When you climb and grab a handhold, your finger flexors work in isometric contraction, since they’re applying an amount of force with its fibers remaining still, not shortening nor stretching, during most of the time this action lasts. When a certain intensity is reached in this contraction type, and this is true for any muscle of the body, the muscle fibers cause an internal compression which leads to a mechanical flattening of the blood vessels in the active muscular area (3,10,19). This critical intensity is known as critical level of occlusion or occlusion threshold (OT) (19).

Any contraction performed at an equal or higher intensity than that threshold, implies the occlusion of the blood vessels in the active area, temporarily depriving its muscle fibers of exogenous (or external) oxygen and nutrients, and impeding the elimination of most of the metabolites (molecules that are generated during the effort). This situation drastically decreases the muscle ability to sustain the contraction in time, unless its intensity is lowered below the OT. Therefore, blood flow is considered as the key element in the duration of isometric efforts (19), and several researches have proven how important it is for the amount of recovery that can be achieved between contractions in climbing (15, 34).

The OT should not be mistaken for the CF (critical force) level (17), which is a lower intensity that would allow sustaining intermittent efforts over a long time, meaning it would be the right intensity for a steady state intermittent effort done at a particular effort:rest ratio. On the graph below derived from the results of a study that has assessed this intensity through an all-out intermittent test done at a 7”:3” ratio, the CF would be reflected in the four rightmost states, where the magnitude of the force applied becomes steady (18).

Source: Giles et al., (2020)

This method to find the OT has already been used by Yamaji et al. (2000, 2002, 2004) (37-39) with protocols of sustained effort (instead of intermittent), but also at maximum intensity. These authors have observed that this critical level of tension, in reference to the OT, revealed itself at the inflection point of the intensity vs time graph (below), where the slope changed from significantly descending (high intensities at the beginning of the exercise) to an almost stable phase (lower intensities). This inflection point also coincided with the moment when maximum levels of deoxygenated hemoglobin (Deoxy-Hb) were reached in the muscle. In this case the measurements were mostly made on the flexor digitorum superficialis using simultaneously a strength sensor (handheld 2 phalanges dynamometry) and near-infrared spectroscopy.

Adapted from a graph of Yamaji et al. (2004)


The OT can be seen as the minimum percentage of the maximum isometric strength (MIS) that a muscle can exert, at which blood would cease to flow in that muscle. This percentage differs between people and varies according to each muscle (3,8,19), since it depends on the percentage of each fiber type that configures it (13,20,29), and on the adaptations that may have been developed at a vascular level, improving the resistance to occlusion (7,23). Since these adaptations, both neuromuscular and vascular, can be achieved through training, it’s important to individually monitor from time to time the possible variations of the OT, since:

  1. the protocols to assess specific endurance in climbing also need the most specific execution conditions (33) reproducing the effort done by the finger flexors during the sections that are most determinant for the performance, which are the most difficult in the climbing routes (25), and that usually require high intensity muscle contractions, most likely implying vascular occlusion (24). Therefore, assessing the local OT of a particular climber allows evaluating his finger endurance in specific conditions and in a controlled hemodynamic context (occlusion), which moreover is the safest way since this doesn’t require attaining maximum intensities.
  2. the lowest working intensity that may induce adaptations improving the OT, is the OT itself (21). This can be used as a safety parameter for training since it allows achieving these adaptations by exercising at below maximum intensities, and as an efficiency tool since it allows greater workout volumes compared to those that could be done at higher intensities. Therefore, training with the knowledge of the OT may allow a better adaptive effect, which is what the pre-configured workouts of the App set to improve the local anaerobic abilities, take advantage of.
  3. the OT also could define the training dynamic to be followed, meaning that climbers with different OT should work differently, especially those with the most extreme values.


The method by which the OT is assessed by self-testing is based on the current evidence (4). It is indirect since it doesn’t measure this parameter directly inside the muscle, but rather through external variables obtained during the tests (in this case the duration of the finger hangs). Indirect procedures are quite common in sports as they are equally functional and much more accessible than laboratory tests which usually need more expensive and complex resources (9). These time-based measurements don’t allow an exact assessment of the OT, but their accuracy is precise enough (±10%) (4). Therefore, this method to approach the OT allows in any case to know at which intensities blood flow exists, and at which it doesn’t, and this is the primary goal of this test (4). The advantage of this procedure is that it only requires having a chronometer and two edge shaped holds, or better, the R-Evolution Training Board with its App.

