Electromechanical delay (EMD) has been defined as temporal delay between the detected onset of muscular activity and the realization of force. Durations for this offset have ranged from 26 to 131 ms and appear to be affected by fiber type, muscle length, fatigue, training, and the speed and type of movement (5,7,9–13,20,22). EMD presents a challenging problem that must be resolved if valid and meaningful relationships between EMG and force, moment, or movement patterns are to be established.
Some investigators have applied constant EMD values in an effort to temporally align EMG and force- or moment-time profiles during dynamic activities (1,3,15–17,19). Implicit in this treatment, however, is the assumption that EMD is manifest as a constant and continuous temporal offset between EMG and force-time records during both unloaded and loaded conditions. This assumption seemingly contradicts the long-held view that EMD is primarily attributable to “taking up slack” in the series elastic component (SEC) of the musculotendon (MT) actuator as suggested by Hill (6), Cavanagh and Komi (2), Komi (8), and Viitasalo and Komi (20). If EMD were primarily attributable to slack in the SEC, it could be argued that EMD would be manifest only at the beginning of an exertion, and it would be reasonable to suggest that once steady-state tension is developed by the MT, the effects of EMD would be restricted to the short delays associated with depolarization of the muscle fiber and propagation of the action potential.
It is difficult to conclude from the current literature whether EMD exists beyond the initial stage of tension development because most investigations involving EMD have been limited to the development of tension from a resting state. If EMD were primarily attributable to removal of slack from the SEC, then the magnitude of EMD would be expected to decline substantially when tension is developed from a nonresting state (i.e., a state of pretension). To our knowledge, only Schmidt and Stull (18) have provided a description of temporal asynchrony between force and EMG activity when force was developed from a state of pretension. Contrary to expectations, however, their results suggested that EMD (as described by motor reaction time) slightly but significantly increased as the level of pretension was increased. Given the unexpected results of Schmidt and Stull, the lack of supporting data from subsequent studies, the conflicting premises of the proposed SEC origin of EMD, and the use of constant temporal shifts to correct for EMD, further evaluation of the EMD during contractions initiated from nonresting conditions is warranted. Therefore, the primary purpose of this study was to test whether EMD is associated exclusively with the onset of tension from a resting state or whether it is manifest continuously throughout a series of intermittent isometric exertions initiated from various tension levels. A secondary purpose of this study was to test whether the duration of EMD remained constant across different rates of force development.
Twenty-four healthy and recreationally active college students volunteered to participate in the experiment (8 men, mean age 24.4 ± 4.0 yr, mean height 178.8 ± 3.5 cm, mean mass 82.2 ± 13.1 kg; and 16 women, mean age 23.7 ± 6.1 yr, mean height 168.1 ± 5.9 cm, and mean mass 68.3 ± 10.1 kg). All subjects were free of recent injury or dysfunction of the upper extremity and provided written informed consent before testing.
Isometric elbow flexion force data were obtained from a strain gauge force transducer calibrated to the nearest 0.1 N (model SM-250–38, Interface, Inc., Scottsdale, AZ). This transducer was attached at one end to the subject’s wrist using a padded leather cuff and at the other end to an immovable, wall-mounted structure. Isometric force data were sampled at 1000 Hz from an A/D board interfaced to a personal computer. Specialized computer programs were written to provide real-time data display and acquisition.
Surface electromyography was used to quantify the relative levels of muscular activity of the biceps brachii muscle using Ag-AgCl differential surface electrodes (Therapeutics Unlimited Inc., Iowa City, IA; preamplification gain = 35; interelectrode distance = 22 mm; electrode diameter = 8 mm). To minimize electrical impedance at the skin-electrode interface, the skin below the electrode site was shaved, abraded, and cleaned with isopropyl alcohol. The electrode was positioned longitudinally above the approximate midpoint of the distal half of muscle and was secured to the skin with a double-sided adhesive pad and an elastic bandage. A single reference electrode was placed on the lateral malleolus of the right fibula.
