effect-of-orthotics-on-vmo-fatigue by Lori@go4-d.com

Journal of Electromyography and Kinesiology 14 (2004) 693–698 www.elsevier.com/locate/jelekin

The eﬀect of foot orthotics on myoelectric fatigue in the vastus lateralis during a simulated skier’s squat Natalie Vanicek, Joanna Kingman , Clare Hencken Department of Sport and Exercise Science, University of Portsmouth, St Michaels Building, Portsmouth PO1 2DT, UK Received 29 April 2003; received in revised form 9 February 2004; accepted 19 March 2004

Abstract Fatigue in the legs is a problem experienced by skiers. It has been suggested that optimal orthotics may reduce muscle fatigue for a given movement task by minimising muscle activity (Med. Sci. Sports Exerc. 31 (1999) S421). The aims were to determine whether EMG would provide an independent method of analysing myoelectric fatigue in the vastus lateralis (VL) during a skier’s squat and whether orthotics could affect this fatigue response. Six skiers performed skier’s squats for as long as possible with no orthotic, low volume orthotics and high volume orthotics in their ski boots. Bipolar, active surface electrodes recorded EMG activity in the VL throughout each squat. Results for the EMG median frequency showed a significant shift in the power density spectrum towards the lower frequencies (P < 0:05) at the end of the contraction, suggesting that myoelectric fatigue was occurring and was measurable using EMG. All conditions displayed a significant decrease in median frequency at the end of the contraction (P ¼ 0:001). The high volume orthotic showed a significant reduction in myoelectric fatigue, however, there was no difference in the duration of squats across the three conditions (P > 0:05). Subjective and objective findings support the use of the high volume foot orthotic for skiers. # 2004 Elsevier Ltd. All rights reserved. Keywords: Orthotics; Myoelectric fatigue; EMG; Median frequency

1. Introduction Muscular fatigue in the lower limbs is a problem commonly experienced by skiers. Muscle fatigue is a difficult term to define, as it is comprised of both subjective and objective aspects. De Luca [7] refers to muscle fatigue as the point at which a contraction can no longer be maintained and is termed the ‘failure point’. There are various biochemical, physiological, psychological and biomechanical variables that can be monitored as fatigue occurs. One of these variables is the electromyogram (EMG). De Luca [7] states that as myoelectric fatigue occurs there is a decrease in the frequency of the firing rate of the muscle and this is evident in the frequency output of the EMG signal. In EMG frequency analysis, a Fast Fourier Transform is Corresponding author. Tel.: +44-2392-842-651; fax: +44-2392842-641. E-mail address: joanna.kingman@port.ac.uk (J. Kingman).

commonly applied to the EMG signal to produce the power density spectrum. Factors such as increased muscle acidosis, interstitial fluid pH, muscle fibre conduction velocity and motor unit recruitment influence the median frequency of the power density spectrum with the occurrence of myoelectric fatigue during a sustained isometric contraction [3,6,7,10]. Subcutaneous (fatty) tissue between the electrode and active muscle fibre may also reduce the frequency values of power density spectrum [7]. Surface EMG is a non-invasive method for monitoring muscle activity. The EMG power density spectrum has recently been recognised as a useful method for analysing localised myoelectric fatigue in static conditions [3,7,10]. The median frequency is referred to as the frequency that divides the power density spectrum into two regions of the same power [1] and it is commonly used to monitor myoelectric fatigue. The downward shift, or decreasing trend, of the frequency content of the electromyographic signal in time has

