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Updated: Mar 19, 2020

Muscle fatigue threshold (MFT) It has been reported in many scientific papers that muscle function is affected by lactic acid accumulation and increased sympathetic nervous system activity when exercise intensity is increased above certain level (e.g. Lucia et al. 1999, Tikkanen et al. 2012, Zuniga et al. 2013). The time point of change in muscle function corresponds to anaerobic threshold measured from ventilation (ventilatory threshold) and occurs usually a bit after lactate threshold or OBLA (onset of blood lactate accumulation, typically 4 mmol / L level) (see figure 1 from Tikkanen et al. 2012 below). Peinabo et al. 2014 suggested that the increase in lactate production mirrors an increased activity in type II, or fast twitch, muscle fibers. According to Burke (1981), the activation of these fibers is preferential once the anaerobic threshold has been reached. The increase in their activity explains the increase in electrical activity of muscles, and supports the idea that the anaerobic threshold can be identified with EMG. This point, because detected with EMG, is called EMG threshold or muscle fatigue threshold (MFT).

Figure 1 (Tikkanen et al. 2012). Location of OBLA, ventilatory threshold (VT) and muscle fatigue threshold (EMG) in recreationally active subjects and endurance athletes in treadmill running. OBLA is onset of blood lactate accumulation (4 mmol / L). EMG is measured here with Mbody. As visible also in figure 1, MFT is detected as change in slope in muscle load / exercise intensity -curve during incremental exercise test. This corresponding intensity as well as muscle load (ML) value itself can be used later in training to categorize training intensities (zones).

Differences between recreational enthusiasts and trained athletes From figure 1 several differences between recreationals and endurance athletes can be noted. First of all, although the comparison of absolute EMG values between individuals can be questioned, it is generally agreed that skillfully coordinated efficient running performance is achieved with less muscle activation. This is seen as lower EMG values in endurance trained athletes even if their running velocities are higher as compared to recreationals. Secondly, OBLA is reached later in test and with higher velocities in endurance athletes due to better basic endurance. In addition, with training the muscle work is maintained even after ventilatory threshold and therefore MFT occurs typically about 4 % after ventilatory threshold in athletes whereas they occur simultaneously in recreationals (Tikkanen et al. 2012). In figure 2 below there are MFTs detected with Mbody from three triathletes during incremental cycling tests. MFT can be detected with Mbody both during running and cycling, and basically all sport types that involve mainly leg muscle work.

Figure 2. Location of lactate threshold and muscle fatigue threshold (MFT) during incremental cycling test until exhaustion. Subjects are three triathletes (2 men, 1 woman).

Relationship between heart rate, blood lactate, muscle load and running pace During incremental exercise there is a linear relationship between heart rate and exercise intensity, as demonstrated in figure 3 during incremental 6 x 1000 m running test. Muscle load (ML), on the other hand, increases typically curvilinearly and mimics the shape of blood lactate accumulation. From muscle loading point of view this means that exercise intensities below MFT are typically quite efficient and the increase in intensity when still below MFT requires little extra muscle work. Thereafter, after MFT sharp decrease in efficiency and increasing demands to muscles loading occur. This is seen as sharp increase in muscle activation and therefore fast fatigue accumulation. This highlights the importance of MFT detection especially in athletes that need to maintain muscle function repeatedly hours after hours (cycling, marathon running, triathlon).

Figure 3. Relationship between heart rate, blood lactate, muscle load and running pace in incremental 6 x 1000 m running test. Myontec Mbody offers solution to detect MFT during standardized exercise tests and based on current scientific literature as well as our own substantial data collection we have developed automatic algorithms for MFT analysis. Therefore, now for the first time, it is possible for athletes and consumers to detect their training threshold noninvasively and use their muscle loading level to classify training intensities. This offers totally new perspectives to training by providing with Training zones based on muscle loading as alternative for zones calculated from Heart rate. These issues are discussed in our next posts.

Authors: Pekka Tolvanen, M.Sc. (Physics), Product Manager, Founder of Myontec Merja Hoffrén-Mikkola, PhD (biomechanics), Content Developer, Myontec

REFERENCES Burke RE. 1981. Motor units: anatomy, physiology and functional organization. In Handbook of Physiology, Section 1: The nervous system. Volume II: Motor control. Edited by Brooks, V.B. Bethesda, Maryland: American physiological society. Lucía A., Sánchez O., Carvajal A. & Chicharro, J.L. 1999. Analysis of aerobicanaerobic transition in elite cyclists during incremental execise with the use of electromyography. British Journal of Sports Medicine 33: 178-185. Peinabo A.B, Rojo J.J., Calderón J. & Maffulli N. 2014. Responses to increasing exercise upon reaching the anerobic threshold, and their control by the nervous system. BMC Sports Science, Medicine and Rehabilitation 6:17. Tikkanen O., Hu M., Vilavuo T., Tolvanen P., Cheng S. & Finni T. 2012. Ventilatory threshold during incremental running can be estimated using EMG shorts. Physiological Measurements 33: 603-614. Zuniga J.M., Bubak M.P., Fisher B.E., Neighbors D.E., Osowski H.S. & Oyen A.N. 2013. Electromyographic and gas exchange fatigue thresholds during incremental treadmill running. Journal of Athletic Medicine 1 (2): 92-103.

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