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Botter A, Oprandi G, Lanfranco F, Allasia S, Maffiuletti NA, Minetto MA (2011) Atlas of the muscle motor points for the lower limb: implications for electrical stimulation procedures and electrode positioning. Eur J Appl Physiol 111:2461–2471 Bilodeau M, Henderson TK, Nolta BE, Pursley PJ, Sandfort GL (2001) Effect of aging on fatigue characteristics of elbow flexor muscles during sustained submaximal contraction. J Appl Physiol (1985) 91(6):2654–2664

It would be tempting to associate the decrease in Ampli FIRST observed during the contraction to changes in the properties of the sarcolemmal membrane. One such membrane property is conduction velocity: however, impulse conduction remained unchanged during the contraction, and thus could not be involved in the decline of Ampli FIRST. Another factor known to have a depressing effect on M-wave amplitude is increased intramuscular temperature ( Rutkove, 2000). However, it appears that muscle temperature could not have a major effect on M-wave size: indeed, muscle skin temperature increased marginally during the first 30s of the contraction, whereas most of the decrease in Ampli FIRST occurred precisely during the initial 30s. Arabadzhiev TI, Dimitrov GV, Chakarov VE, Dimitrov AG, Dimitrova NA (2008) Effects of changes in intracellular action potential on potentials recorded by single-fiber, macro, and belly-tendon electrodes. Muscle Nerve 37(6):700–712. https://doi.org/10.1002/mus.21024

Vyskočil F, Hnik P, Rehfeldt H, Vejsada R, Ujec E (1983) The measurement of K + e concentration changes in human muscles during volitional contractions. Pflugers Arch 399(3):235–237 Gobbo M, Maffiuletti NA, Orizio C, Minetto MA (2014) Muscle motor point identification is essential for optimizing neuromuscular electrical stimulation use. J Neuroeng Rehabil 11:17. https://doi.org/10.1186/1743-0003-11-17 Because it is largely monosynaptic, the latency of an H-reflex depends mainly on the lengths and conduction velocities of the afferent and efferent axons in the peripheral nerve. What is an F wave in ECG? F Wave and H Reflex F waves can be elicited from practically all distal motor nerves. The H reflex is a late CMAP that is evoked regularly from a muscle by a submaximal stimulus to a nerve, and it is due to stimulation of Ia afferent fibers (a spinal reflex). What is H-reflex study? Skin surface temperature was measured during the course of the low-force contraction using a fast-response thermistor (SS6L, BIOPAC, response time 0.6s, accuracy ±0.1°C). The temperature probe was located close to the surface electrodes over the rectus femoris. During the experiments, room temperature was kept at 22±0.2°C. Rating of Subjective Effort

Duchateau J, Hainaut K (1985) Electrical and mechanical failures during sustained and intermittent contractions in humans. J Appl Physiol (1985) 58(3):942–947 The decline in maximal voluntary force (−9%) observed immediately after the 3-min 10% MVC task indicate that such contraction produced muscle fatigue, in agreement with previous observations during low-force contractions ( Søgaard et al., 2006; Smith et al., 2007). Part of this loss of force was due to peripheral factors, as there was a decrease in the amplitude of the resting twitch (~10%) from before to after the contraction. This peripheral fatigue could not be due to an impairment of fiber membrane processes, as both the present results and those of Sjøgaard et al. (1988) indicate that sarcolemmal excitability was preserved throughout the contraction. Therefore, the depression of twitch force was probably caused by altered Ca 2+ handling and/or impairment at the cross bridges level ( Rassier and Macintosh, 2000).Ito K, Hotta Y (2012) EMG-based detection of muscle fatigue during low-level isometric contraction by recurrence quantification analysis and monopolar configuration. Conf Proc IEEE Eng Med Biol Soc 2012:4237–4241. https://doi.org/10.1109/EMBC.2012.6346902 During the prolonged contraction, participants were asked every 30s to score the perceived effort required to produce the target (10% MVC) force on a modified Borg scale from 0 (“nothing”) to 10 (“maximal”; Borg et al., 1985). Data Analysis Rodriguez-Falces J, Place N (2014) Effects of muscle fibre shortening on the characteristics of surface motor unit potentials. Med Biol Eng Comput 52(2):95–107. https://doi.org/10.1007/s11517-013-1112-z