EMG data were conditioned with an EMG-544 amplifier/processor module (Therapeutics Unlimited Inc., Iowa City, IA). Amplification gains were selectable from 1,000 to 100,000 within a bandwidth of 20–4000 Hz. Input impedance was greater than 15 MΩ at 100 Hz with typical input bias current on the order of 3 ηA dc. Common-mode rejection ratio was 87 dB at 60 Hz. EMG data were sampled at 1000 Hz from the system previously described for the acquisition of the isometric force data.
Subjects sat on an adjustable-height chair facing a 19-inch computer monitor. Chair height was adjusted to allow subjects to rest their dominant arm on the surface of a table and to complete a series of isometric elbow flexion tasks in the transverse plane. Throughout the experiment, shoulder and elbow joint positions were maintained at approximately 90° of flexion in the sagittal and transverse planes, respectively. All tests were performed using the dominant arm.
After a brief introduction to the experimental procedures, subjects completed three maximum effort elbow flexion trials developing maximum isometric force gradually in a “ramp and hold” manner over a period of approximately 3 s. Isometric force data were collected for 5 s during these maximum effort trials. A minimum of 30 s was allowed between successive trials to minimize the effects of fatigue. The trial yielding the highest average force in a 250 ms region of the force-time record was defined as the maximum isometric elbow flexion force (MF) and was used to establish submaximal intensities corresponding to 25%, 50%, and 75% MF.
Subjects then completed a series of elbow flexion tasks involving intermittent exertions at various intensities. During each trial, subjects used the real-time display to match and maintain a constant baseline force level corresponding to 0%, 25%, or 50% MF. At 1-s intervals during a 10-s period, subjects received a computer-generated audible cue to produce an elbow flexion pulse force to 25%, 50%, or 75% MF. Subjects were instructed to initiate and release these pulse forces as quickly as possible and were able to assess the accuracy of their pulse intensities using the real-time feedback display. Five combinations of the sustained baseline force and intermittent pulse intensity were tested:
- 1) 0% MF baseline (rest), 25% MF pulse force;
- 2) 0% MF baseline (rest), 50% MF pulse force;
- 3) 0% MF baseline (rest), 75% MF pulse force;
- 4) 25% MF baseline, 50% MF pulse force;
- 5) 50% MF baseline, 75% MF pulse force.
In trials involving the 0% baselines, subjects were instructed to relax between intermittent exertions so that only a minimal, albeit necessary, amount of tension was maintained in the strain gauge system. This was accomplished by asking subjects to maintain a consistent arm position throughout the duration of each trial. Trials during which the relaxation process resulted in a gross change in arm position or led to slack in the strain gauge system were not accepted. In trials involving the 25% and 50% MF sustained baseline exertions, subjects were instructed to maintain the required baseline intensity immediately before and immediately after every intermittent pulse force exertion (see Fig. 1). Trials were monitored closely by the investigators using the real-time feedback display, and verbal feedback was provided when necessary.
Several practice trials were required before subjects became proficient in the experimental procedures. Once proficient, subjects performed three trials of each condition and received a minimum of 2-min rest between trials. Conditions were randomized across subjects. EMG and force data were collected from the dominant arm for 10 s during these sustained baseline/intermittent pulse exertion trials.
EMG data were corrected for gain and dc-bias, and were full-wave rectified. To avoid endpoint problems associated with the Butterworth digital filter, force and EMG records were extended by 100 duplicate samples on either side of the original data series. The extended data series were then filtered with a low-pass, second-order, zero-lag Butterworth digital filter operating at a cutoff frequency of 5 Hz. Duplicate points were eliminated before further processing. Filtered force and EMG data were then normalized by their respective instantaneous maximum values.
For each trial, electromechanical delay was quantified by the temporal shift (τ) that maximized the following normalized cross-correlation function (14) :MATH where R xy was the value of the cross-correlation between the EMG and force records at any time shift, τ;T was the length of the EMG and force-time records (10 s);x and y were the EMG and force-time series, respectively;dτ was the interval between adjacent time shifts (0.001 s); and R xx and R yy were the maximum values of the respective autocorrelations of the EMG and force-time series defined at τ = 0.000. The cross-correlation technique to assess EMD was chosen based upon its documented ability to identify EMD magnitude without relying on subjective criteria for defining EMG or force onsets (21).
