heat adaptation and performance in challenging conditions.

For those not committed to a rigorous training program, it appears they would be a significant dollar and time investment with questionable benefit.

CREATINE PHOSPHOKINASE (CPK or CK)

Assuming that we all respond to exercise differently, significant elevations of a blood CK might suggest overtraining to the point that limits in recovery may outweigh the benefits of the stress-improvement training strategy, and it is time to back off a bit.

In 2023, Bicycling.com published a nice summary on CK and the implications of slightly elevated blood levels. The main points:

• as more cyclists are using blood work to monitor their training, more elevated CK levels are being found.
• an injured muscle cell, even one that is only minimally traumatized by a vigorous workout, will "leak" this enzyme into the blood and elevate blood levels.
• if you have high levels of CK and continue to exercise, you increase your risk of injury, cramps, and fatigue.
• the normal adaptations you'd expect from training will be blunted if your CK is high. Thus adding intentional rest or recovery days would be expected to increase the benefits from your training program.

RHABDOMYOLOSIS

Rhabdomyolysis results when muscle breakdown and damage allow the injured muscles to release their contents into the bloodstream. These in turn can harm the kidneys leading to kidney failure in up to 40% of cases.

Rhabdomyolosis can be seen after trauma, with drug use, with medications such as statins, and with severe dehydration and at extreme temperatures. But it can also be seen after intense exercise that involves repetitive motion of a muscle - and especially a new movement.

Before you get too confident that this is not a worry for you, a regular cyclist, it has also been reported in people who stop cycling for some time and then jump back on the spin bike and try to immediately achieve their old levels of performance. And it can also happen in professional athletes in their sport (snowboarding for example).

The moral here is that that this although this syndrome is rare, you should always be cautious about rigorous exercise when you are just starting out.

Here are few common sense suggestions that might minimize that risk even further:

• Stay well-hydrated before, during, and after the exercise.
• Don't allow your body to get too hot, since being overheated can help bring on rhabdo.
• Don't take your first class in a heated exercise room.
• If you have an instructor or personal trainer, let them know that you are new, and ask for advice when just starting out.
• Take breaks, and work at a slower speed.
• Avoid non-steroidal anti-inflammatory drugs like ibuprofen, since they can also help bring on kidney injury.

A MUSCLE FOUNTAIN OF YOUTH

Prostaglandin E2 (PGE2), one of several molecules released by muscle tissue in response to the injury and stress of training, appears to play a key role in the recruitment of reparative stem cells and re-establishing nerve/muscle cell connections.

Mice genetically engineered without a receptor for prostaglandin E2 displayed noticeably slower muscle regeneration and injured muscle treated with a single dose of PGE2 experienced accelerated repair and recovery.

As we age, the enzyme 15-hydroxyprostaglandin dehydrogenase (15-PGDH) which degrades prostaglandin E2, increases in muscle tissue, decreasing the effectiveness of PGE2. Blocking 15-PGDH in old mice largely reversed age associated loss of muscle and within a month their muscle fibers looked like those of young mice.

Part of the beneficial effect of PGE2 is via facilitating nerve muscle connections. When older mice were treated with an inhibitor of 15-PGDH (which you will remember limits the effectiveness of PGE2) there was an increase in the nerve muscle connections. The net effect of an improvement in muscle cell repair and the increase in nerve/muscle fiber connections was a 15 - 20 % increase in strength.

A decrease in muscle strength (and mass) begins in our 20s and not even rigorous training will prevent this natural process. But now we have the promise of a fountain of youth for muscles which will decrease the morbidity of sarcopenia in the elderly as well as offering athletes of all ages another tool to maximize their performance.

There are other proteins that play a role in muscle cell repair and regeneration. The following summary is from an article in Forbes based on this original work.

Platelet-derived growth factor subunit B (PDGF-B) is a protein involved in the cascade of steps in muscle cell repair. When researchers directly exposed precursor muscle cells (myoblasts and myotubes) to PDGF-B there was a noticeable increase in muscle cell proliferation. The new cells matured more quickly than their untreated counterparts, had an increased thickness when seen under a microscope, and expressed more of a motor protein vital for muscle contraction.

Although there was no aging effect on the impact of this specific protein, the study moves us one step closer to finding a solution to the problem of age related sarcopenia. Platelet-derived growth factor subunit B (or a biosimilar alternative) will have a roll in promoting muscle growth and boosting contractile strength.

THE INEVITABLE - A LOSS OF MUSCLE MASS AS WE AGE.

Muscle mass peaks in an athletes 20s and 30s - and then progressively declines as they age. The rate of loss, on average, is 0.5% per year in the 40s, 1-2% after age 50 years, and nearly 3% per year after age 60. But these are just averages and don't take into account those who work to minimize the damage. We know that physically active individuals - especially those doing resistance training - lose significantly less muscle mass and strength per year as they age. Which makes a case for adopting a proactive muscle preserving routine in your 30s to keep a higher baseline as well as slowing the rate of loss.

This graph demonstrates the benefit of a muscle preserving routine.

An man has from anywhere 77 to 110 lbs of muscle tissue. So for this graph let's assume 100 pounds of muscle. The numbers above are averages for muscle loss, but just averages. There are studies that show total muscle mass can be held steady until age 50 or so (and might actually increase) with focused resistance training. Let's assume that you can at least halve the average rate of loss if you are visiting the gym a couple times a week.

