Running can be considered as a series of repeated bounds that uses the stretch-shortening cycle (SSC) (13). During an SSC, the muscle lengthens, acting eccentrically, before a concentric, shortening action. In a nonfatigued state, this eccentric-concentric coupling results in an increased muscular output during the concentric phase compared to a pure concentric action. Endurance running is essentially a series of repeated submaximal SSCs. Repeated submaximal SSCs cause a decrease in tolerance to stretch loads, evidenced by progressive increases in contact time during braking and push-off phases (16,24,33). Golhoffer et al. (16) further suggested this reduced tolerance resulted specifically from a lower muscle activation in the eccentric phase of the SSC, evidenced by a reduction in electromyography (EMG) values.
A number of studies (1,8,17,18,39) have shown the energetic cost of running at a fixed speed to increase with fatigue, that is, toward the end of a prolonged run. Abe et al. (1) used the eccentric: concentric ratio from EMG as a proxy measure of the ability to use stored elastic energy during SSCs. They found a strong negative relationship (r = −0.702) between increases in oxygen cost during a 90-minute run and changes in the eccentric: concentric ratio. Heise et al. (22) specifically identified the EMG activity of biarticular muscles during the swing phase of running gait, where they are acting eccentrically, had a strong positive relationship with running economy (RE; r = 0.73). Hayes et al (19) examined the role of muscular strength endurance (MSE) in the biarticular hamstrings during a run to exhaustion at the speed at V̇O2max (sV̇O2max). They found that biomechanical changes during the run were inversely related to MSE of the hamstrings as either knee flexors (KFs) or hip extensors (HEs).
To date, little has been published on the impact of MSE upon RE. Westblad et al. (38) found significant but modest correlations (r = 0.50-0.58) between eccentric knee extensor endurance measured isokinetically and RE. The changes in RE that occur during a prolonged run might be related to MSE. The purpose of this study was to investigate the role of MSE on fatigue-induced changes in RE. We hypothesized that any increase in RE would be related to the eccentric MSE of the KFs and HEs.
Experimental Approach to the Problem
Participants completed a total of 4 laboratory sessions: test (a) an incremental treadmill test to determine the speed at lactate threshold (sLT) and lactate turnpoint (sLTP), V̇O2max, and speed at V̇O2max (sV̇O2max); tests (b,c) 2 steady-state runs of equal energy expenditure to assess RE in a fatigued (high intensity exercise [HIE]) and nonfatigued (CON) state; test (d) isokinetic assessment of MSE. The order of tests 2-4 was randomized.
Within the HIE run, there was a 4-minute block at sV̇O2. The purpose of this block was to fatigue but not exhaust the runners. From this, we were able to identify any increase in RE because of fatigue. The hypothesis was tested by examining the relationship between the increase in RE resulting from fatigue and MSE of KF and HE. The RE should be measured during steady-state conditions (14). To ensure this was satisfied, we set the steady running speed in the moderate exercise domain. This was achieved by setting the speed at 20%Δ below V̇O2max.
After institutional ethical approval, 10 well-trained male runners (see Table 1) gave written informed consent to participate in the study. All participants were middle or long distance runners involved in regular training and club competition (>2 years) at the time of the study. Their training consisted primarily of continuous running, with the inclusion of 1 or 2 interval training sessions per week. These runners competed across a range of distances, throughout the year. They were homogenous with respect to V̇O2max (coefficient of variation [CV] = 8.7%), sV̇O2 (CV = 5.6%), and RE measured during the nonfatigued conditions (CON1, CON2, CON3, HIE1) (CV = 6.9%).
Before each visit, participants were instructed to arrive in a fully rested and hydrated state, having consumed no food in the 3 hours before testing, nor having performed any strenuous exercise in the previous 48 hours. Participants were instructed to wear the same clothing and running shoes for each visit. Testing took place at approximately the same time of the day (±2 hours) to minimize diurnal biological variation. Before each test, participants stature (m) and body mass (kg) were recorded. Body mass was recorded wearing shoes to account for the effect of the additional mass on RE. Before each test, participants completed the same warm-up comprising 5-10 minutes of self-paced jogging and stretching.
