It is generally acknowledged that the force-length muscle relationship is not linear, i.e., there is an optimum roughly corresponding to the rest length. Some studies (10,13) have shown that the triceps surae produce more torque when they are at 15° of dorsal flexion during isometric contraction. In addition, during pliometric contractions, the increase of length of the muscle-tendon complex before its shortening changes the mechanicals characteristics in comparison to a concentric contraction. A dorsal flexion exercise performed before a plantar flexion exercise enhances the maximal power (17) and the muscular output (1), compared to plantar flexion without dorsal flexion. However, the mechanisms responsible for the enhancement are still controversial (16). Notably, it has been suggested that during counter movements, active muscles are prestretched and store energy, which is later partly recoiled when muscles contract concentrically (8).
These results have led us to study the influence of shoes with dorsiflexion on performance in physical activity, particularly during vertical jumps (4,9,10). The term dorsiflexion will be defined in this study as any shoe design or external attachment that lowers the heel in relation to the fore-foot. For instance, Larkins and Snabb (10) found a significant effect resulting in an increase in jump height (vertical jumps and jumps with 3-step approach run) at any of the dorsiflexion angles tested. Additionally, their results showed an optimal degree of foot inclination around 3°-4°of dorsiflexion.
Nevertheless, the reasons of this enhancement are unknown. For example, it is not clear at this time why placing the foot in dorsal flexion improves jump height (4,9,10): are the triceps surae able to generate greater force as a result of stored elastic energy; or does this dorsiflexion increase the foot's contact time with the ground, thus causing the jumper to generate greater impulse; or does this foot position induce an increase on a muscular activity, which increases the jumping performance; or is some combination of the above explanations operating? Stefanyshyn and Nigg (15) suggested some explanations for minimizing the loss of energy in sport shoes, notably reducing shoes mass, increasing cushioning, reducing vibrations, increasing stability, and reducing shoe bending. The last point was incurred by reducing the flexion at the metatarso-phalangeal joint by positioning stiff carbon fiber plates, which reduced the energy lost during both running and jumping movements.
Few studies have investigated electromyographic (EMG) activity during fitness exercises like squat, step, lunge, etc. To the best of our knowledge, only 2 studies were dedicated to this purpose. Zimmermann et al. (18) studied the effects of stair-stepping exercises on muscular activity of selected lower extremity muscle groups. Isear et al. (5) analyzed EMG of lower extremity muscle recruitment patterns during an unloaded squat. Nevertheless, no studies have examined the effects of wearing dorsiflexion shoes on muscle activity during these fitness exercises.
The purpose of the present study was then to compare EMG activity of nine lower limb muscles during: (i) exercises usually performed during fitness classes, and (ii) during submaximal running and walking on a treadmill, with 4 different shoes: 2 pairs inducing moderate dorsiflexion and decreasing metatarsus flexion (Springboost shoes), standard fitness shoes, and shoes inducing severe dorsiflexion and decreasing metatarsus flexion (Meridian). The chosen exercises included squats, front and side steps, lunges, plantar flexion, and running and walking on a treadmill.
Experimental Approach to the Problem
This study was designed to examine the effect of specific shoes on muscular activity during fitness exercises and locomotion. Subjects were tested wearing four different types of shoes: Standard fitness shoes (Reebok Revent Mid DMX, around 4° plantar flexion), Special 2° (Springboost B-Fit 2° dorsiflexion), Special 4° (Springboost B-Fit 4° dorsiflexion), and Special 10° (Meridian 10° dorsiflexion). The first 3 types of shoes are alike in terms of mass, while the Meridian shoe is 60% heavier.
Meridian shoes (Figure 1) have a curvature placed in the middle of the sole. This design was specially projected to decrease the metatarsus flexion, which could decrease the energy lost during the stance phase (15). In order to decrease the metatarsus flexion Special shoes (2° and 4°) have a TPU plate in the midsole (Figure 1).
Twelve healthy female subjects (age, 24 ± 4 years; height, 167 ± 6 cm; weight, 60 ± 7 kg) who practiced fitness biweekly, took part in this study. None of the women had a history of neuromuscular disease. All were familiar with the fitness exercises performed in the study (see below). The participants were fully informed of the procedure and the risks involved in this study and gave their written consent. They were also allowed to withdraw from the study at will. The women realized all the tests conditions in one day.
