Jem Arnold

Matthews et al, 2023. Identification of Maximal Steady State Metabolic Rate by the Change in Muscle Oxygen Saturation journals.physiology.org/doi/full/10....

Kirby et al 2021. The balance of muscle oxygen supply and demand reveals critical metabolic rate and predicts time to exhaustion journals.physiology.org/doi/full/10....

Faith Kipyegon & Dr. Brett Kirby using MoxyMonitor #muscleoxygenation #NIRS during training for her 4-min mile attempt coming up now!

They have published how they might be using this information (see links below 👇)

From their promotional bit a few minutes ago

I had the opportunity to contribute to Matt de Neef's wonderful article on iliac artery endofibrosis/FLIA in cyclists

Mostly by knowing where to find the great published diagnostic images, and adding some knowledge we've gained from my recent PhD work on screening and conservative management👇

More on conservative management and return to sport guidelines after surgery

I had the opportunity to contribute to Matt de Neef's wonderful article on iliac artery endofibrosis/FLIA in cyclists

Mostly by knowing where to find the great published diagnostic images, and adding some knowledge we've gained from my recent PhD work on screening and conservative management👇

Why are so many pro cyclists getting iliac artery surgery? We're hearing more about external iliac artery endofibrosis than ever before, but is it actually on the rise? Plus: what even is it? And how is it treated?

As many as 1 in 5 pro cyclists will experience a pelvic artery issue during their career. Learn all about iliac artery endofibrosis in our deep-dive feature. escapecollective.com/why-are-so-m...

This is a really elegant design/result.

Thoughts about running? Will propulsion phase be above the critical occlusion tension in locomotor muscles? During sprints? Middle-distance? Marathon??

I'd expect it probably is in gastroc at least, based on how mNIRS deoxygenates during incremental running in many athletes 🤔

Figure 3D shows total work kJ is basically constant across loads under cuff occlusion, supporting W'bal model

Apart from 80%1RM, in which reps were low enough that a missing fractional last rep would meaningfully reduce total work

@matthewfliss.bsky.social any more thoughts on this?

Oh good thoughts, thanks Cap. Something like that seems likely given motor unit recruitment with fatigue. Would larger MUs recruited at lower forces lower effective COT during a prolonged task?

Is COT prior work dependent, W’bal dependent etc ? It can be manipulated by BFR, so I would think W’bal sensitive

A Critical Occluding Tension Phase Transition Occurs Between 30% and 40% 1RM in Dynamic Knee Extension Exercise Oxygen delivery and utilization to working muscle(s) are essential for sustained exercise performance. Higher contractile loads can occlude intramuscular blood vessels, limiting oxygen perfusion, whi...

A Critical Occluding Tension Phase Transition Occurs Between 30-40% 1RM in Dynamic Knee Extension Exercise

From @matthewfliss.bsky.social @hplburr.bsky.social @martinmacinnis.bsky.social et al 11/11🧵
onlinelibrary.wiley.com/doi/10.1111/...
#NIRS #ExercisePhysiology #SportScience

For his PhD, @matthewfliss.bsky.social will be trying to determine whether COT represents an absolute or a relative load, with high frequency mNIRS and intramuscular pressure recording during knee extension bouts

Watch his work! Fascinating stuff 10/
bsky.app/profile/jema...

Image from a slide showing total work (kilojoules) performed during knee extension bouts from 15 to 80% one rep maximum (1RM), under free flow and occluded conditions. Suggesting that below 25% 1RM is closer to “endurance” exercise, while resistance exercise is above 40% 1RM, and in between is a hybrid or transition between the two.

Lead author Matt Fliss presented this work from his PhD at the recent 2025 Moxy Summit, which is available free to watch 9/
moxy-academy.teachable.com/courses/moxy...

