Jem Arnold
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jemarnold.bsky.social
Jem Arnold
@jemarnold.bsky.social
590 followers 350 following 300 posts
PhD candidate & physiotherapist | 🩸Iliac artery endofibrosis / FLIA | Endurance testing & NIRS.
Treat declarative statements as questions?
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jemarnold.bsky.social
Oh they did at least propose updated estimates by performance level in a subsequent paper
dx.doi.org/10.1055/a-20...
September 10, 2025 at 3:25 AM
jemarnold.bsky.social
Saw a question "what %FTP should my first threshold be at?"

How do we begin to guess? Start with a prediction interval, not a falsely precise point estimate

Data from Sitko et al 2022 helps as a starting estimate (link below 👇)

Any similar datasets? (particularly for female cyclists?)
A boxplot with individual data for FTP (functional threshold power) and LT1 (power at 2.0 mmol/L) in relative power output (W/kg) from 46 competitive male cyclists. LT1 range equals approximately 61-90% FTP, evaluating by rank order across both measures, since paired individual scores are not known. Data digitised from Sitko et al, 2022. Functional Threshold Power as an Alternative to Lactate Thresholds in Road Cycling. https://doi.org/10.1519/jsc.0000000000004070
September 10, 2025 at 3:12 AM
jemarnold.bsky.social
This is really, really good 👀
Model Training and Specification The mean function demonstrated the nonlinear compound curve commonly observed in the MMP data (Figure 1). FPC1 accounted for 79% of the variance and was interpreted as a gain function signifying overall performance (Figure 2, FPC1) as it had the effect of shifting the entire curve up or down. FPC2 accounted for 15% of the variance and was interpreted as a sprint versus endurance bias (Figure 2, FPC2) as it had opposite effects on the curve at short versus long durations. FPC3 accounted for 3% of the variance and was interpreted as a “middle distance” bias (Figure 2, FPC3) as it had opposite effects on the middle durations versus both a short and long durations. It produced right–left shifts in the steep portion of the curve similar to W′. FPC4 accounted for 1% of the variance and lacked a clear interpretation (Figure 2, FPC4). The first 3 FPCs were retained in the F3 model as they were interpretable and represented 97% of the data variance.
August 24, 2025 at 3:08 PM
jemarnold.bsky.social
Matthews et al, 2023. Identification of Maximal Steady State Metabolic Rate by the Change in Muscle Oxygen Saturation journals.physiology.org/doi/full/10....
June 26, 2025 at 6:01 PM
jemarnold.bsky.social
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....
June 26, 2025 at 6:01 PM
jemarnold.bsky.social
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
June 26, 2025 at 6:01 PM
jemarnold.bsky.social
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 showing muscle near-infrared spectroscopy on the gastrocnemius (calf) and quadricep muscle during an incremental treadmill running protocol, where gradient was increased at a constant pace. Gastroc muscle deoxygenates at the walk-run transition to a plateau, which is maintained for the remaining approx. 10 minutes of the incremental test. While quad deoxygenates linearly in proportion to the increasing exercise intensity. Suggesting different patterns of perfusion-extraction mismatch in the two different locomotor muscles.
June 7, 2025 at 1:43 PM
jemarnold.bsky.social
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...
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.
June 6, 2025 at 2:06 PM
jemarnold.bsky.social
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/
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
June 6, 2025 at 2:06 PM
jemarnold.bsky.social
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/
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.
June 6, 2025 at 2:06 PM
jemarnold.bsky.social
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/
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
June 6, 2025 at 2:06 PM
jemarnold.bsky.social
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/
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.
June 6, 2025 at 2:06 PM
jemarnold.bsky.social
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/
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.
June 6, 2025 at 2:06 PM
jemarnold.bsky.social
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/...
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.
June 6, 2025 at 2:06 PM
jemarnold.bsky.social
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
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.
April 24, 2025 at 3:37 AM
jemarnold.bsky.social
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
April 3, 2025 at 3:17 PM
jemarnold.bsky.social
Combined with SmO2 resaturation rate (slope of reoxygenation during rest between each incremental stage). Tends to increase from Sept to Nov (inset), speculatively suggesting greater convective mQ̇O2? Then declining from Nov to Feb 🤔

Not sure how to reconcile the NIRS-BP & resat rate trends here yet
March 22, 2025 at 5:45 PM
jemarnold.bsky.social
NIRS-BP2 'plateau' as represented here, has been argued to suggest diffusion limitation, where additional muscle O2 uptake (mV̇O2) must be driven by convective O2 delivery (mQ̇O2)

Or additional work is being done elsewhere, not under the NIRS probe, in other muscles, heterogenously with muscle depth
March 22, 2025 at 5:45 PM
jemarnold.bsky.social
Love to see change (or lack of) in breakpoints over a competitive season. Maybe surprising LTs didn't change more?

Trend for power @ NIRS-BP2 to decline by ~15 W on average. Meaningful if not significant
March 22, 2025 at 5:45 PM
jemarnold.bsky.social
New longitudinal study in female rowers @drake-eserhaut.bsky.social comparing mNIRS oxygenation breakpoints & resaturation with lactate thresholds

As expected, LTs != mNIRS-BPs for individual athletes.

