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1Human Neurophysiology Laboratory, Centre for Neuroscience, University of Alberta, Edmonton, Alberta; 2Rehabilitation Neuroscience Laboratory, University of Victoria, Victoria, British Columbia; 3International Collaboration on Repair Discoveries, Vancouver, British Columbia; and 4School of Physical Education, University of Victoria, Victoria, British Columbia, Canada
Submitted 5 May 2005 ; accepted in final form 31 August 2005
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ABSTRACT |
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 TOP ABSTRACT MATERIALS AND METHODSRESULTSDISCUSSIONGRANTSREFERENCES |
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-motoneuronal excitability or reduced presynaptic inhibition. Despite a similar increase in strength in the contralateral limb of the exercise group, HA amplitude was unchanged. We conclude that the cross-education effect of strength training may be due to supraspinal to a greater extent than spinal mechanisms. H-reflex; isometric; strength; adaptation; Hebbian synapse
Numerous studies have reported that chronic unilateral motor activity can affect performance of the homologous muscles in the contralateral limb. This phenomenon, termed cross education, occurs during improvements in strength and the learning of motor skills and displays specificity to the training of the opposite limb (12). Structural and functional adaptations due to resistance training are specific to the exercised musculature, and the greatest training effect is found when testing procedures match the training protocol (21). However, cross education of the contralateral limb shows evidence of neuromuscular adaptations despite not being involved in the tasks performed during training. Both supraspinal and spinal factors have been proposed to contribute to the cross education, with no clear consensus on the predominant mechanism.
In an attempt to elucidate neural adaptations to strength training and long-term physical activity, exercise studies have examined reflex pathways by using electromyography (EMG) in conjunction with muscle and nerve stimulation. The H-reflex is possibly the most widely studied reflex because of the ease with which it can be elicited in various muscles. The H-reflex is considered the electrical equivalent of a stretch reflex and is predominantly characterized by the monosynaptic and oligosynaptic projections of group Ia afferents onto homonymous motoneurons (16, 33).
Several exercise studies have examined the H-reflex and proposed various hypotheses as to why it may be potentiated or attenuated because of long-term physical activity (1, 2). Recently, Aagaard et al. (2) showed that the soleus H-reflex was significantly (by 
20%) facilitated after 14 wk of strength training. However, no exercise studies utilizing the H-reflex have examined the influence of cross education on spinal reflex plasticity. It is possible that unilateral strength training affects -motoneuron pool excitability and influences force production capabilities in both limbs. Potentially, training could also affect the magnitude of presynaptic inhibition of Ia afferent feedback. A change in any of these factors would contribute to a change in H-reflex excitability with training. Presently, there is a lack of understanding of the neural adaptations due to strength training as well as cross education. The present experiment is the first to utilize the H-reflex as a tool for examining plasticity of spinal reflexes during cross education. Further understanding of cross education will help to clarify the mechanisms involved in neural adaptations that could be used to potentiate the cross-education effect in exercise and rehabilitation programs.
The purpose of this study was to examine the effects of a 5-wk unilateral, isometric strength-training program on alterations in the H-reflex in both the trained and untrained limbs. It has been suggested that cross education may be mediated by both spinal and supraspinal mechanisms [for review, see Hortobagyi (12)]. For a given leg, whether it be the trained or untrained limb, a parallel change in contractile strength and reflex excitability might be expected in the presence of supraspinal adaptation mechanisms alone. Furthermore, it may be reasoned that the activity state of the limb (that is, whether the muscle is contracting during training) would have a larger effect on the spinally mediated adaptations than the supraspinal ones. That is, during training the muscle activation would generate a barrage of associated somatosensory feedback (e.g., from Golgi tendon organs and cutaneous tactile mechanoreceptors in the foot sole) that is specific to the limb in which the muscle resides. We suggest that this should have a larger effect on spinally mediated adaptations than supraspinal ones because the local feedback is specifically mediated by targeted interneurons affecting local spinal circuits. In contrast, if supraspinal mechanisms dominate the neural adaptations to strength training, we would hypothesize parallel changes in strength gains and H-reflex excitability in both legs after training.