In order to make these indirect assessments reliable, you should perform the initial self-test to find your OT as described in the related HELP texts. Failing to comply with theseguidelines may make you obtain inaccurate results and make the App display an OT that may not be real. This could happen if:

  1. you show signs of fatigue at a local or general level before self-testing, which may be produced by any activity done during the previous days. Although the main recommendation is to rest at least 24 hours prior to it, this time should be adapted to your fatigue level. As the initial self-test is done along several days, if its tests aren’t too demanding, you may experience some progressive recovery over time. This could mean that your actual values could be  underestimated during the first days. If you don’t manage to recover at all, your overall values could be underrated in absolute terms. In both cases the assessment would be inaccurate;
  2. you accumulate an excessive fatigue by doing other activities in addition to self-testing, against which we advise to avoid the aforementioned negative effects, which in this case may imply altered (underestimated) values during the last days. A good indicator of whether you are maintaining a negligible or tolerable level of fatigue during those days, is the maximum strength value, which is measured each day by the first test, and should remain stable. If not, we recommend postponing the testing, and get enough rest (at least 12 hours);
  3. you don’t reach true muscle failure on every finger hang on the first try, which could be caused by a lack of practice with this kind of efforts, either because of being new to the painful sensation that is locally felt (forearm), or because of not being able to maintain the necessary concentration throughout all the tests to perform them properly;
  4. the shape of your fingertips’ skin and/or the ambient conditions (temperature and humidity) suffer significant variations between the testing days. Since these elements affect the friction coefficient between the fingers and the edge’s surface (1,35), they’ll have influence on how long you can keep hanging (16), especially when using the smallest holds (6). Since this external conditions generally depend on the environmental ones, which is the usual case except when laboratory facilities or an air-conditioned training room are available, it’s important that you observe these parameters at the beginning of each self-testing session. If the conditions are too adverse or far from those of the first day, consider postponing the session. This is particularly important for the initial self-test, since it has to be performed along many days, and those variations could make difficult establishing accurate relationships between the results obtained during the different tests (27), which could alter or impede the evaluation of your OT.

It could happen that, even after having strictly followed during self-testing the indications and recommendations described above, which means having performed it under favorable conditions (none of the previous cases), you obtain a resulting OT falling either above or below of what’s been observed for this parameter in all climbing research done so far (4), meaning it may be an extreme OT. In this case, the App would provide an auxiliary OT based on the available scientific evidence, allowing you to build your physiological profile in a valid way, which means letting you know your strength and specific endurance indicators to evaluate the effects of your training and/or to do the pre-configured anaerobic endurance workouts proposed by the App. This may help you to avoid repeating the initial self-test in the short term and the interference this could suppose to the dynamic of your current training.

We advise you to assess your OT with an annual periodicity, except in the case described in the HELP text 2, since its tests usually require 3 days, and because improving the OT normally takes a long time: the adaptations that modify it are antagonistic to the finger strength, which is the main physical quality for climbing (2) and thus the one that, generally speaking, should be primarily developed. This advice is based on the research the author of the App has made over several seasons on climbers with redpoint levels of 7a/5.11d to 9a/5.14d. Future studies on the OT will provide more information on how quickly this parameter evolves in climbers with different levels, so this recommendation may at some point be revised.


Most studies have found an inverse relationship between the OT and the MIS of the muscles (3,19,20,32). In other words, the higher the MIS of a given muscle is, the lower is the percentage of that MIS at which the occlusion of its vessels occurs. This means that the stronger a muscle gets, the sooner occlusion will appear in it during a progressive contraction, which implies an earlier fatigue development.

This could make us believe that having more strength may always mean having less endurance. However, this isn’t entirely true for climbers, at least not among those who have developed vascular adaptations that let them have a higher OT. Therefore, on climbers, more strength doesn’t necessarily imply a lower OT.

Among all the published studies related to the OT carried out so far on the forearm musculature (5,12,19,20,22,26,28-32,36), only one have been made on climbers (4).This fact underlines the scientific gap that still exists in this area of study, which is surprising, since it’s well known that climbers show deep local vascular adaptations (11,15,34) that could play an important role in the occlusive processes (7,23). These adaptations, mainly developed in the muscles that are used to grab the holds, are associated with frequent exposure to ischaemia (lack of blood circulation), and may imply the development of thicker, more resistant, blood vessels that could counteract occlusion at higher effort intensities (21). Hypothetically, achieving these adaptations could improve (elevate) the local OT, allowing the presence of some blood flow at contraction intensities where, before the adaptations, occlusion happened, impeding blood circulation. This improvement could increase the ability to maintain contractions of higher intensity for a longer time, and this is directly related with climbing performance (24).