After a 5-minute warm-up on a cyclo-ergometer at a power chosen by the subjects, EMG data were measured during isometric maximal voluntary contractions (MVC) of the following right lower limb muscles: gluteus maximus (Glut), vastus lateralis (VL), vastus medialis (VM), rectus femoris (RF), biceps femoris (BF), tibialis anterior (TA), gastrocnemius lateralis (GL), gastrocnemius medialis (GM), and soleus (Sol). Several studies have evaluated the EMG muscle activity of the medial and lateral hamstring separately but have not shown significant differences between the 2 groups (12). In order to avoid sensor vibration, women wore nylon hosiery.
For MVC measurements, five contractions were asked of the subjects: knee extension (KE), knee flexion (KF), plantar flexion (PF), dorsiflexion (DF) and hip extension (HE). Thirty seconds of rest were allowed between each trial.
Knee extensor (VL, VM, and RF) MVCs were obtained while subjects were in the squat position with shoulders under a rigid bar. The subjects were asked to attempt to lift up the bar. Biceps femoris MVC was obtained during knee flexion, as the subjects lay down on the ground. The hip was positioned at 0° and the knee flexed 60°. The lower leg was externally rotated. Women attempted flexion of the lower leg. Plantar flexors (GM, GL, and Sol) were obtained while the subjects stood, fixed on the ground, with fully extended knee and women attempted plantar flexion. During tibialis anterior MVC testing, the subjects stood erect and attempted dorsiflexion with the foot fixed. MVC for the gluteus were performed with the women stand over a barrel and attempted extension of the hip.
Then, the subjects performed 7 exercise conditions: squats (sq) at an intensity corresponding to a normal fitness class; steps (ste); submaximal ballistic plantar flexion (bpf); unilateral lunge exercise (lun); side step (sist); treadmill running (run); and treadmill walking (wal), at 10 km·h−1 and 4.5 km·h−1, respectively.
During side and front step, ballistic plantar flexion, and lunge exercises, the women were permitted to perform as many warm-up repetitions as needed until they demonstrated proper technique in cadence given by a metronome. During the lunge exercise, the metronome was set such that one phase of this exercise (i.e., eccentric and concentric) was performed at 0.83 Hz. During the ballistic plantar flexion exercise, the metronome was set such that one complete movement (i.e., eccentric plus concentric) was performed at 1.65 Hz. For the side and front step exercises, the metronome was set such that one complete movement (i.e., step up and step down) was performed at 0.625 Hz.
Each exercise condition was separated by a 30-second rest. For each exercise condition, 4 different types of shoes were tested: Standard fitness shoe, Special 2° dorsiflexion, Special 4° dorsiflexion, and Special 10° dorsiflexion. Special shoes (2°, 4°, and 10°) increase dorsiflexion and decrease metatarsus flexion. Each shoe condition testing was separated by a 3-minute rest. The order of shoes was randomized.
The EMG signal of the nine muscles quoted above were recorded using bipolar silver chloride surface electrodes during MVC and all exercises and shoes conditions. The recording electrodes (Control Graphique Medical, Brie-Comte-Robert, France) were fixed lengthwise over the muscle belly with an interelectrode distance of 25 mm. The reference electrodes were attached to the patella. Low impedance (<5 kΩ) at the skin-electrode surface was obtained by abrading the skin with emery paper and cleaning with alcohol. Myoelectrical signals were amplified and simultaneously digitized online (sampling frequency 2000 Hz).
The characteristics of the acquisition system were: Model: EISA 16.4; CCMB: 90 dB; Input impedance: 200 GΩ; Combined gain: 1000 to 10,000. Stored data were then recorded to a microcomputer with a National Instrument acquisition card (DAQcard-6062E, 16 input/2 output, 500 Ks/s, 12-bits Multifunction I/O, National Instrument Austin, TX) and analyzed with Imago software (locally developed using Labview).
The root mean square (RMS) values were calculated during the MVC trials over a 0.5-second period (RMSmax) as follows:
In order to compare the difference between shoes and muscular activation, EMG readings were normalized to EMG during maximal isometric contraction (2,3,6,5,18). The reason for using this method was to compare different muscle activation in fitness exercises, walking, and running, as well as to make comparisons between shoes. So, all RMS data measured during the fitness exercise and locomotion were presented as a percentage of RMSmax.