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MATERIALS AND METHODS |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONGRANTSREFERENCES |
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Experimental design. The experiment consisted of two groups (exercise n = 10, 6 women and 4 men; control n = 6, 3 women and 3 men) and utilized randomized groups with repeated-measures design (pretest, posttest). Participants were required to participate in two familiarization sessions to become accustomed to performing maximal voluntary isometric contractions (MVICs). A third familiarization session was conducted if a >5% difference was detected between pretraining MVICs performed during the familiarization sessions. After satisfactory completion of the familiarization sessions, participants in the exercise group completed an isometric resistance training protocol using their dominant leg 3 days per week, with a minimum of 2 days rest between training days, for the following 5 wk. The dominant leg was determined by asking participants which leg they preferred to use when kicking a ball.
Familiarization sessions and training programs.
Participants reported to the laboratory, received a random assignment to control or exercise group, and performed a warm-up consisting of three sets of 10 repetitions of unilateral, submaximal, isometric plantar flexions, separated by 3-min rest periods. Participants performed these repetitions at what felt to be 50% of their maximum effort. A warm-up was chosen to reduce the likelihood of a muscular strain during training and to maximize performance. Warm-up repetitions consisting of 50% maximal contractions are unlikely to cause fatigue that could impair the MVIC, especially for triceps surae because this muscle is predominantly made up of slow-twitch, fatigue-resistant muscle fibers. Familiarization with MVIC commenced 5 min after the last warm-up. All training and testing were conducted in the same apparatus. Participants were allowed three attempts at attaining a MVIC with each foot, interspersed by 5 min of rest. MVICs were not held for longer than 6 s. Control group participants did not engage in any form of resistance training program and were asked not to begin a new exercise program for the duration of the study. Exercise group participants performed five sets consisting of eight isometric repetitions held for 6 s at maximal voluntary effort, with a 1-s rest interval between contractions and a 1-min rest interval between sets. This protocol was performed three times per week, for 5 wk. This relatively short training duration was chosen to preclude any morphological adaptations at the muscle level. Cannon and Cafarelli (5) used a similar unilateral training program using isometric resistance training to evaluate neural adaptations due to strengthening of the adductor pollicis muscle. In addition, Shima et al. (25) have demonstrated a significant increase in isometric plantar flexion after a 6-wk unilateral strength-training program.
Protocol.
For training and testing sessions, participants were seated in a chair with their backs supported. Hip, knee, and ankle angles were set at 90, 150, and 90°, respectively. Restraints were placed around the foot to minimize movement. All participants were asked to remain calm and not to alter their posture during testing to control for task dependency of reflex modulation. The temperature, noise, and lighting were held as constant as possible between sessions.
Recordings of MVICs. Torque values were established via strain gauge (Omegadyne model 101-500, range 0–226.7 kg) and amplified by a custom-made high-gain amplifier system. The force was displayed by use of custom-built continuous acquisition software utilizing LABVIEW. Torque was calculated after MVIC by using a consistent moment arm length of 0.15 m (measured from the adjustable heel block to the center of the strain gauge).
EMG recording. EMG was recorded with UNI-GEL single use (Thought Technologies) bipolar surface recording electrodes. A 3-cm interelectrode distance was used. To guard against variations in electrode placement from pre- to posttest, permanent felt markings were applied to all participants. These markings were refreshed approximately every 3 days as needed.
EMG signals were preamplified and band-pass filtered at 30–300 Hz. EMG was collected from the soleus, tibialis anterior (TA), vastus lateralis, and biceps femoris muscles during tibial nerve stimulation. Data were sampled at 2,000 Hz with a 12-bit analog-to-digital converter. For the purpose of calculating muscle activation levels, the EMG signals for all muscles were processed offline. For each signal, any offset was removed and the EMG recordings were full-wave rectified and then low-pass filtered at 40 Hz with a dual-pass Butterworth filter.
The tibial nerve was stimulated with single 1-ms square-wave pulses delivered over the popliteal fossa. The interstimulus interval varied randomly between 3 and 5 s, and the order of testing was randomized between dominant and nondominant leg for each subject. For each M-wave and H-reflex recruitment curve (M-H curves), 75 sweeps of data were collected 20 ms before and 90 ms after the stimulus was delivered.
