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J Appl Physiol 102: 144-148, 2007. First published September 7, 2006; doi:10.1152/japplphysiol.00362.2006 Free Article
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Ia-afferent input to motoneurons during shortening and lengthening muscle contractions in humans

Nicolas T. Petersen,1 Jane E. Butler,2 Mark G. Carpenter,3,5 and Andrew G. Cresswell4,5

1Institute of Exercise and Sport Sciences and Department of Medical Physiology, University of Copenhagen, Copenhagen N, Denmark; 2Prince of Wales Medical Research Institute, Randwick, New South Wales, Australia; 3School of Human Kinetics, University of British Columbia, Vancouver, Canada; 4School of Human Movement Studies and Division of Physiotherapy, The University of Queensland, St. Lucia, Queensland, Australia; and 5Department of Neuroscience, Karolinska Institutet, Stockholm, Sweden

Submitted 23 March 2006 ; accepted in final form 31 August 2006


   ABSTRACT
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 ABSTRACT
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ACKNOWLEDGMENTS
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The central nervous system employs different strategies to execute specific motor tasks. Because afferent feedback during shortening and lengthening muscle contractions differs, the neural strategy underlying these tasks may be quite distinct. Cortical drive may be adjusted or afferent input regulated. The exact mechanisms are not clear. Here, we examine the control of synaptic transmission across the Ia synapse during shortening and lengthening muscle contractions. Subjects were instructed to maintain isolated activity in a single tibialis anterior (TA) motor unit while muscle length was varied from flexion to extension and back. At a fixed interval after a firing of the active motor unit, a single electrical stimulus was applied to the common peroneal nerve to activate Ia afferents from the TA muscle. We investigated the stimulus-induced change in firing probability of 19 individual low-threshold TA motor units during shortening and lengthening contractions. Any change in firing probability depends on both pre- and postsynaptic mechanisms. In this experiment, motoneuron firing rate was similar during both contraction types. There was no difference in the firing probability between shortening and lengthening contractions (0.23 ± 0.03 and 0.20 ± 0.02, respectively). We suggest that there is no contraction type-specific control of Ia input to the motoneurons during shortening and lengthening muscle contractions. Cortical adjustments may have occurred.

motor unit; poststimulus time histogram; concentric; eccentric

DURING NATURAL MOVEMENTS, muscles continuously change their length, and sensory information varies accordingly. The general view is that as a muscle lengthens, muscle spindles increase their firing rate (2, 11), and conversely, when it shortens the spindles decrease their activity. As such, depending on the specific motor task, the nervous system might purposefully adjust the various inputs that drive the motoneurons to perform the required task. Recently, it has been suggested that the motor strategy used to perform lengthening (eccentric) muscle contractions differs from that used during shortening (concentric) muscle contractions (5).

For a given level of torque, the firing rate of motor units is higher during shortening of a muscle than during lengthening (10, 11) because the force produced by a motor unit firing at a given rate during a shortening contraction is less than the force produced by the same unit firing at the same rate during lengthening. Thus, for the same load, motoneurons will receive greater excitatory drive during shortening than lengthening.

Ia-afferent input is unlikely to contribute substantially to the difference in drive to the motoneurons, because direct recordings from muscle spindles in active muscle show lower discharge rates during shortening contractions than lengthening contractions (22), particularly at low contraction strengths (2). Therefore, the increased excitatory drive to motoneurons during shortening is likely to be accounted for by an increase in cortical drive. In support of this, motor-evoked potentials from transcranial magnetic stimulation of cortical cells are larger during shortening than during lengthening muscle contractions (1, 24).

On the other hand, another strategy to control motoneuronal excitability during shortening and lengthening contractions would be to centrally regulate presynaptic inhibition of the afferent input (23). If presynaptic control of transmitter release at the Ia synapse is adjusted according to the task, then other sources that excite the motoneurons, such as cortical drive, may not need to be modulated. At least two mechanisms of presynaptic inhibition are known to influence the amount of transmitter release at the Ia synapse: classic GABAA-mediated presynaptic inhibition caused by primary afferent depolarization (for review see Ref. 21) and homosynaptic postactivation depression (12). Whereas long-lasting homosynaptic postactivation depression is influenced by Ia-activation history and depends only on the properties of the Ia synapse, short-lasting primary afferent depolarization is centrally controlled and may be continuously modulated during muscle activity. Based on changes in H reflexes, Romano and Schieppati (19) suggested that presynaptic inhibition is reduced during shortening contractions; however, mechanisms other than changes in synaptic mechanisms could explain their observations.

