Sajeel Chaudhry

Factors to consider in deducing the efficacy of unstable surface training in sporting movements and its effectiveness amongst the athletic population by Sajeel Chaudhry

This literature investigate the factors to consider in deducing the efficacy of the use unstable devices to enhance athletic performance where the trunk muscle play an important role in transferring torques and angular momentum during the movement (Kibler, Press & Sciasscia, 2006). Thus modes of how the musculature around the trunk achieves core stability & strength shall be explored along with the trunks optimum functionality, and whether unstable surface training has any significant impact on improving athletic performance.The use of unstable surfaces dates back to the 1920s (Bhem, Drinkwater, Willardson & Cowley, 2010a) as neuromuscular training was receiving much attention (Sherrington, 1910). Consequently physiotherapists introduced the use of an unstable surface such as the Swiss ball to promote instability in the core musculature which would require the core to produce a greater stabilising mechanism. Various types of unstable surfaces such as wobble boards, BOSU, inflatable discs, foam tubes and suspended ropes have been introduced to the physiotherapy and physical conditioning industry over the years. This is predominantly due to the research being conducted in the injury prevention and rehabilitation sector, with spinal stability and lower back pain receiving much importance in examining the ability of the spinal stabilising system to resist disturbance from its mode of equilibrium (Axler & MGill, 1997; Leetun et al., 2004; McGill, 1988; McGill & Norman, 1985; Hodge & Richardson, 1997; Nuzzo et al., 2008; Rasmussen et al., 2009). Unstable surfaces have also channelled into the rehab and injury prevention of other anatomical parts of the body such as the ankle and knee increasing joint proprioception and functional balance (Hrysomallis, 2007; Soderman, Werner, Pietila, Engstrom & Alfredson, 2000; Verhagen et al., 2004). Unstable surfaces have now become popular amongst the athletic population in training the trunk region in order to enhance performance, by allowing greater force production in the distal segments via a strong and stable foundation of the core (Willardson, 2007). The core of the body has been identified as the lumbopelvic region of the human body which consists of the spine, hips, pelvis and abdominal structures (Bergmark, 1989; McGill, 2001; Panjabi, 1992; Willardson, 2007, Kibler, 2006). The muscles of the trunk and pelvis are associated with the stability of the spine and pelvis, which acts a foundation for the movement of the upper and lower limbs and in supporting forces or loads which maybe exerted by or upon the body. This lumbopelvic stability is also required for the prevention of injury to the spinal cords and nerves (Panjabi, 1992). Thus the core plays an important role in closed kinetic chain activities where core strength, balance and control of motion maximise and make upper and lower limb movement more efficient (Kibler, 2006).In order to understand how the core is stabilised, Punjabi (1992) presented a conceptual stabilising mechanism which comprised of three subsystems, though mechanistically separate are functionally interdependent. This consists of the passive subsystem, the active subsystem and the neural subsystem. The passive subsystem consists of vertebrae, joint capsules, vertebral discs, facet articulations and spinal ligaments. In a neutral spine the spinal ligaments do not provide any significant stability, and only engage in reactive forces when the spine and individual vertebrae reach end range of the motion. Thus predominantly these components of the passive subsystem act as sensory feedback to the brain for measuring motion and vertebral position of the spine. The active subsystem consists of the muscles and tendons that surround the spine and responsible for generating and transmitting force and providing the required stability to the spine. The neural subsystem comprises of the sensors that relay information about the level of spinal stability in order for the active subsystem to adjust muscle tension for the required level of stability.The core musculature plays an important role in closed kinetic chain activities such as a throwing activities where torques and angular momentum are transferred from the lower limbs to the pelvic girdle, the trunk and then to the upper extremity sequentially. Any weakness in the core musculature may result in the transfer of the torque and angular momentum resulting in a low velocity throw and cause of injury due to overuse, since in a throwing action the shoulder may attempt to compensate by producing more torque (Behm et al., 2010). Thus training the active subsystem of the core is essential in increasing strength of the core musculature. According to Behm (1995) an increase in muscle cross sectional area and neuromuscular coordination both contribute towards improvements in strength. However according to Rutherford and Jones (1986) an enhanced coordination of the agonists, antagonists, synergists and stabiliser muscles provide the neural adaptation as opposed to an increase in recruitment of motor units. Thus an unstable surface would provide the stimuli for an increased proprioception and hence improved coordination of the core musculature. The functionality of the core musculature is another important aspect to consider when forming an understanding of the efficacy of unstable surfaces in training the core for sporting movements. Many closed kinetic chain activities such as overhead throwing of a ball are multi planar, thus the spine needs to be stabilised in the saggittal, frontal and transverse plane. Activating a single muscle to perform abdominal bracing may not be considered functionally viable to stabilise