694

N. Vanicek et al. / Journal of Electromyography and Kinesiology 14 (2004) 693–698

repeatedly been shown during fatiguing static conditions [6,7]. The advantage of this type of analysis is that it can be used as a fatigue index, to monitor and quantify the onset of myoelectric fatigue before the ‘failure point’. An orthotic is a device used to properly align the skeleton, reduce the occurrence of movement-related injuries, improve impact cushioning properties and sensory feedback, as well as increase comfort [14]. The aim of a foot orthotic is to provide optimal functionality in the lower limbs. Foot orthotics are commonly prescribed by clinicians to compensate for anatomic abnormalities [2]. Nigg et al. [14] discuss the concept of minimising muscle work through the use of foot orthotics or inserts. This is based on the notion that the skeleton has a preferred movement trajectory for a specific task and that an intervention that supports this preferred movement path may reduce muscle activity. It has also been suggested that joint stability is a major strategy for reducing muscle activity. However, an orthotic that requires additional muscle work to maintain a stable movement may be the reason for early fatigue. Thus, muscle fatigue may be minimised and performance could increase with an optimal orthotic. Manufacturers of foot orthotics have made similar claims regarding their use during sporting activities, including alpine skiing. A delay in the onset of myoelectric fatigue in the lower limbs during skiing may be a contributing factor to improvements in performance. In recent years, foot orthotics have become more commercially available. However, for logistical and financial reasons many skiers are not able to obtain clinically prescribed or custom-made foot orthotics and therefore end up with a generic, ‘off the shelf’, device. Craton and McKenzie [5] cautioned against the use of prefabricated orthotics; they emphasised the need for individual prescription to suit the wearer’s biomechanics, including foot type, and the sports involved [5]. Skiing is a dynamic sport that takes place in challenging environments where controlled investigation is difficult. Clarys and Cabri [4: p. 422] discuss the difficulty of measuring muscle activity in the field during alpine skiing, stating that it is ‘‘among the most difficult of kinesiological investigations because there are so many influencing factors (e.g. snow conditions, low temperatures) that cannot be controlled’’. It is recognised that to replicate this activity in a controlled laboratory setting is difficult and may have some limitations. The ‘shuss’, or skier’s squat, is the downhill position that the skier aims to maintain in a controlled fashion for as long as possible during a downhill course. In 1972, using a wind-tunnel experiment, Ikai et al. [9] determined that the egg-shaped (ovoid) or ‘shuss’ position produced the least amount of wind resistance for

the skier. The ‘shuss’ was adopted as the preferred position in downhill skiing because it generates greater skier velocity. The skier’s squat can be replicated in a controlled laboratory setting and is a static, isometric contraction that has many advantages for EMG measurement [3,6]. Furthermore, median frequency as a measure of localised myoelectric fatigue has been used during static fatiguing muscle contractions yet, a similar decreasing shift during fatiguing, dynamic conditions, has still to be accepted [13]. During dynamic muscle contractions, muscle length, joint angle and/or muscle force output, as well as the geometrical arrangement between muscle fibres and surface electrodes, can vary leading to the detection of different motor units and changes in the shape of motor unit action potentials [7]. Therefore, for this investigation, the skier’s squat, in a laboratory setting was considered a reasonable method for assessing the relationship between foot orthotics in the ski boot and myoelectric fatigue in one of the knee extensor muscles. Several studies have examined the relationship between the use of foot orthotics in the ski boot, biomechanical alignment of the lower limbs, and control of subtalar joint motion [8,12,15]. It has been assumed that by controlling the position and motion of the foot with the use of foot orthotics in the ski boot, skiing performance and efficiency could be improved [8]. However, no studies that investigate the relationship between foot orthotics and localised myoelectric fatigue have been located. The aims of this investigation were firstly to determine whether EMG would provide an independent method of analysing myoelectric fatigue in the vastus lateralis (VL) during the skier’s squat; and secondly to determine whether generic foot orthotics can reduce myoelectric fatigue in the VL during a skier’s squat.

2. Methods 2.1. Participants Six healthy, experienced alpine skiers volunteered as participants for the study (for participant characteristics see Table 1). Each participant gave written informed consent to participate in the study and ﬁlled in a Pre-test Health Questionnaire. None of the participants were currently experiencing any injuries. Five of the six participants reported having already used foot orthotics while skiing (mean s:d:, 3:6 1:2 years), either based upon recommendation or because of their added comfort and foot support within the ski boot.