In this study, we investigated the effect of Ia-afferent input on active single motor units from tibialis anterior during shortening and lengthening when motoneuron excitability was kept approximately constant. Our interest was to test whether a specific neural strategy regulates the level of presynaptic inhibition of the Ia-afferent synapse to compensate for the differing levels of Ia-afferent activity during shortening and lengthening muscle contractions.


   MATERIALS AND METHODS
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 ABSTRACT
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 ACKNOWLEDGMENTS
 REFERENCES
 
Subjects.   Data were obtained during a total of nine experiments on six healthy subjects (1 woman and 5 men, age 30–47 yr) with no history of neurological injury or disease. All subjects provided informed consent before participating in the study. The experimental procedures were approved by the ethics committee of the local research institute and conformed to the Declaration of Helsinki.

Protocol.   The subject sat reclined on a bench with the right foot securely attached to a microprocessor-controlled torque motor that accurately manipulated ankle displacement, velocity, and acceleration (18). The ankle joint was aligned with the axis of the motor, and the knee of the right leg was flexed ~30° from full extension. Anatomic zero of the ankle was defined as an angle of 90° between the tibia and footplate and positive angles as those moving into extension. A potentiometer attached to the axle of the dynamometer provided ankle angle to an accuracy of 0.3°. Anatomic zero of the ankle was defined as an angle of 90° between the tibia and footplate and positive angles as those moving into extension. A torque transducer (Maywood Instruments, Southwood, Farnborough, UK) located within the axel provided dorsiflexor torque output.

The ankle was continuously moved by the torque motor through a range of motion of 10° (±5° about anatomic zero) with the following movement sequence: 1) 10° of extension at 2°/s (lengthening), 2) hold for 3 s (hold long), 3) 10° of flexion at 2°/s (shortening), and 4) hold for 3 s (hold short) (see Fig. 1A). A weak voluntary contraction of the dorsiflexors was maintained throughout the movement sequence.


Figure 1
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  		Fig. 1. Single subject data. A: experimental protocol showing ankle position, raw tibialis anterior EMG and test (stimulation) and control (no stimulation) events for a single representative subject performing a voluntary dorsiflexion contraction. Stimulus (test) artifacts are evidenced in the EMG signal as large potentials at the time of stimulation. Discriminated single motor unit potentials are overdrawn in the right panel. B: poststimulus time histograms showing the probability of firing (y-axis) of a single motor unit to Ia-afferent stimulation; the histograms are adjusted for the spontaneous firing. The x-axis is the latency of the units firing after a stimulation, this was given at time 0. PSTHs are shown for lengthening (left top), hold-long (right top), shortening (left bottom), and hold-short (right bottom) contractions for a single subject. The cumulative sum is shown for each histogram. Note that the firing probability was similar for each condition.

 
Electromyographic recordings.   An electromyographic (EMG) signal was collected from the tibialis anterior muscles using a pair of 2.5-mm-diameter surface electrodes (Ag-AgCl, Sensor Medics) with an interelectrode distance of 2 cm. The signal was amplified between 20,000 and 50,000 times (model NL 824, Digitimer, Hertfordshire, UK) and band-pass filtered (model NL 125, Digitimer) between 200 and 2 kHz. The EMG signals were analog-to-digital converted at a sampling frequency of 10 kHz using a 16-bit Power 1401 and Spike2 data collection software (Cambridge Electronic Design, Cambridge, UK). Initially, during the performance of an isometric submaximal contraction, the EMG electrode pair was gradually moved over the muscle until a single motor unit could be clearly identified from any signal noise. The electrode pair was then taped to the skin at that location.

For the experiment, subjects were instructed to voluntarily activate the identified single unit in tibialis anterior and to maintain that unit firing at a stable rate during the cyclic movements of the motor, i.e., during shortening and lengthening of the active muscle (Fig. 1A). Only the first recruited units were investigated, and the force levels produced by the contraction never exceeded 2–3% of the subjects maximal voluntary contraction. Auditory and visual feedback of the single unit was given to the subjects to assist their performance. Unit activity was additionally discriminated using a custom-built window discriminator. The discriminated events were fed as a transistor-transistor logic pulse to the data collection system and subsequently used for controlling the timing of online triggering of the electrical stimuli. Unit firing rates were analyzed offline to ensure that there was no clear difference in the background firing rate for each of the four conditions of muscle contraction.

