Most forms of resistance exercise stimulate some degree of strength and conditioning improvements if safe guidelines, consistent with the overload principle, are followed. Eccentric muscle activity is employed in many modes of resistance exercise since it can induce high forces, and muscle responds to repeated bouts of eccentric activity with significant shifts in the structure and function, i.e., increased fiber size, strength, and composition of skeletal muscle (5,9,10,12-14). Many times, however, high force strength and conditioning regimens result in injury and muscle soreness that can retard training. Understanding how muscle adapts to high forces for effective conditioning is essential for a safe progression to high-volume overload regimens that positively affect muscle structure and function.
The patterns and magnitude of the electrical activation of the muscle, as measured by electromyography, also shift in response to eccentric training relative to concentric training (9-11,17,19). Under a maximal voluntary eccentric contraction test condition, after 6-12 weeks of eccentric training, increases in both muscle force production and neural drive (as measured by electromyography) have been reported (9,10,19). The ability to perform graded submaximal eccentric contractions is an important component to normal movement patterns, and the relative electromyographic responses are not clearly defined. This is a renewed area of interest in strength training research since eccentric resistance exercise can induce both profound improvements in muscle conditioning and it may also induce a damage response that can derail training (2,4).
The purpose of this short research report is to describe the integrated electromyographic changes during a submaximal eccentric task in those adapted to high-force chronic (8 weeks) eccentric training and those naive to eccentric training to better describe the adaptations underlying repeated eccentric muscle activity.
Experimental Approach to the Problem
This study was designed to measure and describe the surface integrated electromyogram (IEMG) of the vastus lateralis (VL) while cycling in both a concentric-only mode on a cycle ergometer (concentric bike) and an eccentric-only mode on an eccentric cycle ergometer (eccentric bike) at the same submaximal work rate in eccentrically naive (EN) and eccentrically adapted (EA) subjects.
A sample of convenience composed of 13 healthy male subjects enrolled in a 4-year university, with a mean age of 23.9 years (range, 19-38), categorized as EA (n = 7) or EN (n = 6) following 8 weeks of eccentric or concentric training, respectively, provided written consent to participate in testing in accordance with the institutional review board at the university-based study site. Both the EA and EN subjects had an equivalent mean peak oxygen consumption (V̇o2peak) (group mean = 51.5 mL·kg·min−1, SEM = 4.6) and characterized themselves as recreational weekend athletes who played intramural athletics during the academic school year. All subjects exercised irregularly, i.e., 1-2 times per week on nonconsecutive weeks. Subjects were excluded from participation if they currently participated in resistance or aerobic exercise of ≥20 minutes at least 2-3 times per week on consecutive weeks or if they participated in regular exercise within the past 6 months.
The IEMG assessment followed 8 weeks of concentric-only (EN subjects) or eccentric-only (EA subjects) cycle training. This training has been described in detail previously with this subject cohort (14) and with others (5-7,12,13). Briefly, training was designed to create high eccentric work rates in the EA group (mean power = 489 W by the last week of training) and low concentric work rates (mean power = 128 W by the last week of training) in the EN group despite both groups cycling at an identical percentage of peak heart rate (% HRpeak) starting at 54-59% HRpeak in weeks 1 and 2 and 60-65% HRpeak for weeks 3-8. The training frequency and duration progressively increased over the 8 weeks. During week 1, all subjects rode twice per week for 15 minutes. The EA group training frequency was 3 times per week during weeks 2 and 3 for 25-30 minutes, 4 times per week for 30 minutes during week 4, and 5 times per week for 30 minutes during weeks 5 and 6. The frequency of training was decreased to 3 times per week, but training duration remained at 30 minutes for weeks 7 and 8 due to the EA subject's subjective feeling of fatigue. Pedal rpm was identical for both groups (started at 50 rpm and progressively increased to 70 rpm by week 5).
Because work is the product of a muscle's force production and change (Δ) in length, when an external force is less than the muscle's force production, the muscle Δ length is such that it shortens toward the center (i.e., concentric Δ length). Conversely, when an external force exceeds the muscle's force production, the muscle Δ length is such that it lengthens away from center (i.e., eccentric Δ length). In this report, we use the term work independent of the sign to designate the force generated by the muscle times the distance the muscle shortens (concentrically) or lengthens (eccentrically). Likewise, the term work rate is used independently of the sign as described previously (5-7,12-14). Therefore, those subjects who trained for 8 weeks in a concentric fashion were considered EN, while those that trained in an eccentric fashion were considered EA prior to the measurements of the IEMG under the two (concentric versus eccentric) test conditions.
