Human Hand Muscles Research Articles

Solving musculoskeletal biomechanics with machine learning.

Deep learning is a relatively new computational technique for the description of the musculoskeletal dynamics. The experimental relationships of muscle geometry in different postures are the high-dimensional spatial transformations that can be approximated by relatively simple functions, which opens the opportunity for machine learning (ML) applications. In this study, we challenged general ML algorithms with the problem of approximating the posture-dependent moment arm and muscle length relationships of the human arm and hand muscles. We used two types of algorithms, light gradient boosting machine (LGB) and fully connected artificial neural network (ANN) solving the wrapping kinematics of 33 muscles spanning up to six degrees of freedom (DOF) each for the arm and hand model with 18 DOFs. The input-output training and testing datasets, where joint angles were the input and the muscle length and moment arms were the output, were generated by our previous phenomenological model based on the autogenerated polynomial structures. Both models achieved a similar level of errors: ANN model errors were 0.08 ± 0.05% for muscle lengths and 0.53 ± 0.29% for moment arms, and LGB model made similar errors—0.18 ± 0.06% and 0.13 ± 0.07%, respectively. LGB model reached the training goal with only 103 samples, while ANN required 106 samples; however, LGB models were about 39 times slower than ANN models in the evaluation. The sufficient performance of developed models demonstrates the future applicability of ML for musculoskeletal transformations in a variety of applications, such as in advanced powered prosthetics.

Modeling of Personalized Anatomy Using Plastic Strains

We present a method for modeling solid objects undergoing large spatially varying and/or anisotropic strains, and use it to reconstruct human anatomy from medical images. Our novel shape deformation method uses plastic strains and the finite element method to successfully model shapes undergoing large and/or anisotropic strains, specified by sparse point constraints on the boundary of the object. We extensively compare our method to standard second-order shape deformation methods, variational methods, and surface-based methods, and demonstrate that our method avoids the spikiness, wiggliness, and other artifacts of previous methods. We demonstrate how to perform such shape deformation both for attached and un-attached (“free flying”) objects, using a novel method to solve linear systems with singular matrices with a known nullspace. Although our method is applicable to general large-strain shape deformation modeling, we use it to create personalized 3D triangle and volumetric meshes of human organs, based on magnetic resonance imaging or computed tomography scans. Given a medically accurate anatomy template of a generic individual, we optimize the geometry of the organ to match the magnetic resonance imaging or computed tomography scan of a specific individual. Our examples include human hand muscles, a liver, a hip bone, and a gluteus medius muscle (“hip abductor”).

Effect of fatigue-related group III/IV afferent firing on intracortical inhibition and facilitation in hand muscles.

Fatiguing exercise causes a reduction in motor drive to the muscle. Group III/IV muscle afferent firing is thought to contribute to this process; however, the effect on corticospinal and intracortical networks is poorly understood. In two experiments, participants performed sustained maximal isometric finger abductions of the first dorsal interosseous (FDI) muscle, with postexercise blood flow occlusion (OCC) to maintain the firing of group III/IV afferents or without occlusion (control; CON). Before and after exercise, single- and paired-pulse transcranial magnetic stimulation (TMS) tested motor evoked potentials (MEPs), intracortical facilitation [ICF (12 ms)], and short-interval intracortical inhibition [SICI2 (2 ms), SICI3 (3 ms)]. Ulnar nerve stimulation elicited maximal M waves (MMAX). For experiment 1 (n = 16 participants), TMS intensities were 70% and 120% of resting motor threshold (RMT) for the conditioning and MEP stimuli, respectively. For experiment 2 (n = 16 participants), the MEP was maintained at 1 mV before and after exercise and the conditioning stimulus individualized. In experiment 1, MEP/MMAX was reduced after exercise (~48%, P = 0.007) but was not different between conditions. No changes occurred in ICF or SICI. In experiment 2, MEP/MMAX increased (~27%, P = 0.027) and less inhibition (SICI2: ~21%, P = 0.021) occurred after exercise for both conditions, whereas ICF decreased for CON only (~28%, P = 0.006). MEPs and SICI2 were modulated by fatiguing contractions but not by group III/IV afferent firing, whereas sustained afferent firing appeared to counteract postexercise reductions in ICF in FDI. The findings do not support the idea that actions of group III/IV afferents on motor cortical networks contribute to the reduction in voluntary activation observed in other studies.NEW & NOTEWORTHY This is the first study to investigate, in human hand muscles, the action of fatigue-related group III/IV muscle afferent firing on intracortical facilitation and inhibition. In fatigued and nonexercised hand muscles, intracortical inhibition is reduced after exercise but is not modulated differently by the firing of group III/IV afferents. However, facilitation is maintained for the fatigued muscle when group III/IV afferents fire, but these results are unlikely to explain the reduction in voluntary activation observed in other studies.