Three scenarios.

• First scenario (red color) - no resistance exercise program. Muscle mass drops by 0.5 % of total muscle per year until age 50, then by 2% per year until age 60 and finally 3 % until age 80.
• Second scenario (blue) - assume an exercise program starting at age 50 halves the rate of muscle loss. Start with the same 100 pounds of muscle at age 30, drop by 0.5% per year until age 50 and then, introducing exercise, the rate of loss drops to 1% per year until age 60, and then to 1.5 percent per year until age 80.
• Third scenario (green line) - exercise program starts at age 30. We assume muscle loss can be held off until age 50, then drops 1% per year until age 60, and finally 1.5% per year until age 80.

The graph reinforces the benefits of a lifelong approach to muscular health - regular resistance training (in addition to your regular aerobic routines) and adequate dietary proteins - 1.5 grams of protein per kilogram body weight per day (120 grams for a 165 pound rider) to provide the raw material for muscle cell growth and repair.

The benefits of resistance exercise do not mean bulking up at the gym. Data shows that using lighter weights can build just as much strength as heavier weights, as long as you push to near failure/fatigue in the last few repetitions. There is no need to be intimidated by heavy weights or risk injury by trying to build bulk with a low rep/high weight routine.

And it's never too late to start. A meta-analysis of adults aged 70 years or older (including nonagenarians), regular resistance exercise led to surprising improvements in muscle strength and muscle hypertrophy over 8 to 12 week training programs.

The graph speaks for itself. Visit the gym twice a week and find you own sustainable resistance exercise program. And sooner rather than later. Look at it as a form of old age insurance. And starting now will magnify the long term benefits compared to those of a late starter.

RESISTANCE EXERCISE (MUSCLE STRENGTH) ADDS A LAYER OF AGING PROTECTION.

We’ve long known that

• higher levels of physical activity are associated with lower mortality
• greater muscle strength is associated with lower mortality
• better cardiorespiratory fitness is associated with lower mortality.

But this study is solid evidence that time spent lifting weights provides its own independent survival benefit. It shows that greater muscle strength is associated with longer life - even after accounting for aerobic fitness.

Why might that be? The authors suggest that muscle strength is a marker of resilience. Aging is often marked by unpredictable illnesses, hospitalizations, and injuries. Each of these events can trigger rapid muscle loss, disability, and a downward spiral in health. If you start with more muscle - both in mass and quality - you have a larger reserve. In practical terms, you carry a bit more insurance when inevitable health setbacks occur.

Good aerobic fitness remains a cornerstone of healthy aging. But as we focus on improving VO2max, we should not forget the weight room. Building and maintaining muscle adds another layer of protection against the challenges that come with getting older.

MEASUREMENT OF CYCLING ENERGY OUTPUT (POWER)

Energy output (or work) is expressed as power (the amount of work done during a specified unit of time). Power output can be measured as steady state power output (maintaining a steady speed for minutes to hours) or maximal power output - which require maximal activation of the ATP-CP energy system. The latter reflects the maximal muscle power of the athlete and is limited by the amount of ATP and CP available in the cell - about 6 seconds.

Curt Austin has put together a nice calculator to estimate power output (in Watts - you enter your own parameters) on his website. Malcolm Firth also published some comparative numbers in an online coaching forum. (As the amount of ATP-CP available to the muscle cell is limited, Malcolm's maximum power output over several minutes would be lower than that achievable in a brief sprint lasting 5 to 5 seconds):

"In February 1998 I did a small research project in which a group of 24 cyclists were asked to do two tests on a CompuTrainer (an electromagnetically braked turbo trainer made by RacerMate of Seattle, USA). The first of these was a step increased load test to voluntary exhaustion in which the load began at 100 watts and was increased at approx 20 watts per minute. After a break of at least three hours the cyclists then rode a simulated ten miles time trial on the CompuTrainer with the instruction to complete the distance as quickly as possible. Some of the data is summarized below:

• Average Age: 33.17yr (standard deviation 12.97, range 16yr-61yr)
• Average Max Power for 1 min: 367.46 watts (st dev 62.74w, range 263w-487w)
• Average Max Heart Rate: 187.29bpm (st dev 12.16bpm, range 163bpm-211bpm)
• Average 10 mile Time: 25min 52sec (st dev 1min 50sec, range 29min 09sec - 23min 02sec)
• Average 10 mile Power Output: 286.46 watts (st dev 49.88w, range 215w - 375w)
• Average 10 mile Heart Rate: 177.08bpm (st dev 11.78bpm, range 145bpm-199bpm)

QUESTIONS

Q.I was hoping you could shed some light on how I might be able to shed some upper body muscle mass to lighten up a bit. I have 7% body fat and have not been to the gym in a year, although most of my life has been dedicated to weights until now. I don't want to lose any leg strength in the process.

A. Except for inactivity (accelerated by overall weight loss) , I am unaware of any way to decrease muscle mass. Thus I think you are looking at a long term program of increasing or maintaining lower body stress on the muscles (biking, weight lifting) to maintain or increase mass there while discontinuing all upper body weight work.

ADDITIONAL MUSCLE RELATED CONTENT

• Weight/strength training
• Stretching/massage/muscle cramps
• Muscle pain and soreness
  • Muscle cramps
• Nutrition of post ride recovery including replacing muscle glycogen stores.