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1 School of Kinesiology and Health Science, York University, Toronto, ON, Canada 2 Department of Kinesiology and Health Education, University of Texas at Austin, Austin, TX, USA
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(Received 1 September 2005;
accepted after revision 4 October 2005; first published online 6 October 2005)
Corresponding author L. Griffin: Department of Kinesiology and Health Education, University of Texas at Austin, 1 University Station, D3700, Bellmont 222, Austin, TX 78712, USA. Email: l.griffin{at}mail.utexas.edu
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Introduction |
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Voluntary activation as measured by the twitch interpolation technique has been found not to change (Garfinkel & Cafarelli, 1992; Herbert et al. 1998) or to increase (Knight & Kamen, 2001; Pensini et al. 2002) with resistance training. Few studies have measured motor unit firing patterns during training. Patten et al. (2001) observed increases in maximal motor unit firing rates after just one testing session of the abductor digiti minimi muscle but then no further increase following 6 weeks of isometric resistance training. In a previous investigation, we found that motor unit firing rates during submaximal contractions (50% MVC) of the quadriceps muscle did not change following 8 weeks of isometric resistance training (Rich & Cafarelli, 2000). Yet during dynamic training of the leg muscles, increases in motor unit firing rates during the ramp up (Van Cutsem et al. 1998) and plateau phase (Kamen & Knight, 2004) occurred.
Few studies have investigated multiple neuromuscular parameters simultaneously during the progression of a resistance training programme. Thus the purpose of this study was to measure average single motor unit firing rates during maximal and submaximal (50 and 75% MVC) contractions and to determine the time course of changes in maximal force, EMGmax, M-wave amplitude, percentage activation and agonist–antagonist coactivation production during 3 weeks of isometric resistance training of the quadriceps muscles.
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Twenty male volunteers of age 25.0 ± 5.5 years (mean ± S.D.) were recruited from the university population. None of the subjects had engaged in regular resistance or endurance training for 6 months prior to the start of the experiment and all reported being free of pathology of the knee joint and surrounding musculature. Each gave written informed consent after a complete explanation of the experimental procedures, and each was paid for his participation. The protocol was approved by the York University Human Participants Review Committee and conformed to the Declaration of Helsinki. All subjects came to the laboratory for orientation prior to starting the experiment. During the orientation, the sequence of the protocol was outlined, height and weight recorded, and each subject practised the various manoeuvres involved in the experiment. The highest verified MVC obtained during the orientation was used to match pairs of subjects who were then randomly assigned to either the control (n = 10) or the experimental group (n = 10).
Procedures
Following orientation, the training subjects came to the laboratory three times a week for 3 weeks, giving a total of nine training days, and the control subjects were seen only on training days 1, 5 and 9. Each visit began with a warm-up period of submaximal contractions performed at no fixed rate. This was followed by three to five attempts at MVC, each with superimposed twitches, with at least 90 s rest between contractions. For the training group only, the training protocol followed, which consisted of three sets of 10 MVCs with 2 min rest between sets. During the training, each MVC was held for 3 s with 3 s intervals intervening. We have previously used this protocol successfully to increase isometric force of the quadriceps after only a few weeks of training (Carolan & Cafarelli, 1992).
On training days 1, 5 and 9 single motor unit discharges were recorded from the vastus lateralis muscle at 50, 75 and 100% of the MVC for that day. The subjects held the target by matching the force trace to a horizontal cursor displayed on a monitor. To avoid inducing a training effect, the control subjects did not make 100% MVC contractions. In addition, recordings of force and surface electromyography were made for each training contraction to check that the muscle was being driven at or near maximal levels. Data from the control subjects were obtained only on days 1, 5 and 9 to reduce the possibility of inducing a training effect.
Force measurement
The isometric force exerted by the quadriceps was measured in a dynamometer previously described (Carolan & Cafarelli, 1992). The subject sat in the apparatus with hip and knee joints at 90 deg of flexion. A strain gauge load cell attached to the leg with a cast aluminium cuff 2 cm above the lateral malleolus registered the force output. The strain gauge was calibrated regularly with known weights.
Electrical stimulation
Superimposed and potentiated twitch contractions were evoked by applying a single supramaximal shock to the femoral nerve during and immediately following the performance of the MVCs. These were delivered through carbonized rubber stimulating pads (Medelco, Toronto, ON, Canada); the anode (4 cm x 4 cm) in the inguinal crease and the cathode (12.5 cm x 7 cm) midway between the superior aspect of the greater trochanter and the iliac crest. Maximum stimulus intensity was determined by applying 200 s square-wave pulses of increasing current to the femoral nerve until there was no further increase in evoked force. The stimulus intensity was then increased by 10–20% to ensure maximal effect. Measurements of twitch amplitude and time-to-peak tension as well as M-wave amplitude were obtained from the post-MVC twitches and their accompanying electromyographic recordings. The post-MVC, potentiated twitches were used for pre- and post-training comparison in order to include all muscular adaptations that may have contributed to the increase in muscle force following training.
