emg, force, and power analysis of sprint-specific strength ... · original investigations journal...

ORIGINAL INVESTIGATIONS

JOURNAL OF APPLIED BIOMECHANICS, 1994,10, 1-13 0 1994 by Human Kinetics Publishers, Inc.

EMG, Force, and Power Analysis of Sprint-Specific Strength Exercises

Antti Mero and Paavo V. Komi

This study was undertaken to compare force-time characteristics, muscle power, and electromyographic (EMG) activities of the leg muscles in maximal sprinting and in selected bounding and jumping exercises. Seven male sprinters performed maximal bounding (MB), maximal stepping (MS), maximal hopping with the right (MHR) and left (MHL) legs, and maximal sprint running (MR). These "horizontal" exercises and running were performed on a force platform. EMG activity was telemetered unilaterally from five leg muscles during each trial. The results indicated significant (p < .001) differences among the studied exercises in velocity, stride length, stride rate, flight time, and contact time. Also, significant differences were noticed in reactive forces (p < .01-.001) and power (p < .01) among the performances, whereas only insignificant differences were observed in EMG patterns. The average resultant forces during the braking and propulsion phases in MS, MHR, and MHL were greater (p < .001) than in MR and MB. Stepping and hopping are cyclic and sprint-specific and may be used as strength exercises for sprinters because of great strength demand.

In order to increase their strength levels, sprinters use many kinds of exercises including weight lifting and jumping (bounding) in addition to running. Studies concerning various strength exercises have shown that in weight lifting, for example, forces are great and the time required to produce these forces is very long (e.g., HWinen & Kauhanen, 1986; HWinen, Komi, & Kauhanen, 1987). Some studies have analyzed the characteristics of vertical jumps (see, e-g., Asmussen & Bonde-Petersen, 1974; Bosco & Viitasalo, 1982; Komi & Bosco, 1978; Mero, 1985; Mero & Komi, 1989~). These analyses have concerned force-time characteristics, technical features (e.g., joint angles) and electromyographic (EMG) models. The general trend in these studies is that in vertical jumps the forces are produced at shorter times compared to weight-lifting exercises. However, the contact times in vertical jumps are still 2-4 times longer than in maximal sprinting.

With regard to training for sprinting one could speculate that such exercises should be used where the contact times are very close to those of sprinting and where the forces are also comparable to those in sprinting or higher. Such exercises

Antti Mero and Paavo V. Komi are with the Department of Biology of Physical Activity, University of Jyvaskylii, P.O. Box 35,40351 Jyvaskyla, Finland.

2 Mero and Komi

include runs with light extra loads (Sultanov, 1981) with ranges in contact time from 110 ms to 140 ms, which is still 1.1 to 1.4 times longer compared with maximal sprinting (100 ms; e.g., Mero & Komi, 1985). Also, supramaximal running, where the runner is towed by another runner (Mero & Komi, 1985) or by some apparatus (Mero, Komi, Rusko, & Hirvonen, 1987), has been used. In this type of sprinting, contact time is slightly shorter than in normal maximal sprinting. The forces during the braking phase are greater than in normal sprinting especially in the vertical direction (Mero & Komi, 1989a, 1989b).

Sprinters may also use forward bounding exercises in their training (Sul- tanov, 1981), but no data are available with regard to force-time characteristics and EMG activities of the main leg extensor and flexor muscles. These forward bounding exercises are cyclic jumping exercises, which from a practical point of view seem to have force-time characteristics similar to those of sprint running. Therefore, the purpose of this study was to compare force-time characteristics, power, and EMG activities of the leg muscles in maximal sprinting with the respective variables in each of the three different jumping and bounding exercises. It was expected that this comparison would provide further information about these exercises with practical applications in the training methodology of sprinters.

Methods

Subjects and Tasks

Seven male sprinters served as subjects. Their mean age (LSD) was 23.8 f 1.1 years, height 1.80 f 0.08 m, body mass 74.2 + 7.6 kg, and 100 m record 10.92 + 0.10 s. The subjects performed, in an indoor hall, two maximal runs (MR) and then, in the following order, two maximal bounding (MB) exercises, two maximal stepping (MS) exercises, two maximal hopping exercises with the right leg (MHR), and two maximal hopping exercises with the left leg (MHL). The MR and MB exercises were performed by each subject in such a way that the ball of the foot f i s t made contact with the ground. The MB exercise included a longer stride than that used in running. In both MS and MHs, the heel made first contact. In MS the subject stepped with alternate legs but in MHs either the right or the left leg was used. Figure 1 shows the positions of the foot during the contact phase of the various exercises. All subjects had used these exercises in their training, and in the measurements of the present study they were encouraged to do the exercises "normally" and at a maximum speed.