Stimulation.   The common peroneal nerve (CPN) was stimulated electrically (DS7A, Digitimer) via surface electrodes (4-mm diameter, Ag-AgCl; Medicotest, Ølstykke, Denmark) attached to the skin at the level of the fibula head, with the cathode placed ~2 cm proximal. The duration of the stimulus was 1 ms, and the intensity of the stimulation was set just below threshold for evoking a direct motor response in the muscle. Electrical stimulation at this intensity activates the large-diameter group Ia afferents (6).

Stimuli were given at fixed time intervals following the discharge of the identified single motor unit. This time interval was determined according to the spontaneous firing frequency of the unit and set so that it was sensitive to excitatory inputs (~70 ms after the previous discharge). This method minimizes the number of stimuli given when the unit is refractory and cannot respond (7). "Test" and "control" sweeps (i.e., with and without CPN Ia-afferent stimulation, respectively) were randomly mixed, resulting in interstimulus intervals of 1–5 s.

Analysis.   Single motor unit activity was obtained by using a template-matching algorithm (Spike2 version 4, Cambridge Electronic Design). Each motor unit potential triggered in a sweep was verified by visual inspection to confirm that only spikes from the active single motor unit under investigation were included. Custom-written software (MATLAB) was used for the purpose of unit identification.

Poststimulus time histograms (PSTHs) of 1-ms bin width were constructed for when the active muscle was either shortening, lengthening, held short, or held long. The median value for the number of stimuli that contributed to each PSTH was at least 115 for shortening and lengthening conditions and at least 70 for hold-short and hold-long conditions. The increase in the unit’s firing probability, due to CPN Ia-afferent stimulation, was calculated from the peak observed in the PSTH representing the difference between the "test" and "control" histograms (subtracted bin by bin, see Fig. 1B). Both the test and control histograms were triggered with an ~70 ms delay relative to the previous firing of the motor unit (see Fig. 1A). The probability of firing was calculated as the number of counts in that peak divided by the number of stimuli. The onsets and end times of the peaks were determined visually from the PSTHs and their corresponding cumulative sums.

Sweeps were sorted into sections according to the contraction condition. Thus PSTHs were thus constructed for the following four conditions: "lengthening," "shortening," "hold long," and "hold short."

Statistics.   Wilcoxon tests and sign tests for dependent samples were used to assess the statistical difference in the probability of firing and firing rate, respectively, during lengthening compared with shortening conditions and hold-long vs. hold-short conditions. Differences were considered significant with a P value ≤ 0.05.


   RESULTS
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 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 ACKNOWLEDGMENTS
 REFERENCES
 
The firing probability for tibialis anterior motor units in response to an added Ia-afferent stimulus was the same for shortening and lengthening contractions. This result is shown in the PSTHs and cumulative sums for a single representative subject in Fig. 1B. The average (± SE) firing probability for all 19 units was 0.23 ± 0.03 for shortening, 0.20 ± 0.02 for lengthening (P = 0.18). There was also no difference in firing probability induced by Ia-afferent stimulation for the hold-short (0.24 ± 0.03) and hold-long (0.20 ± 0.03) conditions (P = 0.13). The individual subject and group average data are shown in Fig. 2A.


Figure 2
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  		Fig. 2. Group data. A: solid lines, firing probability of all single units; bullet with SE connected by dashed lines, group means. B: a similar plot illustrating the firing rates of all units and the group means. Left, shortening and lengthening contractions; right, hold-short and hold-long contractions.

 
The mean background firing rate, shown in Fig. 2B, was 7.8 ± 0.3 Hz for shortening compared with 7.4 ± 0.3 Hz for lengthening (P = 0.07), and it was 8.2 ± 0.4 Hz for hold short compared with 7.4 ± 0.2 Hz for hold long (P = 0.07). This indicates that the overall level of motoneuron activity was similar during shortening and lengthening, whereas the motoneuron activity was higher during the hold-short contraction compared with the hold long. Figure 3 shows the relation between firing rate and firing probability. For each unit, the difference in absolute firing rate between shortening and lengthening is plotted against the corresponding difference in absolute probability of firing. The solid circles show the values for shortening and lengthening, and the open circles show the values for hold short and hold long. The correlation coefficients for lengthening minus shortening and hold long minus hold short were –0.07 and 0.30, respectively.


Figure 3
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  		Fig. 3. Group data. For each unit, the difference between the firing rate during lengthening and shortening is plotted against the corresponding difference in firing probability. bullet, Values obtained by subtracting the values during shortening from those obtained during lengthening; {circ}, hold short subtracted from hold long.