Voluntary activation
Voluntary activation of the knee extensors was assessed using the superimposed twitch technique (Merton, 1954) as modified by Allen et al. (1995) and verified by Behm et al. (1996). A supramaximal stimulus was delivered to the femoral nerve during the first 1–2 s of an attempted MVC, when force was constant. The first 180 ms of the force record following the stimulus artifact was then amplified 20-fold. The degree of voluntary activation was calculated as 1.0 minus the ratio of the superimposed to the potentiated twitch.
Electromyography
Surface EMG. The skin over the right vastus lateralis was prepared for surface recording by shaving a small area (4 cm x 5 cm) 7 cm proximal to the superior border of the patella on the anteriolateral aspect of the thigh, then swabbing with a 70% alcohol solution. Bipolar silver–silver-chloride electrodes (1.0 cm diameter, 2 cm interelectrode distance; EQ Inc, Chalfont, PA, USA) were then fixed to the skin with an adhesive patch. The small shaved area served as a reference for future electrode placement. Since the surface EMG is affected by skin preparation, electrode location, electrode orientation and sweating, EMG measures were normalized to the peak-to-peak amplitude of the M-wave recorded during the same test day. A second bipolar surface electrode (1.0 cm diameter, 2 cm interelectrode distance) was placed over the lateral head of biceps femoris half the distance between the ischial tuberosity and the popliteal fossa. The signal from this electrode was used to monitor antagonist coactivation. A water-soaked strap electrode around the proximal thigh was used as the earth electrode. All EMG signals were preamplified at the electrode site and then passed through a variable gain second stage amplifier, before being stored on tape (Vetter, Model 500D, Rebersburg, PA, USA).
Intramuscular EMG. Recordings of single motor unit action potentials were obtained with tungsten microelectrodes (FHC Inc., Bowdoinham, ME, USA). These electrodes have an insulated shaft 10 cm long, a diameter of 270 µm, and an exposed recording tip of 5–15 µm of bare wire. The skin over the vastus lateralis was prepared by shaving and cleansing an area (2 cm x 2 cm) approximately 3 cm distal to the surface electrodes. A 26 gauge hypodermic needle was used to puncture the skin and underlying fascia. The needle was then removed and the microelectrode inserted through the puncture into the muscle. A reference electrode was placed subcutaneously, 3 cm proximal to the patella. During the submaximal contractions, the microelectrode was manually advanced very slowly (0.5–1.0 mm s–1) to record as many short spike trains as possible. Signals from the microelectrode were preamplified near the source and then passed through a second stage amplifier and stored on digital tape for later analysis.
Signal processing
As the force signal was played back off tape, it was acquired by Spike 2 for Windows (version 2.24, CED, Cambridge, UK) at 1000 Hz and smoothed every 25 points. The surface EMG signals from vastus lateralis and biceps femoris were sampled at 5000 Hz and then bandpass filtered between 5 and 5000 Hz. The surface interference pattern was quantified by calculating the root mean square (r.m.s.) of the amplitude of the signal during a 2 s constant force portion of the contraction. Recordings of spike trains from the intramuscular electrodes were first passed through hardware filters with a low pass frequency of 30 kHz (Neurolog NL-126 Filter, Medical Systems, Greenvale, NY, USA) and then A–D converted into Spike 2 at 10 000 Hz. Short segments of the spike train were passed through a window discriminator in order to observe the size, shape and interspike interval (ISI) of successive spikes. Spikes of similar shape were manually overlaid into the same bin, but only bins consisting of a minimum of four spikes and a coefficient of variation of the ISI less than 20% (25% for MVC) were used to calculate motor unit firing rates (see Fig. 1).
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The dependent variables (force, EMG, percentage activation, twitch and M-wave amplitude) were analysed with a 2 x 3 mixed ANOVA with group (trained and control) as the between-subjects factor and training day (1, 5 and 9) as the repeated measures factor. Average motor unit firing rates were analysed with a 2 x 3 repeated measures ANOVA with training day (1 and 9) and relative intensity (50, 75 and 100% MVC) as the independent variables. Average motor unit firing rates from control subjects were analysed with a 2 x 2 repeated measures ANOVA with training day (1 and 9) and MVC intensity (50 and 75% MVC) as the variables. Post hoc comparisons were made with Tukey's test. All statistical procedures were performed with Statistica software (Statistica for Windows, version 5.1, Statsoft, Tulsa, OK, USA).