the spine (Mcgill et al. 2009) since it stiffens the spine at the expense of spinal loading. MGill (2009) suggests that a single muscle should not be activated to its 100% maximum voluntary contraction during a multi planar movement as this would cause an imbalance in the three axis of the spine, causing instability. However if an activity was being performed in a single plane, maximum muscle contraction would not be contraindicated. Thus strength training of the core muscles in isolation may not help stabilise the spine functionally for a sports movement and hence muscle activation of the core must be coordinated and not isolated (Cholewicki et al. 2000). In order to understand the active system of the trunk, Bergmark (1989) proposed a mechanical model of the stability of the lumbar spine. The study showed that the mechanical model of the system was statically indeterminate in equilibrium, and thus this highly complex system required constant nervous control of the muscle forces being applied to stabilise the spine, though this study was modelled around an upright posture with an external load being carried on the shoulders. Similarly McGill et al. (2003) suggests that stability of the core is dynamic and is constantly changing to meet postural adjustments and reactions to external forces. Hence the contribution of each muscle of the core changes as the body goes through motion or during a particular task. In order to form a functional anatomy of the active muscle subsystem Bergmark (1989) classified the active muscle subsystem into global and local muscles. The global muscles (rectus abdominis, transverse adominis, external oblique, internal oblique, illicostalis thoracic portion) are large superficial muscles that have origins at the pelvis and insertions at the thoracic cage, and hence are regarded as being responsible for the transfer of force between the pelvis and thoracic cage. The local muscles (multifidi, psoas major, quadratus lumborum, diaphragm, internal oblique, illiocastalis and longissimus of the lumbar portion) have origins and insertions at the lumbar vertebrae, and hence are responsible for the segmental stability of the lumbar spine for postural adjustment during rest and during movement. The global and local muscles are activated simultaneously and show similar activity patterns (Aroksoki, Valta, Airaksinen & Kankaanpa, 2001; Kavick, Grener, McGill, 2004) during various exercises and movements to stabilise the spine. This suggests that training selection should achieve co activation of the global and local muscles in order to stabilise the spine to provide a stable platform for force transfer and to prevent dysfunction of the subsystems proposed by Punjabi (1992).One particular global muscle that has received major attention in regards to acting as a major stabiliser of the pelvis and spine is the transverse abdominis. Cresswell and Thorstensson (1994) suggested that the transverse abdominis was primarily responsible for the increase in intra abdominal pressure which improves the stability of the trunk by stiffening the whole thoracic segment and hence the trunk acts as a rigid cylinder. Their study also showed that the transverse abdominis was the first muscle that was activated during voluntary and unexpected loading of the trunk and its activation was bilaterally symmetrical. Similar studies by Hodges and Richardson (1997), Hodges(1999); Cresswell and Thorstensson(1993) showed that the transverse abdominis is activated regardless of the direction of the movement of the trunk, acceleration or deceleration, position of the centre of mass of a body or the acting forces on the spine and hence is responsible for the stabilising the spine. A delay in the activation of the transverse abdominis could be results of dysfunction of motor control. However studies by Allison and Morris (2008) shows that the activation of the transverse abdominis is specific to the direction of limb movement and the activation is not bilaterally symmetrical. Though the experiment was performed on subjects using simple movements of the upper extremity such as the arm raise, the findings suggest that the theory that the transverse abdominis acts a primary and bilateral stabiliser in anticipation of a movement should be revisited and that a delayed onset of the transverse abdominis may not necessarily be due to a loss of motor function but is dependent upon movement of the trunk and the dynamic postural adjustment of the spine. McGill(2001) showed that a single muscle such as the transverse abdominis cannot be characterised as a prime stabiliser and that all muscles of the torso are important and the importance of them acting as a prime stabiliser is entirely dependent on the activity performed by the body.So far the literature has identified modes of stabilising the spine, or core stability, and providing a stable platform in order to be able to move and transfer force to the terminal segments from the trunk. Though this is essential, another factor to consider is the ability of the core musculature to produce or withstand force within the athletic population. For this reason Faries and Greenwood (2007) have defined two independent characteristics of the core. These are core stability and core strength. Core strength is the ability of the core musculature to produce force via intra abdominal pressure and contractile forces. Subsequently core stability and core strength both require different types of training modalities. This is essentially due to the different functionalities between local and global muscles, with local muscles responsible for the stability of the spine and global muscles responsible for the production of force. As a result the muscle characteristics of local and global muscles are listed in Table 1. (Faries & Greenwood, 2007).Table 1. Characteristics of local and global muscles