The effect of baseline intensity on electromechanical delay was tested with a 3 × 3 (condition × trial) repeated measures analysis of variance (ANOVA). EMD values from the three trials of conditions 1, 4, and 5 were entered into this analysis. These conditions were selected because the pulse intensities were of the same relative magnitude (25% MF above the sustained baseline intensity) even though the baseline intensities were different. Selection of these conditions also reduced the potential for confounding effects between pretension level and the rate of force development on EMD magnitude. A separate 3 × 3 (condition × trial) repeated measures ANOVA was used to test for differences in EMD due to differences in the relative pulse intensities. Conditions 1, 2, and 3 represented differences between baseline and pulse intensities of 25%, 50%, and 75%, respectively. These conditions provided data to test the effects of the rate of force development on the magnitude of EMD. Significant differences for the main effect of condition were followed up with pairwise comparisons of the estimated marginal means. Significance levels for main effect and post hoc comparisons were adjusted using the Bonferroni procedure to ensure an overall type-I error rate of 1%.
Across conditions, normalized cross-correlation coefficients were consistently high with mean condition values ranging from 0.93 ± 0.04 to 0.96 ± 0.02. Results from cross-correlation analyses of filtered EMG and force data are shown for a representative subject across the experimental conditions (Fig. 2).
Effect of pretension level on EMD.
When force was increased by 25% MF above three different baseline intensities, no significant condition × trial interaction [F (4,20) = 1.30, P = 0.30] was observed. No differences between trials performed at the same intensity [F (2,22) = 3.68, P = 0.04] were observed, but EMD significantly decreased as baseline intensity increased [F (2,22) = 16.31, P < 0.01] (Fig 3). Post hoc analysis of mean condition values collapsed across trials demonstrated that EMD values for pulse forces developed from the 0% MF baseline intensity (condition 1, 83.5 ± 12.9 ms) were significantly longer than those developed from the 25% MF (condition 4, 66.3 ± 11.5 ms) and 50% MF (condition 5, 60.6 ± 16.6 ms) baseline intensities. EMD values were not statistically different when force was developed from the 25% MF and 50% MF baseline intensity conditions.
Effect of rate of force development on EMD.
When force was increased by 25%, 50%, or 75% MF above a 0% MF baseline intensity, no significant condition × trial interaction [F (4,20) = 2.25, P = 0.11] was observed. EMD decreased as pulse intensity increased [F (2,22) = 12.67, P < 0.01] but was not different between trials [F (2,22) = 2.25, P = 0.13]. Post hoc analysis revealed that the EMD associated with a 25% MF increase in force from a resting state was significantly longer than 50% and 75% MF increases in force developed from rest (condition 2, 70.3 ± 10.0 ms; condition 3, 68.9 ± 8.7 ms). EMD values from conditions 2 and 3 were not statistically different (Fig. 4).
The primary purpose of this study was to determine whether EMD would be present when force production was initiated from a state of pretension. It was argued that if EMD was attributable primarily to removal of slack from the SEC, as first suggested by Cavanagh and Komi (2), then the magnitude of EMD would be expected to decline substantially when tension was developed from nonresting conditions (i.e., states of pretension). However, Schmidt and Stull (18) reported a significant increase in motor reaction time from 31.8 ms with a pretension of 2.2 lb to 37.1 ms with a pretension of 37.4 lb. From a mechanical perspective, this 6-ms increase in motor reaction time contradicts the idea that EMD can be explained by removal of SEC slack.
Surprisingly, Schmidt and Stull (18) published the only study, to our knowledge, that employed a design incorporating development of force from a state of pretension, which offered the potential to investigate the origin of EMD. Their unexpected results may have been related to methodological and technological limitations (e.g., precision of the recording instrument and the analysis techniques employed). The current study used a similar design to that presented by Schmidt and Stull but used advanced recording equipment to monitor force and EMG and used an advanced analysis technique (i.e., cross-correlation) to calculate EMD. Contrary to Schmidt and Stull, we found that EMD decreased by up to 16% when additional force was produced from an initial state of pretension (see Fig. 1). These data support previous contentions that removal of slack from the SEC may account for a proportion of EMD. However, contrary to previous assertions that removal of slack from the elastic components of the MT accounted for the significant proportion of EMD (2,8,20), our data suggest that pretensing the muscle, and thus supposedly removing slack in these elastic structures, resulted in a relatively small decrease in the duration of the EMD. Therefore, the origin of EMD may be only partially explained by the removal of slack from elastic structures in the MT. Ongoing research is being conducted to identify more clearly other potential contributors to EMD.