Participants maintained a 10% background EMG contraction during soleus M-H recruitment curve data collection. Visual feedback of the full-wave rectified and low-pass filtered (10 Hz) EMG from soleus was provided to subjects via an oscilloscope. There was a range of torques (7–18% MVC) for subjects who held a 10% EMG background contraction; however, this value was constant for each subject. That is, regardless of the interindividual differences between subject's torque level at a corresponding 10% EMG contraction, this value was the same at posttest. The M-waves and H-reflexes of each participant were normalized to the corresponding Mmax (taken as the single largest M-wave) recorded at that time (see Ref. 33).
Two measures of H-reflex excitability were calculated from each M-H recruitment curve: Hmax and HA. Hmax was calculated as the average of the three largest H-reflexes. HA was calculated as the average of 10 H-reflexes obtained from the ascending limb of the M-H recruitment curve, with a corresponding M-wave of 5% Mmax. The majority of data were collected on the ascending limb of the recruitment curve, and the 10 values closest to 5% Mmax were used to calculate HA (a small range between 4.5–5.5 Mmax). The intensity of stimulation was varied during data collection of the recruitment curve so that no two succeeding pulses were the same. To evaluate the potential confounding effect of changes in antagonist muscle activity on H-reflex amplitude, ratios of soleus to TA EMG were calculated for all conditions.
Statistics.
Repeated-measures ANOVA tests (3 x 2 ANOVAs) were used to examine the effects of isometric resistance training on MVIC and H-reflex amplitudes (Hmax and HA). Fisher's least significant difference test was used to post hoc significant main effects. The significance level was established at P 
0.05.
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RESULTS |
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10% MVIC during all H-reflex recordings that did not significantly differ between testing sessions (P > 0.05). Change in MVIC. Control and exercise groups did not significantly differ in pretest MVICs measures for either leg across groups. However, significant (P < 0.05) interactions were detected for MVICs across groups in the posttest. The exercise group significantly increased MVIC by 15.31% (P = 0.01) and 17.83% (P = 0.03) between pretest and posttest measures for the trained dominant leg and untrained nondominant leg, respectively (see Table 1 and Fig. 3). The control group showed no significant change (P = 0.12) in MVIC strength of the nondominant leg; however, the dominant leg MVIC decreased by 6.3% (P = 0.05, see Table 2).
Soleus/TA EMG ratio. The soleus/TA EMG ratios were obtained from the background contraction held during H-reflex testing. These EMG ratios (derived from absolute µV data) did not significantly differ between groups or between legs for either pretest or posttest. However, when within-group comparisons were made, it was found that the exercise group significantly increased their soleus/TA EMG ratio more than 300% from 1.03 to 3.40 in the trained leg and from 1.03 to 3.15 in the untrained leg (P = 0.0007 and P = 0.002, respectively). This change was not significantly different between legs (P = 0.86). The control group did not significantly change their soleus/TA ratio (dominant P = 0.29; nondominant P = 0.20) from pretest to posttest.
Vastus lateralis and biceps femoris EMG remained unchanged (all values P 
0.4) from pretest to posttest values for both legs of the experimental and control group.
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DISCUSSION |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONGRANTSREFERENCES |
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It is possible that the composition of motor unit types in the motoneuronal pools recruited in the HA and Hmax reflex responses are different. HA reflex responses would presumably recruit the smallest spinal motoneurons, whereas the higher stimulus strength Hmax would recruit larger high-threshold motoneurons. However, because of the low level of activity, the high-threshold motoneurons were likely not activated during the present experiment, and therefore no change in Hmax would be observable. In support of this, maximal soleus H-reflex responses have previously been observed to remain unchanged after training when obtained at rest (2, 22, 28), although significantly increased when recorded during forceful muscle contraction (2, 22), suggesting that training-induced increases in H-reflex excitability occur only during actual activation of the motoneurons. The findings of the present study, in conjunction with previous experiments, suggests that adaptations in the nervous system are due to either increased descending facilitation of motoneuron excitability, and/or altered presynaptic gating and/or postsynaptic facilitation and inhibition. Because there was a significant cross-education effect without a change in HA value for the untrained leg, spinal reflex excitability modulation did not explain the cross-education effect in this experiment.
