21 October 2025, number 107
Athletic Performance Insider
WELCOMEđ
This weekâs Athletic Performance Insider brings you rigorously vetted research with practical takeaways to help you perform better and stay healthy. Our goal is to translate high-quality science into clear, actionable insightsâcutting through noise so you can focus on what matters. Links to original papers (and ResearchGate when the paper is paywalled) let you dive deeper. In this edition;
Hydration alone wonât shield marathonersâ kidneys and guts: Boston data show marked, likely transient organ stress post-raceâmaking cooling, rehydration, and staged recovery nonânegotiable.
Collagen plus eccentric training in elite female masters: preâsession hydrolysed collagen (with vitamin C) amplified patellar tendon growth and rate of force developmentâpromising for sprinting, agility, and braking.
Menstrual cycle and stiffness: peak oestradiol around ovulation coincided with higher leg and joint stiffnessâpotentially advantageous for highâvelocity plyometrics when timed well.
Speak the same intensity language: a unified fiveâlevel scale (very low â very high) aligns prescriptions and communication across endurance and resistance training.
Food first vs. supplement smart: ten rapid-fire verdicts on creatine, gels, sports drinks, coconut water, and moreâwhen everyday foods can substitute and when they canât.
Sleep cements skills: deep and REM stages actively consolidate tactical and motor learning; protect slow-wave sleep to turn practice into lasting performance.
Rethinking âacuteâ injuries: evidence points to mechanical fatigue in hamstring and ACLâprioritise tissue capacity, load management, and recovery.
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RESEARCHđ§
Does staying hydrated protect marathonersâ kidneys and guts? Not enough, Iâm afraid.
McKenna, Z. J., Atkins, W. C., Butts, C. L., Zhao, X., Morris, A. K., Perez, R., Wierick, S. C., Gustus, S. & McDermott, B. P. Biomarkers of organ stress and injury following the Boston Marathon. J. Appl. Physiol. (2025).
Marathons deliver health benefits through training but impose intense, acute stress on the day of the race, potentially affecting the gut and kidneys via reduced blood flow, heat, and fluid shifts.
This study asked whether a competitive marathon elevates biomarkers of gastrointestinal and renal injury, and whether responses differ by sex or hydration status.
Seventy-two qualified, non-elite runners (34 men, 38 women; average age 50 years; average finish time 3 hours 45 minutes) provided blood and urine before and immediately after the 2024 Boston Marathon (average 19°C, 47% humidity). Researchers measured hydration status, intestinal cell injury, skeletal muscle damage, and kidney injury risk.
Hydration markers rose modestly; only 25 to 34% of the participants met post-race criteria for hypohydration [see below*]. Despite this, organ stress was substantial. Intestinal fatty acid binding protein increased by about 4,400 pg/ml, and soluble cluster of differentiation 14 [see below**] also rose. Kidney risk markers surged: the product of tissue inhibitor of metalloproteinase 2 and insulin-like growth factor binding protein 7 increased; 88% of participants exceeded a clinical risk threshold. The increase indicates that the kidneys were under significant stress during the race, highlighting a short-term risk window where monitoring, aggressive rehydration, cooling, and recovery strategies are particularly crucial.
Serum creatinine was above the clinical threshold for acute kidney injury in 96% participants. Responses did not differ by sex, hydration status, age group, or finish time.
Conclusion: Even with generally good hydration, a marathon induces marked, likely transient, gut and kidney stress in competitive recreational runners, without sex-based differences.
Practical applications:
* Plan recovery as seriously as the race: cooling, rest, and staged return to training.
* Hydration is necessary but not sufficient; include heat acclimation, pacing, and gastrointestinal training.
[* hypohydrated = under-hydratedâthe body has a net water deficit compared to its normal (euhydrated) state.]
[** In the context of the marathon study, a rise in soluble cluster of differentiation 14 suggests that the intestinal barrier was stressed enough that small amounts of bacterial components likely crossed into circulation, prompting an immune response. It is a biomarker of gut barrier stress and systemic immune activation, not a diagnosis of infection.]