All running tests were completed on an h/p/cosmos pulsar treadmill (model 3p 4.0, H/P Cosmos Quasar Medical treadmill, Nussdorf-Traunstein, Germany). Expired air was averaged over 60-second periods using a Cardio-Pulmonary Exercise System (Cardio-Kinetics CP-X, Medgraphics Corporation, St. Paul, MN, USA). The gas analysis system was calibrated before each run according to the manufacturer's instructions. Heart rate (HR) was recorded using telemetric HR monitors with a transmitter strapped to the chest and a receiver attached to the treadmill (Polar Electro, Kempele, Finland). Blood lactate concentration was determined from 25-μL samples of capillary blood collected in heparinized capillary tubes. The blood samples were subsequently analyzed for lactate concentration using an Analox P-GM7 Micro-stat (Analox instruments Ltd., London, United Kingdom) automated analyzer, calibrated before use with an 8-mM standard.
Speed at Lactate Threshold and Lactate Turnpoint and V̇O2max
Participants completed a multistage, incremental speed treadmill test to determine sLT and sLTP, followed by an incremental gradient test to establish V̇O2max (35). After warming up, participants completed 6-8 submaximal stages of 4-minute duration with running speed increments of 1 km·h−1 (9). A 1% gradient was used throughout to mimic running outdoors (26). At the end of each stage, participants stood astride the treadmill belt while a finger tip capillary blood sample (∼25 μL) was collected. Participants commenced running within ∼15-20 seconds. The test was terminated after LTP had been passed. In the last 90 seconds of each stage expired air was collected, and HR was recorded during the last 30 seconds.
After termination of the protocol, sLT and sLTP were determined from plots of blood lactate against running speed. The sLT was defined as the final running speed before the first sustained increase in blood lactate. The sLTP was defined as the final running speed before a second sudden and sustained increase in blood lactate. After a 15-minute-recovery, participants completed a 5-minute warm-up at a speed 2 km·h−1 below sLTP. Participants began the V̇O2max test at the speed corresponding to sLTP; the gradient was set at a 0% and increased by 1% min−1 until volitional exhaustion. Expired air was collected continuously, and HR noted at each stage. Participants were given verbal encouragement throughout. V̇O2max was determined using the primary criteria of a plateau in V̇O2, defined as a change in V̇O2 of <± 2.1 ml·kg−1·min−1 with a concomitant increase in exercise intensity. Jones and Doust (26) showed that max measured after a sub-maximal test was not significantly different from max measured on a subsequent day. The speed at V̇O2max was determined by linear extrapolation of the submaximal V̇O2-running speed relationship.
Running economy was assessed in both a nonfatigued state (CON) and after a bout of HIE designed to induce a fatigued but not exhausted state (37). The duration of the 2 RE bouts was equated by energy expenditure determined using the Weir equation (31). The CON run was a continuous 30-minute run at the speed corresponding to 20%Δ below sLTP (20). The HIE run included a 4-minute period at the speed corresponding to sV̇O2max to induce fatigue. Before and after this 4-minute block was an equal duration of running at the speed used during the CON condition. These durations varied in length between participants, but for each participant, the total energy expenditure of CON and HIE was equal.
During HIE, expired air was collected for the 2 minutes immediately before the 4-minute run at sV̇O2max (HIE1), and for the last 2 minutes of the steady-state run after the high-intensity block (HIE2). Within CON, RE was assessed at the corresponding points in time to HIE1 (CON1), HIE2 (CON2), and also the corresponding point of energy expenditure (CON3) to identify whether RE was stable throughout CON (Figure 1). The CON2 and CON3 trials were recorded at similar points in time. Given the stability of our RE scores, we have used the CON2 as the second CON measure because it corresponded to the same point in time as HIE2.
