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The aim of this study was to compare the decomposition results obtained from high-density surface electromyography (EMG) and concurrently recorded intramuscular EMG. Surface EMG signals were recorded with electrode grids from the tibialis anterior, biceps brachii, and abductor digiti minimi muscles of twelve healthy men during isometric contractions ranging between 5% and 20% of the maximal force. Bipolar intramuscular EMG signals were recorded with pairs of wire electrodes. Surface and intramuscular EMG were independently decomposed into motor unit spike trains. When averaged over all the contractions of the same contraction force, the percentage of discharge times of motor units identified by both decompositions varied in the ranges 84%-87% (tibialis anterior), 84%-86% (biceps brachii), and 87%-92% (abductor digiti minimi) across the force levels analyzed. This index of agreement between the two decompositions was linearly correlated with a self-consistency measure of motor unit discharge pattern that was based on coefficient of variation for the interspike interval (R(2) = 0.68 for tibialis anterior, R(2) = 0.56 for biceps brachii, and R(2) = 0.38 for abductor digiti minimi). These results constitute an important contribution to the validation of the noninvasive approach for the investigation of motor unit behavior in isometric low-force tasks.

The aim of the study was to investigate the uniformity of the muscle motor point location for lower limb muscles in healthy subjects. Fifty-three subjects of both genders (age range: 18-50 years) were recruited. The muscle motor points were identified for the following ten muscles of the lower limb (dominant side): vastus medialis, rectus femoris, and vastus lateralis of the quadriceps femoris, biceps femoris, semitendinosus, and semimembranosus of the hamstring muscles, tibialis anterior, peroneus longus, lateral and medial gastrocnemius. The muscle motor point was identified by scanning the skin surface with a stimulation pen electrode and corresponded to the location of the skin area above the muscle in which an electrical pulse evoked a muscle twitch with the least injected current. For each investigated muscle, 0.15 ms square pulses were delivered through the pen electrode at low current amplitude (<10 mA) and frequency (2 Hz). 16 motor points were identified in the 10 investigated muscles of almost all subjects: 3 motor points for the vastus lateralis, 2 motor points for rectus femoris, vastus medialis, biceps femoris, and tibialis anterior, 1 motor point for the remaining muscles. An important inter-individual variability was observed for the position of the following 4 out of 16 motor points: vastus lateralis (proximal), biceps femoris (short head), semimembranosus, and medial gastrocnemius. Possible implications for electrical stimulation procedures and electrode positioning different from those commonly applied for thigh and leg muscles are discussed.

This article is the first section of a review work structured in three parts and concerning a) advances in surface EMG detection and processing techniques, b) recent progress in surface EMG clinical research applications and, c) myoelectric control in neurorehabilitation. This article deals with the state of the art regarding a) the electrode-skin interface (equivalent circuits, skin treatment, conductive gels), b) signal detection modalities, spatial filters and front-end amplifiers, c) power line interference removal, separation of propagating and non-propagating potentials and removal of outliers from surface EMG signal maps, d) segmentation of surface EMG signal maps, e) decomposition of surface EMG into the constituent action potential trains, and f) relationship between surface EMG and force. The material is presented with an effort to fill gaps left by previous reviews and identify areas open for future research.

body mass) was ≈20 % larger, and its angle of application in respect to the horizontal ≈10° smaller, for Bolt, as compared to MLS. Finally, we estimated that, on a 10 % downsloping track Bolt could cover 100 m in 8.2 s. Conclusions The above approach can yield useful information on the bioenergetics and biomechanics of accelerated/decelerated running.Keywords Acceleration · Deceleration · Metabolic power · Mechanical power · Soccer energy expenditure Abbreviations a(t)Acceleration at time t a f Forward acceleration aLaAlactic oxygen debt C 0 Energy cost of running at constant speed on flat terrain (J kg −1 m −1 ) COMCentre of mass C r Energy cost of running (J kg −1 m −1 ) C sr Energy cost of sprint running (J kg −1 m −1 ) Ean Anaerobic energy ED Equivalent distance: distance covered running at constant speed on flat terrain, for a given energy expenditure EDI Equivalent Distance Index: ratio between ED and actual distance covered EM Equivalent body mass ES Equivalent slope = tan (90 − α) F Force F acc Force acting on the subject during accelerated running: M g′ F cost Force acting on the subject during constant speed running: M g gAcceleration of gravity g′ Vectorial sum of af and g: g ′ = a 2 f + g 2 AbstractPurpose To estimate the energetics and biomechanics of accelerated/decelerated running on flat terrain based on its biomechanical similarity to constant speed running up/ down an 'equivalent slope' dictated by the forward acceleration (a f ).Methods Time course of a f allows one to estimate: (1) energy cost of sprint running (C sr ), from the known energy cost of uphill/downhill running, and (2) instantaneous (specific) mechanical accelerating power (P sp = a f × speed).Results In medium-level sprinters (MLS), C sr and metabolic power requirement (P met = C sr × speed) at the onset of a 100-m dash attain ≈50 J kg −1 m −1 , as compared to ≈4 for running at constant speed, and ≈90 W kg −1 . For Bolt's current 100-m world record (9.58 s) the corresponding values attain ≈105 J kg −1 m −1 and ≈200 W kg −1 . This approach, as applied by Osgnach et al. (Med Sci Sports Exerc 42:170-178, 2010) to data obtained by video-analysis during soccer games, has been implemented in portable GPS devices (GPEXE © ), thus yielding P met throughout the match. Actual O 2 consumed, estimated from P met assuming a monoexponential VO 2 response (Patent Pending, TV2014A000074), was close to that determined by portable metabolic carts. Peak P sp (W kg −1 ) was 17.5 and 19.6 for MLS and elite soccer players, and 30 for Bolt. The ratio of horizontal to overall ground reaction force (per kg Communicated by

Experimental methods involving painful electrical stimulation of a peripheral nerve showed the existence of a minimum stimulation frequency capable of inducing cramp, termed "threshold frequency" (TF). Our aim was to test an alternative method to induce fasciculations and cramps electrically. Two daily sessions of electrical stimulation of the abductor hallucis muscle were performed in 19 volunteers on 3 days: stimulation trains of 150 monophasic square pulses (duration 152 s) of increasing frequency (current intensity 30% higher than maximal; frequency of the first trial, 4 pps; recovery between trials, 1 min) were delivered to the main muscle motor point until a cramp developed. Once a cramp was induced the protocol was repeated after 30 min. To verify by electromyography that cramp occurred, a surface electrode array was placed between the motor point and the distal tendon. Ambient and skin temperature were kept constant in all sessions. Fasciculations and cramps were elicited in all subjects. We observed the following median (interquartile range) values of TF: day 1 (session 1), 13 (6) pps; day 1 (session 2), 16 (4) pps; day 2 (session 1), 16 (6) pps; day 2 (session 2), 18 (6) pps; day 3 (session 1), 17 (4) pps; day 3 (session 2), 18 (8) pps. TF intersession intraclass correlation coefficients were 0.82, 0.92, and 0.90 for days 1, 2, and 3, respectively. TF interday intraclass correlation coefficient was 0.85. The absence of pain due to the stimulation and the demonstration of TF reliability support the use of our method for the study of involuntary muscle phenomena.

Cramps are sudden, involuntary, painful muscle contractions. Their pathophysiology remains poorly understood. One hypothesis is that cramps result from changes in motor neuron excitability (central origin). Another hypothesis is that they result from spontaneous discharges of the motor nerves (peripheral origin). The central origin hypothesis has been supported by recent experimental findings, whose implications for understanding cramp contractions are discussed.

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