N. Vanicek et al. / Journal of Electromyography and Kinesiology 14 (2004) 693–698

695

Table 1 Participant characteristics

Mean s.d.

Females

Males

Age (years)

Height (m)

Mass (kg)

Skiing experience (years)

Days skiing/year

Hours skiing/day

29 10.5

1.78 0.06

78.8 11.9

17.5 3.4

31.2 11

5.6 0.6

2.2. Protocol The VL was selected over other knee extensor muscles for this preliminary analysis using surface EMG because of its large superficial position on the lateral aspect of the thigh and due to its function as a singlejoint knee extensor muscle. Thus, with the knee angle kept constant, variations in hip angle would not affect the muscle length of the VL as with the rectus femoris. The participants performed a warm-up including stretches to prevent possible injury. The VL was identified on the participant’s dominant leg, using vision and palpation and the middle of the contracted muscle belly was marked [4]. The skin was prepared by shaving (if necessary) and wiping with an isopropyl alcoholic swab. Bar, bipolar active surface electrodes of 10 1 mm dimensions (DelSys Bagnoli-4 EMG system, US) were attached in-line with muscle fibre direction using adhesive tape. The electrode material was 99% silver and the inter-electrode distance of 10 mm was fixed. Following similar skin preparation one passive disk reference electrode (10 mm radius) was positioned on the participant’s patella. Electromyographic activity was differentiated by preamplifiers and recorded via an on-line, cabled DelSys Bagnoli-4 EMG system (US), with an input impedance of less than 1015/0.2 X/pF, a common mode rejection ratio at 60 Hz of greater than 80 dB, a noise level of 1.2 lV, a gain of 10 þ 2% and a bandwidth range from 0 to 500 Hz. Muscle activity was sampled at 1024 Hz via a 16 bit DAQ-516 A/D card and stored on a Compaq Armada laptop computer using the EMGWorks Acquisition 1.1.0.1. Beta data programme (DADLL National Instruments 516 Series Version 1.0.0.3., DelSys Inc, 1997–2000). The raw EMG data were filtered using a Chebyshev Type 2 high pass filter. The participant placed himself or herself in their comfortable skier’s squat position on a non-slippery v incline of 13 , with the elbows flexed at waist height. v Their knee angle (mean s:d:, 84:2 3:0 ) was measured using a goniometer once they assumed their squat position. The goniometer was positioned on the lateral side of the rotational axis of the knee. Anatomical landmarks used for goniometric measurement included the greater trochanter and lateral malleolus of the fibula as the end-points of the thigh and shank segments, respectively. Electromyographic activity from

the VL was recorded continually while the participant held the skier’s squat position for as long as possible. Verbal encouragement was given throughout the test. The test was stopped once the participant could no longer hold the squat or when the goniometer regisv tered a variation in knee angle greater than 10 through either flexion or extension of the knee joint. Each squat was timed and video recorded. Each participant performed the squat under three conditions. During the two experimental conditions, foot orthotics fitted by size were placed into the participant’s ski boots, who then put on their ski socks and ski boots, buckling them as they normally would whilst skiing. Superfeet Trim-to-Fit Synergizer1 Green and Blue foot orthotics were used, they have a patented shape, rear-foot and mid-foot control points, patented support bridge and a natural shock absorption system. They are made from Long-Wearing Trocellen1 foam1 and a New Etc.1 Top cover. The Superfeet Trim-to-Fit Synergizer1 Green foot orthotic is designed for high volume footwear with removable inner soles and will be referred to as the high volume foot orthotic throughout. The Superfeet Trim-to-Fit Synergizer1 Blue foot orthotic uses less foam than the high volume foot orthotic and therefore is more suitable for lower volume footwear with non-removable inner soles and will be referred to as the low volume foot orthotic throughout. The third condition was used as a control; this consisted of the participants performing the skier’s squat for as long as possible without any foot orthotics in the ski boot. During this condition, there was no support under the longitudinal arch of the foot. The order in which the three conditions were performed was randomised for each participant. An adequate rest period was allowed between each squat. 2.3. Data analysis The EMG data for each participant during each condition was processed using a power density spectrum for every second of the entire duration of each squat. The EMGworks software uses a Fast Fourier Transform to calculate the power density spectrum. The median frequency, in Hertz (Hz), was then processed every second for the duration of each skier’s squat. Participants maintained the skier’s squat for different