We used Hopkins spreadsheet for reliability (www.sportsci.org/resource/stats/index.html) to compare CON2 and CON3. The mean change was −0.7 ml·kg−1·km−1 (confidence interval [CI]90% = −1.7 to 0.3 ml·kg−1·km−1; technical error of measurement [TEM] = 1.2 ml·kg−1·km−1) or −0.3% (CI90% = −0.8 to 0.2%; TEM = 0.6%). Saunders (personal communication Saunders, P. June 17, 2009.) reported TEM values for RE, recorded at the Australian Institute of Sport, ranging from 1.23 ml·kg−1·km−1 at 14 km·h−1 to 1.60 ml·kg−1·km−1 at 18 km·h−1.
Muscular Strength Endurance
All tests were conducted using a Cybex Norm dynamometer (Phoenix Healthcare, Nottingham, United Kingdom), which was calibrated before each test using the manufacturer's instructions. All participants conducted a familiarization program at least 48 hours before testing. The MSEs of the hip and knee flexors and extensors were assessed through the work done (WD) during 2 sets of 20 maximal eccentric actions at 180°·s−1. Eccentric muscle actions were assessed because of the relationship previously shown between the MSE of these muscle groups and kinematic changes during a run to exhaustion at sV̇O2 (19). In our laboratory, the reliability of the isokinetic endurance task has an intraclass correlation coefficient of r = 0.85 and a CV 7.4%.
Before the isokinetic measurements, participants ran for 10 minutes at a self-selected speed and completed 3 submaximal followed by 3 maximal efforts on the Cybex at test velocity. Two minutes' rest was given between each set, with 6 minutes of active recovery allocated between each joint.
Changes in Running Economy
To identify whether RE changed because of the HIE intervention, we adopted the approach recommended by Hopkins (23) and compared the SD of the pre-post changes for both conditions (ΔCON and ΔHIE) as an absolute change (ml·kg−1·km−1), a relative change (%), and an effect size using the method of Cohen (10). Hopkins (23) also recommended comparing the TEM of each condition to show the ‘error free’ magnitude of change as an SD by using
. Batterham and Hopkins (3) magnitude based inferences were also calculated.
Relationship between Running Economy and Muscular Strength Endurance
The change in RE for each condition (ΔCON and ΔHIE) was calculated along with the global change in RE (ΔRE) calculated as the difference in ΔCON and ΔHIE. Relationships between ΔRE and MSE were determined using Pearson correlations and also partial correlations. Previously Billat et al. (6) have shown that in a sample of runners homogeneous for V̇O2max (CV = 6.0%), there was considerable interindividual variation in time that sV̇O2max could be sustained on 2 separate occasions (run 1 CV = 25.0%, range 262-598 seconds; run 2 CV = 28.1%, range 295-632 seconds). The fatiguing task in this study was a 4-minute bout at sV̇O2max. The further exercise is conducted above the boundary marking the start of the severe exercise domain, the greater the increase in lactate (34) and the shorter the time to exhaustion (32). For this reason, we used partial correlations to account for (LTP as a % V̇O2max.) the extent to which the 4-minute run at sV̇O2max exceeded the boundary between the heavy and severe domains.
Descriptive statistics are represented as mean (SD). Statistical significance was set at p ≤ 0.05. Correlations and partial correlations were conducted using SPSS (version 16, SPSS Inc., Chicago, IL, USA).