Because the duration of movement was not always the same, integrated EMG (iEMG) was also calculated as follows:
In order to characterize the different phases of the 6 exercises conditions (squat, step, lunge, side step, walk, and run), a goniometer was placed on the right knee and synchronized with the EMG (Figure 2). The locally developed goniometer was composed with two sections, an industrial potentiometer (model 157, Vishay Spectrol, Singapore) with ±0.25% of precision, and a rigid bar fixed on the rotary part of the potentiometer. For the ballistic plantar flexion, the EMG signal was cut from the gastrocnemius medialis muscular activity. For the step, side step, and walking exercises, the concentric phase of the movement was also analyzed separately. In addition, the eccentric and concentric phase and swing phase of the step during running was analyzed.
Descriptive statistics are presented as mean values ± standard deviation (SD). Each EMG value was compared between shoe conditions using a one-way analysis of variance (ANOVA) with repeated measures. Fisher's PLSD post-hoc tests were applied to determine between-means differences when the analysis of variance revealed a significant main effect. A p value of ≤0.05 was accepted as the level of statistical significance.
For clarity, the exercises were pooled as follows: (i) submaximal fitness exercises: squat (Sq), step (step), lunge (lun), side step (sist); (ii) exercise focused on plantar flexion: submaximal ballistic plantar flexion (bpf); and (iii) locomotion: walking (walk), running (run).
Also for clarity, since similar trends were found for VM, VL, and RF on the one hand, and GM, GL, and Sol on the other hand, these muscles were pooled on knee extensors (KE) and plantar flexors (PF), respectively.
Submaximal Fitness Exercises
During the fitness exercises, there was a general trend toward a higher tibialis anterior activity with shoes increasing dorsiflexion and decreasing metatarsus flexion (Table 1). For the whole movement (Table 1) and concentric phase (Figure 3) of the squat exercise, knee extensor activity was significantly lower for shoes with large dorsiflexion (Special 10°). However, since there was a tendency toward a longer duration of the movement with the Special 10° shoes, there was no significant difference between shoes in terms of iEMG (data not shown).
During the lunge exercise, there was a tendency toward higher plantar flexors activity for Special shoes 2 and 4° (Table 1). No differences were detected between shoes for gluteus and for any submaximal fitness exercises.
Exercise focused on plantar flexion
This exercise was dedicated to specifically focus on the leg muscles, i.e., triceps surae and tibialis anterior. Surprisingly, Special shoes (2° and 4°) induced higher activity for knee flexor and knee extensors in comparison to standard fitness shoes (Table 2).
As expected, differences were also noted for the triceps surae and tibialis anterior muscles. For the tibialis anterior muscle, activation was significantly lower for the Standard shoes to compare with the three other pairs. For the plantar flexor muscles, shoes with large dorsiflexion (Special 10°) induced a lower activation than both Special (2° and 4°) and Standard shoes. A tendency was observed toward higher activation with Special shoes (2° and 4°) to compare with the Standard condition (Table 1).
During the locomotion, there was a clear increase of plantar flexors activity when the dorsiflexion increased. This higher activation during running as dorsiflexion increased is particularly true during the eccentric phase (Table 3). Similar results were found for iEMG data but less marked (data not shown).
As shown in Figure 4, there was a difference of activation toward lower RMS for Special shoes (2 and 4°) for knee extensors during running. During walking, Special 4° (0.18 mV·s−1 ± 0.04) induced lower iEMG compared to the 3 other pairs of shoes (Standard: 0.20 mV·s−1 ± 0.05, p < 0.05; Special 10°: 0.20 mV·s−1 ± 0.05, p < 0.05; Special 2°: 0.19 mV·s−1 ± 0.05 p < 0.05). The differences that existed for RMS during running were not significant any more when considering the iEMG values for knee extensors.
During both walking and running, there was a nonsignificant tendency toward a higher RMS as dorsiflexion increases for gluteus (data not show). This was particularly true for the concentric phase of running (Standard: 35%RMSmax versus Special 4°: 42% RMSmax, p = 0.11). No differences were noted among shoes for biceps femoris for both RMS and iEMG values.
The main results of the present study are: (i) shoes increasing dorsiflexion and decreasing metatarsus flexion reorganize the motor pattern during fitness exercises, walking, and running; (ii) these shoes increase tibialis anterior activity during all exercises tested here; and (iii) an optimal dorsiflexion exists for some exercises.