The eccentric bike was constructed locally with the power train of a standard Monarch cycle ergometer (Figure 1).
The eccentric bike has an adjustable recumbent seat and is driven by a 3-horsepower direct current (DC) motor with 4 idlers between the motor and the flywheel. The gear ratio from the flywheel to the pedal crank is 1:3.75. All components are mounted on a steel frame. A DC motor controller, with a 0- to 10-V output for both motor speed and load, controls the motor speed and a magnetic sensor monitors pedal rpm (which is displayed to the rider during the training session).
The voltage and amperage outputs from the controller are monitored through an analog-to-digital board and dedicated computer. The ergometer was calibrated by using the original standard ergometers friction band and applying known loads (via weights) as the motor moved the flywheel in a forward direction at a fixed rpm and reading the amperage/voltage of the motor. Therefore, for a fixed load and rpm, the calibration performed in the forward direction also served to calibrate the reverse direction of the flywheel. Hence, the work rate was maintained by the subject resisting the pedal motion at a fixed rate.
Two silver silver/silver chloride surface electrodes were placed 1 cm apart on the mid-belly of the VL. The VL electrode placement site was confirmed during a maximal knee extension maneuver whereby the mid-belly of the VL could be palpated in a region midway between the greater trochanter and the lateral femoral condyle. The ground electrode was placed over the fibular head. The EMG signals were amplified 1000 times and filtered with low- and high-band pass filters set at 1 Hz and 5 KHz, respectively. The analog signal was then converted to a digital signal and processed as a raw electromyographic signal that was then rectified and integrated as integrated electromyography. Three separate integrated electromyographic measurements were recorded on the same day during the data collection session in the following order:
- The IEMG during a maximal voluntary isometric knee extension maneuver on a Cybex dynamometer (Cybex, Ronkonkoma, NY) with the hip and knee at 90° of flexion was recorded and full-wave rectified.
- The IEMG from a concentric cycling test session of 2 minutes at 200 W.
- The IEMG from an eccentric cycling test session of 2 minutes at 200 W.
Therefore, the submaximal test conditions (measurement test sessions 2 and 3 above) for both the EA and EN subjects were composed of the 2-minute, 200-W concentric and eccentric cycling test sessions. All integrated electromyographic measurements were recorded 1 week after the completion of 8 weeks of concentric-only training (EN subjects) or eccentric-only training (EA subjects). Electrodes, electrode placement, amplification, and filtering remained unchanged for all measurements.
The IEMG during the submaximal conditions (a 2-minute, 200-W, concentric cycling test and a 2-minute, 200-W, eccentric cycling test) were normalized relative to the IEMG obtained during the maximal voluntary isometric knee extension maneuver on the Cybex dynamometer. The integrated electromyographic intensity for the test conditions, IEMG per pedal cycle, is the product of the IEMG burst duration per pedal cycle and the IEMG per burst.
A 2-way repeated-measures ANOVA was used for the IEMG analysis. In all cases, the α level of significance was set at 0.05. Tukey's test for all pairwise multiple comparisons was performed when significance was noted.
The raw EMG examples (Figures 2 and 3) demonstrate apparent differences in both the duration and amplitude of the electromyographic signal of the EN and EA subjects when tested under the submaximal conditions during the concentric and eccentric cycling events.
Several consistent patterns emerged when comparing mean integrated EMG changes in both groups.
IEMG Burst Duration per Pedal Cycle
The EN subjects activated their VL during 78% of the pedal cycle, while the EA muscle was activated only 41% (p < 0.001) when both groups were tested eccentrically. In contrast, the duration of the IEMG burst duration per pedal cycle was almost identical for both groups when tested concentrically (49% EA, 51% EN).
IEMG per Burst
There was a significant difference in the IEMG per burst within each training group under the 2 test conditions. In both groups, IEMG per burst decreased (p < 0.001) when cycling concentrically versus cycling eccentrically. The EN subjects went from 61% to 34%, and the EA subjects went from 48% to 24%. There was no difference between groups under the 2 test conditions.
