The electrical activity of eight lower limb muscles was monitored with pairs of surface Ag/AgCl electrodes (Blue Sensor; Ambu Ltd., Copenhagen, Denmark) placed on the skin with a 2-cm interelectrode distance. The electrodes were placed longitudinally with respect to the underlying muscle fiber arrangement and located according to recommendations by SENIAM (Surface EMG for the Non-Invasive Assessment of Muscles) (10). Before electrode application, the skin was shaved and cleaned with alcohol to minimize impedance. The wires connected to the electrodes were well secured with tape to avoid movement-induced artifacts. Raw EMG signals were preamplified (with a gain of 375) close to the electrodes, band-pass-filtered (8-500 Hz), amplified (ME6000P16; Mega Electronics, Ltd., Kuopio, Finland), and analog-to-digital converted at a sampling rate of 1 kHz. The data logger of the ME6000 biomonitor recorded the signal of the FSR sensors, allowing temporal synchronization with EMG signals.
Raw EMG data were high-pass-filtered at 20 Hz using a dual-pass, fourth-order Butterworth filter to eliminate possible movement artifacts. Muscular activity was quantified using the EMG signal recorded during the different phases of the stride. This signal was full-wave-rectified and the root mean square (RMS) was computed with a 20-ms moving window. The EMG of each muscle was measured at 250 m for INC and 300 m for LEV running. From these recordings, five strides were analyzed (from the 10th to the 15th stride after the start of the exercise, which corresponded to the passage of the runner in the shot of the camera). The data from these five strides were averaged to obtain a mean RMS envelope for each muscle (Fig. 3). Then, the RMS values of the contact and flight phases were calculated for each muscle. During the contact phase, the RMS of braking and push-off phases was calculated. Furthermore, the RMS of monoarticular muscles (SOL and VL) was also calculated for concentric and eccentric phases of the contact, which were determined from the changes in knee and hip joint angles obtained from video analysis. Lastly, integrated electromyography (iEMG) was also calculated for each of the aforementioned conditions.
Descriptive statistics were calculated for the selected kinematic, foot motion pattern, and EMG values. All data are presented as mean ± SD. After a normality test, a Wilcoxon test was used to test the main effect of running condition on each parameter. All significant differences reported are at P ≤ 0.05 unless otherwise noted.
Running velocity and step kinematics.
Subjects reached a significantly lower velocity in the INC condition (7.56 ± 0.41 vs 6.28 ± 0.38 m·s−1). This lower velocity was associated with significantly lower SR and SL in the INC condition than in the LEV condition (Table 1). The absolute T c was longer during the INC condition compared to LEV condition, which was associated with an increase in push-off time (+26.4%; Table 1). T f was not different between INC and LEV.
All the results concerning kinematics are presented in Table 1. Compared to LEV, INC induced several significant (P ≤ 0.05) changes in stance kinematics: a lower FFP, a lower FHF, a higher knee flexion at foot strike, and a higher hip flexion when the foot is directly below the hip (Table 1). In other words, lower limbs were more flexed during contact in INC running. When focusing on the braking phase only, the range of motion (ROM) of the knee in the INC condition is smaller than in the LEV condition. However, during the push-off phase, no difference was observed in the ROM between INC and LEV conditions (Table 1).
EMG activity over the five averaged strides.
The EMG signal of GM could not be analyzed because too much noise was recorded on this specific channel for a part of the subjects. The analysis of the five averaged strides showed that no difference was observed in RMS or iEMG for all the muscles studied.
EMG activity during contact and flight phases.
A lower RMS value was found for BF and ST during the contact phase in the INC compared to the LEV condition (Fig. 4A). However, iEMG of both muscles was not significantly lower in the INC condition. During the flight phase, the RMS of BF and ST was not significantly different between LEV and INC running. Only the RMS of VL was significantly lower in the INC condition (Fig. 4B). The iEMG of the VL was also significantly lower (35.2 ± 30.7 vs 25.7 ± 21.8; P < 0.05).
EMG activity during braking and push-off phases.
The analysis of the braking and push-off phases during contact showed no significant difference in the RMS between LEV and INC conditions for all muscles (Fig. 4C, D). Only the RMS of BF during push-off was slightly (but not significantly) lower in INC running (261.0 ± 199.7 vs 167.4 ± 143.9 mV, P = 0.08; Fig. 4D). No significant differences were observed in iEMG during these phases.
EMG activity during concentric and eccentric phases (monoarticular muscles).
During the concentric phase of the VL, the RMS was lower in INC than in LEV running condition (57.9 ± 62.0 vs 70.2 ± 65.1 mV; P ≤ 0.05). A similar tendency was observed for the SOL (163.8 ± 47.8 vs 237.1 ± 86.5 mV; P = 0.08).
