Downhill, freeride, enduro, and cross-county mountain biking all require longs periods of isometric (no movement) contractions, as your arms act as both shock absorber and steerer. All while you are landing jumps, banking sharp turns, and negativing down steep, narrow trails. It’s no wonder many mountain biker’s forearms hurt.

Sore forearms while mountain biking and 4 major factors:

• Improper bike set-up/bike-fit

• Arm-Pump

• Over-use

• Vibrations

3 minutes of mountain biking while gripping the handlebars with 15% effort will reduce the oxygen-carrying capacity of blood through your forearms by 50%.

Sore forearms, bike fitting, & brake levers

This is the easiest solution and most likely the main cause of forearm discomfort if you are new to mountain biking or are unfamiliar with “bike fitting.”

If you have to adjust your hand-grip while braking or shifting, then you need to make modifications to the position of your levers.

You should be able to ride with your hands on the handlebar grips at all times while only extending a finger or two to brake, shift gears, or adjust your dropper post…..if not, then it’s time for a bike-fit.

In addition to lever positioning, saddle height and handlebar height are important. The less upright you sit, the more weight is being supported by your arms and hands. On a properly fitting mountain bike, about 25% of your weight is supported by your hands, while the rest is supported by the saddle and pedals.

Arm Pump & mountain biking

When you’re barrelling down technical terrain or steep singletrack your forearms start to burn and ache. As you continue the descent, the ache intensifies, soon enough, you can barely hold onto the handlebars, and pulling the brake levers becomes a bitch…….this is called “arm pump.” 

Medically, “arm pump” is known as Chronic Exertional Compartment Syndrome (CECS) of the forearm.

While mountain biking, especially downhilling in a bike park or riding down steep and technical singletrack, both forearms are continuously exposed to heavy vibrations while maintaining balance on rough and uneven terrain, without an opportunity to relax.

Prolonged isometric contractions are necessary to absorb shocks and vibrations caused by the roughness of the trail and stabilization during the handling and landings.

“Arm pump” is also common in motocross (the throttling and significantly heavier bike exacerbate it) and is a problem for many mountain bikers, especially downhillers. The longer, steeper, rougher, and faster you go all work together to fatigue your muscles and if you’re riding a heavy DH mountain bike, this only amplifies it…..as does cold weather.

Arm Pump Explained

All your muscles are surrounded by a thin, inflexible sheath of connective tissue called fascia.

Mountain biking, especially downhill, makes your forearm muscles work extra-hard, which results in increased blood flow and pressure. The higher the pressure gets, the more your veins and arteries get squished, which results in less blood moving to and from your forearms, causing blood to get trapped. 

Consequently, an imbalance occurs, where your forearms consume more toxic waste-products than nutrients. Because the fresh blood is unable to get to the muscle, while toxic blood is unable to leave, these muscles will not receive nutrients, such as electrolytes or oxygen, and are left to sit a pool of de-oxygenated blood. 

In the short-term, inflammation and soreness is the by-product. In the Long-term, without any interventions or modifications, the result could be arm-pump (Chronic Exertional Compartment Syndrome of the forearm).

Mountain biking & tips to reduce arm pump

Most MTB handlebar grips have a diameter of 28 to 40mm with the “standard” grip being 30mm.

However, these sizes are too small for many riders. Larger diameter grips allow for a more powerful and stable grip.

So what is the optimal diameter?

• Men - 35mm to 45 mm

• Women - 30 to 40 mm

• Just changing the diameter by 5mm up or down from your optimal size, results in -25% difference in your grip strength

Things to keep in mind:

• Gloves - Wearing gloves adds thickness, which increases the effective grip diameter

• Levers - Bigger handlebar grips may require you to adjust the position of your levers

Gloves affect grip strength, comfort, force, and even alter the recruitment and activation of forearm muscles. Many gloves negatively impact grip strength by nearly 25%.

Gloves change the effective grip diameter, and they have a huge impact on your ability to maintain grip strength during a ride and how long it will take before your “gripping” muscles get fatigued.

Thicker and stiffer gloves cause a larger decrease in peak grip force than thinner, more elastic, or flexible gloves. This means that when you wear full-fingered gloves, you need to grip about 10 to 25% more forcefully to actually equal the same amount of force without a glove. This translates to over-gripping and much quicker forearm fatigue and “arm pump.”

