Neuromuscular characteristics and muscle power as... : Medicine & Science in Sports & Exercise (original) (raw)
Many previous studies have indicated that maximal oxygen uptake (V˙O2max), lactate and ventilatory thresholds, and running economy are important determinants of distance running performance (5,6,8,26,33,39). However, many coaches have observed that some endurance athletes are unable to perform well in a given sport event although their oxygen transport and utilization capacity is high, and it has further raised the question what the athletes should do to improve their performance capacity.
Although endurance sport events require high aerobic power, the athletes must also be able to maintain a relatively high velocity over the course of a race. This emphasizes the role of neuromuscular characteristics that are related to voluntary and reflex neural activation, muscle force and elasticity, and running mechanics (18), as well as the role of anaerobic characteristics in elite endurance athletes. Bulbulian et al. (4) and Houmard et al. (15) have shown that anaerobic characteristics can differentiate well-trained endurance athletes according to their distance running performance. Some "taper" studies (e.g., 17,36) have found an improvement in endurance performance with reduced training without changes in V˙O2max. Strength training (e.g., 13,22,24,31) has been shown to improve endurance performance without any changes in V˙O2max. All these data suggest that also neuromuscular and anaerobic characteristics are important for endurance performance.
Noakes (27) and some other researchers (e.g., 12) have suggested that endurance performance may not only be limited by central factors related to oxygen uptake but also by so called muscle power factors affected by an interaction of neuromuscular and anaerobic characteristics. In the present study, muscle power is defined as an ability of the neuromuscular system to produce power during maximal exercise when glycolytic and oxidative energy production is high and muscle contractility may be limited. However, relatively few studies have investigated the significance of neuromuscular characteristics or muscle power as determinants of endurance performance. In contrast to the aerobic system, the muscle power generated that cannot be accounted for by oxygen consumption is a difficult metabolic construct to measure (35), and there are no generally accepted methods to measure or evaluate the muscle power of endurance athletes. Peak velocity reached during the V˙O2max treadmill running test has been shown to be a good predictor of endurance performance in middle- and long-distance running events (e.g., 16,28). Noakes (27) has suggested that peak treadmill running velocity could also be used as a measure of muscle power in endurance runners. However, the aerobic processes (e.g., V˙O2max) are very important for peak treadmill running velocity during the V˙O2max test (e.g., 14) although neuromuscular and anaerobic characteristics may also be involved.
Peak velocity (VMART) in the maximal anaerobic running test (MART) has been shown to correlate with 100-m, 400-m, 800-m, and 1500-m time on the track and with cross-country ski performance (34,35), as well as with maximal accumulated oxygen deficit (23), blood lactate concentration in the MART (peak BlaMART), and maximal 30-m running velocity (e.g., 30). Vuorimaa et al. (41) and Nummela et al. (30) have further found that sprinters and middle-distance runners have significantly higher VMART than long-distance runners. Consequently, Rusko and Nummela (35) have surmised that VMART is influenced both by the anaerobic power and capacity and by neuromuscular characteristics without the influence of V˙O2max and that VMART could be used as an indicator of muscle power especially in sprinters.
The purposes of the present study were to investigate 1) the importance of neuromuscular characteristics and muscle power as determinants of 5-km running performance in male endurance athletes and 2) whether VMART can be used as a measure of muscle power in endurance athletes.
METHODS
Subjects. The participants were 17 elite male orienteers (cross-country running in the forest and overground). The basis for selection of the subjects was that their physiological endurance characteristics during an incremental treadmill running test to exhaustion as well as their training background were as homogeneous as possible. Their age and years of training ranged between 19-30 yr (24 ± 4) and 5-15 yr (9 ± 3), respectively, and their height, body mass, and percentage of body fat were 181.3 ± 5.8 cm, 70.6 ± 6.1 kg, and 9.3 ± 1.9%, respectively (mean ± SD). Informed consent was obtained from all the subjects, and the study was approved by the Ethics Committee of the University of Jyväskylä, Jyväskylä, Finland. The subjects were familiarized with treadmill and track running before the measurements.