The results of this study showed that INC sprint running induced a decrease in running velocity caused by a decrease in both SL and SR (14.2% and 7.4%, respectively) and a 26.4% increase in push-off time. Kinematic parameters were also modified: the knee angle at foot strike, the hip angle when the foot is directly below the hip, and the ROM of the knee and the ankle during contact were reduced in INC running. In other words, lower limbs were more flexed during INC running. However, the variation in kinematics did not induce large modifications of the foot path diagram. Concerning muscular activation, the major findings concern the decrease in the RMS of ST and BF during contact and the decrease in that of VL during its concentric phase in INC sprint running. However, the level of neuromuscular activity of the other muscles studied remained similar between LEV and INC.
Our data showed that, compared to level running, uphill sprint running induced a 4% reduction in knee angle at foot strike, a 15% reduction in hip angle when the foot is directly below the hip, and 38% and 17% reductions in the ROM of the knee and of the ankle, respectively, during the braking phase. It should be noted that these data were obtained from a limited data sample because only one complete stride cycle could be analyzed for each condition (LEV and INC). That said, these results are similar to those obtained by Paradisis and Cooke (19) who have demonstrated a decrease in the trunk, shank, and thigh angles during the braking phase. In the latter study, these modifications induced a significant 22.5% decrease in foot strike position relative to the vertical position of the hip (FSP in our study) during uphill sprint running. Contrary to these authors, we did not observe any significant decrease in FSP despite the decrease in both hip and knee angles. This discrepancy can be attributed to the method of FSP estimation used by Paradisis and Cooke who computed the distance between the foot and a line perpendicular to the running surface, which was the vertical projection of the center of mass at touchdown and not that of the theoretical middle of the hip (as defined in the present study, i.e., the great trochanter).
Furthermore, the 17% increase in T c during INC running observed in the present study is much larger than that of the 3% previously reported by Paradisis and Cooke (19). This difference could be attributed to the lower slope they used (3% vs 5.4% in the present study). The lower slope used by Paradisis and Cooke induced a lower decrease in running velocity during INC than that observed in the present study. Indeed, with T c being inversely related to the running velocity (5), one could have expected an increase in T c with decreasing velocity. Thus, a slight change in the slope seems to have a great influence on both running velocity and T c. Therefore, it seems that coaches and athletes have to be very careful when choosing the slope during INC sprint running training sessions.
On the field, sprinting on incline surfaces is often used as a form of specific strength and power training that is considered by coaches to provide a highly specific forward sprint training load, more specific than that reached through other forms of resistance exercise such as weight training. Considering the decrease in maximal running velocity and the increase in T c observed in the present study, it is not obvious that a positive transfer may occur from INC sprint training to horizontal LEV sprint performance. An interesting result is the increase in push-off time in the INC condition although the SL was shorter than in the LEV condition. Indeed, the higher relative part of the propulsive phase during contact in uphill sprinting and the increase in resistance associated with the slope may overload the muscles, which contributes to the force production during push-off.
In contrast with the latter data, the decrease in RMS for BF and ST during the contact phase associated with the decrease in RMS for VL during its concentric phase did not support the aforementioned hypothesis of an increase in muscular loading during the push-off phase. Aiming to compare similar velocities during INC and LEV running, Gottschall and Kram (8) have shown that the normal (i.e., in the direction perpendicular to the ground) impact force and the parallel (i.e., in the direction parallel to the ground) braking impulse are lower and that the parallel propulsive impulse is higher during INC running. This increase was associated with a longer push-off time and a higher peak propulsive parallel force. As a consequence, the increase in propulsive parallel impulse allowed subjects to maintain a similar velocity during INC and LEV running. However, in our study, the fact that running velocity decreased during INC versus LEV running could be associated with a decrease in propulsive parallel impulse (8). Hence, with push-off time being longer for an equivalent impulse, it seems reasonable to hypothesize that the peak and/or mean propulsive parallel force applied to the ground is lower during INC sprint running. As a consequence, the decrease in RMS of BF and ST during the contact phase may be partly associated with this decrease in propulsive parallel force. However, the increase in push-off time could also lead to an increase in the time of activation of BF and ST. This is confirmed by the stability of iEMG of both muscles during contact and push-off phases. Thus, during INC running, BF and ST could be less activated (as shown by the lower RMS values obtained during the contact phase) but for a longer time. That being said, gluteus muscles and the other heads of the hamstrings have to be considered to fully complete this comparison. Future studies could also examine the level and timing of the activation of the other lower limb muscles.