696

N. Vanicek et al. / Journal of Electromyography and Kinesiology 14 (2004) 693–698

durations, therefore, to establish a significant shift in the median frequency response of the muscle between the start and the finish of the contraction, the first 20 s of each contraction was compared with the last 20 s. To ensure that assumptions for parametric analysis were addressed, the data obtained from the first and last 20 s were tested for normality. This indicated that the Wilcoxon, Signed Ranks test would be the appropriate test to determine whether EMG would provide an independent method of analysing myoelectric fatigue in the VL. To determine the effect of each condition, the median frequency response between the first 20 s and the last 20 s was statistically compared across conditions using a repeated measures analysis of variance (ANOVA). Since the sample size within each condition was small, a power analysis using recommendations by Lenth [11] was conducted after first establishing the desired effect size . This indicated a Power of 0.7334, which, given the unequal variance in the control conditions, was considered reasonable. A TUKEY post-hoc analysis was undertaken to examine within subjects and between subjects differences, to determine whether generic foot orthotics could reduce myoelectric fatigue in the VL during a skier’s squat. The alpha level was set at P < 0:05 and all statistical analysis was performed using the Statistical Package for Social Sciences software (SPSS Version 10.1 programme for Windows, Chicago, Ill).

Table 2 Mean median frequency values (s.d.) obtained from the VL during the ﬁrst and last 20 s of the skier’s squat (in Hz)

No orthotic Low volume orthotic High volume orthotic

0–20 (s)

Last 20 (s)

66.03 (9.08) 67.31 (6.71) 68.98 (10.92)

63.22 (8.42) 60.94 (7.98) 65.88 (12.86)

Denotes signiﬁcant diﬀerences between 0–20 and last 20 s within each condition (where P < 0:05).

all conditions. Overall, a Wilcoxon Signed Ranks test revealed that there was a significant difference between the first and the last 20 s (Z ¼ 8:627, P ¼ 0:000). Further statistical analysis revealed that the mean median frequency in the last 20 s was significantly lower in all three conditions than that reported in the first 20 s (F 2;357 ¼ 6:953, P ¼ 0:001). The TUKEY post-hoc analysis revealed that there were differences between conditions, with the high volume foot orthotic condition showing significantly less reduction in median frequency at the end of the contraction compared to the low volume and control condition (P ¼ 0:032 and P ¼ 0:009, respectively). No significant differences were found between the control condition and the low volume orthotic with regard to the myoelectrical decline between the first and last 20 s of the skier’s squat.

3. Results

4. Discussion

The duration that each participant maintained the squat is shown in Fig. 1. Across conditions there was no significant difference in the duration that participants were able to maintain the squat (F ¼ 0:2ð2Þ , P ¼ 0:98). For all participants, Table 2 reveals the mean EMG median frequency values obtained from the VL during the first 20 s and the last 20 s of the skier’s squat across

The first aim of this investigation was to determine whether EMG provided an independent method of analysing myoelectric fatigue in the VL during the skier’s squat. It can be stated that during all conditions fatigue occurred, as the participants could no longer maintain the squat; this was deemed as mechanical failure. However, analysing fatigue based on mechanical failure alone provides little insight into the physiological and biomechanical events occurring during an isometric contraction. The median frequency of the power density spectrum revealed a significant shift in the skewness towards the lower frequencies at the end of the contraction. This result suggests that EMG provided an independent method of analysing myoelectric fatigue in the VL during the skier’s squat. The second aim of this investigation was to determine whether generic foot orthotics could reduce myoelectric fatigue in the VL during a skier’s squat. The results showed that the foot orthotics were not beneficial in prolonging the duration of the squat, since there was no significant difference in the time to mechanical failure between conditions. The median frequency values (Table 2) for all conditions displayed a significant fatigue effect in the VL. However, the high

Fig. 1. Squat duration (s) for all participants across all conditions.