Intensity: IEMG per Pedal Cycle
The only significant between-group differences for electromyographic intensity occurred when subjects performed eccentric cycling (p < 0.001). Within-group differences also occurred under the eccentric cycling condition (p < 0.001) only. The EA group's IEMG per pedal cycle was 10% when cycling eccentrically versus 27% for the EN group. The EA group also decreased the maximal IEMG per pedal cycle from 23% to 10% when cycling eccentrically vs. concentrically.
Eccentric muscle activity occurs in almost all resistance exercise regimens, from low-repetition, high-force overload activities to high repetition, low-force activities. Safe and effective training progression toward high-intensity resistance exercise with eccentric muscle activity requires an understanding of how the neuromuscular system adapts and protects musculotendinous structures.
In this study, we quantify how shifts in a muscle's activation pattern (EMG) for a submaximal eccentric task vary after chronic eccentric exercise. Since eccentric muscular work is both ubiquitous in nature and typically submaximal, we focused specifically on the effects after training, in those considered eccentrically naive and compared them to those considered eccentrically adapted during a submaximal eccentric task. The electromyographic intensity, i.e., the IEMG per pedal cycle (Figure 4), which is the product of the duration of IEMG over an entire pedal cycle and the IEMG per burst, best reflects the way in which the VL is used while pedaling in a concentric vs. an eccentric fashion. electromyographic intensity in the EA group was one-third that of the EN when cycling eccentrically. Both the IEMG duration per pedal cycle and IEMG per burst of the EA subjects is greatly reduced relative to the EN group during an identical submaximal eccentric task. This differing response of electromyographic intensity (IEMG per pedal cycle) between groups seems to be related to the fact that during each pedal cycle on the eccentric ergometer, the EA activated their VL only half as long as the EN, perhaps the result of a motor learning response.
This is the first study to look at the EMG elicited with a submaximal task after chronic eccentric training. Other studies have reported the electromyographic patterns during maximal effort tasks after a chronic eccentric training or a general resistance training regimen (1,10). After eccentric training, greater increases in electromyographic activity were noted during maximal voluntary effort test conditions as compared to the increases in electromyographic activity after concentric training (9,10,19). More recently, Seger and Thorstensson (20) noted no major electromyographic training-effect differences between those doing pure concentric or eccentric resistance exercise; and Michaut et al. (17) suggested neural adaptations mitigated exercise-induced torque loss during eccentric muscle actions.
In this study, we saw a definitive neural response with a decrease in electromyographic intensity after training with the EA subjects, yet no change in the EN subjects, under the submaximal eccentric testing condition. This study, however, was clearly underpowered to detect small differences in the EMG. Therefore, we caution the reader to interpret the nonsignificant findings with an understanding that >800 subjects would have been required to see a statistically significant difference at a power of 0.80 and an α level of 0.05. The descriptive nature of this short report and the inherent limitations of studies with small samples should also serve as a caveat to the reader. Nevertheless, we describe a statistically significant decrease in neural drive occurring during a submaximal eccentric task once subjects become adapted to eccentric muscular work. Others have proposed the presence of a neural adaptation commensurate with the protective effect (16) due to a shift in the muscle fiber recruitment patterns toward nonsusceptible fibers, which could have occurred in this study (8). With that, we also speculate the electromyographic adaptations that occurred were either via a lower level of activation distributed across the entire population of motor neurons, the activation of only a subset of the entire motor neuron population within the VL muscle, or both.
Rigorous strength and conditioning regimens require keen monitoring of the training overload so as to induce skeletal muscle hypertrophy and increases in strength/power while avoiding levels of muscle damage, injury, and soreness that can delay training. The described electromyographic changes in this study followed repeated exposure to eccentric muscle work and may be a component of the protective effect. Practically speaking, high levels of muscle overload in training may be better tolerated if preceded by progressive and repeated exposures, i.e., a ramping up of high forces without muscle injury or soreness via eccentric muscle activity (3,15,18,21). We speculate that shifts in patterns of muscle activation could contribute to the progressive accommodation to high muscle force exercise. As well, we suggest that prior to progressing toward rigorous strength training, clients and patients should become adapted to high eccentric loads and forces to avoid injury and a potential delay in strength and conditioning training regimens.
Supported in part by a grant from the Foundation for Physical Therapy, the ARCs Foundation and National Science Foundation IBN9714731. The authors acknowledge there is no known conflict of interest in presenting this material.
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Keywords:© 2008 National Strength and Conditioning Association
muscle; adaptation; work; overload; electromyogram