Treadmill measurements. The measurements were done during two days (Table 1). On the first day, after the anthropometric measurements were obtained, the subjects performed three running tests on the treadmill: a submaximal running economy test (REtread), a maximal aerobic power test, and a maximal anaerobic running test (MART) (34). REtread was measured as a steady-state oxygen uptake (V˙O2, U mL·kg−0.75·min−1) (38) during the last min of submaximal 5-min runs at the velocity of 3.67 (REtread1) and 4.17 (REtread2) m·s−1. V˙O2 was measured for every 30-s period using a portable telemetric oxygen analyzer (Cosmed K2, Rome, Italy) (7).
Chronological presentation of the measurements performed on day 1 and 2.
V˙O2max and respiratory compensation threshold (RCT) were determined during the maximal aerobic power test, which was an incremental test to exhaustion. For the warm-up the initial velocity and inclination were 2.22 m·s−1 and 1°, respectively. The velocity was increased by 0.28 m·s−1 after every 3-min stage until the velocity of 4.75 m·s−1 except that two velocities increased by 0.56 m·s−1 in the middle of the test. After completion of the 3-min stage at 4.75 m·s−1, the velocity was kept constant but the inclination was increased by 1° every min until exhaustion. Fingertip blood samples were taken after each stage during a 20-s stop of the treadmill between stages to determine blood lactate concentrations (Bla) by an enzymatic-electrode method (EBIO 6666, Eppendorf-Netheler-Hinz GmbH, Hamburg, Germany). Ventilation (V˙E) and V˙O2 were measured using the portable telemetric oxygen analyzer (Cosmed K2) for every 30-s period. The RCT was determined as the velocity of the stage being just below the point in which the FEO2 (%) distinctly increased, FECO2 (%) distinctly decreased and the linearity of the V˙E and V˙E/V˙CO2 curve disappeared (2,32,37). Maximal oxygen uptake (Treadmill V˙O2max) was taken as the highest mean of two consecutive 30-s V˙O2 value. Because the inclination of the treadmill was increased and the velocity was kept constant during the last stages of the test, peak treadmill running performance was calculated not as the peak velocity but as the peak oxygen demand of running (V˙O2max demand) from the time of the last completed 1-min run and from the exhaustion time of the following faster run using the formula of the American College of Sports Medicine (1): (Equation) where v = the speed of the treadmill and grade = the slope of the treadmill expressed as the tangent of the treadmill angle with the horizontal.
Twenty minutes after the maximal aerobic power test, the subjects performed the MART, which consisted of a series of 20-s runs on a treadmill with a 100-s recovery between the runs. A 5-s acceleration phase was included in the 100-s recovery. The first run was performed at the velocity of 3.71 m·s−1 at a 4° grade. The velocity of the treadmill was increased by 0.35 m·s−1 for each consecutive run until exhaustion. The velocity of the first run as well as the increase in velocity between the runs were selected so that blood lactate concentration would not increase over 3 mmol·L−1 after the first run and so that exhaustion would be attained within 12 runs. Exhaustion in the MART was determined as the time when the subject could no longer run at the speed of the treadmill and fell off. A harness connected to an emergency break was used to ensure the safety of the subjects. VMART was determined from the velocity of the last completed 20-s run and from the exhaustion time of the following faster run so that each additional 2 s after 10 s running increased the VMART by 1/6 of the velocity increase between the runs (34). Fingertip blood samples were taken at rest, 40 s after each run, and 2.5 and 5 min after exhaustion to determine the highest blood lactate concentration in the MART (peak BlaMART). Blood samples were analyzed using the Eppendorf (EBIO 6666) lactate analyzer.
