Models of recruitment and rate coding organization in motor-unit pools.

“Structure-based models, for example, can now describe the generation of the surface EMG in complex volume conductors (31) as a consequence of various control strategies (35, 46, 84).”

“For that purpose, mechanical properties of MUs obtained using the spike-triggered averaging method recorded in the tibialis anterior (Van Cutsem et al. 1997) were inserted into the model developed by Fuglevand et al. (1993).”

“The more segmented aspect of the EMG activity during these contractions could be explained by a tendency of motor units to pulse at a similar frequency after training (Fuglevand et al. 1993).”

“In this simulation, motor units were recruited and derecruited according to the size principle (44) during each ramp (Fig.”

“In the standard model, ISI variability was modeled using a Gaussian distribution and a constant CV of 0.2 (Fuglevand et al. 1993).”

“The computer model comprised 120 motor neurons with systematic variations in recruitment threshold, range of discharge rates, innervation ratio, conduction velocity of sarcolemmal action potentials, motor unit territory, amplitude and duration of the twitch force, and the specific shape of the force-frequency relation…”

“To quantify the outputs of the model, we postulated the existence of direct relationships between the number of recruited axons and the overall muscle activation (Fuglevand et al., 1993).”

“plemented using the exponential threshold method described in [35].”

“This trend was similar to that observed in other studies on the first dorsal interosseus (Burnett et al. 2000; Galganski et al. 1993; Laidlaw et al. 1999, 2000) and quadriceps femoris (Christou et al. 2002; Tracy and Enoka 2002) muscles.”

“The distribution of recruitment thresholds for the motor neurons was modelled with an exponential function with many low-threshold neurons and progressively fewer high-threshold neurons, as described by Fuglevand et al (1993).”

“It has been suggested that variation in recruitment range may contribute to the different EMGforce relations observed experimentally (3, 21).”

“This finding indicates that the amplitude of the surface enerated by motor units were simulated with the planar volume conductor model by Fuglevand et al. (1993).”

“Because of the size principle, the increase in force with additional MU recruitment is predicted by modeling to be more than proportional (Fuglevand et al., 1993).”

“Simulation studies appear to support an association between motor neuron discharge rates and rapid force production, although there are no direct experimental observations for this association (Fuglevand et al. 1993; Harwood & Rice, 2012).”

“For example, when using as a window the function proposed by Fuglevand et al. (1993) for describing the motor unit twitch force (with contraction time = 90 ms) instead of the Hann window, the association between the FCC and force signal increased to 71.8 ± 13.1% (average over all contraction…”

“Moreover, the relation between amplitude and excitation level depends on the pattern of motor unit activation (10), electrode location in relation to innervation zones and tendon regions, and crosstalk (Fig.”

“The discharge rates at the force peak (10% MVC) were significantly larger than at recruitment and derecruitment and were inversely correlated to recruitment threshold (De Luca et al., 1982; Fuglevand et al., 1993; Hogrel, 2003; Monster and Chan, 1977).”

“The relationship between the number of motor units in a muscle and muscle noise was examined using a previously tested model of force generation by a single muscle under isometric conditions (Fuglevand et al. 1993; Jones et al. 2002).”

“8 and 9 show examples of simulated surface EMG signals detected during electrically elicited and voluntary contractions [13], [29], [41].”

“Both modeling studies (Fuglevand et al., 1993; Heckman, 1994) and reflex experiments in human subjects (see Cathers et al. (2004) for a review) indicate that this ‘‘recruitment nonlinearity” mechanism is effective only up to about 30–50% of maximum voluntary force, as most of recruitment is…”

“Model structure and modeling strategy We modified the model of a motoneuron pool described in detail by Fuglevand et al. (1993) (see also Fuglevand 1989; Fuglevand and Bigland-Ritchie 1993) so that it could simulate interpolated twitches of the adductor pollicis muscle.”

“The motor neuron parameters were the same as used by Cisi & Kohn (2008; their Table 2) and selected according to an exponential distribution over the pool of motor neurons (Fuglevand et al. 1993).”

“In [46], different motor unit discharge patterns were simulated (including those following or violating”

“Because the relative contribution of single motor units to net muscle force declines as the number of active motor units increases (13), however, it is unlikely that differences in the steadiness of limb muscles are attributable to the behavior of single motor units.”

“We do not believe that such a model is currently feasible, largely because existing detailed mechanistic models of EMG apply to isometric contractions only [30] and often fail to realistically replicate fundamental features of EMG [31].”

“For the FDI muscle, the number of motor units (n) is 120 (Feinstein et al. 1955), the range of recruitment (RR) is 0 to 0.67 (De Luca and Hostage 2010), and, according to Fuglevand et al. (1993), the recruitment threshold distribution is exponential, as shown in Fig.”

“The relationship between force and EMG signals can be nearly linear under isometric conditions.(3,17,20) Our data were normalized to a maximum voluntary contraction on a Cybex dynamometer at three different angles.”

“Because the force contributed by a single motor unit to the net muscle force declines as the number of active motor units increases (Fuglevand et al. 1993), a muscle with fewer motor units might exhibit greater force fluctuations.”

“How the number of active motoneurons, their discharge rate and the degree of their synchronization is related to the mean value of the rectified EMG is not known, but computer models bearing on this issue are currently being developed (Fuglevand et al., 1993)”

“[18] modeled motor-unit pool organization and concluded that these differences may be due more to firing rate behavior differences between muscles.”