MAXIMAL STEPPING MAXIMAL BOUNDING MAXIMAL RUNNING MAXIMAL HOPPING

Figure 1 - Foot positions during contact in various performances.

EMG, Force, and Power 3

Data Collection

The performance distance of each task (exercise) was 45 m. The test order (sprinting, bounding, stepping, and hopping) was the same for all subjects. Recov- ery periods between two similar exercises were from 3 to 4 min and between different exercises from 7 to 8 min. All performances occurred on a long (10 x 1.2 m) force platform system placed in the middle of a level and straight indoor runway (35-45 m). The platform was covered with a Tartan mat and the subjects used their normal spiked shoes.

Average constant performance velocity (V) was measured by photocells over 10 m (3545 m) after the acceleration distance of 35 m. Stride length (SL) was measured on a thin paper spread over the Tartan mat (two successive strides). Force-time curves of the horizontal and vertical ground reaction forces (1,000 Hz) were stored on a magnetic tape. The five leg muscles, which are active in sprinting (see, e.g., Mero & Komi, 1987), were selected for EMG collection. The muscle activity from the m. gastrocnemius cap. lat. (GA), biceps femoris cap. long. (BF), gluteus maximus (GM), rectus femoris (RF), and vastus lateralis (VL) was unilaterally recorded by telemetry (Medinik AB Model IC-600-G) using surface electrodes (Beckman miniature skin electrodes). Bipolar (interelectrode distance 20 mm and recording diameter 4 mm) EMG recording was employed. The electrodes were placed longitudinally over the motor point areas of the stronger leg (all subjects chose the right leg) determined by an electrical stimulator (DISA). EMGs were amplified and a band width of 10 Hz to 1 kHz was used. The data were stored simultaneously with the force on a magnetic tape.

Data Analysis

The formula V = SL x SR was used to calculate stride rate (SR). The formula P = F x SL x SR (Luhtanen, Mero, & Bosco, 1989) was used to calculate mechanical power output (P) separately for braking and propulsion phases. F refers to average horizontal reaction force.

The mean force data (expressed as average horizontal, vertical, and resultant forces) for two trials for each exercise of every subject were analyzed. Each trial consisted of two right foot and two left foot contacts in the phase of approximately 3 5 4 5 m. The contact phase was divided into braking and propulsion phases according to the negative horizontal force. EMG signals were rectified and integrated (IEMG) for each time period and expressed for 1 s. The IEMG values were calculated according to the principles shown by Mero and Komi (1987) and presented in the phases of preactivity (50 ms), braking, propulsion, postactivity (50 ms), and minimum (50 ms). The durations of the braking and propulsion phases ranged in the different exercises and are shown in Table 1.

Statistics

A multivariate analysis procedure (MANOVA) (e.g., Monison, 1976) produced the F statistic used to determine the presence of a significant difference among maximal sprinting and sprint-specific strength exercises in measures of stride variables, force and power production, and EMG. Pillai's trace, Hotelling's trace, and Wilks's lambda provided tests of the null hypothesis, acceptable at the p < .O1 level of rejection. The Student's paired t-test used subsequent to the MANOVA

Table 1 Velocities, Stride Lengths, Stride Rates, and Durations of the Various Stride Phases in the Exercises

Variable

Stride Stride Flight Contact Braking Propulsion Velocity length rate phase phase phase phase (m . s-') (m) (Hz) (ms) (ms) (ms) (ms)

Maximal running M SD

Maximal bounding M SD

Maximal stepping M SD

Maximal hopping M right leg SD

Maximal hopping M left leg SD


provided a measure of the significance between each possible pair of exercises. A probability of p < .05 was required for significance.

Results

Stride Variables

The kinematic results of striding are presented in Table 1. Significant differences (p < .001) existed among the studied exercises in measures of velocity, F(4,3) = 3,592; SL, F(4, 3) = 601; SR, F(4, 3) = 2,806; flight time, F(4, 3) = 13,173; contact time, F(4,3) = 14,773; braking time, F(4,3) = 113; and propulsion time, F(4, 3) = 402. Student's t-test indicated that both MR and MB differed clearly (p < .001) from the other three exercises in the kinematic variables. The differences between MR and MB were clear as follows: in velocity (p < .001), in SL (p < .OOl), in SR (p < .001), in flight time (p < .01), and in contact time (p < .01).

