Ankle strength, endurance, and stability are often components of training programs aimed at improving sport performance or preventing or recovering from ankle injury. Both static and dynamic components of many sport activities are predicated on optimal ankle function. Training programs that incorporate ankle conditioning often include exercises specifically targeting the peroneus longus (PL)-the primary source of musculotendinous stability to the lateral lower leg and ankle (1,6,8).
The biomechanical function of the PL is complex and often misunderstood, potentially leading to less than optimal training. As a consequence, it is often inadequately or inappropriately addressed in training of the ankle (1). The complexity of function of the PL is noted in its anatomy. The PL originates from the proximal and lateral fibula making 3 turns before reaching its insertion on the plantar base of the first metatarsal and medial cuneiform: (a) the tendon passes posterior and inferior to the lateral malleolus, (b) it passes posterior to the trochlear process on the lateral aspect of the calcaneus, and (c) it makes an acute turn around the lateral aspect of the cuboid, where it is maintained in a groove on the plantar aspect of the cuboid by the long plantar ligament (3,9). The relationship of the PL with the lateral and plantar aspect of the cuboid has been termed the “cuboid pulley” (2). In weight-bearing conditions, the PL, via the cuboid pulley, acts to stabilize the first ray (medial cuneiform, first metatarsal, and great toe) for push off by exerting a plantarflexion force at the ankle (11). This creates a rigid lever for push off assisting in propulsion. The magnitude of the plantarflexion force is dependent on the length-tension relationship of the PL, and the cuboid pulley assists in aligning the PL tendon for optimum length and function (11).
Common exercises for the PL often include activities performed while non-weight bearing, in the unidirectional movement of ankle eversion, and with elastic resistive bands or cuff weights. However, previous data show that the primary biomechanical function of the PL is not unidirectional ankle eversion, nor does it occur when non-weight bearing (7). Electromyographic (EMG) data show the primary role of the PL to be stability and propulsion, as peak PL activity occurs during the later half of stance phase when the weight of the body is plantarflexed onto and over the forefoot (5,7,10). This lack of agreement between conventional training exercises and biomechanical function of the PL may limit the effectiveness of some ankle programs, which target the PL using non-weight-bearing unidirectional exercises.
Exercises that more closely address the specific biomechanical functions of the PL were originally described by Bellew and Dunn (1) in 2002 in the Strength and ConditioningJournal, but quantifiable EMG assessment of PL activity during these exercises was not reported. As the need for evidence-supported training increases, quantifiable data examining the efficacy of such interventions are desired. Therefore, the purpose of this study was to quantify and compare the electrical activity of the PL during these biomechanically specific exercises with a unidirectional, resisted, ankle eversion exercise. The study hypothesis stated that electrical activity of the PL would be greater during the biomechanically specific exercises than during the ankle eversion exercise.
Experimental Approach to the Problem
Electromyographic data from the PL of the self-reported dominant leg were examined during 2 ankle exercises previously described by Bellew and Dunn (1) and during a unidirectional resisted ankle eversion, commonly used to strengthen the ankle. Because the primary interest of the study was to examine and compare the magnitude of EMG activity (the dependent variable) across exercises (the independent variable), normalization of PL activity within subjects was necessary. To normalize data within each subject, performance on each of the exercises of interest was interpreted relative to the EMG activity during a reference unilateral heel raise (RHR). The RHR was chosen as this exercise is commonly found in ankle training programs and would be a familiar motor task for all subjects. The reference heel raise (RHR) was a standard unilateral heel raise in which subjects plantarflexed the ankle rising onto the forefoot. Subjects were instructed to rise onto their toes and return to the starting position, and subjects were permitted fingertip contact with a chair to maintain stability throughout the concentric and eccentric phases. The order of performance of the 3 test exercises was randomized across subjects, and all subjects were tested during a single session. For all exercises, subjects performed 5 consecutive repetitions to a verbal count of 2 seconds for each of the concentric and eccentric phases.
Twenty (10 men and 10 women) college-aged students (age 24.8 ± 2.7 years; range 22-30 years) were recruited from the local campus. This sample size of 20 was based on pilot data from our laboratory examining anticipated effect size between conditions, an assumed power greater than 0.80, and an alpha level of significance of 0.05. Exclusion criteria included history of ankle injury within 2 years, any neuromuscular pathology that impaired the ability to stand on the self-reported dominant leg for at least 5 seconds, or any reports of unexplained giving way or looseness of the ankle. All subjects were involved in recreational activity, but none were trained athletes or presently engaged in any type of regular training program. All procedures of this study were approved by the Institutional Review Board of the Louisiana State University Health Sciences Center, and all subjects gave signed informed consent before participation.