“5A) (Fuglevand et al. 1993).”

“Initially the motoneurons fire repetitively, but with time, some motoneurons slow their firing rate and others stop [37,38].”

“Because the recruitment of one motor unit adds relatively little to the force being exerted by a muscle (43), even weak contractions require the concurrent activation of many motor units.”

“The number of MU fibers was uniformly distributed between 50 and 450, and the MU fiber density was 20 fibers/mm2 (19).”

“The number of fibers of each MU varied between 50 and 450, and the MU fiber density was 20 fibers/mm(2) [20, 21].”

“Because the discharge rate is typically high (e.g., 100 Hz) in the case of an MVC task (Fuglevand et al., 1993; Herbert and Gandevia, 1999) and the number of motor units is supposed to be sufficiently large, the assumption of a continuous voluntary drive (B) is adequate for our force data fitting.”

“…LeFever and De Luca 1982; Mambrito and De Luca 1984; McGill et al. 1985; ), or when analyzing signal properties in computer simulations or mathe- atical models of EMG (Day et al. 1996; Fuglevand et al. 1993; Libkind 1968, 1969; Milner-Brown and Stein 1975; Person and Libkind 1970; Yao et al. 2000).”

“2010) that represented the motor unit recruitment and rate coding characteristics (Fuglevand et al. 1993) of the first dorsal interosseus muscle (FDI; index finger abductor) (Barry et al.”

“…of muscle activation, a combination of motor unit recruitment and rate coding or increased frequency of activation of already active motor units (Fuglevand et al., 1993), dictates the energy cost since the rate of energy use depends on the number of fibers activated, the cross-bridge turnover…”

“This effect of the unfused tetani on the force fluctuations is greatest at low forces, when the relative contribution of a motor unit to the net force is greatest, and is greater in muscles with a narrow recruitment range (17), such as occurs with the decline in motor unit number with age (7, 11).”

“…CSTwiththeimpulseresponseofacriticallydampened, second-ordersystem f ( t ), which has been used extensively to model the motor unit twitch force (Fuglevand et al. 1993): Using this equation, a brute force optimization procedure was used to optimally scale both P , the peak twitch force, from…”

“1992), with an exponential distribution across the population (Fuglevand et al. 1993; Kernell, 1992).”

“Altogether, five different realizations of a muscle with elliptical cross-section (25 (transversal) 16 (depth) mm) were simulated using planar volume conductor model [5] and the MU recruitment model [10].”

“Nonetheless, the amplitude of a surface EMG signal can provide a useful approximation of the amplitude component of the neural drive to muscle during some controlled conditions (8, 18).”

“The motor neuron parameters were the same as used by Cisi & Kohn (2008; their Table 2) and selected according to an exponential distribution over the pool of motor neurons (Fuglevand et al. 1993).”

“The study involved computer simulations that were based on a model of a population of motor units (28) with the addition of a surface EMG model that incorporated muscle fibers with a finite length and a volume conductor with skin, subcutaneous, and muscle layers (25).”

“The motor-neuron pool, controlling the motor units, was inspired by the model presented by Fuglevand et al. (1993).”

“The motor neuron parameters were the same as used by Cisi & Kohn (2008) and selected according to an exponential distribution over the pool of motor neurons (Fuglevand et al. 1993).”

“Large motor units (50) and higher discharge-rate variabilities of motor units (51) have been associated with greater variability in force.”

“However, the rate of recruitment of alpha motor neurons may follow an exponential function (Fuglevand et al. 1993; Jones 2005).”

“Given that there are distinct contributions of neuromuscular mechanisms at various portions of the force production curve (Fuglevand et al. 1993; Milner-Brown et al. 1973), the age-force level variability interaction has lead to the proposition that various neurophysiological mechanisms are…”

“Irrespective of the precise cause for this change, it would significantly increase the amplitude of the multiunit EMG signal (e.g. Fuglevand et al. 1993; Keenan et al. 2006), and may be sufficiently large to account fully for the previously reported difference in multiunit EMG between the control…”

“Table 1 Description of the parameters used to simulate interference EMGs (SD means standard deviation; Fuglevand et al., 1993).”

“glevand and colleagues [95], which has been used extensively”

“Mean firing rates with >30 % coefficient of variation were excluded for further analysis (Fuglevand et al. 1993).”

“Regardless of the underlying mechanism, based on the correlation between EMG and MEP, it could be argued that the increase in EMG, per se, ‘facilitated’ the MEP size in the FENT condition (Fuglevand et al. 1993; McNeil et al. 2011) and therefore masked the fall seen in the CTRL condition (Fig.”

“This non-linear relationship could reflect physiological, i.e. motor unit recruitment and rate coding strategy (Fuglevand et al., 1993), and non-physiological factors, spatial inhomogeneity in the location of different types of motor unit within a muscle in the dimension of depth (Knight and Kamen,…”

“It is further worth noting that, in the earlier model (Fuglevand et al. 1993), the diameter of muscle fibers of all motor units was assigned the same value, while bigger motor units contained more muscle fibers.”

“The aforementioned results nicely agree with models describing the activation behaviour at fibre or motor unit level, see e.g. Fuglevand et al. (1993); Ding et al. (1998) and Herbert and Gandevia (1999).”

“At 10% MVC force therefore, the last-recruited motor unit in the hand muscle would have a larger relative peak-to-peak force than would the proximal arm muscle (Fuglevand et al. 1993).”