Monday, June 23, 2014

pH Threshold: More Useful than Lactate

Lets's imagine we have a race or time trial coming up. Maybe it's an 8K run, maybe it's 20K on the bike. But we expect the time trial to last 26-28 minutes.

Now, what will be the physiological governors of our performance?

We could create a long, long list or flow chart of factors that govern exercise performance. But in the simplest sense, it comes down to the maximum sustainable power (work/time) that an athlete can generate for the given race duration. The more power, the faster the time trial (less time) and vice versa.

 So, what governs maximum sustainable power? Here again, a number of factors (VO2max, substrate availability, economy, a central governor?). Assuming these factors are consistent, what about lactate threshold?

For years coaches and physiologists have been proclaiming lactate threshold to be the maximum sustainable pace or power for race distances of ~60:00. Assuming above this threshold, lactate accumulates exponentially and associated hydrogen ions lead to acidosis. But why then can athletes complete shorter races 30-50:00 at paces and powers greater than the lactate threshold?

Powers and paces above lactate threshold do not always induce acidosis.
We now know that the old perceptions of lactic acid are not correct. Instead of being a culprit of acidosis, lactate production consumes protons, preventing or delaying acidosis and enables high intensity exercise (Morris & Shafer, 2010; Robergs, 2011).

Lactate production consuming a proton and regenerating NAD+
But while lactate production can prevent or delay H+ accumulation, we've always known that acidosis occurs at high intensities and decreases in pH can limit performance. So, if lactate isn't a contributor to acidosis, what is?

ATP hydrolysis generates hydrogen ions, as do a number of mechanisms involved in glycolysis. These protons created in glycolysis can be consumed by pyruvate kinase and lactate dehydrogenase (lactate production). But during intense exercise the pathways for proton buffering and consumption can be overwhelmed because of an very high demand for ATP. At this point, ATP hydrolysis exceeds the rate of ATP resynthesis, the athlete cannot create enough lactate to consume all of the H+ released so H+ accumulation (acidosis) occurs.

What implication does this biochemistry have on training for athletes?

STOP TRAINING AT LACTATE THRESHOLD
and start training above pH threshold

Research from Morris and Shafer (2010) has demonstrated that acidosis, or a decrease in blood pH, does not occur at lactate threshold. Instead, acidosis occurs at a greater workrate. Table 1 below demonstrates that while lactate threshold occured at 219W, blood pH did not change until 261W was reached. Logically then, the athlete should be able to maintain a workrate just below pH threshold for the duration of a time trial. And that's exactly what Morris and Shafer found.
(Morris & Shafer, 2010)

Increase pH Threshold to Increase Sustainable Power 

Since pH threshold is a reliable indicator of sustainable power or pace, an athlete should not train to improve his lactate threshold, but he should train to improve his pH threshold. By training at intensities above the pH threshold, one can increase his pH threshold by training the muscle fibers' ability to create lactate, transport lactate/H+, and buffer H+ (Juel, 2008; Juel and Halestrap, 1999; Pilegaard et al., 1999). As Morris and Shafer (2010) note, the most effective way to improve physiological capacity is to train at an intensity that exceeds current capacity. One way to do this is to separate bouts of training above your current capacity with periods of recovery. This is nothing profound; it's just high intensity interval training. But, the key being - train at intensities above the pH threshold. Do not underestimate your sustainable pace or power.

Of course, pH threshold as a function of pace or power can also be increased through extensive endurance training. By increasing mitochondrial density, these mitochondria consume H+ while performing oxidative phosphorylation. Shifting the curve further to the right. I would never advocate that an "endurance athlete" forgo endurance training all together. Rather, endurance athletes should incorporate high intensity training into their schedules.

What about performances that are not limited by acidosis, like the marathon?

The marathon may not be limited by acidosis. Instead, it is often an issue of economy, substrate availability, or mechanical breakdown. An athlete should be able to maintain a pace just under his pH threshold as long as he can maintain adequate glycogen stores or blood glucose, but what we often see is that marathons are run just below lactate threshold. Here, the athlete is more efficient with glycogen and glucose. High intensity intervals can increase VO2max and pH threshold. I've written on the value of high intensity intervals for the marathon runner in the past, and I stand by that philosophy.