The first biomechanically correct exercise, the quarter heel raise (QHR), was a modified version of the traditional heel raise performed with the subject standing barefooted on the dominant limb with the foot flat on the floor. To emphasize stabilization of the first metatarsal head by the PL, a quarter was placed under the first metatarsal head to serve as a tactile cue. Subjects were instructed to perform a single-leg heel raise motion with effort given to maintaining firm pressure of the first metatarsal head on the quarter (Figure 1). Subjects were instructed to rise up on the toes while trying to push down on the quarter as much as possible. To assure stability during single-limb stance, subjects were permitted to maintain fingertip contact with the back of a chair placed in front of them.
The second exercise, the band heel raise (BHR), also required the subject to perform a single-limb heel raise. However, a resistive elastic band was positioned around the mid foot and anchored laterally, away from the body, perpendicular to the long axis of the foot (Figure 2). Before each session, the length of the band was set so that the measured tension placed across the mid foot was consistently 5 lb for each repetition and subject. The pull of the band in the lateral direction and away from midline imparts a force to the foot that induces supination/inversion of the foot as the subject attempts to rise onto the toes. This supination/inversion results in unweighting of the first metatarsal, that is, a force that is opposite the function of the PL. In this manner, the force of the band acts as a perturbation to the PL and was designed to potentiate PL activity. The test motion was a heel raise, that is, rise onto the toes, while attempting to maintain first metatarsal contact with the floor despite the tension of the elastic band. With the foot flat on the ground, the supination/inversion force is minimal. However, as the heel rises and weight is transferred to the forefoot, the supination/inversion force of the elastic band increases requiring increased stabilization of the first metatarsal from the PL. Subjects were likewise offered fingertip touch to the back of a chair for stability.
The conventional ankle eversion exercise was performed with the subject side lying on a padded examination table, the dominant limb resting atop the contralateral limb, and both feet extending over the edge of the table. Subjects were permitted to slightly flex the hips and knees as desired for comfort. Subjects were instructed to lift (evert) their ankle vertically through the full range of motion and then slowly return to the starting position. To provide resistance consistent with that applied by the elastic band in the BHR activity, a standard 5 lb canvas cuff weight was placed across the mid foot.
The EMG test apparatus was a digital data acquisition system converted from an analog polysomnograph machine featuring Grass amplifiers (Model 9; Grass Instruments, West Warwick, RI, USA). The software interface used was LabView (version 8.2; National Instruments, Austin, TX, USA). Ag/AgCl adhesive surface electrodes, 15 mm diameter, were used for the study. After denuding a portion of the test subject's lateral leg, swabbing with alcohol, and allowing the test site to dry, the examiner palpated the PL and placed one electrode directly over the muscle belly, with a second electrode placed 1 in. distally along the PL. A ground electrode was attached to the subject's medial malleolus. The bipolar EMG measurements were amplified, band-pass filtered (3-1,000 Hz), and sampled at 5 kHz. Data were captured from a floor switch fixed to the surface under the subject's heel during the closed chain exercises. Removal of the heel pressure from the switch indicated the initiation of the exercise and triggered the onset of data acquisition within the repetition. Subsequently, data acquisition for each trial was terminated when body weight was returned to the ground, indicating completion of the repetition.
The root means square (RMS) value was determined from the electrical activity for the first second of the middle 3 repetitions, and values were averaged to yield a mean RMS score for each exercise. To make meaningful comparisons of amplitude, each of the subject's values were normalized to its reference heel raise to give a relative percent change that served as the basis for statistical inference. A single-factor analysis of variance (ANOVA) was calculated using the relative percent change for each exercise. Post hoc testing was performed using least significant difference method. SPSS v14.0 for Windows (SPSS, Inc., Chicago, IL, USA) was used for all analyses. An alpha level of criterion of 0.05 was established. The observed power for this model was calculated at 0.965.
Table 1 depicts the mean RMS values for each of the 4 exercises. There was no significant difference between the average RMS of the 4 exercises (F = 2.277, p = 0.086). Table 2 presents the percent change of each exercise relative to the RHR, that is, the percent difference in magnitude of electrical activity relative to the reference heel raise. The overall ANOVA indicated there was a significant difference in percent change between exercises (F = 8.85, p < 0.001). Post hoc testing showed that the second biomechanically specific exercise, the BHR, yielded 7.73% greater electrical activity than the RHR and this was significantly greater than the conventional eversion exercise, which demonstrated 32.77% less EMG than the RHR (p < 0.001). To our surprise, the first biomechanically specific exercise, the QHR, showed 4.18% less activity than the RHR. However, despite being less than the RHR, the magnitude of PL activity during the QHR was still 28.59% greater than the conventional eversion exercise (p = 0.006). When comparing the magnitude of PL activity between the 2 biomechanically specific exercises, no significant difference was noted in the percent change between BHR and QHR (p = 0.22).