A hierarchy of corticospinal plasticity in human hand and forearm muscles.

Key points Pairing stimulation of a finger flexor or extensor muscle at the motor point with transcranial magnetic stimulation (TMS) of the motor cortex generated plastic changes in motor output.Increases in output were greater in intrinsic hand muscles than in the finger flexor. No changes occurred in the finger extensor. This gradient was seen irrespective of which muscle was stimulated paired with transcranial magnetic stimulation.Intermittent theta‐burst stimulation also produced increases in output, although these were similar across muscles.We suggest that intrinsic hand and flexor muscles have a higher potential to show plasticity than extensors, although only when plasticity is induced by sensory input. This may relate to differences seen in recovery of function in these muscles after injury, such as post‐stroke. The ability of the motor system to show plastic change underlies skill learning and also permits recovery after injury. One puzzling observation is that, after stroke, upper limb flexor muscles show good recovery but extensors remain weak, with this being a major contributor to residual disability. We hypothesized that there might be differences in potential for plasticity across hand and forearm muscles. In the present study, we investigated this using two protocols based on transcranial magnetic brain stimulation (TMS) in healthy human subjects. Baseline TMS responses were recorded from two intrinsic hand muscles: flexor digitorum superficialis (FDS) and extensor digitorum communis (EDC). In the first study, paired associative stimulation (PAS) was delivered by pairing motor point stimulation of FDS or EDC with TMS. Responses were then remeasured. Increases were greatest in the hand muscles, smaller in FDS and non‐significant in EDC, irrespective of whether stimulation of FDS or EDC was used. In the second study, intermittent theta‐burst rapid rate TMS was applied instead of PAS. In this case, all muscles showed similar increases in TMS responses. We conclude that the potential to show plastic changes in motor cortical output has the gradient: hand muscles > flexors > extensors. However, this was only seen in a protocol that requires integration of sensory input (PAS) and not when plasticity was induced purely by cortical stimulation (rapid rate TMS). This observation may relate to why functional recovery tends to favour flexor and hand muscles over extensors.

Age-related differences in short- and long-interval intracortical inhibition in a human hand muscle.

Effects of age on the assessment of intracortical inhibition with paired-pulse transcranial magnetic stimulation (TMS) have been variable, which may be due to between-study differences in test TMS intensity and test motor evoked potential (MEP) amplitude. To investigate age-related differences in short- (SICI) and long-interval intracortical inhibition (LICI) across a range of test TMS intensities and test MEP amplitudes. In 22 young and 18 older subjects, SICI and LICI were recorded at a range of test TMS intensities (110%-150% of motor threshold) while the first dorsal interosseous (FDI) muscle was at rest, or producing a precision grip of the index finger and thumb. Data were subsequently compared according to the amplitude of the MEP produced by the test alone TMS. When pooled across all test TMS intensities, SICI in resting muscle and LICI in active muscle were similar in young and older adults, whereas SICI in active muscle and LICI in resting muscle were reduced in older adults. Regrouping data based on test MEP amplitude demonstrated similar effects of age for SICI and LICI in resting muscle, whereas more subtle differences between age groups were revealed for SICI and LICI in active muscle. Advancing age influences GABA-mediated intracortical inhibition, but the outcome is dependent on the experimental conditions. Age-related differences in SICI and LICI were influenced by test TMS intensity and test MEP amplitude, suggesting that these are important considerations when assessing intracortical inhibition in older adults, particularly in an active muscle.