However, in the only study carried out so far with climbers on the OT by Bergua et al., (2020) (4), this threshold has been detected at relative intensities going from 45% and 75% of the MIS, with the median placed at 65.6% (± 8.9%). However, no significant relationship was found, in the whole analyzed sample, between the strength, experience or the climbing level and the OT, although in the subgroup with the higher level there was a positive relationship between the experience and the OT. This could suggest some rather belated adaptations of this parameter in the climbers, although they would not imply by themselves a higher performance. Therefore, even if boosting finger strength in conjunction with adaptations achieving a higher OT may seem important training goals for climbing, the lack of current evidence and the acknowledgment of how essential is the local hemodynamic profile assessment in climbers (14), guarantee further research on the OT related to this sport.


(1) Amca AM, Vigouroux L, Aritan S, Berton E. The effect of chalk on the finger-hold friction coefficient in rock climbing. Sports biomechanics 2012 Nov;11(4):473-479.

(2) Baláš J, Pecha O, Martin AJ, Cochrane D. Hand–arm strength and endurance as predictors of climbing performance. European Journal of Sport Science 2012;12(1):16-25.

(3) Barnes WS. The relationship between maximum isometric strength and intramuscular circulatory occlusion. Ergonomics 1980 Apr;23(4):351-357.

(4) Bergua P, Montero-Marin J, Gomez-Bruton A, A. Casajús J. The finger flexors occlusion threshold in sport-climbers: an exploratory study on its indirect approximation. European Journal of Sport Science 2020:1-21.

(5) Bonde-Petersen F, Mørk A, Nielsen E. Local muscle blood flow and sustained contractions of human arm and back muscles. Eur J Appl Physiol Occup Physiol 1975;34(1):43-50.

(6) Bourne R, Halaki M, Vanwanseele B, Clarke J. Measuring lifting forces in rock climbing: effect of hold size and fingertip structure. Journal of applied biomechanics 2011 Feb;27(1):40-46.

(7) Boushel R. Muscle metaboreflex control of the circulation during exercise. Acta physiologica 2010;199(4):367-383.

(8) Carlson BR. Level of maximum isometric strength and relative load isometric endurance. Ergonomics 1969.;12(3):429-435.

(9) Chicharro JL, Laín SA. Transición aeróbica-anaeróbica: concepto, metodología de determinación y aplicaciones. : Master Line; 2004.

(10) Demura S, Nakada M, Yamaji S, Nagasawa Y. Relationships between force-time parameters and muscle oxygenation kinetics during maximal sustained isometric grip and maximal repeated rhythmic grip with different contraction frequencies. Journal of physiological anthropology 2008;27(3):161-168.

(11) Ferguson RA, Brown MD. Arterial blood pressure and forearm vascular conductance responses to sustained and rhythmic isometric exercise and arterial occlusion in trained rock climbers and untrained sedentary subjects. European journal of applied physiology and occupational physiology 1997;76(2):174-180.

(12) Fitzpatrick R, Taylor JL, McCloskey D. Effects of arterial perfusion pressure on force production in working human hand muscles. The Journal of physiology 1996;495(3):885-891.

(13) Frey Law LA, Avin KG. Endurance time is joint-specific: a modelling and meta-analysis investigation. Ergonomics 2010;53(1):109-129.

(14) Fryer S, Stone KJ, Sveen J, Dickson T, España-Romero V, Giles D, et al. Differences in forearm strength, endurance, and hemodynamic kinetics between male boulderers and lead rock climbers. European Journal of Sport Science 2017 07/28:1-7.

(15) Fryer S, Stoner L, Lucero A, Witter T, Scarrott C, Dickson T, et al. Haemodynamic kinetics and intermittent finger flexor performance in rock climbers. International Journal of Sports Medicine 2015;36(2):137-142.

(16) Fuss FK, Burr L, Weizman Y, Niegl G. Measurement of the Coefficient of Friction and the Centre of Pressure of a Curved Surface of a Climbing Handhold. Procedia Engineering 2013;60(0):491-495.

(17) Giles D, Chidley J, Taylor N, Torr O, Hadley J, Randall T, et al. The Determination of Finger Flexor Critical Force in Rock Climbers. International journal of sports physiology and performance 2019:1-24.

(18) Giles D, Hartley C, Maslen H, Hadley J, Taylor N, Torr O, et al. An all-out test to determine finger flexor critical force in rock climbers. International Journal of Sports Physiology and Performance 2020.