Figure showing femoral blood flow velocity during knee extension at a tension of 10% maximal voluntary contraction (MVC). Blood flow reduces close to zero during contraction phases, and increases during relaxation phases. https://physoc.onlinelibrary.wiley.com/doi/10.1113/jphysiol.1988.sp017332

Even in “pure” endurance sport like cycling, there are momentary intramuscular occlusions during every pedal stroke above a certain power

Pushing on the pedals squeezes blood out of muscle like a sponge. Relaxing the muscle allows blood to flow back in 8/

This seems immediately relevant for hybrid sports like CrossFit and HYROX, in which performance occurs on right around COT with both resistance- and endurance-like demands

@wod_science on instagram has great resources on using mNIRS in these sports 7/
www.instagram.com/p/DCELdTaIRlH/

COT represents a phase transition across a range of loads, as occlusive tension increases heterogenously within and between muscles

As likely are all metabolic “thresholds” 👇

COT is muscle & task-specific. How might this apply in sport? 6/
bsky.app/profile/capt...

Figure showing muscle near-infrared spectroscopy (mNIRS) oxygen saturation (SmO2) with increasing loads (percent one rep maximum, 1RM) knee extension from 15% to 80% in free-flow and occluded conditions. SmO2 remains near zero at all loads during occluded exercise, but remains progressively higher at lower loads below 30% 1RM in free-flow conditions, implying that oxygen delivery is better able to maintain metabolic energetic balance with oxygen extraction at those lower loads, until the muscle reaches critical occlusion tension (COT) at higher loads, and self-occludes against blood flow.

mNIRS muscle oxygenation showed improved perfusive matching of O2 delivery to extraction (mQ̇O₂ ~ mV̇O₂) at loads <COT

Implying how mNIRS might be used for internal load monitoring in resistance training (more to come on this 👀) 5/

This pattern of results suggests greater maximal strength provides work capacity advantages at loads above the COT transition, but at loads below this transition, there is a greater advantage from having an oxidative muscle phenotype… ET individuals showed greater work capacity compared to both UT and RT groups only with loads ≤ 25% 1RM in the free flow state, likely due to effective usage and/or delivery of oxygen in circumstances when mQ̇O 2 is minimally impaired. Finally, strength appears to be an important factor for work capacity when loads are ≥ 40% 1RM or for any load when external occlusion is present. Once loads reach ≤ 30% 1RM, there is a shift where having a more oxidative phenotype is advantageous for total work capacity

Endurance trained 🚴(ET) had greater work capacity below COT, while resistance trained 🏋️(RT) had greater work capacity above COT

Work capacity <COT was correlated with ⬆️VO2max & ⬆️capillarisation

Work capacity >COT was correlated with ⬆️peak strength 4/

Figure showing number of repetitions for knee extensions at various loads percent of one rep max (1RM) for untrained (UT), endurance trained (ET), and resistance trained (RT) participants. Where reps and total work performed increases exponentially at lower loads in free-flow condition, up to a ceiling at 300 reps (20 minutes of continuous knee extension exercise). While reps and total work linearly increase only up to around 50 reps under cuff-occluded condition, where an external occlusion cuff is inflated during exercise, stopping the inflow of blood.

To find COT, knee extension max reps were compared in 'free flow' and with external cuff occlusion

At loads below COT, reps increased exponentially in free-flow conditions, compared to cuff-occluded

At loads above COT, there were no differences between occluded & free-flow 3/

Schematic diagram of a capillary blood vessel compressing under increasing muscle contraction loads and intramuscular tension, causing progressive partial then complete occlusion against blood flow.

Intramuscular pressure from contractions will occlude vessels and reduce muscle blood flow & O₂ exchange

Starting at the smallest capillaries and increasing occlusive tension with contraction force