Maximal voluntary contraction training yields comparable strength gains in both trained and untrained legs.
Subjects in the exercise group significantly increased their force production capacity in both the trained and untrained limb, therefore demonstrating a large cross-education effect (see Table 1). The exercise group significantly increased isometric plantar flexion strength by 15.31% (P = 0.01) in the trained dominant limb and 17.83% (P = 0.03) in the untrained nondominant limb. However, this difference in strength increase between the trained and untrained leg was not significant (P = 0.24). Although our training protocol was of sufficient length to induce a significant cross-education effect, most other studies have been 3–4 wk longer. Because the time course of the cross-education effect is not known, it is possible that a larger strength increase in the trained limb only would have occurred had our subjects continued for a longer training duration. The risk with continuing a training study of this nature for too long is that peripheral adaptation such as muscular hypertrophy begins to set in at 8 wk, which we wished to avoid to properly evaluate the neural components. Previous cross-education studies have reported increases of 5–35% in force production for the untrained limb during concentric and isometric contractions using training periods of 8–10 wk (34). Eccentric contractions have been found to induce greater cross-education effects compared with isometric and concentric contractions. For example, Hortobagyi et al. (13) found cross-education effects as large as 104% during eccentric, isokinetic knee extension training. The time course of the cross-education effect due to strength training has not yet been examined. In addition, this is the first experiment to utilize isometric plantar flexion as the training method. Our findings suggest that the response of isometric plantar flexion to strength training and the cross-education effect is similar to other muscles using concentric and isometric contractions but smaller than outcomes imposed by eccentric training. Control subjects did not significantly increase their force production capacity in either leg, although they showed a slight strength decrease in the dominant leg during the 5-wk interval (see Table 2).
Plasticity in the soleus H-reflex pathway is seen only in the trained leg.
Control group subjects did not display significant changes in H-reflex amplitude for either leg between pretest and posttest measures. However, the exercise group experienced a 35% increase (P = 0.04) in HA values in the trained leg in the absence of a corresponding effect on reflex excitability in the untrained leg. This increase in HA amplitude of the trained leg reflects plastic adaptations in the Ia spinal reflex pathway leading to increased reflex excitability. This could allow for greater motor unit recruitment and force-generating capabilities. Training-induced adaptive plasticity of H-reflex excitability has been well studied in the monkey and rat with both facilitation ("up training") and suppression ("down training") being observed (see Ref. 33 for review). However, there are few longitudinal (e.g., "training") studies in humans. A recent study (2) involved a comprehensive evaluation of neural adaptation arising from 14 wk of heavy resistance training of the human ankle extensor muscles and reported a significant increase in soleus H-reflex amplitude of 20% exclusively during maximum muscle contraction and not at rest. Within a session of balance board training, human subjects reduced the soleus H-reflex amplitude by 25% (27). It has also been shown that the H-reflex can progressively adapt to training involving walking backward and that this training can persist for months after the training (24). Our data showing a 35% increase in H-reflex amplitude are thus comparable to the available longitudinal human studies. It is of note that the neural plasticity associated with the H-reflex was only observed in the trained leg. The lack of H-reflex plasticity in the untrained leg (despite a comparable increase in force production) suggests that two loci of control may be present. Possibly, not only spinal but also supraspinal contributions occurred for the trained leg, whereas supraspinal changes alone would be involved for the untrained leg.
Invariance of the Hmax-to-Mmax ratio. Previous studies have examined changes in evoked H-reflex amplitude induced by resistance training (2, 22). The Hmax-to-Mmax ratio has been shown to display a certain degree of plasticity to regular physical exercise (7, 18). However, to our knowledge this is the first experiment to utilize H-reflex measures when examining the cross-education effect due to strength training.