Considering the different phases of the stride, the decrease in muscular activity of BF and ST during the contact phase is not in accordance with previous studies showing a similar or higher activation of the hip extensor muscles during INC running (21-23). These discrepancies could be explained by the different running velocities reached. As previously mentioned, the running velocities reported in these studies were set at the same value during LEV and INC running. In the present study, which was based on all-out efforts, the lower maximal running velocity during INC (compared to LEV) could explain the decrease in muscular activity of BF and ST (9,12,13,16).
During the stride cycle, VL is activated concentrically at the end of the flight phase and eccentrically at the beginning of the contact phase (9,12,16). The major function of VL during running is to extend the knee during the terminal swing and to possibly stabilize the patella (16). Thus, the activity of VL before maximal front leg extension has been demonstrated to decrease as a function of running velocity (12,16). The present results confirm this observation, showing that, during its concentric phase, the RMS of VL decreases by 35% in the INC condition. Moreover, the decrease in VL activity could be associated with the decrease in front leg extension and the lower knee angle at foot strike during INC running. All the aforementioned differences in kinematics and muscular activity levels seem to be more directly related to running velocity than to incline. As a matter of fact, other studies should be undertaken to better understand the relationships between running kinematics, muscular activity, running velocity, and incline and to provide evidence-based arguments to figure out whether the changes observed are due to the slope of the terrain and/or to the changes in running velocity it induces and to what extent.
That said, it is interesting to note that despite the decrease in running velocity in the INC condition, the muscular activity of all other muscles remained similar to that measured in LEV sprint running, particularly concerning GA that has been shown to be more activated during the push-off phase of LEV running (14,15).
Why are INC sprint training sessions used?
Contrary to our hypothesis, the neuromuscular activity of some of the lower limb muscles recorded was lower in the INC condition than in the LEV sprint running. From a neuromuscular activity point of view, the increase in slope during a sprint running session did not compensate for the decrease in running velocity induced. At that point, a question remains to be answered: what is the rationale for coaches to use INC sprint training sessions?
Different hypotheses can be made considering the different adaptations occurring during INC running. An explanation can be found in the mechanical similarity between INC running and the acceleration phase of a sprint (6,7). Indeed, INC running induced an increased push-off time; an adaptation also observed during the acceleration phase of a sprint (15). Thus, it can be put forward that the increase in push-off time could be beneficial to performance during the acceleration phase of the sprint start, in that the longer time available for force application and hence high positive impulses production might lead to their increase at each step (at least during the onset of the sprint). A second explanation can be found in the decrease of activity of the hamstrings. Indeed, this decrease may have, for consequence, to protect the runner from hamstrings injuries. The high prevalence of hamstrings strains in sprinters traumatology is well documented and high-velocity running is particularly associated with a risk of hamstrings injury (1,2). These authors demonstrated that all subjects of a group of 18 elite sprinters had been injured during competitive sprinting and that the primary injuries were all in the long head of BF muscle. Hamstrings injuries are often detrimental to the athlete, causing prolonged absence from competition and high rates of injury recurrence. Thus, it can be hypothesized that INC sprint running makes a maximal commitment of the runner possible, concomitantly inducing a lower level of activity of the hamstrings than that observed during LEV sprinting. On the one hand, that could lead to a decrease in the constraints applied to the hamstrings during training and hence partly prevent injuries. On the other hand, it can also be assumed that this decrease in activity of the hamstrings does not prepare this muscular group for the solicitation induced by LEV maximal running and especially during competitions. Therefore, INC sprint training sessions could be coupled with LEV sprint training sessions to better prepare hamstrings to be maximally activated. Another explanation could be found regarding the amount of work. The decrease in the activity of the hamstrings associated with the decrease in vertical impact force (8) could allow a greater amount of training load than in LEV running. To confirm this last hypothesis, the effect of fatigue during INC sprint running remains to be investigated (personal data in preparation).
In conclusion, INC sprint running induced a decrease in running velocity and led to numerous modifications of the running pattern. Step kinematics were modified, with decreasing SL and SR and increased push-off time, and the lower limbs were more flexed during INC running. Concerning muscular activation, the major findings concerned the whole decrease in the RMS for ST and BF during the contact phase and in the RMS for VL during its concentric phase. Hamstrings muscles were less activated but for a longer time during contact. These results suggest that INC sprint running could be beneficial to performance of the specific acceleration phase of a sprint start and to help reduce the level of activation of the hamstrings during maximal sprint session.
The authors are grateful to the French Athletics Federation and the French Ministry of Sport for their financial support.
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Keywords:©2008The American College of Sports Medicine
MAXIMAL VELOCITY RUNNING; KINEMATICS; EMG; ATHLETICS TRAINING