Can You Determine Your Optimal Training Volume Using NIRS & Muscle Oxygenation ?

If you want to boil water on the stove you wouldn’t put the flame on the lowest setting — the water would never reach a rolling boil no matter how much time you gave it. Instead, you would set the flame to the appropriate intensity and then lend it the necessary time it needs to work its magic and make the water boil.

In training terms for hypertrophy, if you’re not pushing your sets close enough to failure, or training with enough intensity, it doesn’t matter how high your weekly set volume is because it will ultimately be a waster effort. That being said, assuming intensity is ‘optimal’, total sets taken to near failure within the ~35–85% 1RM range, is likely the greatest determiner for building muscle (4, 8).

However, as volumes get higher, it appears that we need to drive frequency up to see gains or even prevent a backslide from occurring. Based on the current body of evidence, it seems that the most productive sets you can do in a session for a given body part range from ~8–14 sets on average (7). The exact optimal volume in a session is likely a product of the proximity to failure for each work set, the specifics of the training plan, the muscle group being trained, and individual factors like recovery, genetics, work capacity, physiological predisposition.

So, for example, if you’re only doing 10 sets of bicep work per week, you’re probably fine doing all of your volume in a single session (though splitting it up may actually allow for better training quality, and subsequently better gains). But, if you’re training 20 sets of biceps per week, it is ill-advised to do all of that in one session, and you’d probably fare better spreading that out over 2–3 sessions.

It makes intuitive sense that you can only stimulate so much muscle growth, or any other adaptation, within a single workout. The bodies adaptive capacities are limited, and adaptations are only desirable against repeated stresses over time. However, if we apply these transient stressors with enough volume and frequency for an extended duration they will effectively become environmental stressors which are potent stimuli that will elicit structural adaptations. Which, are changes to the muscle, bone, mitochondria, and so forth.

Another reason why higher frequency may be desirable as training volume, defined as total work sets per week for a given muscle group, is that there is a limit to the amount of quality training you can do in one session. One of the more obvious reasons for this is that neuromuscular fatigue will accumulate across a workout, which will reduce muscle activation and subsequently mechanical tension. Another reasons is that we will accumulate more muscle damage with each set performed.

This ties into the concept of repeated bouts, which says that the magnitude of adaptation you get from each subsequent sets gets smaller and smaller, so past a given point each additional work set provides such little benefit that it is not worth the cost of being performed. If you keep pushing past that point each set may not only provide little benefit, but may actually be counter productive as it may result it a negative protein balance, due to muscle protein breakdown, without stimulating more muscle growth or muscle protein synthesis. If this is done frequently enough over time you may end up in a net negative protein balance which can lead to losses in muscle mass.

A maximum productive training volume per workout would also explain why some studies find benefits of higher training frequencies, but others do not. Most of the studies that find benefits of higher training frequencies are in trained lifters with higher than average weekly training volumes. Conversely, there are many studies where training frequency does not seem to matter independent of training volume and these studies are mainly done with training volumes below 10 sets per week for a given muscle group.

We already discussed that the evidence points to ~8–14 sets per muscle group being the maximum productive volume in a single session, but that still leaves us with a pretty big range which doesn’t have a lot of practical value.

This then leads me to my next point… why?

Why would 8 sets be the low end maximum productive volume per session on average and 14 the top end cut off? And is this a moving target that can be skewed upwards or downwards based on individual factors? Additionally, is there a practical way to know what the optimal intra-session volume is on a given day so we can maximize productive volume across a day, week, micro-cycle, macro-cycle, or training career? If we only have so much time, energy, and ‘adaptation currency’ why waste it on training that won’t bring us closer to our goal ?

I believe NIRS (near-infrared spectroscopy) technology, and muscle oxygenation monitoring, can give us a highly effective, and minimally invasive, way of figuring this out.

What is muscle oxygenation and subsequently, what is NIRS?
Muscle oxygenation is a measurement of how much hemoglobin is carrying oxygen in the capillaries of the muscle and the subsequent transfer of oxygen to myoglobin, the oxygen carrying molecule located within the muscle. Muscle oxygenation is a localized measure that depends on level of blood flow, and changes in the hemoglobin dissociation curve. Muscle oxygenation is measured optically with near-infrared light, so it is completely non-invasive. The differing absorption spectra of the infrared light passing through the muscular tissue can be used to identify the relative amount of hemoglobin and myoglobin bound oxygen compared to the amount that is not. It is expressed as a percentage from 0 to 100 and is abbreviated SmO2.