N. Vanicek et al. / Journal of Electromyography and Kinesiology 14 (2004) 693–698

volume condition revealed significantly less decline in median frequency when compared to the other conditions. It is interesting to note the lack of relationship between myoelectric fatigue and squat duration, as the reduction in myoelectric fatigue in the high volume condition did not lead to a significant increase in squat duration. It is therefore, suggested that mechanical failure during this condition occurred as a result of fatigue in different muscles or due to other biochemical or physiological factors. Further research is warranted in this area to establish the relationship between myoelectric fatigue and mechanical failure during isometric contractions. During this investigation, participants reported a subjective preference towards the high volume orthotics, stating that it felt more comfortable and more supportive. The notion of comfort of a foot orthotic is difficult to define objectively yet it has been recognised as an important factor related to fit and joint stability during dynamic activities and during fatigue [14]. The high volume orthotic also showed a significant reduction in myoelectric fatigue in the VL, but did not prolong the duration of the skier’s squat. In this study, no consideration was made for the biomechanical alignment of the lower limbs and it is therefore not known whether the participants required an orthotic device or the type of support needed. Foot orthotics are often recommended to increase comfort and neutralise lower leg alignment. The situation in this study is similar to that faced by the recreational skier when considering purchasing generic foot orthotics for ski boots. There are many intrinsic and extrinsic factors that may affect the EMG signal and the frequency response [1]; these factors were acknowledged and controlled for wherever possible. Furthermore, the study was performed at the beginning of the ski season and none of the participants had actively engaged in skiing within a 6-month period prior to testing. Therefore, a training effect was not deemed to have influenced squat duration. Gender differences have been noted in the median frequency content of the EMG signal [7,10]. These are likely to be due to the larger amount of subcutaneous tissue between the surface electrode and the active muscle fibres in women. This greater thickness presents a low-pass filtering effect, thereby reducing the high frequency components of the EMG signal. These factors would be of importance for cross-gender comparisons, however, this investigation focused on within subject comparisons. In conclusion, a significant shift in the median frequency of the power density spectrum was observed at the end of the contractions, suggesting that EMG provided an independent method of assessing myoelectric fatigue in the VL during the skier’s squat. All con-

697

ditions revealed a significant myoelectric fatigue response when comparing the first and last 20 s of the squat. The high volume orthotic displayed a significant reduction in myoelectric fatigue compared to the other conditions. However, there was no significant difference in the time to mechanical failure of the skier’s squat across conditions. The lack of relationship between myoelectric fatigue and mechanical failure requires further investigation. Based on the objective and subjective findings from this study, the high volume foot orthotic is recommended as an alternative to individualised, custom-made orthotics for skiers.