 

   DISCUSSION
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
ACKNOWLEDGMENTS
 REFERENCES
 
The main result of the study was that the size of the Ia excitatory postsynaptic potential onto active single motoneurons is unchanged between shortening and lengthening contractions. The evidence for this is that the peak in the PSTHs induced by electrical stimulation of Ia afferents did not change with the contraction type (see Ref. 14). This suggests that, at least for the Ia synapse, there is no differential central control of the peripheral receptor input, which relates to the type of contraction performed. For this reason, if muscle spindle firing is greater during lengthening than shortening contractions (see Ref. 2), our findings point toward varying cortical control of the motoneuron pool to maintain a similar motor unit firing during both types of contractions.

Changes in Ia activity between lengthening and shortening.   The range of plantar flexion and dorsiflexion movements of the ankle joint used in this experiment produces changes in muscle length in tibialis anterior at rest and during voluntary contractions (8, 9), with more than half of the muscle-tendon length change occurring in the muscle fibers themselves (0.53 mm/°) (9). It is to be expected that the approximate 5-mm muscle fiber length changes that occurred in this study are detected by the muscle spindles and that the discharge rate of the afferent fibers will be different between contraction types (2). In fact, despite increased fusimotor drive during shortening voluntary contractions, spindle afferents have been found to have higher discharge rates during lengthening compared with shortening contractions producing the same torque (2).

Presynaptic mechanisms.   Changes in presynaptic inhibition of the Ia-afferent synapse have been shown to accompany voluntary contractions (13, 15, 16). More recently, central control of presynaptic inhibition has been highlighted as an important component of strategies for motor function in primates (20, 23). To control the sensory input from the Ia afferents during voluntary tasks, presynaptic inhibition might be differentially modulated during the shortening and lengthening phases of a movement. In our study, this was not the case, as illustrated by the similar stimulus-induced increases in motoneuron firing probability during the shortening and lengthening contractions. This finding also contradicts the earlier result of Romano and Schieppati (19), who, on the basis of differences in H reflexes, suggested that the amount of presynaptic inhibition differed between shortening and lengthening contractions. However, the H reflex is not purely monosynaptic (3), and its size can be affected by excitability changes in the nonmonosynaptic pathways. Ib afferents, for instance, would presumably discharge more in lengthening contractions, giving rise to greater inhibition, particularly when the EMG is matched.

Synaptic transmission from Ia afferents to the motoneurons is strongly influenced by the history of its activity. However, this homosynaptic postactivation depression appears to be most evident when measured at rest (12, 17). In the weak contractions used in the present study, muscle spindle firing rate would be greater compared with rest (25) and therefore it is unlikely that homosynaptic postactivation depression would be a strong candidate to modulate Ia input.

Neural strategies during shortening and lengthening contractions.   It appears, at least for early recruited motor units (with similar firing rates), that presynaptic inhibition of Ia afferents is operating free of specific regulatory control during shortening and lengthening contractions. Therefore, the difference in spindle afferent input to the motoneurons between lengthening and shortening contractions has to be compensated for by other mechanisms. Several lines of evidence suggest that this compensation arises in the cortex (for review see Ref. 4). Responses to transcranial magnetic and electrical stimulation over the motor cortex increase during shortening contractions and decrease during lengthening contractions compared with during isometric contractions (1, 24). Such cortical changes support our findings that presynaptic inhibition is not altered between shortening and lengthening contractions at low levels of activation, and thus changes in cortical drive and/or other neural pathways can ensure constant output from the motoneuron pool when muscle length is changing.


   ACKNOWLEDGMENTS
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 ACKNOWLEDGMENTS
REFERENCES
 
This study was supported by the Novo Nordic Foundation (to N. T. Petersen), the National Health and Medical Research Council of Australia (to J. E. Butler), the Swedish Research Council (to A. G. Cresswell), and the Karolinska Institute.


   FOOTNOTES
 

Address for reprint requests and other correspondence: N. T. Petersen, Univ. of Copenhagen, Dept. of Medical Physiology and Institute of Exercise and Sport Sciences, Blegdamsvej 3, 2200 Copenhagen N, Denmark (e-mail: nicolas{at}mfi.ku.dk)

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.


   REFERENCES
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 ABSTRACT
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 DISCUSSION
 ACKNOWLEDGMENTS
 REFERENCES
 

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