Ascending beta oscillation from finger muscle to sensorimotor cortex contributes to enhanced steady-state isometric contraction in humans

Objectiveβ-Band corticomuscular coherence is suggested as an electrophysiological mechanism that contributes to sensorimotor functioning in the maintenance of steady-state contractions. Converging evidence suggests that not only the descending corticospinal pathway but the ascending sensory feedback pathway is involved in the generation of β-band corticomuscular coherence. The present study aimed to investigate which pathway, descending vs. ascending, contributes more to the stability of muscle contraction, especially for human intrinsic hand muscles. MethodsIn this study, we assessed directed transfer function (DTF) between magnetoencephalography signals over the sensorimotor cortex (SMC) and rectified electromyographic (EMG) signals recorded during steady-state isometric contraction of the right thumb muscle (flexor pollicis brevis, FPB) or right little finger muscle (flexor digiti minimi brevis, FDMB) in 15 right-handed healthy subjects. Resultsβ-Band DTF was statistically significant in both descending (SMC→EMG) and ascending (EMG→SMC) directions, and mean phase delays for each direction were in agreement with the conduction time for the descending corticospinal and ascending sensory feedback pathways. The strengths of the β-band DTF (EMG→SMC direction) were greater in the FPB muscle than in the FDMB muscle, while the strengths of the β-band DTF (SMC→EMG direction) were not different between the two muscles. Moreover, the β-band DTF (EMG→SMC direction) was greater in the “Stable” period than in the “Less Stable” period within the FDMB muscle. Greater DTF (EMG→SMC direction) was positively associated with the stability of muscle contraction. ConclusionsOur findings suggest that ascending β-band oscillatory activity may promote a steady-state isometric contraction by efficiently transmitting sensory feedback from finger muscles to the sensorimotor cortex. SignificanceThe results show the differential contribution of the ascending part of the corticomuscular network depending on the functional organization.

Modulation of short- and long-interval intracortical inhibition with increasing motor evoked potential amplitude in a human hand muscle

ObjectiveThe aim of the current study was to investigate the effect of increasing test motor evoked potential (MEP) amplitude on short- (SICI) and long-interval intracortical inhibition (LICI) at rest and during activation of the first dorsal interosseous (FDI) muscle. MethodsIn 22 young subjects, a conditioning-test transcranial magnetic stimulation (TMS) paradigm was used to assess SICI and LICI at 5 different test TMS intensities (110–150% motor threshold) in resting and active FDI. In 9 additional subjects, SICI and LICI data were quantified when the test MEP amplitude represented specific proportions of the maximal compound muscle action potential (Mmax) in each subject. ResultsTest TMS intensity influenced SICI and LICI in rest and active FDI muscle. The normalised test MEP amplitude (%Mmax) did not influence SICI at rest, whereas there was a decrease in LICI at rest and an increase in SICI in active FDI with an increased normalised test MEP amplitude (%Mmax). ConclusionsOur results demonstrate differential effects of normalised test MEP amplitude (%Mmax) on SICI and LICI in resting and active FDI muscle. SignificanceEstimation of SICI and LICI under some circumstances may be influenced by the normalised test MEP amplitude in subject populations with different Mmax characteristics.

P 233. Robot-assisted and image-guided randomized TMS mapping of human and primate hand muscle