Local MusclesGlobal Muscles

Deeply PlacedSuperficial

Slow twitch natureFast twitch nature

Active in endurance activitiesActive in power activities

LengthenShorten and tighten

Activated at low resistance levels (30-40% MVC)

MVC= Maximal Voluntary Contraction

By taking into account the function and characteristics of local and global muscles of the core, Comeford and Mottram (2001) have been able to make recommendations to the type of loading required to recruit the two types of muscles. Local muscles require low load threshold training and global muscles require high load threshold training. Their study also stated that any imbalance in the way that muscles are recruited may lead to the dysfunction of the trunk region and consequently injury. Thus the amount of load and duration are important factors in how the motor units of the muscle are recruited in order to train the core for stability or strength. An inappropriate recruitment of the core musculature with insufficient strength and endurance training can be associated with the dynamic instability of the spine (OSullivan, Twomey & Allison, 1997). In order to induce strength benefits an activation of >60% Maximum voluntary Contraction is suggested and 25% MVC which may not efficiently give stability adaptation. The remaining 3 exercises did not really have any significant difference between the means of stable vs. unstable exercises. However the local muscles were within the stability training activation zone with only a few global muscles within strength activation zone of %MVC. This may be due to the fact that these exercises have been chosen for core stabilisation even though the literature has not specifically distinguished the co activation of global and local muscles which should be for strength and stability respectively. Thus these exercises almost independently either target strength or stability. Also the experiment lacks multi planar exercises thus its effectiveness in sporting activity cannot be fully deduced although the elbow toe exercise on an unstable surface gives a balanced adaptation towards strength and stability training of the core.Escamilla et al. (2010) performed a laboratory study to test the ability of a swiss ball in activating the core musculature. Nine male and nine female subjects were used from a non athletic background with no injury with no prior experience of using a swiss ball however were all experienced in performing the bent knee sit up and crunch. Surface electrodes were placed to measure activity of the global muscles (upper and lower rectus abdominis and external obliques) and local muscles (internal oblique and lumbar paraspinal) to record electromyography (EMG) data. Eight swiss ball exercises were selected shown in Appendix B (roll out, pike, knee up, skier, hip extension right, hip extension left, decline push up and sitting march). The EMG data was normalised for each of the muscle sites using a maximum voluntary isometric contraction (MVIC) for 3 seconds and the data is presented in Table 4.Table 4. Mean (SD) of %MVIC for each muscle and exercise

Upper RALower RAEOIOLP

Roll out63 (30)53 (23)46 (18)46 (21)6 (2)

Pike47 (18)55 (16)84 (37)56 (22)8 (3)

Knee up32 (15)35 (14)64 (39)41 (16)6 (3)

Skier38 (17)33 (8)73 (40)47 (18)6 (3)

Hip Ext. Right43 (21)44 (11)56 (32)40 (26)7 (3)

Hip Ext. Left41 (24)39 (19)39 (19)45 (25)6 (3)

Decline push up38 (20)37 (16)36 (24)33 (18)6 (2)

Sitting march7 (6)7 (6)14 (6)16 (11)5 (2)

Abbreviations: RA, rectus abdominas; EO, external obliques; IO, internal obliques; LP, lumbar paraspinal

With all of the eight swiss ball exercises co activation of the global and local muscles was achieved with no muscle achieving 100% MVC. With the activation levels of the global muscles for strength the pike seems to be a particular exercise of significance on the swiss ball since it also incorporates movement in the transverse plane. However the internal oblique which is a local muscle achieved much higher activation levels compared to the lumbar paraspinal. Hence the efficacy of a swiss ball with these exercises to provide strength and stability recruitment simultaneously is questionable since activation levels of the local muscle is >25% MVC which may cause a dysfunction of the core whilst adapting to the muscle recruitment. With global muscles achieving 60% MVC the swiss ball provides strength benefits with these exercises performed. The sitting march on the other hand only provides stability benefits to the local muscle with the global muscles not targeting strength gains. Table 5. Provides a summary of the muscle activation levels relative to the exercise.Table 5. Relative muscle recruitment

>60% MVIC41-60%MVIC21-40%MVIC0-20%MVIC

Upper RARoll-out

Crunch, pike, hip extension

Bent-knee sit-up, skier, decline push-up, knee-up

Sitting march right

Lower RAPike, roll-out, hip extension

Hip extension, crunch, decline push-up, knee-up, skier, bent-knee sit-up

Sitting march right

EOPike, skier, knee-up

Hip extension, roll-out

Hip extension, decline push-up, bent-knee sit-up, crunch

Sitting march right

IOPike, skier, roll-out, hip extension, knee-up

Hip extension, decline push-up, crunch, bent-knee sit-up

Sitting march right

LPPike, roll-out, hip extension, crunch, decline push-up, knee-up, skier, bent-knee sit-up, sitting march right

