We all have dealt with the revered arm-pump. This horrible mechanism comes to haunt you in the late stages of a race and can cause your lap times to seriously increase and you lose positions. This mechanism is fairly complex and we have some real challenges in dealing with it. The first thing that I will state is why I don't think it's lactic-acid.
I have heard this before from people and lactic-acid is argued to potentially not even exist in the human body because, it requires a level of acidity the body cant achieve or maintain life at. Regardless we know that Lactate does exist, which helps to shuttle out negative bi-products from exercise and also free Hydrogen ions (H+). These H+ cause your body to be more acidic and can cause that burning sensation you feel. Additionally, the presence of increased H+ concentrations can cause muscles to fatigue faster by inhibiting part of the process that allows them to contract or by competing with part of the complex chemical process of initiating a muscle contraction.
So now.....we all in the motocross world know that your suspension can increase this arm pump and your grip choice for your bars can reduce some of it. The key is why? Vibration!!
Your bike itself when you roll the throttle creates a certain amount of vibration. This vibration then resonates through your body. Additionally, a rough track with bad suspension will put a large volume of high impact stress into your arms from lack of effective absorption from the forks themselves. This in essence acts similar to vibration when the bars are jerking around in your hands, though this would be at a small scale level. This is insanely critical to your performance and this is why.
Bovenzi and Griffin (1997), found that high frequency vibration caused a reduction in blood flow to the fingers, when the hands were tested for blood flow during different vibration frequencies. Additionally Bovenzi et al. (1999), found a reduction in grip strength from the vibration caused by loggers using hand saws. This research shows that high vibration can potentially reduce blood flow to your hands, deprive you of required nutrients to maintain strong muscular contractions and lead to fatigue. This is one potential reason. Some conflicting evidence states a different idea.
According to Nakamura et al. (1995), blood circulation was increased when exposed to whole body vibration and grasping bars. Now additionally what we know in the training world is that high frequency vibration can cause greater amounts of motor neuron activation. In layman's terms; the higher the rate of vibration the greater amount of muscle activation over all and the faster your body learns to do that. This is of course is only true and studied up to somewhere around 80Hz or slightly more.
According to Hazell et al. (2007), higher frequencies of vibration increased muscle EMG activity in general. An average of 3.5% increase in muscle activity was observed across multiple exercises for different parts of the body. This then supports the findings of Nakamura et al. (1995) that blood flow is most likely increased during vibration. Increased blood flow during increased muscle activity seems logical. This though means a shorter time to fatigue from working harder and an increase in H+ that leads to faster fatigue/muscle acidity from more work.
Think of when you were hurt and hadn't been on the bike for a long time. When you first got on and those first few weeks riding, you get arm pump pretty bad until you ride more and it slowly dissipates. The rougher the track the worse it is but, overall it is less the more frequently you ride. It's just like weight training, the more you work the more your body adapts to the stress.
Now this isn't revolutionary science. We've all played a game or been in some situation where you held onto something that was vibrating and it caused your hands to cramp up. I remember playing an Adams Family game at the arcade where you had to hold onto the handles as long as you could while they vibrated at higher and higher frequencies. Eventually it was too intense and you couldn't bear it. Then your hands hurt for a while after. It's the exact same thing just on a larger scale and for greater duration's of time.
In conclusion, I believe that the evidence more clearly supports the idea that the vibration from the bike increases the amount of work the muscles are doing in a very short amount of time, which leads to greater fatigue based on the mechanisms mentioned above. Your arms work harder and create more H+ to increase the muscle acidity. The time to fatigue is already naturally going to be less but, the now secondary effects of increased H+ makes things far worse. The kicker is that your body can only produce so much lactate and transport bi-product out of your body so fast. While the process to get rid of this bad stuff moves along at about the pace of a little Chevy Geo, your muscles are working at about the pace of a Ferrari 458.
Considering all of this and the simplicity of it, you can see why bike setup becomes so critical. You could inadvertently sabotage your own physical performance on the track. The key is to take this simple idea and apply it to your bike setup, to optimize your performance.
References:
Bovenzi M., Zadini A., Franzinelli A., & Borgogni F. (1991). Occupational musculoskeletal disorders in the neck and upper limbs of forestry workers exposed to hand-arm vibration. Ergonomics, 34, 547-562.
Bovenzi M., & Griffin M.J., (1997). Haemodynamic changes in ipsilateral and contralateral fingers caused by acute exposures to hand transmitted vibration. Occupational Environmental Medicine, 54, 566-576.
Bovenzi M., Lindsell C.J., & Griffin M.J., (1999). Magnitude of acute exposures to vibration and finger circulation. Scandinavian Journal of Work Environment Health, 25, 278-284.
Hazell, T.J., Jakobi, J.M., & Kenno, K.A. (2007). Effects of whole-body vibration on upper and lower body EMG during static and dynamic contractions. Journal of Applied Physiology, Nutrition and Metabolism, 32, 1156-1163
Nakamura H., Ariizumi M., Okazawa T., Nagase H., Yoshida M., & Okada A. (1995). Involvement of endothelin in peripheral circulatory induced by hand-arm vibration. Central European Journal of Public Health, 3 (suppl), 27-30.