Force and Power Production

Table 2 shows the horizontal force and power production of the exercises. Signifi- cant differences existed among the exercises in average force during braking phase, F(4, 3) = 275, p < .001; in average force during propulsion phase, F(4, 3) = 28, p < .01; in force impulse during braking phase, F(4, 3) = 36, p < .01; in force impulse during propulsion phase, F(4, 3) = 83, p < .01; in power during braking phase, F(4,3) = 42, p < .01; and in power during propulsion phase, F(4, 3) = 41, p < .01. When measuring differences between each pair of exercises we noticed that the average braking force was smaller (p < .05) in MB compared with all other exercises. In the propulsion phase the average forces of MR and MB were greater ( p < .05) than the respective values in the other exercises. The braking impulse was smaller (p < .001) in MR and MB than in the other exercises, but in the propulsion impulse there were no differences. The power value related to body weight was very high in MR in both the braking and propulsion phases of the performance, and it was during braking greater (p < .001) than in the other exercises. In the propulsion phase both MR and MB had higher (p < .001) power values than the other exercises.

In the vertical force production (Table 3) there were also significant differences among the exercises as follows: in average force during braking phase, F(4, 3) = 47, p < .01; in average force during propulsion phase, F(4, 3) = 71, p < .01; in force impulse during braking phase, F(4, 3) = 1,603, p < .001; and in force impulse during propulsion phase, F(4, 3) = 519, p < ,001. In MR and MB the forces were very similar but in the other exercises the forces were much greater in the braking phase (p < .001) and in the propulsion phase (p < .05). Vertical force impulse during braking and propulsion was greater (p < .001) in MS, MHR, and MHL compared with running and bounding.

Figure 2 presents the average resultant forces and their directions. The MANOVA showed significant differences (<.001) among the exercises in both phases in average resultant force: braking, F(4, 3) = 8,140; propulsion, F(4, 3) = 2,140. The forces in MS and MHs were in both contact phases greater (p < .001) than in MR and MB. There were no differences in the directions of the forces among the exercises in the braking phase (mean values ranged from 79 to 84"). Whereas in the propulsion phase there were differences in the MANOVA, F(4,

Table 2 Horizontal Reaction Forces, Force Impulses, and Powers of the Various Exercises

Variable Average force (N) Force impulse (Ns) Power (W . kg-')

Braking Propulsion Braking Propulsion Braking Propulsion







Table 3 VerticaPReaction Forces and Force Impulses of the Exercises

Variable Average force (N) Force impulse (Ns)

Braking Propulsion Braking Propulsion






"ody weight is included in vertical forces.

3) = 56, p < .01, and in the pair comparison, the direction of MR (76") and MB (74") was more (p < .01) forward than in the other exercises (84").

EMG

The mean EMGs of the studied muscles are shown in Figures 3-7. In the GA muscle (Figure 3) the peak activity occurred in MR and MB during the braking phase but in the other exercises during the propulsion phase. In MR and MB the peak activities of the RF muscle occurred during the contralateral contact, whereas in the other exercises they occurred during the ipsilateral contact (Figure 7). In the amount of EMG activity there were no differences in any studied muscle among the exercises in the MANOVA.

Discussion

The velocity in maximal running was 14.9% greater than in maximal bounding and 38.9453% greater than in the other exercises. Also SR was the highest in running but SL was, as expected, greater in the studied strength exercises. The contact time was longer in the exercises compared with running according to the following coefficients: 1.19 for MB, 1.81 for MS, and 1.94-1.98 for MHs. The values are smaller than those measured in vertical jumps (e.g., Mero, 1985). This implies that the studied "horizontal" exercises are closer than the "vertical" exercises to the force production time used in sprint running.

The horizontal force impulses during braking were great in the exercises where the first contact on the ground occurred on the heel. When an athlete is running and bounding on the ball of the foot the braking force impulse may be likely reduced by either decreasing the braking force or shortening the braking time. Also the combination of both is possible. In the propulsion phase the horizontal forces were greater in running and bounding and the total force produc-

Mero and Komi

AVERAGE RESULTANT FORCE ( N )

MR = MAXIMAL RUNNING MB = MAXIMAL BOUNDING MS = MAXIMAL STEPPING - MHL MHR= MAXIMAL HOPPING

(RIGHT LEG) MHL= MAXIMAL HOPPING

(LEFT LEG)

BRAKING PHASE PROPULSION PHASE

Figure 2 - Mean values for the average resultant forces and their directions in various performances (MR = maximal running, MS = maximal stepping, MB = maximal bounding, MHR = maximal hopping right leg).

tion was directed more horizontally than in the other performances. In economical sprint running, the runner should produce the smallest horizontal braking force possible and the greatest horizontal propulsive force possible (Mero, Komi, & Gregor, 1992). When the total contact time is short, the power will also be very high. The MB exercise seems to be a specific strength exercise for sprinters compared with the other strength exercises because of its short contact time, great propulsive horizontal force, and consequently high power.