The PL is the primary source of musculotendinous support to the lateral ankle and is often the focus of training programs aiming to increase dynamic stability to the lateral ankle. The anatomical and biomechanical complexity of the PL has lead to misunderstanding of its function as evidenced by the lack of agreement between common PL exercises and data from biomechanical studies (5,10). Traditional ankle exercises aimed at strengthening the PL often use isolated unidirectional motions of eversion or plantarflexion while non-weight-bearing positions, but much data are available that report that peak PL activity occurs in weight bearing when body weight is stabilized over the forefoot (5,7,10). Use of exercises specific to the observed biomechanical function of the PL may facilitate more effective training outcomes from programs targeting the PL. This study provides quantifiable EMG data from 2 exercises for the PL that more accurately address its biomechanical function. Furthermore, this is the first study of its kind to specifically examine EMG activity of training exercises designed to target the PL. The information provided from this investigation suggests that the activation of the PL during these exercises is greater than that of a more traditional ankle eversion exercise often used to strengthen the PL.
The PL is considered the primary provider of dynamic musculotendinous stability to the lateral ankle through a response mechanism whereby proprioceptive input from the ankle joint potentiates, or increases, motor activity of the PL (6,8,10). Functional ankle instability, characterized by perceived giving way of the ankle and/or recurring ankle sprains, is a common consequence after injury to the lateral ankle (10) and has been strongly associated with weakness of the PL and dysfunction of this sensory-motor relationship (6,8,12). If the PL imparts active stability to the lateral ankle through a sensory-input motor-output response, and if the underlying mechanism precipitating functional instability is weakness of the PL, it follows then that exercises that result in greater activity from the PL may be useful additions to training programs for the ankle. Although it is beyond the scope of this investigation, it may be proposed that training of the PL may be used to increase the sensitivity of the proprioceptive afferent input response and serve as a preventative measure against inversion ankle sprains. Although this study did not assess PL activity in subjects with a history of ankle injury, previous data do show that the onset of PL activity in such subjects with ankle injury is delayed when compared with the non-injured ankle (4,13). Thus, the potential for these PL exercises as preventative training remains a viable consideration.
Common to both of the biomechanically specific exercises, yet absent from non-weight-bearing eversion, is an emphasis on stabilization of the forefoot in the weight-bearing and plantarflexed ankle position during dynamic movement. As described in these exercises, this emphasis on stabilization is elicited by either tactile cuing (quarter under the first toe) or external perturbation (elastic band pulling across mid foot). This emphasis on maintaining firm contact between the forefoot and ground is consistent with the known closed kinetic chain function of the PL and likely underlies the increased magnitude of electrical activity noted in the PL. Increased activity of the PL during dynamic activity with the body weight supported over the forefoot is consistent with previous data. Louwerens et al. (7) reported PL activity to peak in the third quarter of the stance phase of gait when body weight is transferred over the forefoot. With increasing speed of gait, the peak activity of the PL is maintained through the third quarter of the gait cycle, into the early portion of the fourth quarter where propulsion of body weight occurs (7). These data and the data of the present study lend support to the inclusion of exercises that incorporate not simply weight bearing but dynamic weight bearing on the forefoot where body weight is transferred over the forefoot.
Exercises that maintain consistency with prior evidence from biomechanical studies of PL function are more likely to increase efficacy of ankle training. Although the effect of these exercises on a subject population with known impairment of the ankle was not examined, these data nonetheless offer significant new findings to the literature to guide decision making in populations where training of the PL is desired. To date, there is a paucity of studies specifically addressing the activity of the PL during exercises in healthy non-injured subjects. Information from this study can be used to guide exercise programming and provide greater evidence-based options for addressing PL function. The data of this investigation provide such evidence to support the use of these biomechanically specific exercises in addition to, or in lieu of, conventional ankle eversion as is commonly used in many ankle training programs.
Effective training of the ankle often includes exercises to increase stability of the lateral ankle by strengthening the PL. Non-weight-bearing ankle eversion with resistive bands or weights is commonly used to target the PL. This study presents 2 biomechanically specific exercises that are shown to elicit greater activity from the PL and are performed in weight bearing where previous biomechanical studies show peak activity of the PL. Adherence to exercises with quantifiable data supporting their use is prudent and will likely improve training outcomes. Incorporation of the exercises described in this study in a comprehensive ankle training program offers the training and rehabilitation professional greater options in muscle-specific training.
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Keywords:© 2010 National Strength and Conditioning Association
ankle; strengthening; foot; biomechanics