In our experiment the Hmax-to-Mmax ratio did not change in the exercise or control group for either leg. Available literature on the sensitivity of the Hmax-to-Mmax ratio to training is conflicting. Casabona et al. (7), as well as Maffiuletti et al. (15), found decreased Hmax-to-Mmax ratios in athletes trained for explosive movements. Possibly, these findings can be explained by differences in fiber-type composition, because explosive-type athletes, owing to a likely higher proportion of high-threshold type motor units, would be expected to show a lesser H-reflex response at a given stimulus intensity, particularly on the ascending limb of the M-H recruitment curve. Casabona and colleagues suggested that intensive training utilizing primarily type II fibers decreased the synaptic strength of type Ia excitatory afferents on small and intermediate motoneurons. This implies that strength training may result in a small to large motoneuron transformation that can be detected via the H-reflex. Perot et al. (19) found increased Hmax-to-Mmax ratios in subjects who undertook 8 wk of endurance training. Reduced Hmax-to-Mmax ratios have also been found after 20 days of bed rest (30) and in highly trained ballet dancers (18). These seemingly conflicting findings may be due either to motoneuronal transformation or to different methodologies because H-reflex measures are sensitive to influences such as limb position, muscle activity, and size of the test reflex (33). In particular it is clear that measures taken at rest may not reflect neural adaptations to activity (1, 28). Alternatively, it could be theorized that an increased Hmax-to-Mmax ratio reflects increased -motoneuron excitability and increased reflex excitability via the Ia pathway, leading to a greater recruitment of motoneurons. Our findings do not support that increases in strength are associated with an altered Hmax-to-Mmax ratio, because both limbs of the exercise group improved significantly in force-production capacity. Possibly, the Hmax-to-Mmax ratio is not a sensitive enough measure to detect subtle neurological changes at the level of the motoneuron during initial stages of strength training. Past experiments have utilized the Hmax-to-Mmax ratio almost exclusively during exercise studies; thus analyzing the H-reflex on the ascending limb of the recruitment curve (e.g., at an M-wave of 5% as used here) may give additional insight into neurological adaptations.
Possible mechanisms.
Both spinal and supraspinal mechanisms have been proposed to influence the cross-education effect. With respect to supraspinal mechanisms, traditional views hold that the contralateral primary motor cortex (M1) controls the unilateral motor commands and actions. Kristeva et al. (14) hypothesized that excitation of one motor cortex during voluntary contractions of a muscle might produce an effect on the contralateral motor cortex. Kristeva et al. examined magnetic fields accompanying voluntary movement in both motor cortices during left and right unilateral and bilateral finger flexions. Magnetic fields were similar in both the left and right motor cortex regardless of the nature of the task (bilateral or unilateral). This finding suggested the presence of a bilateral generator and that unilateral voluntary movements involved activation of the contralateral motor cortex. Furthermore, 15% of corticospinal fibers cross to the contralateral side (4), and thus coactivation of homologous muscles has been suggested to be caused by an overflow of descending signals from the ipsilateral motor cortex. Yue and Cole (31) provided evidence of supraspinal involvement in cross education when they demonstrated an 11% increase in the strength of the homologous, untrained hand muscle due to imagined contractions of the ipsilateral hand. A popular hypothesis of cross education has been that unilateral muscle contractions increase the excitability of nondecussated descending motor fibers to the homologous ipsilateral muscle. However, as discussed in a recent review by Hortobagyi (12), there are monosynaptic and polysynaptic projections from M1 to some ipsilateral muscles. The direct projection involves 10% of the pyramidal tract fibers that descend nondecussated in the lateral corticospinal tract and project to motoneurons innervating upper arm and trunk muscles but do not reach the muscles of the extremities (4). Thus, from the original data of Brinkman and Kuypers (4), Hortobagyi suggested that, because these nondecussated paths do not reach the muscles of the extremities on the ipsilateral side, it appears unlikely that cross education occurs through this path. Because descending commands from supraspinal structures can modulate the H-reflex response (16, 23, 33), it is possible that supraspinal adaptations are responsible for the strength gains and changes in H-reflex amplitudes we observed after training. Large unilateral contractions have been shown to disinhibit the ipsilateral M1 (8) so that the excitability of the contralateral corticospinal system is potentiated, possibly requiring less