Introduction Coil positioning is a crucial factor for transcranial magnetic stimulation (TMS). Manual positioning of flat TMS coil against the curved head allows for errors in translation and rotation. As scalp markings are an ineffective way to ensure consistency in coil positioning, image-guided neuronavigation is used to reduce errors. However, as manual neuronavigation increases time for coil positioning it is less suitable when the coil has to be moved between pulses during TMS mapping. The use of a robot for coil positioning may overcome these drawbacks of manual neuronavigation. Objectives We aimed at assessing the practicability and efficacy of the using a robot for a neuronavigated TMS mapping of the cortical representation of the first dorsal interosseus (FDI) of humans and rhesus monkey. Materials and methods For FDI mapping, 12 supra-threshold stimuli (120% RMT) were applied in a randomized order to each grid point of a 7 × 7 grid with a spacing of 10 mm in 8 subjects (mean age 26 y, 6 male) with a Magstim 200 stimulator and a standard 70 mm figure-8 coil. Surface EMG was recorded from contralateral FDI. An Adept Viper s850 robotic arm moved the coil to each grid point on an MRI-based head model in a random order. An IR camera obtained actual positional data of the head. This allowed tracking of head movements and respective online movement compensation by the robot. Centers of gravity were projected onto the brain surface and co-registered to an MNI152 T1 1 mm brain template. A similar setup was used for FDI mapping in a male adult rhesus monkey on a 4x5 grid with 5 mm spacing. TMS was applied with a Magstim 25 mm figure-8 coil. The mapping was performed during multiple sessions during which the monkey’s head was attached to a frame with frontal and occipital head posts. Relaxation of FDI was rewarded with water after each pulse. Results TMS Mapping sessions took about 80–90 min in each subject with a pulse interval of 8 s. Preparations lasted about 20 min. A closely spaced TMS representation of FDI could be identified in each subject within the precentral gyrus. The shape of the representation area resembled ovals with a mean maximum MEP amplitude of 1.35 mV (SD: 0.52). Mean stereotaxic coordinates of the centers of gravity were (−36, −12, 67) in MNI152 space. In the animal experiment, the monkey adapted to the setup within 4 weeks. The preparation time for each session was about 10 min. Across all sessions muscle relaxation was present during 64% of the pulses. The pulse interval was 13.4 s. Four repetitions of a complete grid could be recorded in a single session of about 45 min duration. MEPs from contralateral FDI were elicited from a highly defined elliptical area (1 × 1.5 cm) within the precentral gyrus. Conclusion Due to the combination of fast high-precision coil positioning and online head tracking, robot-assisted and image-guided TMS is exquisitely suited to perform randomized TMS mappings in the human with acceptable preparation times. In addition, this method allows for precise TMS mapping of the considerably smaller rhesus brain where high-precision coil positioning with a high inter-session reliability is required for multiple session mappings with grid sizes in the millimeter scale.

Evolution and homologies of primate and modern human hand and forearm muscles, with notes on thumb movements and tool use

In this paper, we explore how the results of a primate-wide higher-level phylogenetic analysis of muscle characters can improve our understanding of the evolution and homologies of the forearm and hand muscles of modern humans. Contrary to what is often suggested in the literature, none of the forearm and hand muscle structures usually present in modern humans are autapomorphic. All are found in one or more extant non-human primate taxa. What is unique is the particular combination of muscles. However, more muscles go to the thumb in modern humans than in almost all other primates, reinforcing the hypothesis that focal thumb movements probably played an important role in human evolution. What makes the modern human thumb myology special within the primate clade is not so much its intrinsic musculature but two extrinsic muscles, extensor pollicis brevis and flexor pollicis longus, that are otherwise only found in hylobatids. It is likely that these two forearm muscles play different functional roles in hylobatids and modern humans. In the former, the thumb is separated from elongated digits by a deep cleft and there is no pulp-to-pulp opposition, whereas modern humans exhibit powerful thumb flexion and greater manipulative abilities, such as those involved in the manufacture and use of tools. The functional and evolutionary significance of a third peculiar structure, the intrinsic hand structure that is often called the ‘interosseous volaris primus of Henle’ (and which we suggest is referred to as the musculus adductor pollicis accessorius) is still obscure. The presence of distinct contrahentes digitorum and intermetacarpales in adult chimpanzees is likely the result of prolonged or delayed development of the hand musculature of these apes. In relation to these structures, extant chimpanzees are more neotenic than modern humans.