Isear et al. (5) analyzed EMG of lower extremity muscle recruitment patterns during an unloaded squat. Values on the concentric phase records in this study were similar to our data with standard fitness shoes for the biceps femoris, vastus medialis, vastus lateralis, and gastrocnemius. However, differences between theses 2 studies existed for the gluteus (14% versus 27% for Isear et al. and the present study, respectively) and rectus femoris (21% versus 48%) and for the average value of all muscles (23.5% versus 28.5%). The difference noted in the motor pattern could be explained by the exercise position: our subjects kept the hands on their hips whereas the subjects of Isear et al. (5) had the arms positioned in the horizontal. The balancing role of arms could influence the recruitment patterns.
The shoes tested in the present study reorganize the motor pattern but this reorganization differs according to fitness exercises. For instance, it was observed that the use of these shoes tended to be stable in the knee extensor activity during step and lunge exercise but to decrease during squats.
In the present experiment, the tibialis anterior activity increased with Special shoes (2°, 4°, and 10°). This increase could be explained by the balancing role of these muscles; indeed, one can presume that dorsiflexion shoes activate this muscle causing the body to readjust its center of gravity, inducing a greater activity on this dorsal flexor.
The present results also showed that an optimal dorsiflexion exists for specific exercises. For instance, muscular activation increased for tibialis anterior activity for the basic submaximal plantar flexion or for the squat at moderate dorsiflexion (Special 2° and 4°) and then decreased for the Special 10°. Similarly, plantar flexors and knee extensors tended to show an optimum for moderate dorsiflexion shoes for the lunge and the squat exercise, respectively. It can be speculated that the reciprocal intervention of inhibition and activation due to peripheral afferences (mainly muscles spindles and Golgi tendon organs) can explain this latter observation.
Data of the submaximal fitness exercises showed that Special shoes tend to increase muscular activity of plantar flexors during lunge, knee extensors during ballistic plantar flexion, and knee flexors for all of these exercises.
During walking and running, when metatarsus flexion decreased and dorsiflexion in the shoe increased, there was a clear indication of an increase in the triceps surae RMS. Interestingly, this higher muscular activation when running in dorsiflexion was particularly noted during the eccentric phase (Table 3). As shown in this table, the triceps surae activation is only significantly different between Standard fitness shoes and the three other pairs during the eccentric phase. This could be an important point regarding the economy of running because this would probably lead to higher leg stiffness (11,14). However, this hypothesis still has to be tested.
However, the plantar flexor EMG activity during the concentric phase was not significantly modified. It can be speculated that the running velocity tested here (10 km·h−1) was too low to really allow the differences noted during the eccentric phase to play a role in the modification of the muscular recruitment during the concentric phase. Higher running velocities must be performed in future experiments.
Data of the locomotion were particularly important; indeed, there was a decrease of knee extensor RMS with Special shoes (2° and 4°) and a tendency toward a higher RMS for gluteus. Thus, the Special shoes reorganized the motor pattern during locomotion too.
In conclusion, the present results show that Special shoes affect muscle recruitment and reorganize the motor pattern. As a consequence, these shoes can activate lower limb muscles differently than standard shoes during both fitness exercises and locomotion. However, this modification of coordination depends on the type of exercises studied. The only general tendency was that the tibialis anterior activity increased with Special shoes. The present results also show that, during submaximal fitness exercises, there sometimes exists an optimal dorsiflexion. Future research should focus on testing: (i) the influence of shoes with moderate dorsiflexion on lower limbs muscular activity during running at different velocities; and (ii) the influence of theses shoes on performance and energy cost of locomotion and fitness exercises. Thus, the combination of EMG data with the measurement of oxygen consumption and forces during jumping and running would facilitate a more complete analysis of the effects of this shoes.
Since Special shoes increase the muscular activity of tibialis anterior, this could be an interesting point because this muscle is not often solicited, although it is necessary in training to exercise both agonist and antagonist muscles. Thus, these shoes could help to balance the development of calf muscles and dorsiflexor muscles.
Also, Special shoes reorganize the motor pattern, so these shoes could be used in training programs in order to work differently, particularly to increase the stimulation of the calf muscles. The combination of training with Special shoes and Standard shoes could represent a good compromise to stimulate the lower limbs differently.
This study was supported by a grant from Springboost SA (Lausanne, Switzerland), a manufacturer who can benefit from the results of the present study. The authors wish to express their gratitude to the Center of Analysis Sport and Health to have made available the necessary material to perform this study.
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Keywords:© 2008 National Strength and Conditioning Association
EMG; locomotion; training