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Results |
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MVC and twitch forces
Figure 2 displays the daily changes in knee extensor MVC for all training and testing days. There was no significant difference in the absolute values of MVC force on day 1 between the training (761 ± 77 N) and control groups (848 ± 77 N). Knee extensor MVC increased significantly after only four training days (P < 0.05) and was 35% greater than the initial value by the ninth training day (1031 ± 78 N, P < 0.002). The MVC for the control group was not significantly different from day 1 on day 9 (835.1 ± 70 N). Day 1 values for twitch amplitude and time-to-peak tension did not differ significantly between training (122.7 ± 11.2 N, 126 ± 2.0 ms) and control groups (156.7 ± 9.6 N, 121 ± 4.0 ms). Figure 3 displays the twitch amplitude data. There appears to be an increase by day 9; however, this was not statistically significant.
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Initially, both groups were able to activate vastus lateralis to the same degree during MVC (trained 95.7 ± 1.83% and control 94.3 ± 0.997%) and by the end of 3 weeks both groups also showed a small but significant increase (P < 0.05) in their ability to drive this muscle during an attempted MVC (trained 98.44 ± 0.658% and control 96.8 ± 1.3%; P < 0.005).
Single motor unit firing rates were significantly higher at 75 versus 50% and at 100 versus 75% MVC (P < 0.05). There was no significant difference in motor unit firing rates across all force levels between training and control subjects on day 1. The average motor unit firing rates for all subjects for all contractions on training days 1 and 9 are displayed in Fig. 4. Motor unit firing rates were not recorded during 100% MVC in the control group to avoid a training effect. The total number of spike trains recorded for all subjects is shown in parentheses beside each data point in Fig. 4. A total of 511 spike trains were recorded. Training group motor unit firing rates at 50, 75 and 100% MVC were: 15.51 ± 1.48, 20.23 ± 1.85 and 42.25 ± 2.72 Hz, respectively, on day 1; 14.71 ± 1.18, 20.61 ± 1.37 and 40.18 ± 4.14 Hz, respectively, on day 5; and 16.23 ± 1.56, 21.57 ± 1.84 and 43.46 ± 2.63 Hz, respectively, on day 9.
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The magnitude of vastus lateralis EMGmax is displayed in Fig. 5. There was no significant difference in the absolute values of EMGmax r.m.s. on day 1 between the training (102 ± 9 mV) and control groups (154 ± 30 mV). Vastus lateralis EMGmax r.m.s. increased to 132 ± 19 mV by day 5 and significantly (P < 0.05) to 149 ± 26 mV by day 9. There was no significant change in the antagonist, biceps femoris, EMGmax in the training group from day 1 (92 ± 24 mV) to day 9 (89 ± 17 mV).
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Discussion |
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Motor unit firing rates
Few studies have investigated changes in motor unit firing rates during resistance training. Van Cutsem et al. (1998), in an attempt to increase contraction speed, trained subjects using high velocity, low intensity dynamic contractions of the ankle dorsiflexors. After 12 weeks, they observed a more frequent occurrence of short interspike intervals at the onset of contraction. Kamen & Knight (2004) found a 15% increase in motor unit firing rates at contractions levels of 100% MVC but not at 10 or 50% MVC following 6 weeks of dynamic training of the quadriceps muscles. However, no change in maximal motor unit firing rate occurred following isometric resistance training of the abductor digiti minimi muscle (Patten et al. 2001). Average motor unit firing rates at 50% MVC did not change after 8 weeks of isometric training in vastus lateralis muscles (Rich & Cafarelli, 2000). In the present study, mean motor unit firing rates at 50, 75 or 100% MVC did not change after isometric resistance training despite a significant increase in absolute force during each level of contraction. Thus, there was actually a leftward shift in the force–frequency curve, since the firing rates at the same absolute levels of muscle force would have been expected to be lower. These data taken together would indicate that maximal motor unit firing rates increase in response to dynamic but not isometric resistance training.
A few studies have measured changes in percentage maximal activation following resistance training using the twitch interpolation technique. In the present study we found a small but significant increase in percentage maximal activation in both groups on the second testing day (training day 5). This is consistent with the findings of other investigations (Knight & Kamen, 2001; Pensini et al. 2002). This indicates that there may have been an increase in the ability to recruit single motor units following resistance training. An increase in motor unit recruitment may have contributed to the increase in surface EMG activity and may have occurred independently of increases in mean motor unit firing rates. There appeared to be a reduction in antagonist activity in the present study, but it was not statistically significant. Reductions in coactivation have been proposed to be a contributing factor to the increase in agonist MVC with training (Carolan & Cafarelli, 1992). The small changes in activation and coactivation observed in these studies are not likely to be sufficient to solely account for the large increases in MVC force during the first few weeks of training. It is possible that other neural adaptations may also have occurred. Such adaptations include changes in the control of the synergistic muscles (Rutherford & Jones, 1986) and motor unit firing rate synchronization (Semmler & Nordstrom, 1998).