Track measurements. On the second day, the subjects ran three tests on a 200-m indoor track: a maximal 20-m speed (V20m) test, a submaximal running economy (REtrack) test, and a 5-km time trial (5K) (Table 1). After a 20-min warm-up, the subjects performed three maximal 20-m runs with the running start of 30 m. The runs were separated by a brief recovery. Twenty-meter running time was measured using two photocell gates (Newtest Inc., Oulu, Finland) connected to an electronic timer, and the highest velocity of 20-m runs (V20m) was recorded. The runs were performed over a special 9.4-m-long force platform system, which consisted of five two-dimensional (2D) and three 3D force plates (0.9 × 1.0 m each, TR Test Inc., Finland, natural frequency in the vertical direction 170 Hz) and one Kistler 3D force platform (0.9 × 0.9 m, Honeycomb, Kistler, Switzerland, 400 Hz) connected in series and covered with a tartan mat. Each force plate registered both vertical (Fz) and horizontal (Fy) components of the ground reaction force. Fz, Fy, and contact times (CT) were recorded by a microcomputer (Toshiba T3200 SX) using an AT Codas A/D converter card (Dataq Instruments, Inc., Akron, OH) with a sampling frequency of 500 Hz. Stride rates (SR) were calculated by using CT and flight times (FT) [1/(CT + FT)]and stride lengths by using velocity and SR (V/SR). Each 20-m run included two to three contacts of the right leg on the force platform system. The horizontal force-time curve was used to separate the contact time and both Fz and Fy force components into the braking and the propulsion phases (25). The integrals of both force-time curves were calculated and divided by the respective time period to obtain the average force for the whole contact phase and for braking and propulsion phases, separately.
Ten minutes after the V20m test, the subjects performed the REtrack test (2 × 5 min) at the same velocities as on the treadmill. The velocity of the runs was guided by a lamp speed control system ("light rabbit," Protom, Naakka Inc., Finland). Oxygen uptake during the runs was measured using the portable telemetric oxygen analyzer (Cosmed K2), and track running economy was calculated as a steady-state V˙O2 (mL·kg−0.75·min−1) during the last minute of running at 3.67 m·s−1 (REtrack1) and 4.17 m·s−1 (REtrack2).
Ten minutes after the REtrack test, each subject performed alone the 5-km time trial (5K) on the 200-m indoor track. The split times were given to athletes during the 5K. Fingertip blood samples for the determination of the highest blood lactate concentration (peak Bla5K) were taken at rest, immediately after the 5K, and after 1, 3, and 5 min of recovery. The mean velocity of the 5K (V5K) was calculated. At the beginning of the 5K and after 2.5 and 4 km, the subjects ran one 200-m lap at the same constant velocity of 4.55 m·s−1 through the photocell gates and over the force platforms for the determination of the same force and stride parameters as in the V20m test. The velocity of the constant velocity laps was guided by the lamp speed control system (Protom).
Statistical methods. Means, standard deviations, coefficients of variation and Pearson correlation coefficients were calculated using normal parametric statistics. In addition, a stepwise multiple linear regression analysis was used to predict 5K (V5K) and peak treadmill running (V˙O2max demand) performance. The independent variables (RCT, REtrack2, and VMART) that correlated most significantly with the V5K and V˙O2max demand were entered into a stepwise procedure.
RESULTS
As shown in Table 2, the subjects of the present study were relatively homogeneous with regard to aerobic power and economy variables (V˙O2max, RCT, REtrack, and REtread), as well as muscle power (VMART) and neuromuscular variables (V20m, CT and SR in maximal 20-m run, and CT in constant velocity laps of 5K). The subjects were also closely clustered in their 5K track running time and treadmill running performance (V˙O2max demand) (Table 2).
Mean, standard deviation, and coefficients of variation (CV) of performance characteristics during the track and treadmill tests.