neuronal excitatory activity required to execute a certain task. Even viewing a mirror image of the ipsilateral hand has been found to excite the ipsilateral M1 pathway (9), as can simple finger movements (20). In contrast, some studies have also suggested that both inhibition and excitation may be present owing to unilateral movement (26, 32). A recent review by Carson (6) assesses the means by which excitatory and inhibitory interactions take place at multiple levels of the neuraxis during unilateral and bilateral tasks. Carson suggests that promoting movement in a nonimpaired limb may serve to sensitize a damaged motor cortex to subsequent or concurrent training-induced modifications of a damaged limb. Aagaard et al. (2) measured changes in both H-reflex (for spinal excitability) and V-waves (putative reflection of descending drive) during a resistance training program. Potentiation of both waveforms was observed after training, and it was suggested that supraspinal and spinal adaptations collectively contributed to the changes in neural output that arose. However, the lack of change in HA values of the untrained leg of the exercise group in this study suggests that, if supraspinal adaptations are responsible for the cross-education effect, they do not exert an effect on -motoneuronal or reflex excitability in the contralateral limb. It is also possible that altered presynaptic inhibition could be responsible for an elevated H-reflex. However, because we only observed changes in HA and not Hmax, a reduction in presynaptic inhibition would be acting preferentially on nerve terminals associated with lower threshold motoneurons only. Possibly, the untrained limb is affected more by supraspinal adaptations, whereas the trained limb may incur adaptations that are both supraspinal and spinal.
Evidence for spinal mechanism involvement in cross education has come primarily from experiments using functional electrical stimulation (FES) in healthy and spinal cord-injured patients. FES artificially stimulates muscle and consequently eliminates supraspinal control of muscle activity. Hortobagyi et al. (13) found that FES-evoked eccentric contractions evoked greater cross education strength in the contralateral limb when tested using FES-evoked eccentric contractions than compared with voluntary eccentric contractions. It has also been found that hand muscle strength displayed a cross-education phenomenon when stimulated with FES in persons with spinal cord injuries (10). It has been hypothesized that because FES can simultaneously activate sensory afferent fibers and -motoneurons, a cross-education effect may be induced at the spinal level by increasing the excitability of motoneurons and interneurons affecting the contralateral limb (34). It has been suggested that a lack of EMG activity from the contralateral muscle during unilateral training sessions is a strong indicator that spinal mechanisms are responsible for the cross-education effect (13). It was reasoned that, because of the bilateral topography of the motor cortices, any supraspinal influence would have been seen as elevated EMG amplitude of the homologous, contralateral muscle. However, because EMG measures only muscle activity and not descending efferent volleys, it is possible that descending supraspinal commands may still cause a cross-education effect without elevated EMG in the contralateral muscle.
Our results do not support the suggestion that spinal mechanisms that alter the H-reflex or -motoneuron excitability are responsible for the cross-education effects. In contrast, the fact that the HA values in the trained soleus increased may be due to increased -motoneuron and reflex excitability. Possibly, the trained limb experienced increased somatosensory feedback during training compared with the untrained limb. Because somatosensory stimuli can alter the H-reflex response (33), this may be the mechanism that differentiates the trained and untrained leg responses. We propose that increased somatosensory stimuli generated by the trained limb, in conjunction with descending supraspinal commands, function synergistically to potentiate Ia spinal reflex pathways, contributing to an increased force generating capacity. Furthermore, the contralateral untrained limb, void of such direct stimuli, likely increases force-generating capacity through supraspinal mechanisms. This form of "paired" or associative conditioning is reminiscent of Hebbian plasticity, originally put forth by Dr. Donald Hebb (11), who suggested that enhanced synaptic efficacy can arise because of simultaneous paired inputs. Conditioned changes in synaptic efficacy are similar to what has been suggested to occur in cortical neural adaptations (29). Whether the Ia--motoneuron synapse can be considered a Hebbian synapse is a speculation requiring further experimental support.
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FOOTNOTES |
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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.
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