References [1] R. Bartlett, Introduction to Sports Biomechanics, E & FN Spon, London, 1997. [2] R. Bartlett, Sports Biomechanics: Reducing Injury and Improving Performance, E & FN Spon, London, 1999. [3] P. Bonato, S.H. Roy, M. Knaflitz, C.J. De Luca, Time– frequency parameters of the surface myoelectric signal for assessing muscle fatigue during cyclic dynamic contractions, IEEE Transactions on Biomedical Engineering 48 (7) (2001) 745–753. [4] J.P. Clarys, J. Cabri, Electromyography and the study of sports movements: a review, Journal of Sports Sciences 11 (1993) 379–448. [5] N. Craton, D.C. McKenzie, Orthotics in injury prevention, in: P.A.F.H. Renstrom (Ed.), Sports Injuries: Basic Principles of Prevention and Care, Blackwell Scientific, London, 1993, pp. 417–428. [6] C.J. De Luca, Myoelectrical manifestations of localized muscular fatigue in humans, Critical Reviews in Biomedical Engineering 11 (4) (1984) 251–279. [7] C.J. De Luca, The use of surface electromyography in biomechanics, Journal of Applied Biomechanics 13 (2) (1997) 135–163. [8] S.B. Greenberg, D.J. Sanderson, J.E. Taunton, J.G. Macintyre, Control of subtalar motion with the use of ski boot footbeds, Clinical Journal of Sports Medicine 1 (1991) 188–192. [9] M. Ikai, K. Watanabe, T. Fukunaga, Motion analysis and telemetering electromyography of alpine skiing, in: N. Mutsuo (Ed.), Proceedings of the International Congress of Winter Sports Medicine, Sapporo Medical College, Sapporo, 1972, pp. 106–110. [10] L.S. Krivickas, A. Taylor, R.M. Maniar, E. Mascha, S.S. Reisman, Is spectral analysis of the surface electromyographic signal a clinically useful tool for evaluation of skeletal muscle fatigue? Journal of Clinical Neurophysiology 15 (2) (1998) 138–145. [11] R. Lenth, Some practical guidelines for effective sample-size determination, The American Statistician 55 (2001) 187–193. [12] J.G. Macintyre, G.O. Matheson, Clinical biomechanics of skiing, Canadian Family Physician 34 (1988) 107–114. [13] D. MacIsaac, P.A. Parker, R.N. Scott, The short-time Fourier transform and muscle fatigue assessment in dynamic contractions, Journal of Electromyography and Kinesiology 11 (2001) 439–449. [14] B.M. Nigg, M.A. Nurse, D.J. Stefanyshyn, Shoe inserts and orthotics for sport and physical activities, Medicine and Science in Sports and Exercise 31 (1999) S421–S428. [15] J.P. Santoro, V.V. Cachia, G.E. Tilley, D. Cartwright, N.A. Grumbie, Effect of the orthosis on performance in alpine skiing: a preliminary report, Podiatric Sports Medicine 79 (2) (1989) 93–99.

698

N. Vanicek et al. / Journal of Electromyography and Kinesiology 14 (2004) 693â&#x20AC;&#x201C;698

Natalie Vanicek received her BSc (Hons) in Sports and Exercise Science from the University of Portsmouth, UK, in 2001. She is currently completing her MSc in Biomechanics at the University of British Columbia Vancouver, Canada. The research focus for her Masterâ&#x20AC;&#x2122;s thesis is investigating the temporal-distance and kinematic adaptations to a novel walking task when lower limb mechanics are altered with the use of a unilateral prosthetic simulator. Other research interests include the functional aspects of variability of movement patterns and biomechanics related to sporting movements and injury prevention. Joanna Kingman received her PhD in Sports Biomechanics in 1999 at University College Chichester, where she also completed her undergraduate BSc (Hons) Sports Science. Her PhD focused on electromyography applied to a sports speciďŹ c situation, roller hockey match play. Kingman works at the University of Portsmouth in England and is also the deputy course leader for the new MSc in Enhancing and Maintaining Sports performance. She teaches Sports Biomechanics units to Undergraduate and Post-

graduate Students within the Sports Science Department. She has a strong research interest in electromyography and has a PhD student working in this area. Clare Hencken completed her PhD in exercise Physiology in 2000 at the University of Alabama USA, where she did an MSc in Human Performance. She has a background in Physical Education and Sports Psychology having completed her undergraduate degree at St. Lukes, Exeter University. Hencken works at the University of Portsmouth in England and is also the course leader for the new MSc in Enhancing and Maintaining Sports Performance. She teaches both physiology and psychology units within the department. She has a strong research interest in anthropometry and is currently working with a premiere League Football team, providing anthropometric assessments and studying the link with performance.