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129
muscle groups. The AVRs elicited in all three conditions since these express the whole motor activity, whereas
were recorded in the Extensor carpi radialis ECR and motor unit parameters each account for only a part of
Abductor pollicis longus APL muscles. These muscles muscle activity.
were chosen because they have different biomechanical properties. In isometric contractions, the APL is mainly
involved in thumb and wrist abduction, while the ECR are
2. Material and methods
prime movers for wrist extension as well as for wrist abduction [2]. However, they contribute relatively more to
With the approval of the local Ethics Committee, wrist extension than to wrist abduction [21]. We therefore
experiments were performed on 13 healthy human particip- expected that, were the motor responses to be related to the
ants 3 women and 10 men aged 18–52 years, selected direction of the illusory movement, the two muscles would
because they experienced clear kinesthetic illusions during be involved to a different extent according to this direction.
tendon vibration. All the subjects gave informed consent to To evidence any such difference, we analyzed the relations
the experimental procedure as required by the Helsinki between the site of application of the tendon vibration, the
declaration 1964. direction of the illusory movement, and the parameters of
The subjects were seated in an adjustable armchair with the AVR at an overall level surface EMG location, latency
their left forearm placed in a cushioned groove to ensure and amplitude, and at a more detailed level motor unit
that a stereotyped position was kept from one experiment types and discharge characteristics, to check whether
to another. The hand was held in a semi-prone position by motor unit recruitment and activation patterns according to
a U-shaped device with an adjustable width, which also their slow or fast type could be related to the direction of
maintained the back of the hand in contact with a high the illusory movement.
sensitivity isometric force transducer Fig. 1. Moreover, to describe the sensory codes underlying the
conscious perception of movement, Roll and Gilhodes [23] proposed that the proprioceptive coding of direction and
2.1. Tendon vibration velocity could be based on a vector coding similar to the
population vector described by Georgopoulos et al. for Vibration was delivered via two rack-mounted electro-
directionally tuned neurons in different areas in the brain magnetic devices Ling dynamic system, type 101 in-
[7,8,15,16], i.e., the direction of the population vector dependently driven by two sinewaves generated by soft-
would represent that of the movement, and its modulus ware Cambridge Electronic Devices, Spike 2. The am-
would represent the velocity of the movement. The results plitudes and frequencies of the displacements of the heads
of Roll and Gilhodes [23] showed that separate vibration of the vibrators were constantly monitored using infrared
of two adjacent muscle groups evoked illusory sensations emitting-detecting photocells fixed on the vibrators. The
of orthogonal movements, while their co-vibration evoked initial pressure exerted by the vibrators on the tendons
illusory sensations in a direction intermediate between the resulted in a displacement of the vibrating rods. The
two orthogonal ones. The vectors representing these inter- resulting deflection in the signal of the infrared photocells
mediate illusory sensations corresponded to the sum of the was measured and used as the baseline of the signal for the
vectors representing the illusory sensations in the two experiment. The vibration amplitude could thus be kept
orthogonal directions.
These results
supported the
constant. The vibration amplitude was adjusted 0.2 to 0.5 adequacy of the vectorial model for describing the per-
mm, peak to peak to induce optimal kinesthetic illusions ceptual integration of proprioceptive messages.
at 80 Hz and then kept constant throughout the experiment. We hypothesized that the AVR could also be modeled on
Vibration was applied perpendicularly to the distal the basis of a vectorial model, similar to that used to
tendons of the left wrist flexor and adductor muscles, either describe the sensation of illusory movement but reversed,
separately or in combination. Each condition was repeated
since, accordingly, the « motor vector » coding for a motor
five times. response involving several muscles would be broken down
in as many subvectors corresponding to the individual motor commands directed to each of the motoneuron pools
2.2. Kinesthetic illusion quantification involved.
To test this hypothesis, we looked whether vectorial The subjects were requested to relax and keep their eyes
representations could account for the integration of prop- closed throughout the experimental session. They were
rioceptive signals from several muscles, and for the asked to carefully reproduce the direction and velocity of
organization of the elementary motor commands leading to the hand illusory movement they felt during muscle
a single directionalized motor response, by attempting to vibration by simultaneously drawing with their contralater-
model the illusory sensations of movement and the motor al hand on a digitizing table. The means of the directions
responses recorded in this study. The motor responses and of the average velocities of the kinesthetic illusions
were modeled on the basis of the surface EMG recordings, were calculated over the five trials for each combination.