Changes in Running Economy
The relationship between RE in HIE1 and HIE 2 is shown in Figure 2; this highlights the increase in RE after the 4-minute run at sV̇O2max. Changes in RE were evident as ΔCON increased by 2.2 ± 3.6 ml·kg−1·km−1 (1.1 ± 1.8%) while ΔHIE increased by 6.0 ± 4.4 ml·kg−1·km−1 (3.0 ± 2.2%) (Figure 3). The greater change in the SD of the differences during the HIE condition (ΔCON ± 3.6 ml·kg−1·km−1 or ± 1.8% vs. ΔHIE ± 4.4 ml·kg−1·km−1 or ± 2.2%) is indicative that HIE did induce an increase in RE. This is further supported by the effect size of these changes (ES = 0.94) and the ‘error free’ magnitude of change (+ 2.5 ml·kg−1·km−1 or + 1.3%). The qualitative inference for ΔCON was an 80% likelihood that the change was trivial, however ΔHIE was 96% very likely to be a positive increase.
Relationship between Δ Running Economy and Muscular Strength Endurance
The bivariate and partial correlations between ΔRE and MSE of the KFs working eccentrically (KFecc) ranged from (r = −0.567 to r = −0.798; p = 0.088-0.010). The use of partial correlations strengthened the relationships with ΔRE (Table 2). Additionally, when MSE was mass corrected, some of the relationships showed a modest strengthening.
No significant partial correlations were found between ΔRE and MSE of the HEs working eccentrically (HEecc) in set 1 (r = −0.526; p = 0.146), set 2 (r = −0.476; p = 0.195) and both sets combined (r = −0.598; p = 0.089). Similarly to KFecc, these relationships strengthened slightly when MSE was expressed relative to body mass to become (r = −0.598; p = 0.089), (r = −0.565; p = 0.113) for sets 1 and 2, respectively. There was no effect when the combined total of sets 1 and 2 was expressed relative to body mass (r = −0.511; p = 0.160). The partial and bivariate correlations for HEecc variables were of similar strength.
The main findings of this study were RE deteriorated after a bout of high speed running when compared with a control condition. This change in RE was strongly related to the eccentric MSE of the KFs, particularly when controlling for LTP as a % V̇O2max. The partial correlations strengthened all of the relationships between the change in RE and isokinetic measures for the KFs but not for HEs.
The magnitude of ΔRE after 4 minutes running at sV̇O2max suggests that this intervention successfully fatigued, but did not exhaust, our runners. Both the quantitative values (error-free magnitude of change, effect size, and comparison of SD of differences), and qualitative based inferences suggest that a real change in RE took place. This is in line with previous studies that found increases in RE ranging from approximately 4% after 90 minutes running at 65% V̇O2max (39) to 13% after a marathon (18,29). Our change in RE is similar to exercise at 65% V̇O2max but not surprisingly smaller than the change postmarathon. These previous studies involved running in the moderate to heavy exercise domain, but unlike our study, they were often to exhaustion. Given the intensity of our intervention, it would seem that RE deteriorates as athletes fatigue regardless of the exercise domain.
To date, there has been very little research examining the relationship between MSE and fatigue in endurance running. A number of possible explanations for the relationships between ΔRE and eccentric MSE of KF exist. Gazeau et al. (15) suggested that the capacity of the KFs to act as a brake, that is, work eccentrically, was critical in maintaining stride mechanics during a run to exhaustion at sV̇O2max. Subsequently, we (19) demonstrated that the changes in stride mechanics were strongly related to WD isokinetically by the KF and HEs acting eccentrically. Kyrolainen et al. (28) found a relationship (r = 0.48; p < 0.05) between oxygen consumption and EMG of the bicep femoris during the braking phase of the running cycle. Collectively, these findings suggest that the KF also play a role in regulating RE as runners begin to fatigue. In contrast, to our previous work HEecc measures did not show significant relationships with RE. Biewener et al. (4) examined the joint moment at both the knee and hip across a range of movement speeds from walking to fast running. Knee joint moment increased markedly on the transition from walk to a slow run, whereas hip joint moment increased steadily as movement speed increased. In this study, the speed of movement was in the moderate exercise domain, and most likely, the hamstrings were affected more by knee joint moments than hip joint moments. This might explain why KF rather than HE was strongly related to changes in RE.