Above a Critical Occluding Tension (COT), work capacity (reps) is limited 2/

ABSTRACT Oxygen delivery and utilization to working muscle(s) are essential for sustained exercise performance. Higher contractile loads can occlude intramuscular blood vessels, limiting oxygen perfusion, while lighter loads minimize occlusion and allow perfusion. This study explored how external load impacted relative muscle endurance, work capacity, and muscle oxygenation in free-flow and cuff-occluded states to estimate the load at which a muscle occludes itself. Twenty-nine participants (10 untrained (UT), 9 endurance-trained (ET), 10 resistance-trained (RT); 14 females, 15 males) completed repetitions to task failure at 15%, 20%, 25%, 30%, 40%, and 80% of their unilateral knee-extension 1RM in free-flow and cuff-occluded states, with muscle oxygenation (SmO 2 ) measured using near-infrared spectroscopy (NIRS). Compared to cuff-occlusion, greater work capacity and SmO 2 were observed in the free-flow state at 15%–30% 1RM (p < 0.05), but no differences were seen at 40% and 80% 1RM (p ≥ 0.05). In the free-flow state above 40% 1RM, RT participants performed more work than UT and ET (p < 0.05), with no differences between UT and ET (p ≥ 0.05). At 25% and 30% 1RM, ET and RT participants performed more work than UT (p < 0.05), with no differences between ET and RT (p ≥ 0.05). At 15% and 20% 1RM, ET participants performed more work than UT and RT (p < 0.05). These results suggest a load range of > 30% but ≤ 40% 1RM may represent a phase transition in critical occluding tension of the quadriceps, with contractile induced occlusion occurring above this range, and perfusion occurring below.

When does #ResistanceExercise become #EnduranceExercise? 🏋️↔️🏃

It might depend on the critical occluding tension in our muscles 💪🩸

🧵/11👇

A Critical Occluding Tension Phase Transition Occurs Between 30-40% 1RM in Dynamic Knee Extension Exercise
onlinelibrary.wiley.com/doi/10.1111/...

Desktop on the left shows muscle oxygen saturation (SmO2) in the vastus lateralis from Train.Red mNIRS, showing deoxygenation during repeated knee extensions to maximal task tolerance. Desktop on right shows intramuscular pressure from the same muscle, showing peaks during each contraction, where the magnitude of the peaks tends to increase during the set until near the end where fatigue limits the contraction pressure.

Experimental set-up with intramuscular pressure catheter inserted into right vastus lateralis under the mNIRS sensor.

Ever wonder what intramuscular contraction pressure and mNIRS look like during knee extensions to max task tolerance? 🦵🔦

mNIRS SmO2 on left. IMP on right. Look closely! 👀
@matthewfliss.bsky.social PhD project
#SportScience #NIRS #ExercisePhysiology

A tantalizing near-win sends Pogačar back to the forge Undone by an atavistic race that harshly punishes even the smallest mistake, Tadej Pogačar learned that it's not as if he can't win Paris-Roubaix; he simply didn't.

wrote about pogačar's roubaix experiment for @escapecollective.bsky.social escapecollective.com/a-tantalizin...

Editorial in International Journal of Sports Physiology and Performance, (Ahead of Print) from author Jos J. de Koning (Department of Human Movement Sciences, Vrije Universiteit Amsterdam, Amsterdam, The Netherlands) titled "Cooperation Takes Us Further" DOI: https://doi.org/10.1123/ijspp.2025-0100

Collaboration is hard and rewarding. Antagonism is easy and unproductive.

Thanks to IJSPP for this editorial on collaboration in sport & exercise science

I hope we will continue to share knowledge with such generous and excellent colleagues in science

journals.humankinetics.com/view/journal...

Editorial in International Journal of Sports Physiology and Performance, (Ahead of Print) from author Jos J. de Koning (Department of Human Movement Sciences, Vrije Universiteit Amsterdam, Amsterdam, The Netherlands) titled "Cooperation Takes Us Further" DOI: https://doi.org/10.1123/ijspp.2025-0100

Collaboration is hard and rewarding. Antagonism is easy and unproductive.

Thanks to IJSPP for this editorial on collaboration in sport & exercise science

I hope we will continue to share knowledge with such generous and excellent colleagues in science

journals.humankinetics.com/view/journal...

A number of big name riders in the Women's & Men's peloton who have returned from surgery(ies) for iliac artery endofibrosis in recent years, in light of the news on Fabio Jakobsen (article in Dutch)

Ah, sorry to hear this but glad he can get treatment

Bilateral flow limitations are thought to be much less common in cyclists (<15%) but we think there may be higher prevalence with the "unaffected" leg still showing some functional limitations
👉https://www.mdpi.com/2077-0383/11/24/7462

Smart people discussing how to model power-duration data from first principles 👇