If you recall from Billat (2002), as elite marathon runners completed high intensity intervals in a "pre-competitive" phase, they were able to decrease their all out 1000m run times. I believe all that was happening here was the athletes were improving their pH threshold and subsequently, maximal sustainable pace. If we could take a group of elite runners, say sub-2:08 marathoners and have them run an all out 800, 1500, and 5000m, I suspect that the finishing places would correlate well with marathon performances. Meaning, between a group of similar abilities (< 2:08 marathon), pH threshold would be a reliable indicator of marathon performance. I do not believe, however, that all 800m, 1500m, or 5000m times can be correlated to marathon performance because you will have people with different fiber type compositions and body types.

Can pH threshold be determined outside of the laboratory?

No, not exactly. But, from what I've seen and what Dave Morris has shared with me, pH threshold occurs roughly around 30:00 power or a pace that sustainable for 30:00-40:00. For some this may be 10K race pace. 

Sample pH Threshold test involving treadmill running at 1% gradient.
pH threshold is indicated by the arrow, occurring at 10 mph or 6:00/mile pace.
For this athlete, the 10K race pace would fit well. The athlete's 10K best is 36:48. The determined pH threshold on the treadmill was 6:00/mile or 37:17 for 10K. So, even if you do not have access to a lab and the equipment to do pH threshold analyses, you can approximate pH threshold pace or power if you have data from a 10K race or a 30:00-40:00 time trial.

Take that pace or power and design interval workouts, modulating work to recovery intervals so that the athlete can sustain paces or powers greater than his current pH threshold and create large amounts of lactate. This will improve the athlete's ability to create lactate to buffer H+ and transport that lactate and H+ in and out of muscle fibers - increasing maximal sustainable pace or power.

Practical application: Workouts

Increasing pH threshold requires an increased ability to create lactate. Remember that H+ inhibits glycolysis, so to be able to continue to run glycolysis and create lactate, we must clear H+ from the cytosol; this is where monocarboxylate transporters (MCTs) come in. These transporters on the muscle cell membranes transport H+ and lactate in and out of cells. MCT concentrations are effected by training, particularly high intensity training where low pH values are seen (Juel, 2006).

Therefore, workouts need to be completed at power outputs or paces greater than the athletes current pH threshold. There is no formula for calculating the optimal work to recovery times or total volume, but in general, the work and recovery needs to be manipulated so that the athlete can achieve powers/paces greater than pH threshold. For example, if an athletes pH threshold is 300W, 10:00 intervals at 300W or even 310W may not provide an adequate stimulus for MCT transcription. But, if the athlete can break the workout into 3 sets of 3 x 3:00 with 3:00 recoveries between reps; maybe then the athlete can complete the intervals at 350 or 360W, induce acidosis and trigger MCT transcription. But MCT transcription is not the only adaptation that could come from intervals of this length at these intensities; VO2max may also improve. Together these adaptations increase maximum sustainable pace.

So then, why can't athletes maintain paces or powers below pH threshold, but above lactate threshold for longer durations (1-2+ hours)? Well, it turns out using glycolysis and faster twitching muscle fibers is not the most efficient way to oxidize glucose and glycogen for ATP resynthesis. So, glycogen and blood glucose availability becomes an issue later in the race. Further, muscle damage, decreased economy of movement and/or the slow component may play a role.

References

Billat, V., Demarle, A., Paiva, M., & Koralsztein, J. P. (2002). Effect of training on the physiological factors of performance in elite marathon runners (males and females). Int J Sports Med, 23(5), 336-341.
Juel, C. (2006). Training-induced changes in membrane transport proteins of human skeletal muscle. Eur J Appl Physiol, 96(6), 627-635.
Juel, C. (2008). Regulation of pH in human skeletal muscle: adaptations to physical activity. Acta Physiol (Oxf), 193(1), 17-24.
Juel, C., & Halestrap, A. P. (1999). Lactate transport in skeletal muscle — role and regulation of the monocarboxylate transporter. J Physiol, 517(3), 633-642.
Morris, D. M., & Shafer, R. S. (2010). Comparison of power outputs during time trialing and power outputs eliciting metabolic variables in cycle ergometry. Int J Sport Nutr Exerc Metab, 20(2), 115-121.
Pilegaard, H., Domino, K., Noland, T., Juel, C., Hellsten, Y., Halestrap, A. P., & Bangsbo, J. (1999). Effect of high-intensity exercise training on lactate/H+ transport capacity in human skeletal muscle. American Journal of Physiology - Endocrinology And Metabolism, 276(2), E255-E261.
Robergs, R. A. (2011). Nothing ‘evil' and no ‘conundrum' about muscle lactate production. Experimental Physiology, 96(10), 1097-1098.

No comments:

Post a Comment