130 S
Fig. 1. Experimental setup. The subjects sat with their left forearms maintained in a cushioned groove. With their right hand, they reproduced the kinesthetic illusion by drawing on a digitizing table. Vibrations were applied perpendicularly to the distal tendons of the wrist adductor muscles Av and of
the wrist flexor muscles Fv. Motor activities were recorded in the wrist extensor and in the thumb abductor muscles through paired surface electrodes and metallic microelectrodes. The subjects kept their eyes closed throughout the experimental series.
2.3. Surface and motor unit EMG recordings spike-triggered averaging method. The motor units were
characterized by their twitch contraction time CT. We The overall electromyographic activities EMG of the
thereby could compare the contractile properties of the wrist Extensor carpi radialis longus and brevis ECR and
motor units recruited during the AVR and tentatively of the Abductor pollicis longus APL were recorded
classify them as fast or slow motor units. through paired surface electrodes. Simultaneously, the
action potentials of single muscle fibers representing single motor unit activity were recorded in those muscles through
2.5. Data analysis two monopolar metal microelectrodes impedance 1 M
V, tested at 1000 Hz. The microelectrodes were inserted
Single motor unit recording was checked on-line and transcutaneously in the muscles and then moved in minute
off-line by software permitting single motor units to be steps until the recordings of easily identifiable single motor
identified by shape analysis Cambridge Electronic Design unit activities were stable. The global and unitary muscle
Spike 2. activities were amplified by AM 502 Tektronix amplifiers,
Three parameters were used to describe the activity of a with the band pass limited to 300–3000 Hz.
motor unit: its response latency, measured between the The integrated surface EMGs were quantitatively esti-
onset of the vibration and the first action potential; its mated using a computer-based program measuring the area
mean maximal frequency; and the initial slope of its between the zero muscle activity level and the integration
frequency curve, i.e., the velocity of the initial increase in curve during the whole vibration duration, i.e., 10 or 20 s.
frequency. Motor unit discharges were expressed as instantaneous
firing rates. The instantaneous frequency was transformed 2.4. Muscle force recording
into a frequency curve by a computer-based program. Maximal mean frequency was computed between cursor 2
The net force produced by wrist extension, calibrated in and cursor 3 Fig. 2. The best fit line was computed
Newtons, was recorded as a direct signal DC and as a between cursor 1 and cursor 2 Fig. 2 using the least
filtered signal AC, band-pass 0.3 Hz–100 Hz, which was squares error method. The coefficient of correlation R
used off-line to extract the motor unit twitch by the was calculated for every frequency curve. The mean
S . Calvin-Figuiere et al. Brain Research 881 2000 128 –138
131
Fig. 2. Motor responses during tendon vibration at 80 Hz on the wrist flexor muscles. The recordings show from bottom to top: The surface EMGs of the Abductor pollicis longus muscle APL emg and of the Extensor carpi radialis muscles ECR emg. The integrated EMGs of the same muscle groups ECR
int. emg; APL int. emg. ECR motor unit discharge ECR MU, its instantaneous frequency curve Inst. Freq., and its fitted curve. APL motor unit discharge APL MU, its instantaneous frequency curve Inst. Freq., and its fitted curve. The slope of the phase of frequency increase was calculated
between the vertical dotted lines 1 and 2.
calculated for all the trials was R 50.97. The slope S of
3. Results