Abbreviations: RA, rectus abdominas; EO, external obliques; IO, internal obliques; LP, lumbar paraspinal

Closed kinetic chain sports such as a baseball pitch and the tennis serve have a unilateral action, and for training specificity unilateral exercises may prove to be more beneficial. By providing a unilateral resisted action, the trunk is further de stabilised due to torque being applied to the body (Behm, Leonard, Young, Boney & MacKinnon, 2005). However resistance training exercises are predominantly bilateral whether performed on a stable or unstable surface. Thus Behm et al. (2005) conducted an experiment to compare the core muscle activity on stable and unstable surfaces in trunk strengthening and resistance exercises. Six men and five women were chosen who were actively engaged with resistance training on stable and unstable devices. The resistance exercises were performed unilaterally and bilaterally as it was hypothesised that unilateral resistance exercises would activate the trunk muscles more than bilateral resistance exercises. Six trunk exercises (Bridge, pelvic tilt, alternate arm and leg extension, parallel hold, side bridge and superman) were selected which were performed on the floor and a swiss ball. Two resistance exercises (chest press and shoulder press) were performed bilaterally and unilaterally on a bench and a swiss ball. Surface electrodes were placed over the upper lumbar erector spinae (positioned to measure activity to emphasize more as a global muscle), lumbosacral erector spinae (positioned to measure activity to emphasize more as a local muscle) and lower abdominal (positioned to measure activity to emphasize more as a global muscle). The results of the muscle activity was displayed as a ratio as the muscle sites were normalised to a 3 second maximum voluntary isometric contraction test (Upper lumbar erector spine and lumbo sacral erector spinae were referenced to as back extension; Lower abdominal was referenced as abdominal hollowing).For the upper lumbar erector spinae, there was no significant difference between the stable and unstable surface muscle activity, whilst performing the six trunk exercises. However the superman exercise had significantly greater activation than the bridge (51.3% of superman), pelvic tilt (48.0%), alternate arm and leg extension left (49.1%), parallel hold (12.3%), side bridge right (30.8%), and side bridge left (51.9%). The shoulder press had no significant difference between the stable and unstable muscle activity. However the unilateral shoulder press had greater activation over the bilateral (91.1% of unilateral right arm; 66.6% of unilateral left arm). During the chest press, significantly greater activation was observed in the unstable surface (37.7%). The unilateral chest press had greater activation over the bilateral (71.7% of unilateral).