In the past, the argument for neural factors being involved in strength increases hinged on increases in muscle activation observed in the surface electromyogram. At maximal efforts this adaptation was interpreted to mean a more complete recruitment of the entire motor unit pool or increased motor unit firing rates. Yet in a previous investigation we found no increases in surface EMG of the quadriceps muscles (Cannon & Cafarelli, 1987). Similar discrepancies occur in the tibialis anterior muscle; some have observed an increase in EMG (van Cutsem et al. 1998), while others found a decrease (Holtermann et al. 2005). However, M-waves were not measured in these studies. The present study found that increases in surface EMG from the vastus lateralus muscle occurred with increases in the muscle force from the quadriceps as a whole. Thus the increase in EMG occurred in the absence of increased muscle activation or mean motor unit firing rates, but rather was correlated to a potentiation of the M-wave.
M-wave amplitude
The increased EMG activity seen in the early stage of adaptation was correlated with a potentiation of the M-wave and not with an increase in motor unit firing rates. It is also possible that other factors in addition to the increase in M-wave amplitude contributed to the increase in surface EMG activity. Hicks et al. (1992) also found an increase in the amplitude of the M-wave in older subjects after training. They proposed that the enhanced excitability of the sarcolemma was due to increased sodium pump activity brought on by the training (Hicks & McComas, 1989). Upregulation of both Na+- and K+-ATPases are known to occur as consequence of training (Clausen, 1996). However, changes in intramuscular water content and subcutaneous adipose tissue could also affect the amplitude of the surface electromyogram (Krotkiewski et al. 1979). It is also possible that small changes in muscle fibre size could affect the amplitude of the M-wave. Sale et al. (1983) found no change in the amplitude of the M-wave with resistance training, but maximal EMG was not measured in that study. Thus, it is possible that the increase in surface EMG observed in some studies but not others may be due to differences in sarcolemmal excitability or electrical transmission through the tissue rather than to differences in motor unit firing rates. There was no significant change in EMG when normalized to the M-wave amplitude in the present investigation.
Twitch amplitude and hypertrophy
There was no significant change in twitch amplitude in response to short-term resistance training. Others have also either found no change (Davies & McGrath, 1982; Young et al. 1985; Sale et al. 1992) or a small increase (Duchateau & Hainaut, 1984; Rich & Cafarelli, 2000) in twitch amplitude or tetanic force (Davies et al. 1985) following resistance training. It has been reported that increases in force and muscle cross-sectional area can occur in the absence of an increase in twitch amplitude (Alway et al. 1989). Thus, twitch amplitude may not be a reliable indicator of changes in muscle size or strength, especially when hypertrophy is sleight, as might be expected if it did occur in the first few weeks of training. Despite the difficulty of verifying slight hypertrophy with evoked twitch responses in whole human muscle, the likelihood that increased protein synthesis contributed to the increase in MVC observed in the present study is quite possible.
Increases in the area of type II fibres have been observed after 5 (Krotkiewski et al. 1979) and 8 weeks of resistance training (Ishida et al. 1990). Techniques including ultrasound (Young & Bilby, 1993), magnetic resonance imaging (Housh et al. 1992; Narici et al. 1996) and computerized tomography (Garfinkel & Cafarelli, 1992) have revealed increases in muscle cross-sectional area during the first 8 weeks of training. The rate of protein synthesis in the biceps brachii muscles of weightlifters increases by 50% within 4 h of one training session (Chesley et al. 1992) and 109% by 24 h (MacDougall et al. 1992), and protein turnover is reduced following acute resistance exercise (Phillips et al. 1997). Thus changes within the muscle during the initial stages of training may be sufficient to account for the increase in muscle strength observed during this period. However, a combination of mechanisms, including increased protein synthesis, changes in quadriceps synergistic muscle activation and synchronization, and increases in agonist versus antagonist activation, may contribute to the increase in maximal muscle force output during the first few weeks of resistance training. The results of this study indicate that the primary ‘neural adaptation’ to training that contributes to increases in maximal force output following isometric resistance training is not an increase in average motor unit firing rate.
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) are plotted as a percentage of initial (day 1) values. There was a main effect of training days in the trained group, but not for the controls. By the ninth training day MVC had increased by 35%.
0.05 compared to day 1.