A secondary purpose of this investigation was to assess EMD when the rate of force development was altered. When force was increased to 50% MF or 75% MF from a resting state over approximately the same time, EMD decreased but remained relatively large (69–70 ms) compared with the EMD recorded when force was raised to 25% MF from a resting state (84 ms). These lower EMD values closely matched EMD values recorded for 25% MF pulse force increases developed from nonresting states (25% and 50% MF).
Our data indicate that EMD was reduced when force was increased from a nonresting state and when the rate of force development increased, further underscoring the point that explaining EMD as an exclusive function of slack removal from the SEC was not completely correct. Viitasalo and Komi (20) recognized the complex nature of EMD and suggested that the duration of EMD was likely related to the rate of force development. Rate of force development depends on the change in force (magnitude) and the time over which this change occurs. Our experimental design held constant the time over which force was developed (1-Hz cadence) and varied the pulse force magnitude. Therefore, these data support Viitasalo and Komi’s suggestion that EMD varies with rate of force development but that the value of EMD approaches a constant value as the rate of force development increases.
The methodology used in this experiment and its effect on the absolute values of EMD deserves comment. Our research question required exertions to be initiated from various nonresting levels. We chose two nonresting levels to test the hypothesis that EMD is associated with “taking up slack” in the SEC. Because the EMG waveform is highly variable, identifying EMD during exertions initiated from nonresting levels was impossible using traditional threshold-based techniques. In most previous investigations (2,4,7–13,18,20), EMD was measured by establishing “thresholds” above which EMG and force onsets were defined. When exertions are initiated from rest, this method is probably suitable, although still potentially biased by the definition of the threshold, because there is usually a clear distinction in the EMG activity between rest and nonrest conditions. However, when EMG data are measured during nonresting levels, the variability of the waveform precludes one’s ability to establish meaningful thresholds. Therefore, we chose the cross-correlation technique and developed an experimental protocol involving a series of intermittent “baseline-to-pulse” exertions to avoid the limitations posed by the traditional threshold-based methods of measuring EMD. The rectified EMG data were filtered at 5 Hz to improve the reliability of the EMD measures extracted from the cross-correlation analysis. This cutoff frequency presented a suitable compromise to avoid oversmoothing, which occurs at lower cutoff frequencies, and undersmoothing, which can reduce between-trial and cross-correlation reliability during higher-intensity exertions. For the sake of comparison, a reanalysis of our data using 3- and 10-Hz filtering is presented in Table 1.
Across conditions, the average difference in EMD between the various cutoff frequencies was 2.47 ms, therefore demonstrating how the absolute values for this parameter need to be considered in light of the specific methodology used. However, because the average difference in EMD was 16.91 ms between conditions that were significant in this experiment (using the 5-Hz filtered data), we conclude that there was a clear and marked treatment effect in the data analysis.
In conclusion, EMD was present and approached a relatively constant value regardless of initial tension levels and rate of force requirements. However, these EMD values were observed under two different scenarios (i.e., nonzero baseline intensities and increasing rates of force development), suggesting that the temporal alignment of EMG and kinetic data is influenced by at least two factors: the state of pretension and the rate of force development. The results of this study would suggest that EMD is present and therefore must be considered during exertions initiated from both resting and nonresting states. Our results also suggest that incorporating a constant temporal offset to align EMG and kinetic data may be reasonable if the actions of interest are performed from nonresting conditions or if the rate of force development is relatively fast. The value of this constant offset should be determined in a manner that seeks to replicate the conditions under which the kinetic data will be collected.
Address for correspondence: Peter F. Vint, Research Integrations, Inc., 9280 S. Kyrene Rd., Suite 101, Tempe, AZ 85284; E-mail: email@example.com.
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