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A positive Hoffman sign indicates an upper motor neuron lesion and corticospinal pathway dysfunction likely due to cervical cord compression. What causes positive Hoffman’s?

Vagg R, Mogyoros I, Kiernan MC, Burke D (1998) Activity-dependent hyperpolarization of human motor axons produced by natural activity. J Physiol 507(Pt 3):919–925 Stalberg E (1966) Propagation velocity in human muscle fibers in situ. Acta Physiol Scand Suppl 287:1–112 Place N, Maffiuletti NA, Ballay Y, Lepers R (2005) Twitch potentiation is greater after a fatiguing submaximal isometric contraction performed at short vs. long quadriceps muscle length. J Appl Physiol (1985) 98(2):429–436. https://doi.org/10.1152/japplphysiol.00664.2004

Rozand V, Cattagni T, Theurel J, Martin A, Lepers R (2015) Neuromuscular fatigue following isometric contractions with similar torque time integral. Int J Sports Med 36(1):35–40. https://doi.org/10.1055/s-0034-1375614 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 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 The progressive decrease in M-wave duration observed during the low-force contraction could not be due to conduction velocity, as this parameter was seen to remain unchanged. Another possible candidate to explain the narrowing of the M wave, an increase in intramuscular temperature, can also be ruled out: the reason is that Dur FIRST and Dur SECOND decreased markedly during the first 30s of the contraction, while muscle temperature was found remained constant during the same period. We believe that factors related to muscle architecture might have been involved in the reduction of M-wave duration. Specifically, we hypothesize that during the initial phase of the 10% MVC task, the length of muscle fascicles decreased. Our hypothesis is supported by a study of Mademli and Arampatzis (2005), who showed that during an isometric contraction of the plantar flexors at 40% MVC, the fascicle length shortens, this shortening being marked at the initial part of the contraction. Thus, it is conceivable that a muscle fascicle shortening would also occur during a sustained 10% MVC task, since tendon stiffness is lower at low contraction forces ( Hodges et al., 2003). In conclusion, the fascicle length would decrease progressively over the first 30s of the contraction as the tendon progressively stretches, leading to a decrease in M-wave duration. Amplitude of the M-Wave First Phase

Rodriguez-Falces J, Place N (2017a) New insights into the potentiation of the first and second phases of the M-wave after voluntary contractions in the quadriceps muscle. Muscle Nerve 55(1):35–45. https://doi.org/10.1002/mus.25186 Narici MV, Binzoni T, Hiltbrand E, Fasel J, Terrier F, Cerretelli P (1996) In vivo human gastrocnemius architecture with changing joint angle at rest and during graded isometric contraction. J Physiol 496(Pt 1):287–297 Kranz H, Williams AM, Cassell J, Caddy DJ, Silberstein RB (1983) Factors determining the frequency content of the electromyogram. J Appl Physiol Respir Environ Exerc Physiol 55(2):392–399 Masuda T, Miyano H, Sadoyama T (1985) The position of innervation zones in the biceps brachii investigated by surface electromyography. IEEE Trans Biomed Eng 32:36–42 Place N, Yamada T, Bruton JD, Westerblad H (2010) Muscle fatigue: from observations in humans to underlying mechanisms studied in intact single muscle fibres. Eur J Appl Physiol 110(1):1–15. https://doi.org/10.1007/s00421-010-1480-0

For each M-wave potential, the amplitude, duration, and area of the first (Ampli FIRST, Dur FIRST, and Area FIRST) and second (Ampli SECOND, Dur SECOND, and Area SECOND) phases were computed as in previous studies (see Figure 1 and Rodriguez-Falces and Place, 2017a, b). The onset for Dur FIRST was determined by a deviation greater than 2 SDs of the baseline. The onset for Dur FIRST was determined by a deviation greater than 2 SDs of the baseline noise from the baseline, whereas the end-point corresponded to the baseline-crossing point. This crossing point marked the onset of the second phase. The end-point for Dur SECOND was determined by a deviation less than 2 SDs of the baseline noise from the baseline. The area parameters were calculated as the integral of the absolute value of the M wave over the above-defined phases. Ampli PP was computed as the sum of Ampli FIRST and Ampli SECOND. Area TOTAL was calculated by adding the areas of the first and second phases. Dur PP was computed as the time interval between the first and second peaks of the M wave. For the quadriceps twitch, the peak force was measured.

Bigland-Ritchie B, Johansson R, Lippold OC, Woods JJ (1983) Contractile speed and EMG changes during fatigue of sustained maximal voluntary contractions. J Neurophysiol 50(1):313–324 Abstract. The Hoffmann (H-) reflex is an electrical analogue of the monosynaptic stretch reflex, elicited by bypassing the muscle spindle and directly stimulating the afferent nerve. Studying H-reflex modulation provides insight into how the nervous system centrally modulates stretch reflex responses. How does a reflex arc function?Kuchinad RA, Ivanova TD, Garland SJ (2004) Modulation of motor unit discharge rate and H-reflex amplitude during submaximal fatigue of the human soleus muscle. Exp Brain Res 158(3):345–355. https://doi.org/10.1007/s00221-004-1907-0 In the present study we did not measure changes in fascicle length, and thus we can only speculate that the changes observed in M-wave parameters were attributable, to a certain degree, to a shortening in fascicle length. On the other hand, in the present study we adopted a contraction duration of 3min, which was considerably shorter that than the duration (400s) selected by previous investigators ( Arendt-Nielsen et al., 1989). This difference in the contraction duration hampers the comparison of the results. Conclusion The second remarkable observation is that, similarly to Ampli FIRST, the largest change in Ampli SECOND occurred during the initial 30s of the contraction. The parallel time course of changes in these parameters suggests that they were affected by the same mechanism. Our hypothesis of a shortening of muscle fascicle length could explain the concurrent increase in Ampli SECOND and decrease in Ampli FIRST. Certainly, muscle shortening would affect the M-wave second phase, as this phase is generated upon the extinction of the action potentials at the fiber ends ( Rodriguez-Falces and Place, 2018). Specifically, a reduction in fascicle length would cause a more synchronous arrival of the action potentials at the tendons, thus provoking an increase in Ampli SECOND ( Rodriguez-Falces and Place, 2014). As for the M-wave first phase, the bulging of the muscle caused by muscle shortening would make Ampli FIRST to decrease, as commented above. Recovery Period 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 McComas AJ, Galea V, Einhorn RW (1994) Pseudofacilitation: a misleading term. Muscle Nerve 17(6):599–607. https://doi.org/10.1002/mus.880170606

Rodriguez-Falces J, Place N (2017b) Muscle excitability during sustained maximal voluntary contractions by a separate analysis of the M-wave phases. Scand J Med Sci Sports. 27(12):1761–1775 Plaskett CJ, Cafarelli E (2001) Caffeine increases endurance and attenuates force sensation during submaximal isometric contractions. J Appl Physiol (1985) 91(4):1535–1544Garland SJ, Garner SH, McComas AJ (1988) Reduced voluntary electromyographic activity after fatiguing stimulation of human muscle. J Physiol 401:547–556