The present results have shown that a decrease in Ampli FIRST during a low-force contraction can occur even in the absence of changes in fiber membrane excitability. This result is of relevance as it implies that, when a muscle contraction is sustained, there exists “non-sarcolemmal” factors which influence the magnitude of the M-wave first phase. Some of these factors (muscle shortening; electrode-to-fiber distance; intramuscular temperature) have been discussed above. It is important to note that the effects of these “non-sarcolemmal” factors on Ampli FIRST are of limited extent: indeed, the decrease in Ampli FIRST was only of ~7%. It is likely that, when a contraction of moderate or high intensity is sustained, these non-sarcolemmal factors would continue exerting a depressing effect on Ampli FIRST, but this effect would be probably counteracted and masked by the increase in extracellular K + concentration, which acts to increase Ampli FIRST ( Rodriguez-Falces and Place, 2017b). Rodriguez-Falces J, Place N (2016) Differences in the recruitment curves obtained with monopolar and bipolar electrode configurations in the quadriceps femoris. Muscle Nerve 54(1):118–131. https://doi.org/10.1002/mus.25006 Fortune E, Lowery MM (2009) Effect of extracellular potassium accumulation on muscle fiber conduction velocity: a simulation study. Ann Biomed Eng 37(10):2105–2117. https://doi.org/10.1007/s10439-009-9756-4 The increase in Ampli SECOND observed during first ~30s of the low-force contraction was interpreted to be due to the shortening of muscle fascicles and is consistent with our previous observations ( Rodriguez-Falces and Place, 2017a, b). Thus, it appears that even for a very low contraction intensity, muscle shortening exerts a noticeable effect on Ampli SECOND. This finding lends support to the idea that the M-wave second phase is highly sensitive to changes in muscle length ( Rodriguez-Falces and Place, 2014), and thus this phase should be excluded for the analysis of membrane excitability. Limitations of the StudyRoeleveld K, Stegeman DF, Vingerhoets HM, Van Oosterom A (1997a) Motor unit potential contribution to surface electromyography. Acta Physiol Scand 160(2):175–183. https://doi.org/10.1046/j.1365-201X.1997.00152.x Hultman E, Sjoholm H (1983) Electromyogram, force and relaxation time during and after continuous electrical stimulation of human skeletal muscle in situ. J Physiol 339:33–40 Crone C, Johnsen LL, Hultborn H, Orsnes GB (1999) Amplitude of the maximum motor response (Mmax) in human muscles typically decreases during the course of an experiment. Exp Brain Res 124(2):265–270 Lexell J, Henriksson-Larsen K, Sjostrom M (1983) Distribution of different fibre types in human skeletal muscles. 2. A study of cross-sections of whole m. vastus lateralis. Acta Physiol Scand 117(1):115–122. https://doi.org/10.1111/j.1748-1716.1983.tb07185.x Plaskett CJ, Cafarelli E (2001) Caffeine increases endurance and attenuates force sensation during submaximal isometric contractions. J Appl Physiol (1985) 91(4):1535–1544

To better appreciate the changes in the duration of the M wave, various responses evoked during and after the 3-min contraction are plotted in superimposed fashion (bottom panel). It can be seen that M-wave duration decreased noticeably during the first minute of the contraction (from the first to the sixth response). Noteworthy, M-wave duration increased only slightly from the M wave evoked at the end of the 3-min contraction (18th response) to the M wave elicited after 5s of rest (Post-5s). M-Wave Parameters During the Low-Force Contraction Mademli L, Arampatzis A (2005) Behaviour of the human gastrocnemius muscle architecture during submaximal isometric fatigue. Eur J Appl Physiol 94(5–6):611–617. https://doi.org/10.1007/s00421-005-1366-8The innervation zone and muscle fibers’ direction were identified in each muscle using a dry linear array of 16 electrodes (5mm inter-electrode distance) during gentle isometric contractions. The array was connected to a multichannel amplifier (OT Bioelettronica, Torino; bandwidth 10–500Hz) and electromyographic (EMG) signals were recorded in single-differential (bipolar) configuration. The location of the innervation zone was determined by observing the channel of the array showing minimum amplitude or phase reversal ( Masuda et al., 1985). The direction of the muscle fibers was identified by choosing the orientation of the array that yielded optimal propagation of action potentials between the innervation zone and tendon regions ( Farina et al., 2002). Electromyographic Recordings Radicheva NI, Kolev VB, Peneva NE (1993) Influence of intracellular potential and conduction velocity on extracellular muscle fibre potential. J Electromyogr Kinesiol 3(2):95–102. https://doi.org/10.1016/1050-6411(93)90004-G The H-reflex can normally be seen in many muscles but is easily obtained in the soleus muscle (with posterior tibial nerve stimulation at the popliteal fossa), the flexor carpi radialis muscle (with median nerve stimulation at the elbow), and the quadriceps (with femoral nerve stimulation). What does a positive Hoffman test mean?