(19) Heyward VH. Influence of static strength and intramuscular occlusion on submaximal static muscular endurance. Research Quarterly 1975(46):393-402.

(20) Humphreys P, Lind A. The blood flow through active and inactive muscles of the forearm during sustained hand‐grip contractions. The Journal of physiology 1963.;166(1):120-135.

(21) Hunt JE, Walton LA, Ferguson RA. Brachial artery modifications to blood flow-restricted handgrip training and detraining. Journal of applied physiology 2012 Mar;112(6):956-961.

(22) Hunter SK, Enoka RM. Sex differences in the fatigability of arm muscles depends on absolute force during isometric contractions. Journal of Applied Physiology 2001.;91(6):2686-2694.

(23) Hunter SK, Schletty JM, Schlachter KM, Griffith EE, Polichnowski AJ, Ng AV. Active hyperemia and vascular conductance differ between men and women for an isometric fatiguing contraction. Journal of applied physiology 2006 Jul;101(1):140-150.

(24) MacLeod D, Sutherland DL, Buntin L, Whitaker A, Aitchison T, Watt I, et al. Physiological determinants of climbing-specific finger endurance and sport rock climbing performance. Journal of sports sciences 2007 Oct;25(12):1433-1443.

(25) Michailov M, Staszkiewicz R, Szyguła Z, Ręgwelski T, Staszkiewicz R, editors. The importance of aerobic capacity in rock climbing. Hypoxia Symposium; 13-9-2013; Zakopane, Dolina Chocholowska: Medicina Sportiva & Polish society of Sports Medicine; 2013.

(26) Nakada M, Demura S, Yamaji S, Minami M, Kitabayashi T, Nagasawa Y. Relationships between force curves and muscle oxygenation kinetics during repeated handgrip. Journal of physiological anthropology and applied human science 2004;23(6):191-196.

(27) Phillips K, Noh B, Gage M, Yoon T. The effect of cold ambient temperatures on climbing-specific finger flexor performance. European Journal of Sport Science 2017:1-9.

(28) Rohter FD, Hyman C. Blood flow in arm and finger during muscle contraction and joint position changes. J Appl Physiol 1962;17(5):819-823.

(29) Royce J. Isometric fatigue curves in human muscle with normal and occluded circulation. Research Quarterly.American Association for Health, Physical Education and Recreation 1958;29(2):204-212.

(30) Sadamoto T, Bonde-Petersen F, Suzuki Y. Skeletal muscle tension, flow, pressure, and EMG during sustained isometric contractions in humans. Eur J Appl Physiol Occup Physiol 1983;51(3):395-408.

(31) Sjøgaard G, Savard G, Juel C. Muscle blood flow during isometric activity and its relation to muscle fatigue. European journal of applied physiology and occupational physiology 1988;57(3):327-335.

(32) Start K, Holmes R. Local muscle endurance with open and occluded intramuscular circulation. Journal of applied physiology 1963;18:804-807.

(33) Staszkiewicz R, Ruchlewicz T, Szopa J. Handgrip strength and selected endurance variables. Journal of Human Kinetics 2002;7:29-42.

(34) Thompson EB, Farrow L, Hunt JE, Lewis MP, Ferguson RA. Brachial artery characteristics and micro-vascular filtration capacity in rock climbers. European journal of sport science 2015;15(4):296-304.

(35) Tomlinson S, Lewis R, Carré M. Review of the frictional properties of finger-object contact when gripping. Proceedings of the Institution of Mechanical Engineers, Part J: Journal of Engineering Tribology 2007;221(8):841-850.

(36) Wigmore DM, Befroy DE, Lanza IR, Kent-Braun JA. Contraction frequency modulates muscle fatigue and the rate of myoglobin desaturation during incremental contractions in humans. Applied Physiology, Nutrition, and Metabolism 2008;33(5):915-921.

(37) Yamaji S, Demura S, Nagasawa Y, Nakada M, Yoshimura Y, Matsuzawa Z, et al. Examination of the parameters of static muscle endurance on sustained static maximal hand gripping. Jpn J Phys Educ 2000;45:695-706.

(38) Yamaji S, Demura S, Nagasawa Y, Nakada M. Relationships between decreasing force and muscle oxygenation kinetics during sustained static gripping. J Physiol Anthropol Appl Human Sci 2004;23(2):41-47.

(39) Yamaji S, Demura S, Nagasawa Y, Nakada M, Kitabayashi T. The effect of measurement time when evaluating static muscle endurance during sustained static maximal gripping. J Physiol Anthropol Appl Human Sci 2002;21(3):151-158.


More on how to make self-testing compatible with training