For the lumbosacral erector spinae, there was significantly greater activation (4.7%) of unstable surface muscle activity compared to stable activity whilst performing the six trunk exercises. The superman exercise had significantly greater activation than the bridge (84.6% of the superman), pelvic tilt (50.3%,), alternate arm and leg extension right (65.6%), alternate arm and leg extension left (62.7%), parallel hold (12.5%), side bridge right (34.5%), and side bridge left (61.0%). The shoulder press had no significant difference between the stable and unstable muscle activity. However the unilateral shoulder press had greater activation over the bilateral (86.3% of unilateral right arm; 33.3% of unilateral left arm). ). During the chest press significantly greater activation was observed between in the unstable surface (54.3%). The unilateral chest press had greater activation over the bilateral (50% of unilateral right arm; 66.6% of unilateral right arm).For the lower abdominal, there was significantly greater activation (27.9%) of unstable surface muscle activity compared to stable activity whilst performing the six trunk exercises. The side bridge exercise had significantly greater activation than the bridge (30.6% of side bridge), pelvic tilt (33.8%). alternate arm and leg extension right (46.5%), alternate arm and leg extension left (25%), parallel bold (37.3%), side bridge right (48.2%), and superman (13.9%). The shoulder press had no significant difference between the stable and unstable muscle activity nor any different between unilateral and bilateral shoulder press. During the chest press significantly greater activation was observed between in the unstable surface (37.8%). The unilateral chest press had greater activation over the bilateral (24% of unilateral right arm; 33.2% of unilateral right arm).The results indicate that co activation does exist between the local and global muscles and that unstable surfaces provide more of a stimuli, though the electromyography sites werent accurately placed. However the results do not accurately depict the individual muscles sites activation levels relative to the normalisation test, hence it is unclear what activation levels they achieve i.e. strength or stability. The experiment does suggest that incorporating unilateral limb movement may provide more stimuli for strength adaptation of the global muscles and add specificity training benefits.Similar studies show that performing resistance exercises on unstable surfaces provide increased activation of the core musculature as compared to stable platforms (Marshall and Murphy, 2006; Anderson and Behm, 2005). However performing exercises and movements may not provide sufficient training specificity. According to Hamlyn, Behm & Young (2007) activating the trunk musculature from a supine or prone position whether on stable or unstable surfaces may not provide training specificity since most sports and daily activities are performed in an upright an erect position. Neither does the training action replicate the muscular action or velocities experienced in a closed kinetic chain movement. Thus Hamlyn et al. (2007) performed an experiment to examine the activation of the trunk muscles during dynamic weight training exercises such as the squat and deadlift and isometric instability exercises such as the superman and sidebridge on a swiss ball. Electromyography activity was measured from the lower adominal (LA), external obliques (EO), upper lumbar erector spinae (ULES) and lumbar sacral erector spinae (LSES) muscle groups. The results showed that performing an 80% 1 RM squat exceeded the LSES activity by 65.5% and 53.1% compared to the superman and sidebridge respectively. Performing an 80% 1 RM deadlift exceeded the ULES activity by 69.3% and 68.6% compared to the superman and sidebridge respectively. However there was no significant change in the EO or LA activity. The results show that stability muscles of the trunk achieve greater activation whereas the global muscles achieve relatively the same activation. Thus performing upright, resisted, dynamic exercises can provide higher trunk muscle activation and provide greater training specificity as opposed to unstable surfaces. The studies discussed so far, mainly comprised of relatively untrained subjects compared to well trained subjects in resistance exercises. A study by Wahl and Behm (2008) showed that unstable devices did not provide an increase in muscle activation in the lumbosacral erector spinae (LES) or lower abdominals compared to stable surfaces in highly resistance trained individuals, suggesting that resistance training with free weights provides sufficient trunk muscle activation and that unstable surfaces are unable to provide further stimulus to trained individuals. Studies using the intervention of unstable surfaces to improve athletic performance have shown no concrete evidence on well trained subjects either. A 6 week swiss ball training programme had a positive impact on the core stability test score, however had no impact on the running economy, posture, vertical and horizontal ground reaction forces of the subjects (Stanton, Reaburn & Humphries, 2004; Sato & Mokha, 2009). Similarly a swiss ball training programme for trained swimmers did not have any significant improvement in vertical jump heights, medicine ball throws and 100 yard swim time compared to the control group (Schibek, Guskiewicz, Prentice, Mays & Davis, 2001). However in relatively untrained subjects various studies have shown that unstable surface training has been able to improve performance in terms of vertical jump heights, take off velocity and balance (Behm et al., 2010a). In conclusion unstable surface training is able to target the points highlighted in Table 2. but not necessarily simultaneously or in a multi planar action. Unstable surfaces do target the local and global muscles but predominantly either target local or global muscles independently, in terms of activation levels. Neither have the studies made any recommendations on the duration or type of movement to be used. For instance local muscles are slow twitch fibres hence longer durations with low stimulus are required, whereas with global muscles on and off type muscle activation with high stimulus is required. The evidence of transfer of training effect into the athletic population isnt substantial either. Since S&C coaches tend to work with trained athletes the efficacy of unstable surfaces in providing further adaptation and improvement is not clear and it is suggested that the improvements in the trunk stability are skills specific and traditional free weights training provides sufficient core training (Willardson, 2007). Further more if athletic performance is the target then movements such as the squat, deadlifts, pulls, cleans and snatches should be used which are multi joint exercises targeting force, power and velocity profiles (Behm et al., 2005). However unstable surface training can be beneficial as part of a periodised training program where the training goal is embedded within the mesocycles and the requirement can be switched to target either strength or endurance of the trunk region (Willardson, 2007). This can be achieved if an S&C coach is aware of the activation levels for local and global muscles along with the exercises that activate these muscles independently for strength and endurance adaptations. Further research into synthesising exercises on unstable devices that provide specific adaptations of endurance and strength would be highly beneficial. Despite these training recommendations ultimately the best way of improving stability and force transfer in a sporting movement is to practise the sport itself on the same surface as the sport (Willardon, Behm et al., 2005).

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APPENDIX A

APPENDIX B

Start position for the pike, knee up, skier, decline push up and hip extensions

End position for the pike

End position for the knee up

End position for the skier

End position for the decline push up

End position for the hip extension

Start position for the roll out

End position for the roll out

Sitting march

1