The vertical force during impact was clearly greater in MS and MHs than in running and bounding. The forces were 1.64-1.93 times greater than in running indicating that the exercises may be effective strength exercises in order to develop leg extensor muscles to sustain impact effects (see, e-g., Mero, Komi, & Gregor, 1992).

During the propulsion phase the forces were 1.54-1.82 times greater in MS and MHs compared with running, showing that not only during braking but also during propulsion the average vertical force can be high. The resultant force,

EMG, Force, and Power

- PREACTlVlTY BRAKING PROPULSION POSTACTNITV MINIMUM

IPSILATERAL CONTACT Figure 3 - Mean values for the IEMG activity of the gastrocnemius muscle in various performances (MR = maximal running, MS = maximal stepping, MB = maximal bounding, MHR = maximal hopping right leg).

PREACTlVlN BRAKING PROPULSION POSTACTIVITY MINIMUM

IPSILATERAL CONTACT

Figure 4 - Mean values for the IEMG activity of the vastus lateralis muscle in various performances (MR = maximal running, MS = maximal stepping, MB = maximal bounding, MHR = maximal hopping right leg).

Mero and Komi

PREACTIVITY BRAKING PROPULSION POSTACTIVITY MINIMUM

IPSILATERAL CONTACT

Figure 5 - Mean values for the IEMG activity of the biceps femoris muscle in various performances (MR = maximal running, MS = maximal stepping, MB = maximal bounding, MHR = maximal hopping right leg).

--

PREACTIVITY BFAKING PROPULSION WSTACTIVITY MINIMUM

IPSILATERAL CONTACT Figure 6 - Mean values for the IEMG activity of the gluteus maximus muscle in various performances (MR = maximal running, MS = maximal stepping, MIS = maximal bounding, MHR = maximal hopping right leg).

EMG, Force, and Power

l EMG

1600 (pV. sl

L 1 i . .

PRE BRA PRO POST FLIGHT PRE BRA PRO POST PHASE

CONTRALATERAL IPSILATERAL CONTACT CON TACT

Figure 7 - Mean values for the IEMG activity of the rectus femoris muscle in various performances. (MR = maximal running, MS = maximal stepping, MB = maximal bounding, MHR = maximal hopping right leg).

on the other hand, of the studied exercises was not only great but was directed vertically due to the small horizontal component. The direction of the force production in these exercises is similar to the actual sprinting.

In MR and MB the first contact to the ground occurs on the ball of the foot, and this seems to achieve a different EMG model compared to the other exercises where the first contact occurs on the heel. The MB was the only exercise that simulated the EMG model of actual sprinting. The RF muscle in MR and MB demonstrated peak activities during the contralateral contact when the muscle was acting as a hip flexor. However, the timing of the peak activity of RF occurred in MR in the braking phase, supporting the earlier findings of Mero and Komi (1987), but in MB the peak activity was located in the propulsion phase perhaps due to the longer lever of the leg in MB. The results in MR and MB emphasize the role of the RF muscle in bringing the thigh of the swinging leg forward during the contralateral contact. In the braking phase of sprint running the thigh of the swinging leg is the only segment to produce accelerative forces forward (Mero, Luhtanen, & Komi, 1986).

The programming of strength training for sprinters has the same general principles in many countries (e.g., Bauersfeld & Schroter, 1986; Dintiman, 1984; Levchenko, 1984; Mero, Peltola, & Saarela, 1987). However, it seems that the use of the sprint-specific strength exercises is not well documented in the litera-

12 Mero and Komi

ture. The results of the present study reveal possibilities for training strength of sprinters in a sprint-specific way using horizontal bounding and jumping. This might be especially important at the end of the preparatory phase before the competition period when the objective is to try to train the neuromuscular system to the specific event performance. If the transfer from weight training to fast sprint running occurs too abruptly there are also risks for muscular injuries. The sprint-specific strength exercises may help the sprinter to better train his or her neuromuscular system to perform the actual competitive sprint trials.

In summary, according to the biomechanical analysis there are clear differences in stride variables and in force and power production among the studied exercises (maximal sprinting, bounding, stepping, and hopping), although no differences were observed in EMG patterns of the five leg muscles between the performances. The studied exercises are cyclic and sprint-specific, and especially stepping and hopping may be used as strength exercises for sprinters because of great strength demand.

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