Superposition of Motor Commands in the Course of Creating 'Two-Joint' Static Efforts by Human Hand Muscles

Superposition of Motor Commands in the Course of Creating 'Two-Joint' Static Efforts by Human Hand Muscles

Mechanical properties and neural control of human hand motor units

Motor units serve both as the mechanical apparatus and the final stage of neural processing through which motor behaviours are enacted. Therefore, knowledge about the contractile properties and organization of the neural inputs to motor units supplying finger muscles is essential for understanding the control strategies underlying the diverse motor functions of the human hand. In this brief review, basic contractile properties of motor units residing in human hand muscles are described. Hand motor units are not readily categorized into the classical physiological types as established in the cat gastrocnemius muscle. In addition, the distribution of descending synaptic inputs to motor nuclei supplying different hand muscles is outlined. Motor neurons innervating intrinsic muscles appear to have relatively independent lines of input from supraspinal centres whereas substantial divergence of descending input is seen across motor nuclei supplying extrinsic hand muscles. The functional significance of such differential organizations of descending inputs for the control of hand movements is discussed.

Human hand function: the limitations of brain and brawn

Human hand function: the limitations of brain and brawn

Long-interval intracortical inhibition in a human hand muscle

When two motor cortical stimuli are delivered with an interstimulus interval of 50-200ms, the response (motor evoked potential; MEP) to the second stimulus is typically suppressed. This phenomenon is termed long-interval intracortical inhibition (LICI), although data from one subject suggest that facilitation is possible. Moreover, we recently showed that suppression can be mediated at a spinal level. We characterized LICI more fully by exploring a broad range of contraction strengths and test stimulus intensities. MEPs were evoked in first dorsal interosseous by transcranial magnetic stimulation over the motor cortex. Single test and paired (conditioning-test interval of 100ms) stimuli at intensities of 100-160% resting motor threshold were delivered at rest or during brief contractions of 10, 25, or 100% maximal voluntary force. Inhibition or facilitation was quantified with the standard ratio in which conditioned MEPs were expressed as a percentage of unconditioned MEPs. Inhibition was greatest at weak-moderate contraction strengths and least at rest and during maximal efforts. Both at rest and during maximal efforts, MEPs evoked by strong stimuli were facilitated. In a subset of subjects, cervicomedullary stimulation was used to activate the corticospinal tract to identify possible spinal influences on changes to MEPs. Contraction strength and test stimulus intensity each had different effects on unconditioned and conditioned MEP size, and hence, LICI is highly dependent on both factors. Further, because motoneurons are facilitated during contraction but disfacilitated after a strong conditioning stimulus, the standard ratio of LICI is of questionable validity during voluntary contractions.

Interhemispheric inhibition between primary motor cortices: what have we learned?

Interhemispheric inhibition between primary motor cortices: what have we learned?

Differential contributions of spinal and cortical motoneurons to input–output properties of human small hand muscle

Objective: It is not fully understood how the stimulus/response curves obtained by transcranial magnetic stimulation (TMS) reflect the function of the cortico-motoneuronal (CM) and spinal motoneuronal (SM) systems in healthy subjects. To understand these response functions, we studied patients with amyotrophic lateral sclerosis (ALS) whose upper but not lower motor neurons were affected.Methods: First, we determined the effects of voluntary muscle contraction and intensity of TMS on the motor evoked potentials (MEPs) of the first dorsal interossei (FDI) muscle in ten healthy control subjects (mean age: 35.1 ± 6.7 years) and two older female subjects (60 and 64 years old). Second, we investigated whether this relationship was altered in two ALS patients (60-year-old woman and 69-year-old man). The MEPs were recorded at different degrees of voluntary contraction by threshold stimulus intensities (TSIs) or at different levels of TMS with the muscle at rest.Results: In the controls, the MEP amplitudes of the FDI muscle elicited by TSI increased linearly with muscle contraction (right: Y = 0.068X + 0.754; left: Y = 0.0670X + 0.807), whereas the MEP amplitudes elicited by different TMS intensities increased sigmoidally with a flexion point at 1.10–1.15 TSI: right: Amp = 2.21/{1 + exp[(S50-Stim)/K]}, S50 = 10.73, K = 4.70; left: Amp = 2.90/{1 + exp[(S50-Stim)/K]}, S50 = 13.64, K = 5.98. The latter was abnormal only on one side of each ALS patient.Discussion: From these results, we suggest that the sigmoidal TMS intensity–size curves reflect mainly CM activities, while the linear contraction–size curves reflect SM activities.