Correlation coefficients of the selected aerobic power and running economy variables with the distance and treadmill running performance are seen in Table 3. Significant correlation was observed between V5K and V˙O2max demand. Both V5K and V˙O2max demand correlated with V˙O2max, RCT, REtread, and REtrack(Table 3). No correlation was observed between V5K and Bla5K.
Correlation coefficients for the selected aerobic power and running economy variables and the distance and treadmill running performance.
Correlation coefficients of selected neuromuscular characteristics and VMART with the distance and treadmill running performance are seen in Table 4. Both V5K and V˙O2max demand correlated with VMART, but only V5K correlated with V20m, CT, and SR in the maximal 20-m run and the mean CT of the constant velocity laps during the 5K (Table 4). Both V5K and V20m correlated with the duration of ground contact braking phase in the maximal 20-m run (r = −0.54, P < 0.05 and r = −0.62, P < 0.01, respectively). Furthermore, REtrack2 correlated with the mean CT of constant velocity laps during the 5K (r = 0.64, P < 0.001). No correlations were observed between V5K and ground reaction forces of the maximal 20-m run or the constant velocity laps. VMART correlated with peak BlaMART (r = 0.59, P < 0.05), V20m (r = 0.87, P < 0.001), and CT in the maximal 20-m run (r = −0.61, P < 0.01) but not with V˙O2max. Examples of individual data are presented in Tables 5-7.
Correlation coefficients for the selected neuromuscular and muscle power variables and the distance and treadmill running performance.
The performance characteristics of two well trained subjects, A and B, with different 5-km run times and subjects C and D who had similar 5-km run times.
The selected 20-m maximal run stride parameters and ground reaction forces of two well trained subjects A and B with different 5-km run times.
The selected 5-km mean CVL run stride parameters and ground reaction forces of two well trained subjects A and B with different 5-km run times.
The stepwise multiple regression analyses using V5K and V˙O2max demand as the dependent variables showed that the combination of RCT (r2 = 0.55), REtrack2 (r2 = 0.70), and VMART (r2 = 0.85) as independent variables accounted for 85% of the variance in V5K (P < 0.01) whereas RCT (r2 = 0.65) and REtrack2 (r2 = 0.83) accounted for 83% of the variance in V˙O2max demand (P < 0.01).
DISCUSSION
The present study showed clearly that, in addition to aerobic power and running economy, the neuromuscular characteristics were also related to the 5-km running performance. Although no correlation was observed between V5K and ground reaction forces, the relationships of V5K with the CT and SR of maximal 20-m run as well as with the mean CT of the constant velocity laps during the 5K suggest that the ability to fast force production during maximal and submaximal running was related to the 5-km running performance.
There are several explanations for the differences in ground contact times between the athletes. The differences in velocity affect the contact times and stride rates (19). However, factors other than velocity must have influenced the biomechanical parameters measured during the constant velocity laps in the present study. High stride rates and short ground contact times, especially in the braking phase, have been found to influence the sprint running performance (e.g., 21,25). Williams (42) has found that also during submaximal running ground contact times differ between a rearfoot and a midfoot striker. Some other kinematic or anthropometric factors (e.g., 42) might also explain the differences in the contact times between athletes. Although the present endurance athletes were relatively homogeneous with regard to physiological and running performance characteristics, their muscle fiber compositions might be different and could explain the differences in the neuromuscular characteristics (e.g., 11). Viitasalo and Komi (40) have shown that the force-time characteristics differ between subjects with different composition of fast- and slow-twitch fibers in their muscles. Force output of muscle contraction is also known to depend on the rate and force of myofibrillar cross-bridge cycle activity and effective storage and release of elastic energy during stretch-shortening cycle exercises (e.g., 3,10).