The thickest of gloves also changes the length-strength relationship of the flexor and extensor muscles. This loss of gripping ability results in a lowered efficiency and a decrease in performance over time.

I personally feel “naked” on my MTB if I’m not wearing full-finger gloves.

Fingerless gloves are a quick fix, but if you’re like me and only ride with fingered-gloves, then look for the thinnest, most flexible, and best fitting glove.

Mountain biking, especially navigating new technical singletracks and extreme steeps can cause you to have anxiety, which is your body's way of focusing your attention and amping you up, to prepare you for the ride. While this sounds psychological (mental), which it is, it still alters your physiological (physical) response:

As a result of these thoughts, your body will:

• Produce more catecholamines: Namely adrenaline (epinephrine) and noradrenaline.  This will make you more alert and ready, but it also raises your heart rate, blood flow, makes you sweat more, and breathe harder……and as a result, you will grip with significantly more force without even realizing it.

As you continue on your ride, thoughts such as “I hope the next drop is smaller” or “who is riding up behind me?” just cause your body to keep dumping adrenaline (and related hormones) into your body. Once thoughts like these enter your head, it's a great time to stop…...look around, breathe, relax, enjoy the view, and clear your mind.

If you don’t, your ‘death grip’ will cause your heart to pump even more blood, which will get trapped even quicker into your forearms, which just causes them to swell and “burnout.”

However, keep in mind that just gripping the handlebars with 10 to 20% effort will reduce the oxygen in your forearms by 50% within ~10 minutes. But that is significantly better than gripping with two-thirds of your power, as your forearms won’t be able to hold on for more than a minute.

While you can use a hand-clamp/grip exerciser to target your forearm muscles, there are much better exercises.

Deadlifts, farmers walk, power-clean, hammer curls, finger-tip push-ups, squats, bent-over rows, and pull-ups & hangs will do wonders for your overall conditioning and grip strength. If you want to step it up a notch, try using an extra thick barbell (3-inch bar) and turn every lift into an intense gripping motion.

The grip muscles in your hands and forearm are primarily composed of slow-twitch fibers, allowing you to go long periods before exhaustion. You use these gripping muscles all day long during routine tasks of daily living. Because of this, you need to train these muscles for long duration while using heavy weights (dead-lifts are perfect).

Want to know what MTB means, as per the inventor of the mountain bike? Read my article and find out!

Bikes & vibrations

Vibration is a mechanical movement that is a form of a mechanical wave, like all waves……..it transfers energy. Due to the consequences of these vibrations, standards, and guidelines in the workplace have been developed to define exposure limits and allow for preventive action. Most Hand-arm Vibrations (HAVs) are transmitted from power tools, but they are also passed-on from the handlebars to the cyclist.

Numerous disorders are associated with prolonged or intense exposure to vibrations. Carpal Tunnel Syndrome (CTS) and Repetitive Stress Injuries (RPI) are the two most common in bicycling. Carpal Tunnel Syndrome is so common that cycling has it own term for it……Cyclist’s Palsy.

Riding on a flat, rough road, will cause your forearm muscles to work at half-capacity. After a couple hours of pedaling up a rough fire road or old mining road to access singletrack, you forearms are already fatigued from the vibrations. Before bombing down the singletrack, take some time to rest those forearms, even if they don’t feel tired.

How do you know if you’re affected by HAVS?

Generally, your dominant hand absorbs more vibrations, thus, you will tend to have more pain/symptoms from that hand/arm.

Cyclists on road bikes experience 2 to 4 times more vibrations compared to mountain bikers riding on the same surface. Why? Mountain bikes have larger tires running at a lower PSI, one or two shock absorbers (suspension), and MTBs don’t have top end gears (more speed on rough roads = more vibrations).

Jesse is Director of Pedal Chile and lives in Valdivia, Chile. Jesse has a Master of Science in Health & Human Performance and a Bachelor of Science in Kinesiology. Hobbies: MTBing, snowboarding, reading, taster of craft beers, researcher, & compression sock wearer.

More articles from Pedal Chile

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