Sensory modulation of voluntary and TMS-induced activation in hand muscles

Nociceptive suppression of tonic voluntary electromyographic (EMG) activity in human hand muscles (cutaneous silent period, CSP) is in its functional organization consistent with a spinal protective reflex. Motoneuronal excitability and its modulation may also be investigated by conditioned motor evoked potentials (MEPs). To date, effects of exteroceptive stimuli on tonic EMG and on MEPs have been compared mainly using innocuous stimuli, while noxious stimuli have not been studied in great detail. In ten subjects, we recorded CSPs induced in volitionally activated flexor pollicis brevis muscle (FPB) by noxious digit II (D2) stimulation, and in first dorsal interosseous muscle (FDI) following noxious D2 and digit V (D5) stimulation. Then, transcranial magnetic stimulation (TMS) was used to evoke MEPs in the same hand muscles at rest--conditioned by equal noxious D2 or D5 stimulation and individually delayed--so that the MEPs occurred at times corresponding to immediately before, during, and immediately after the CSP in each subject. Immediately before the CSP, there was no significant difference between nociceptive MEP modulation and tonic EMG modulation in any muscle-finger-combination. In the middle of the CSP, noxious finger stimulation exerted suppression of TMS-induced MEPs in all the three muscle-finger-combinations, but less so as compared to corresponding tonic EMG levels. After the CSP, MEPs remained suppressed when corresponding tonic EMG levels were significantly enhanced. Notably, MEPs were also suppressed in cases when occurring at times corresponding to the excitatory long-loop reflex. Incomplete MEP suppression during the CSP may allow for an "emergency grip" even during noxious stimulation. MEP suppression outlasting the CSP is compatible with a "passive" re-synchronization of volitionally activated motor units rather than an "active" reflex involving recruitment of corticospinal motoneurons. The differences in tonic EMG and MEP modulation favors an effect of noxious digital nerve stimulation on interneurons responsible for presynaptic inhibition rather than a postsynaptic inhibitory effect on the motoneuron pool. The present findings caution against the use of nociceptive MEP modulation at rest to substitute for tonic EMG modulation as tested in CSP studies.

Contraction coupling efficiency of human first dorsal interosseous muscle.

During working contractions, chemical energy in the form of ATP is converted to external work. The efficiency of this conversion, called 'contraction coupling efficiency', is calculated by the ratio of work output to energy input from ATP splitting. Experiments on isolated muscles and permeabilized fibres show the efficiency of this conversion has a wide range, 0.2-0.7. We measured the work output in contractions of a single human hand muscle in vivo and of the ATP cost of that work to calculate the contraction coupling efficiency of the muscle. Five subjects performed six bouts of rapid voluntary contractions every 1.5 s for 42 s (28 contractions, each with time to peak force < 150 ms). The bouts encompassed a 7-fold range of workloads. The ATP cost during work was quantified by measuring the extent of chemical changes within the muscle from (31)P magnetic resonance spectra. Contraction coupling efficiency was determined as the slope of paired measurements of work output and ATP cost at the five graded work loads. The results show that 0.68 of the chemical energy available from ATP splitting was converted to external work output. A plausible mechanism to account for this high value is a substantially lower efficiency for mitochondrial ATP synthesis. The method described here can be used to analyse changes in the overall efficiency determined from oxygen consumption during exercise that can occur in disease or with age, and to test the hypothesis that such changes are due to reduced contraction coupling efficiency.

Reflex responsiveness of a human hand muscle when controlling isometric force and joint position

Objective This study compared reflex responsiveness of the first dorsal interosseus muscle during two tasks that employ different strategies to stabilize the finger while exerting the same net muscle torque. Methods Healthy human subjects performed two motor tasks that involved either pushing up against a rigid restraint to exert a constant isometric force equal to 20% of maximum or maintaining a constant angle at the metacarpophalangeal joint while supporting an equivalent inertial load. Each task consisted of six 40-s contractions during which electrical and mechanical stimuli were delivered. Results The amplitude of short and long latency reflex responses to mechanical stretch did not differ significantly between tasks. In contrast, reflexes evoked by electrical stimulation were significantly greater when supporting the inertial load. Conclusions Agonist motor neurons exhibited heightened reflex responsiveness to synaptic input from heteronymous afferents when controlling the position of an inertial load. Task differences in the reflex response to electrical stimulation were not reflected in the response to mechanical perturbation, indicating a difference in the efficacy of the pathways that mediate these effects. Significance Results from this study suggest that modulation of spinal reflex pathways may contribute to differences in the control of force and position during isometric contractions of the first dorsal interosseus muscle.