In addition to the neuromuscular characteristics, anaerobic power and capacity may also be important for distance running performance in well-trained endurance athletes (4,15). Moreover, Noakes (27) and some other researchers (e.g., 12) have suggested that endurance performance and V˙O2max may be limited, not only by central factors related to oxygen transport and utilization, but also by the so-called muscle power factor that is related to neuromuscular and anaerobic characteristics. The previous studies have shown that during fatigued conditions an increased H+ ion concentration that is related to the increased blood lactate concentration impairs the contractile properties of the muscles (e.g., 20). During middle-distance running and uphill cross-country skiing, energy expenditure may exceed maximal aerobic power, and the athletes must be able to maintain a relatively high velocity over the course of a race although their muscle and blood lactate concentrations are high (9,29, see also 35). This further emphasizes the importance of muscle power factor (the ability of the neuromuscular system to produce power during maximal exercise when glycolytic and oxidative energy production is high and muscle contractility may be limited) in endurance sports (35). However, relatively few studies have investigated the muscle power as the determinant of endurance performance, and there is no generally accepted method to measure the muscle power in endurance athletes.
Noakes (27) has suggested that the peak velocity sustained for 1 min at the end of an incremental treadmill running test to exhaustion could be used as a measure of the muscle power of distance runners. In the present study, the correlation between V5K and V˙O2max demand indicates that peak treadmill running performance is a good predictor of track running performance, which is in agreement with some previous studies (e.g., 16,28). As could be expected, the results of the correlation and regression analyses showed that aerobic power and economy parameters predominated as determinants of V˙O2max demand. This supports the previous suggestions (e.g., 14) that although neuromuscular and anaerobic characteristics may also be involved in peak treadmill running performance, the role of aerobic processes is very important. Consequently, it has been suggested (35) that VMART, which is influenced both by anaerobic power and capacity and by neuromuscular characteristics and not by aerobic power factors, could be used as a measure of muscle power at least in sprinters. In the present study, neuromuscular characteristics and VMART were strongly related to the V5K, and VMART correlated significantly with peak BlaMART, CT in the maximal 20-m run, and with V20m. These data suggest that interaction of neuromuscular and anaerobic characteristics contribute to VMART. In contrast to peak treadmill running velocity or V˙O2max demand, VMART was not related to V˙O2max or other aerobic power variables. Therefore, VMART is suggested to be a better measure of muscle power of endurance athletes than peak treadmill running performance although the biomechanical, neuromuscular, and anaerobic characteristics influencing VMART are not yet fully understood.
Examples of individual data also illustrate the importance of the neuromuscular characteristics and muscle power for distance running performance in homogeneous endurance athletes who have attained a high maximal aerobic power (Tables 5-7). There were no differences in V˙O2max or RCT between subjects A and B, but subject B had better running economy and VMART as well as better V20m and other neuromuscular characteristics that could explain his faster 5-km running time (Tables 5-7). Another example (Table 5) shows that individual performance may be associated with a different combination of aerobic power, running economy, neuromuscular characteristics, and muscle power. Subject C could have achieved his 5-km running time with the combination of higher V˙O2max and RCT than subject D, whereas subject D had relatively low aerobic capacity but better running economy, V20m, and VMART compared with subject C. Although the running economy did not correlate significantly with VMART or V20m, the individual examples above, together with significant correlation between the REtrack2 and the mean CT of CVL during the 5K, suggest that improving the running economy by increasing neuromuscular characteristics cannot be excluded as partial contribution to the running performance.
In conclusion, in addition to aerobic power and running economy, neuromuscular characteristics and muscle power were also related to the 5-km running performance in well-trained endurance athletes who were relatively homogeneous with regard to physiological and running performance characteristics. The results of the present study suggested that interaction of neuromuscular and anaerobic characteristics contribute to VMART and that VMART can be used as an indicator of muscle power in endurance athletes.
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Keywords:
DISTANCE RUNNING PERFORMANCE; MART; RUNNING ECONOMY; V˙O2MAX; RESPIRATORY COMPENSATION THRESHOLD; ENDURANCE ATHLETES
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