The cause of the change in RE with fatigue has not been clearly identified. Several studies (11,12,30) have found a reduction in voluntary force production postexercise. Lepers et al. (30) found that eccentric actions were effected more than concentric actions. Our data would seem to support this because the runners completing more work during the isokinetic test experienced the smallest gains in RE. One possible explanation could be that during the isokinetic test, those completing more work eccentrically were better able to withstand repeated eccentric actions during running without detrimental effects; they were more fatigue resistant. Alternatively, it might be that during the 4 minutes at sV̇O2max, these runners were less able to tolerate the higher intensity SSCs, or at least the eccentric component. The relationship between this and the ability to withstand the eccentric actions during prolonged lower intensity running requires further research. Previous research (25,40) has shown that not only plyometric training in particular, but also resistance training, can result in improved RE. This may be because of improved eccentric capability.
The inability to maintain stretch tolerance has been found after low-intensity long duration exercise and may be related to our hypothesis that KFecc reflects a form of fatigue resistance. Avela et al. (2) and Nicol et al. (33) found decreases in stretch-resistive force and reflex sensitivity, whereas ground contact time and oxygen consumption increased postmarathon. The inability to maintain stretch tolerance was thought to increase knee flexion angle during ground contact thereby increasing contact time, reducing elastic energy use and increasing oxygen cost. During repeated submaximal SSCs, the eccentric component seems to be more affected than concentric action (24). This is thought to be because of either muscle damage; reduced central drive (27); the stimulation of type III or IV receptors through biochemical changes, most likely acidosis, resulting in α motorneuron inhibition (5); or reduced γ loop support causing disfacilitation of α motorneurons (7). How performance in the KFecc test might reflect greater fatigue resistance through these hypothesized mechanisms is however unknown and warrants further research.
It is possible that greater MSE could limit the extent of any change in muscle recruitment strategy. During submaximal running, any reduction in contractile function would presumably be offset by a change in muscle fiber recruitment pattern, with the possibility of less metabolically efficient type II fibers being recruited. In support of this is the work of Abe et al. (1) who found a decrease in the eccentric: concentric (ecc:con ratio) integrated elecrotmyography (iEMG) ratio after 90 minutes of running, because of an increase in iEMG during the concentric phase. The change in the ecc:con had a strong negative relationship with RE (r = −0.702). During a 30-minute submaximal run, greater coactivation of biarticular leg muscles was related to better RE (21). Furthermore, fatigue has been shown to change muscle coordination strategy (36).
We found RE increased because of fatigue induced by a 4-minute run at sV̇O2max. When interindividual differences in submaximal aerobic fitness were accounted for, these changes were strongly related to the WD during eccentric isokinetic KF measures. Greater MSE appears to limit the extent of the increase in RE. From the data collected, it is not possible to determine the underlying causal mechanism(s). Previous studies on fatigue during repeated submaximal SSCs suggest an inability to maintain stretch tolerance. We hypothesize that this ability to maintain stretch tolerance is in some way related to MSE. Further research is required to investigate this relationship.
The results of this study indicate that eccentric strength expression may have a potential regulatory role in determining running performance. Of note, eccentric muscular endurance of the KFs attenuates fatigue-induced increases in RE in well-trained runners; represented by enhanced fatigue resistance during steady-state trials. On the basis of these findings, it appears that conditioning work, which focuses on augmenting eccentric muscular endurance of the legs may offer beneficial adaptations that promote fatigue resistance.
Our results suggest that coaches and athletes could effectively implement conditioning strategies that challenge eccentric muscle actions. These strategies include plyometrics, resistance training with an emphasis on eccentric portion of repetitions, down-hill running and over-speed training. The findings of this study suggest that any such investment in these methodologies would be a valuable addition to the training portfolio of the middle or long distance runner, because they would serve to enhance RE and ultimately delay the early onset of fatigue mechanisms.
There was no external body funding for this study.
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