Task‐related changes in propriospinal excitation from hand muscles to human flexor carpi radialis motoneurones

This study addresses whether there is excitation from human hand muscles to flexor carpi radialis (FCR) motoneurones mediated through propriospinal circuits and, if so, whether it is used in specific motor tasks. Electrical stimuli to the ulnar nerve at wrist level produced an excitation in FCR motoneurones with characteristics typical of a propriospinally mediated effect: low threshold (0.6 x motor threshold (MT)), a group I effect that was not reproduced by purely cutaneous stimuli, long central delay (4.1 +/- 0.4 ms in single units), suppression when the stimulus intensity was increased, and facilitation of the corticospinal excitation at the premotoneuronal level. Ulnar-induced propriospinally mediated excitation was compared during selective voluntary contractions of the FCR and, at equivalent level of FCR EMG, during tasks in which the FCR was activated automatically in postural contractions rather than voluntarily (grip, pinching and pointing). The excitation was significantly greater during grip (and pinching) than during voluntary FCR contractions and pointing, whether measured in single motor units or tonic EMG activity, or whether the response to motor cortex stimulation was assessed as the compound motor-evoked potential or the corticospinal peak in single units. The discrepancy between the tasks appeared with ulnar intensities above 0.8 x MT and was then present across a wide range of stimulus intensities. This suggests a reduction in the corticospinal control of 'feedback inhibitory interneurones' mediating peripheral inhibition to propriospinal neurones during grip and pinching. The resulting more effective background excitation of propriospinal neurones by the peripheral input from hand muscles could contribute to stabilizing the wrist during grip.

Location specificity of plantar cutaneous reflexes involving lower limb muscles in humans

It is known that cutaneous reflexes in human hand muscles show strong location-specificity dependent on the digit stimulated. We hypothesized that in lower leg muscles the cutaneous reflex following tactile sensation of the plantar surface of the foot is also organized in a location-specific manner. The purpose of the present study was to test this hypothesis. Middle latency reflexes (approximately 70-110 ms, MLR) following non-noxious electrical stimulation to different locations on the plantar foot were recorded from 16 neurologically intact volunteers (15 males, 1 female). Electrical stimulation was given to the fore-medial (f-M), fore-lateral (f-L) and heel (HL) regions of the plantar surface of the right foot while the subjects performed isometric dorsiflexion (tibialis anterior, TA), plantarflexion (soleus, Sol and medial gastrocnemius, MG), eversion (peroneus longus, PL) and knee extension (vastus lateralis, VL) while sitting and standing. In the Sol and MG, an excitatory response was observed following HL stimulation, which was switched to an inhibitory response following f-M or f-L stimulation (P < 0.001). A reciprocal pattern in contrast to Sol was observed in the TA. In the PL, MLR exhibited significant excitation following both f-L and HL stimulation, which, however, was switched to an inhibitory response following f-M stimulation (P < 0.001). Moderate inhibition of the MLR was seen in the VL for all stimulated positions. Systematic stimulation along the lateral side of the plantar foot demonstrated that the reflex reversal occurred around the middle of the plantar foot in the Sol and TA. In all muscles tested, the slope of the regression line between the magnitude of the MLR and background electromyographic activity significantly decreased during standing compared with sitting except for the PL following f-L simulation. These results suggest that reflex effects from cutaneous nerves in the plantar foot onto the motoneurons innervating the lower leg muscles are organized in a highly topographic manner in humans. The organization of these reflexes may play an important role in the alteration of limb loading and/or ground contact in response to tactile sensation of the plantar foot while sitting and standing.