We need more longitudinal mNIRS research like this! But where are the performance outcomes? 👀
March 22, 2025 at 5:45 PM
jemarnold.bsky.social
Studies have repeatedly observed that muscle oxidative function (measured from biopsy or more recently non-invasively with mNIRS) shows high explanatory value for endurance TT performance; from 30-90+%

Equalling or even beating traditional models which include VO2max & efficiency 12/
Figure showing explained variance in 25km time trial performance among a group of trained cyclists in this study. The time constant for the recovery of skeletal muscle oxygen uptake, measured with mNIRS (muscle near infrared spectroscopy) explains 92% of the total variance alone. While traditional models of performance predictors, which include VO2max, percent of VO2max at metabolic threshold, and cycling economy (or other measures of metabolic efficiency) explain around 70-80% of the total variance. Batterson PM, Norton MR, Hetz SE, et al. (2020) Improving Biologic Predictors of Cycling Endurance Performance with near-Infrared Spectroscopy Derived Measures of Skeletal Muscle Respiration: E Pluribus Unum. Physiological Reports. https://doi.org/10.14814/phy2.14342
March 20, 2025 at 2:43 PM
jemarnold.bsky.social
Recent studies have focused more on peripheral skeletal muscle determinants of endurance performance

Fibre structure, capillarisation, [myoglobin], oxidative capacity; all factors of "last-millimetre" O2 transport which can be a rate-limiting step 11/
A schematic figure connecting contributing factors to endurance performance: Performance oxygen uptake; metabolic efficiency and mechanical economy; lactate threshold; VO2max; and the interrelated mechanistic factors of peripheral muscle, including: fiber type, fiber cross-sectional area, oxidative capacity, capillarisation, myoglobin concentration, physiological muscle cross-sectional area. van der Zwaard S, Brocherie F, Jaspers RT. (2021) Under the Hood: Skeletal Muscle Determinants of Endurance Performance. Frontiers in Sports and Active Living. https://www.frontiersin.org/article/10.3389/fspor.2021.719434
March 20, 2025 at 2:43 PM
jemarnold.bsky.social
If endurance performance is improving despite no change in VO2max, what else is contributing?

Some studies observe that Δ metabolic efficiency (GE, running economy) is inversely related to ΔVO2max, between athletes and related to Δperformance across a training season 10/
A figure showing directional change in 4 variables across a training season for a group of elite cross-country skiiers: oxygen cost (metabolic efficiency of external work/pace) tends to decrease; VO2peak remains flat; accumulated O2 deficit (an estimate of “anaerobic capacity”) tends to increase; and 1000m time-trial time tends to decrease (improved performance). Other studies show similar directional changes and relationships across a training season and between participants. However, there are some statistical confounding issues which should weaken (but not entirely overturn, in my opinion) our confidence in these relationships. Losnegard T, Myklebust H, Spencer M, et al. (2013) Seasonal Variations in Vo2max, O2-Cost, O2-Deficit, and Performance in Elite Cross-Country Skiers. The Journal of Strength & Conditioning Research. https://journals.lww.com/nsca-jscr/fulltext/2013/07000/seasonal_variations_in_v_combining_dot.5.aspx
March 20, 2025 at 2:43 PM
jemarnold.bsky.social
.. an athlete starting with already high VO2max and TT probably has less capacity to improve those outcomes more

So a strong cross-sectional relationship at one time point turns into a low/negative longitudinal relationship

(I'm glazing over some confounding factors here) 9/
Three figures showing a strong positive correlation between VO2max and time-trial performance both before and after a training intervention in a group of participants. However, the relationship between the change in VO2max and change in TT performance becomes negative/non-significant for those same individuals. Vollaard NBJ, Constantin-Teodosiu D, Fredriksson K, et al. (2009) Systematic Analysis of Adaptations in Aerobic Capacity and Submaximal Energy Metabolism Provides a Unique Insight into Determinants of Human Aerobic Performance. J Appl Physiol. https://doi.org/10.1152/japplphysiol.91453.2008
March 20, 2025 at 2:43 PM
jemarnold.bsky.social
Does this seem as unintuitive to you as it did to me?

I suspect because we think about endurance performance & VO2max *cross-sectionally* rather than *longitudinally*, which is a kind of Simpson's Paradox

Between individuals, VO2max strongly predicts TT performance, but... 8/
Simulated data representing a classic case of Simpson’s Paradox, where the relationship of two variables (y ~ x) may be positive across heterogenous groups of individuals, but may be flat or negative within each of those groups, where individuals are more homogenous. In real terms, cross-sectionally between individuals picked off the street, we would likely find that VO2max strongly predicted endurance performance. But within a group of trained athletes at the same performance level (say, Cat 2 cyclists or 3-hr marathoners), there is likely to be a less strong relationship between VO2max and performance. Since all of them have likely improved their VO2max close to their full potential, and performance differences are more related to other factors.
March 20, 2025 at 2:43 PM