“Near-infrared light can penetrate biological tissues with less scattering and absorption than visible light and consequently offers advantages for imaging and quantitative measurements. In its simplest form, a NIRS device consists of a light-source emitting two or more wavelengths of light in the near-infrared range (650–1000 nm) into the tissue of interest and a detector placed at a known distance from the source(s). The chromophores haemoglobin (Hb) and myoglobin (Mb) are oxygen carriers in blood and skeletal myocytes respectively and their absorbance of near infrared light differs depending on whether they are in an oxygenated or deoxygenated state.” (5).

What does NIRS measure?
“NIRS measurements reflect the balance of O2 delivery to working muscles and muscle O2 consumption in capillary beds” (9).

“Combining NIRS with simple physiological interventions, such as venous or arterial occlusions,allows quantitative measurements to be made from skeletal muscle. This provides a tool for assessing two major determinants of the capacity of muscles to exercise: O2 delivery and O2 utilization. The non-invasive nature of NIRS makes it an appealing technique for use in adynamic environment and for activities of daily living.” (5)

As coaches and athletes, why should we care?
“During quick, short bursts, of high intensity or high load movement such as strength training the muscle is reliant on the creation of energy molecules, ATP, from phosphocreatine (PCr), to maintain cellular energy balance. It is now known that the replenishment of this energy system is based primarily on energy synthesis from aerobic (O2consuming) means. Muscle oxygenation responds almost instantaneously with the onset of exercise, indicating that high intensity strength training is intimately tied to oxygen availability to the muscle, specifically this could represent the almost instantaneous start of replenishment of ATP via aerobic means. Therefore, monitoring oxygen consumption can be a very useful tool to aid in strength training. It should be noted that the human body can be energetically limited in the ability to combust ATP-PCR for a 1-rep max, which is independent of the Moxy-SmO2 measure that looks at replenishment. Therefore, Muscle Oxygen Consumption measures may be limited for a 1-rep max, but become exponentially more useful as the rep count, and reliance on O2 consumption, goes up.”
-Phillip Batterson, Moxy Monitor

Figure I. Here we see an athlete’s response to repeated sprinting. The red columns show muscle deoxygenation and the blue show the reoxygenation periods (at rest). You can calculate the rates of deoxygenation and reoxygenation, as well as the magnitude of each, which applies not only to repeat sprint training, but strength training as well.

NIRS is the first hope for us coaches monitoring stress reactions in real-time, so we can try and figure out how much productive volume an athlete can handle in a given day such that we can maximize it across a week, or cycle.

It’s been demonstrated that for a given skeletal muscle force output, muscle activation increase as SmO2 (muscular oxygen saturation) decreases and that this response is rapidly reversible upon muscle reoxygenation (2). Why is this relevant for hypertrophy training? Well, when we utilize oxygen at a faster rate than it is delivery there will be an impairment in the sensitivity of actin-myosin myofilaments to calcium ions, which appears to drive peripheral fatigue — peripheral fatigue will then cause increases in motor unit recruitment, which will lead to increases in mechanical tension (1,2,3).

In simplistic terms: local muscle desaturation → peripheral fatigue → increased mechanical tension → hypertrophy.

Figure II. An athlete’s response to a “10 reps x4 sets” of back squatting at a moderate intensity. In each set you can see a rapid drop in muscle oxygen saturation down to ~10% SmO2. This indicates increased motor unit recruitment across the set as the athlete approaches failure. Note that they were able to desaturate to the same low point on each set, as well as recover oxygen to baseline during rest, indicating that they have no reach a point of diminishing return.

When i’m monitoring muscle oxygenation, during hypertrophy training, one of the things i’m looking at is whether or not someone can desaturate the target muscle when we put it under load. If we are using a high enough percentage of the athletes 1RM, and they are pushing a set close to their failure point, I generally see muscle oxygen saturation dropping in a near linear manner across the work set. However, after a certain number of work sets I not only see a drop in performance, but also in inability to desaturate the target muscle, which tells me we probably aren’t getting sufficient mechanical tension to drive hypertrophy.