Collagen supplementation plus eccentric training boosts tendon growth and explosive force in elite female masters
Nulty, C. D. & Erskine, R. M. Collagen Supplementation Augments Strength Training-Induced Gains in Tendon Size and Rate of Force Development in Elite Female Master Field Hockey Athletes. Int. J. Sport Nutr. Exerc. Metab. 35, 510â519 (2025).
Tendons are crucial for force transmission and explosive actions, yet they adapt slowly. Collagen provides amino acids for tendon synthesis, suggesting that supplementation could boost tendon remodelling during resistance training. The study asked: Does adding hydrolysed collagen to eccentric lower-limb training enhance tendon size and explosive force in elite female Masters field hockey players?
A group of 22 premenopausal women from the national squad with an average age of 37, took part in an 8-week study. They did lower-limb resistance exercises three times a week, including two off-site sessions and one supervised flywheel squat. One group took 30 grams of hydrolysed collagen with 500 mg of vitamin C 30 minutes before each workout, while the other took a placebo (maltodextrin). Researchers measured the size of the patellar tendon and performance skills, like the mid-thigh pull, in a mobile lab at the national training centre.
Maximum voluntary force increased 13%, and vastus lateralis muscle thickness increased 5%, with no group differences. Compared with placebo, collagen supplementation produced greater increases in patellar tendon size (4% vs. 2%; collagen vs. placebo) and isometric peak rate of force development (28% vs. 17%). Eccentric braking impulse during countermovement jump improved with collagen but not with a placebo; countermovement jump height did not change meaningfully.
Conclusion: Eight weeks of eccentric training plus collagen enhanced tendon hypertrophy and explosive force development versus training alone in elite premenopausal athletes.
Practical applications: For elite female field athletes, ingest 30 grams of hydrolysed collagen with 500 milligrams of vitamin C about 30 minutes pre-session, three times weekly, alongside structured eccentric lower-limb work (including flywheel squats) to target tendon remodelling and rate of force development, potentially benefiting sprinting, agility, and braking actions in play.
Ovulation ups the spring: Oestradiol boosts lower-limb stiffness in elite women
McGrath, M. D., Brown, N. A. T., Radcliffe, C. R., McKay, A. K. A., Bull, J., Harris, R., Minahan, C., Ackerman, K. E., Burke, L. M. & Coltman, C. E. Do fluctuations in estradiol and progesterone across the menstrual cycle affect mechanical stiffness in female athletes? J. Sci. Med. Sport (2025).
Mechanical stiffness is how much the body resists changing shape when force is applied. It affects how athletes use energy and might also impact their chances of getting hurt. Fluctuating sex hormones throughout the menstrual cycle may influence neuromuscular control and stiffness, but evidence in elite settings remains limited. This study asked whether changes in oestradiol and progesterone are associated with mechanical stiffness in female athletes.
Twenty-four National Rugby League Indigenous Womenâs Academy players (18â29 years) completed testing at three menstrual phases: early follicular, late follicular, and mid-luteal. Blood samples quantified oestradiol and progesterone. Athletes performed three drop jumps onto dual force plates while motion capture quantified vertical, leg, and joint (hip, knee, ankle) stiffness.
Higher oestradiol was significantly linked to increased knee joint and leg stiffness, as well as higher ankle stiffness. Progesterone was positively associated with ankle stiffness. Hip and vertical stiffness showed no significant associations. Practically, ankle, knee, and leg stiffness were elevated when oestradiol peaks, typically around ovulation (Phase 2).
Conclusion: Lower limb dynamic stiffness appears higher during the high-oestradiol window, potentially as a protective neuromuscular response, though causality requires confirmation.
Practical applications:
* Monitor cycle phase and consider testing or high-velocity plyometrics near late follicular when stiffness may be advantageous.
* Integrate individualised menstrual tracking with performance and wellness data to guide progression and recovery.
PRACTICAL đ¤
Speak the same language on exercise intensity?
