Muscle-Pair Specific Distribution and Grip-Type Modulation of Neural Common Input to Extrinsic Digit Flexors

Abstract

. Musclepair specific distribution and grip-type modulation of neural common input to extrinsic digit flexors. J Neurophysiol 96: 1258 -1266, 2006. First published May 24, 2006 doi:10.1152/jn.00327.2006. To gain insight into the synergistic control of hand muscles, we have recently quantified the strength of correlated neural activity across motor units from extrinsic digit flexors during a five-digit object-hold task. We found stronger synchrony and coherence across motor units from thumb and index finger flexor muscle compartment than between the thumb flexor and other finger flexor muscle compartments. The present study of two-digit object hold was designed to determine the extent to which such distribution of common input among thumbfinger flexor muscle compartments, revealed by holding an object with five digits, is preserved when varying the functional role of a given digit pair. We recorded normal force exerted by the digits and electrical activity of single motor units from muscle flexor pollicis longus (FPL) and two compartments of the m. flexor digitorum profundus (FDP2 and FDP3; index and middle finger, respectively). Consistent with our previous results from five-digit grasping, synchrony and coherence across motor units from FPL-FDP2 was significantly stronger than in FPL-FDP3 during object hold with two digits [common input strength: 0.49 Ϯ 0.02 and 0.35 Ϯ 0.02 (means Ϯ SE), respectively; peak coherence: 0.0054 and 0.0038, respectively]. This suggests that the distribution of common neural input is muscle-pair specific regardless of grip type. However, the strength of coherence, but not synchrony, was significantly stronger in two-versus five-digit object hold for both muscle combinations, suggesting the periodicity of common input is sensitive to grip type.

Activation of individual extrinsic thumb muscles and compartments of extrinsic finger muscles

“…It has been demonstrated that interactions exist and that coactivations and motor unit synchronizations occur between the muscles of the thumb and those of the fingers during multidigit grasping tasks (Fuglevand 2011;Hockensmith et al 2005;Johnston et al 2005;Gandevia 1993, 1994;Winges et al 2006). If these muscles were used to control an artificial hand, the coordinated activities of both sets of muscles would cause simultaneous motions of the thumb and fingers.…”

Coordination of intrinsic and extrinsic hand muscle activity as a function of wrist joint angle during two-digit grasping

“…The intrinsic muscles recorded from were the First Dorsal Interosseous (FDI) and First Palmar Interosseous (FPI). A detailed description of wire electrode preparation, insertion, and verification of electrode placement in the target muscles is presented elsewhere ([13,25,26]). After the experimental session, we recorded EMG during isometric maximum voluntary contraction (MVC) of each muscle.…”

“…For each trial, the average EMG amplitude of each muscle was expressed as a percentage of its MVC (%EMG). For analysis of the coordination of multi-muscle EMG activity we constructed an n-dimensional vector (n = 5 muscles) defined by the %EMG amplitude of each muscle (termed muscle activation pattern [25]; MAP). A unit MAP vector was computed for each trial and then averaged across trials within subject, wrist angles and conditions [4,20,21].…”

“…Previous studies of single motor units of hand muscles have revealed correlated neural drive to motor neuron pools of intrinsic [16] and extrinsic [9,12,13,14,24,25] hand muscles as well as to both sets of hand muscles [26]. Analysis of correlated variability in multi-unit EMG amplitude of hand muscles has further revealed diffused, broad co-activation of intrinsic and extrinsic hand muscles [19].…”

Synchronization of motor unit firings: an epiphenomenon of firing rate characteristics not common inputs

“…Dozens of reports have since adopted this notion, including the following, among others: 1) within the same muscle (Garland and Miles 1997;Kamen and Roy 2000;Keen et al 2012;Laine and Bailey 2011;Mellor and Hodges 2005;Nordstrom et al 1990;Schmied et al 1993); 2) across synergistic muscles (Barry et al 2009;Bremner et al 1991a, b;Carr et al 1994;Gibbs et al 1997;Keen and Fuglevand 2004;McIsaac and Fuglevand 2007;Powers et al 1989;Winges and Santello 2004); 3) during tremor (Halliday et al 1999;Logigian et al 1988;Semmler and Nordstrom 1995); 4) in various neuromuscular pathologies (Baker et al 1992;Farmer et al 1990Farmer et al , 1993Kirkwood et al 1984;Mottram et al 2010;Schmied et al 1999); 5) during various muscle contraction paradigms, such as precision grip tasks (Hockensmith et al 2005;Huesler et al 2000;Kilner et al 2002;Santello and Fuglevand 2004;Winges et al 2006); 6) in exercise training (Boonstra et al 2009;Dartnall et al 2008Dartnall et al , 2011Griffin et al 2009;Mochizuki et al 2005;Schmied and Descarreaux 2010); and 7) during muscle fatigue (Boonstra et al 2008;Grönlund et al 2009;…”