This figure shows data from a strength workout comprised of 15 sets. For each set, the reps are continued until reaching a low plateau. Sets 1–3 show the low plateau reaching the Performance Baseline. Sets 4–7 show that the low plateaus are a bit above the Performance Baseline. Sets 8–11 are after an extended recovery and again show the low plateau reaching the Performance Baseline. The final 4 sets show that the athlete cannot get close to the Performance Baseline, which may indicate that they have crossed the threshold of diminishing returns where these sets are not only no longer effective, but also counter productive

When the cell is not in the ready state, it will not be able to take up and utilize O2, and we will actually see increase in muscle oxygen saturation during the work bout. Knowing this we can monitor set to set blood flow, muscle oxygenation, and desaturation trends trends to see when we go from being able to utilize oxygen in the muscle during work bouts to no longer being able to utilize oxygen.

My speculation is that the point at which we can no longer utilize oxygen in the tissue is the same point at which the repeated bout effect is occuring and we at a point where additional training volume is no longer effective. As a result, monitoring muscle oxygen saturation in live time may be a viable way of figuring out our maximum productive volume on any given day, which can an add unprecedented layer of precision trying to hit the moving target of ‘optimal’ training volume.

To wrap it up, the key to maximizing volume over an extended duration is all about walking the razors of ‘just enough’ before we start to see detriment, while simultaneously being able to drive progressive overload as a proxy for ensuring we’re getting muscle growth. NIRS may, in fact, be a way of doing this with more precision, but there certainly needs to be more investigation into this before I hang my hat upon it. In the interim, some practical takeaways…

Training volume should be optimized with training frequency in mind, not separately. Training volume should also be considered on a per-workout basis, not just on a total weekly basis. Training your chest two times per week with ten sets per session may have worse results than training three times per week with six to seven sets per session or even 4 sessions per week with five sets per session. That being said, when training with lower volumes (<12 sets per week per muscle group), manipulating frequency doesn’t appear to be nearly as important than when training with 15–30 sets for a given muscle group.

The crux then becomes figuring out which of the above options are optimal. Increases in muscle mass as very difficult to measure, and even the rate of strength progression may not be a reliable way to know if we’re managing volume and frequency ‘optimally’.

My belief is that NIRS and muscle oximetry may prove to be a valid, non-invasive, and practical way to determine how much volume we can handle on a given day.

Work Cited:
1. Allen D. G., Lamb G. D., Westerblad H. (2008). Impaired calcium release during fatigue. J. Appl. Physiol. 104 296–305. 10.1152/japplphysiol.00908.2007

2. Drouin, Patrick & Kohoko, Zach & Mew, Olivia & Lynn, Mytchel & Fenuta, Alyssa & Tschakovsky, Michael. (2019). Fatigue-Independent Alterations in Muscle Activation and Effort Perception During Forearm Exercise: The Role of Local Oxygen Delivery. Journal of Applied Physiology. 127. 10.1152/japplphysiol.00122.2019.

3. Fitts R. H. (2008). The cross-bridge cycle and skeletal muscle fatigue. J. Appl. Physiol. 104 551–558. 10.1152/japplphysiol.01200.2007

4. Jenkins, Nathaniel & Housh, Terry & Cochrane, Kristen & Bergstrom, Haley & Hill, Ethan & Smith, Cory & Buckner, Samuel & Cramer, Joel & Schmidt, Richard & Johnson, Glen. (2015). Neuromuscular adaptations after 2- and 4-weeks of 80% versus 30% 1RM resistance training to failure. The Journal of Strength and Conditioning Research. In Press. 10.1519/JSC.0000000000001308.

5. Jones S, Chiesa ST, Chaturvedi N, Hughes AD. Recent developments in near-infrared spectroscopy (NIRS) for the assessment of local skeletal muscle microvascular function and capacity to utilise oxygen. Artery Res. 2016;16:25–33

6. Kamga, C., Krishnamurthy, S., & Shiva, S. (2012). Myoglobin and mitochondria: a relationship bound by oxygen and nitric oxide. Nitric oxide : biology and chemistry, 26(4), 251–258. doi:10.1016/j.niox.2012.03.005

7. Henselmans, M. “Is there a maximum productive training volume per session.” Retrieved from https://mennohenselmans.com/maximum-productive-training-volume-per-session/

8. Schoenfeld, B.J., Peterson, M.D., Ogborn, D.I., Contreras, B., & Sonmez, G.T. (2015). Effects of Low- vs. High-Load Resistance Training on Muscle Strength and Hypertrophy in Well-Trained Men. Journal of strength and conditioning research, 29 10, 2954–63 .

9. Ufland P, Lapole T, Ahmaidi S, Buchheit M. Muscle force recovery in relation to muscle oxygenation. Clin Physiol Funct Imaging. 2012;32(5):380–387.