Bishop DJ, Beck B, Biddle SJ, Denay KL, Ferri A, Gibala MJ, Headley S, Jones AM, Jung M, Lee MJ, Moholdt T. Physical Activity and Exercise Intensity Terminology: A Joint American College of Sports Medicine (ACSM) Expert Statement and Exercise and Sport Science Australia (ESSA) Consensus Statement. Med. Sci. Sports Exerc. 57, 2599â2613 (2025).
For many years, studies have shown that physical activity and exercise have many health benefits. Many guidelines have been created to promote health and fitness. However, a common problem is that people use different terms to describe exercise intensity. This makes it harder to compare research, give clear advice, and get people to follow it. To fix this, an international group has suggested a standard way to describe exercise intensity that can be used for people of all ages, genders, fitness levels, health conditions, and types of activity.
The group recommends five exercise intensitiesâ
*very low,
*low,
*moderate,
*high,
*very high
âand pairs them with aligned perceptions of effort:
*very easy,
*easy,
*somewhat hard,
*hard,
*very hard
To ensure applicability in both endurance and resistance contexts, they advise avoiding terms like light, heavy, weak, and strong, which can be misinterpreted as load-specific. While acknowledging some fieldsâ established traditions, the authors argue that this common lexicon will sharpen comparisons, clarify prescriptions, and enhance translation from lab to field.
Practical applications:
*Program design: Map session goals to the five intensities and coach cues to the corresponding effort descriptors.
*Communication: Align athletes, coaches, and clinicians on a shared scale for monitoring and progression.
[the original text has lovely graphics which relate these categories to physiological states]
Food first or supplement smart? Ten myths athletes need to rethink
Wardenaar, F. C., Aussieker, T., Dolan, E., FernĂĄndez-Campos, C., Kavouras, S. A., Ormsbee, M. J., Rawson, E. S., Saunders, B., Schott, K. D., Sekiguchi, Y. & Burke, L. M. Summary of the 2025 Professionals in Nutrition for Exercise and Sport â10 Questions/10 Expertsâ SessionâCan Everyday Foods Replace Some Ergogenic Supplements and Commercially Available Sports Foods? Int. J. Sport Nutr. Exerc. Metab. 1â10 (2025).
At the 2025 American College of Sports Medicine Annual Meeting, a rapid-fire â10 Questions/10 Expertsâ session tested the practicality of a food-first philosophy in sports nutrition, asking when everyday foods can replace supplements and commercial sports products. The article recaps key takeaways and verdicts from each myth-busting prompt.
1) Do people get enough dietary creatine from everyday foods?
Verdict: Often noâvegetarian and low-meat diets provide inadequate amounts; supplementation can aid performance and health.
2) Are non-sugar sweeteners useful to reduce energy intake in athletes?
Verdict: Possibly in select cases, but evidence in athletes is limited; not a primary strategy.
3) Do beef jerky or chocolate milk provide enough leucine to replace branched-chain amino acid supplements (BCAAs)?
Verdict: Whole foods can supply leucine, but hitting the target may add extra calories; when calorie control is a priority, choose a complete protein like whey rather than isolated BCAAs.
4) Can honey, applesauce, or other foods replace carbohydrate gels?
Verdict: Yesâfood-based fuelling can be a substitute during and after exercise when nutrients are matched.
5) Is a homemade sports drink from juice and salt as effective as commercial drinks?
Verdict: Potentially, depending on needs and composition; commercial options remain convenient.
6) Do vegetarians need to supplement carnosine and carnitine?
Verdict: Routine supplementation is unnecessary; targeted use may benefit some athletes.
7) Is teff grain a rich source of iron?
Verdict: Variableâhigh values often reflect soil contamination; ârichâ depends on meeting âĽ20% daily value per serving.
8) Is coconut water sufficient to replace electrolyte drinks?
Verdict: Flavour aids intake; add sodium to match electrolyte targets.
9) Do B vitamins affect urine colour hydration checks?
Verdict: They can darken urine and confound colour scoring; more data are needed in dehydrated states.
10) Does pre-exercise food-sourced calcium alter acute bone metabolism?
Verdict: Sufficient dairy before endurance exercise can support bone turnover markers; long-term effects remain unclear.
[the orginal paper goes into detail and provides evidence for each answer]
INSIGHTS đĄ
Are you training your skills while you sleep? The evidence says yes.
Lutz, N. D., Harkotte, M. & Born, J. Sleepâs contribution to memory formation. Physiol. Rev. 106, 363â483 (2025).
Sleep takes up about a third of our lives, but many athletes cut it short for extra training, travel, or screen time. This review pulls together 10 years of studies in people and animals to show that sleep isnât just âtime off.â During sleep, the brain actively strengthens and reorganises memories that support learning skills, making decisions, and executing tactics.
This review shows that during deep non-rapid eye movement (non-REM) sleep, the brain replays the day and locks in new skills and tactics using slow waves [see below*], spindles, and brief ripple bursts [see below**]. This âsystems consolidationâ strengthens important memories and trims the junk, boosting recall, accuracy, and flexibility. REM sleep then fine-tunes memories tied to emotion and rewards, helping motivation, resilience, and strategic thinking.
With sleep widely disrupted and declining with age, understanding how sleep shapes memory can guide strategies to protect and improve the cognitive and motor skills vital for training and competition.
The evidence shows sleep actively turns training into lasting performance gains, especially for complex skills, tactics, and emotionally charged situations.
The authors conclude that sleep has a causal role in consolidating and organising memory, and that disturbed sleep (e.g., insomnia) impairs these processes. Emerging approachesâlike non-invasive brain stimulation during sleep and targeted memory reactivationâmay enhance sleep-dependent memory beyond medications.
Practical applications:
* Prioritise sleep duration and quality around learning and skill acquisition sessions; schedule high-cognitive-load training to allow same-day nocturnal sleep afterwards.
* Protect slow-wave sleep: reduce late caffeine and alcohol, manage light exposure, and consider naps strategically when nights are short.
* For older athletes or patients, consider sleep-focused interventions to mitigate slow-wave sleep decline and related memory deficits
* Monitor and treat insomnia promptly to preserve learning efficiency.
[*Slow-wave sleep is the deepest stage of nonârapid eye movement sleep, marked by high-amplitude, low-frequency brain activity (0.5â4 Hz) and very low muscle tone.]
[** In sleep science, brief ripple bursts (often called hippocampal sharpâwave ripples, or SWRs/ripples) are very short, high-frequency brain oscillations that occur mostly during deep non-REM sleep and quiet rest.]
Rethinking hamstring and ACL injuries: Fatigue, or freak accidents?
Hooren, B. V. Rethinking Acute Sports Injuries: Evidence for an Overuse Mechanism in Hamstring and ACL Injuries. Scand. J. Med. Sci. Sports 35, e70146 (2025).
Acute and overuse have long been regarded as separate categories in sports injury classification, with âacuteâ designated for incidents linked to a clear event and âoveruseâ for those without such an incident.
This review challenges that dichotomy, suggesting that many hamstring and anterior cruciate ligament injuries often labelled as acute may actually result from an overuse process driven by mechanical fatigue.
The author combines data from human observation, animal experiments, and tissue studies. They found that there are no visible changes in movement just before hamstring injuries, indicating that injuries occur during normal movement. The location and microscopic damage of the injuries are similar to those seen in animal models after repetitive stretching. Additionally, repeated small jumps can tear the anterior cruciate ligament (ACL) in cadavers, and similar damage is seen in patients with non-contact ACL injuries. Overall, these findings suggest that repeated stress weakens tissues over time, leading to injuries at loads less than the maximum they can withstand.
Conclusion: Recognising mechanical fatigue reframes prevention and rehabilitation, prioritising tissue capacity management and recovery over solely incident-focused risk factors.
Practical applications:
Periodise exposure to high-risk movements; monitor cumulative tissue load using modelling and wearables; integrate recovery windows; progress eccentric and plyometric loading judiciously; and individualise return-to-play based on tissue readiness rather than symptom resolution alone.
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