The influence of ‘Slacklining’ on quadriceps rehabilitation, activation and intensity

Objectives: To determine and compare the level of quadriceps activation for knee injured participants during kinetic open-chain, closed-chain and composite-chain (Slackline) clinical exercises. Quadriceps activation is a critical component of lower limb movement and subsequently, rehabilitation. However, selective activation can be difficult due to pain, loss of function and impaired neuro-motor activation.

Design: Repeated measures (within-subjects) ANOVA.

Methods: Consecutive physiotherapy outpatients (n = 49, 41.8±16.8 years, range 13–72 years, 57%female) with an acute (<2 weeks) knee injury were recruited. Participants were assessed for quadriceps activation using skin mounted electromyography during five separate clinical quadriceps activation

exercises: two open-chain, inner range quads and straight leg raise; two closed-chain, step down and step up; and a composite-chain, slacklining step-up. Outcome measures were: median score on electromyography as measured in microvolts (V); and perceived exertion on an 11-point numerical rating scale.

Results: Median scores of the open- and closed-chain exercises showed no statistical difference,

while composite-chain Slackline exercise showed significantly (p < 0.0001) higher quadriceps activation

(F(2.52, 121.00) = 21.53, p < 0.0001) at significantly lower exertion (F(1.62, 77.70) = 26.88, p < 0.0001).

Conclusions: The use of Slackline rehabilitation training can provide significant increases in activation and recruitment of the quadriceps for composite-chain exercises in the clinical setting. This activation occurs spontaneously at significantly lower levels of perceived exertion. This spontaneous quadriceps activation in a selective and simple manner is a valuable adjunct exercise for lower limb rehabilitation programmes.

This is of particular relevance for the outpatient setting and circumstances where the quadriceps is inhibited and activation is required.

© 2013 Sports Medicine Australia. Published by Elsevier Ltd. All rights reserved.

1. Introduction

   Activation of the quadriceps muscle is a critical aspect of kneeand lower limb function and when required, the rehabilitationprocess. It is recognised that following injury or surgery to thelower limb, incomplete voluntary activation of the quadricepsleads to weakness and subsequent secondary problems1in boththe affected and unaffected limb.2These problems can includeloss of function,3failure to return to sport4and premature onsetof osteoarthritis.5The presence of increased pain6can cause upto a 34% decrease in maximal isometric knee extension torque.7However, the use of weight-bearing (WB) or non-weight-bearing(NWB) knee-extension exercises does not acutely change quadri-ceps torque output or the levels of activation.8Selective muscle activation can also be difficult due to pain inhibition, loss offunction and impaired neuro-motor activation. Consequently, therequired voluntary activation has been found to correlate highlywith knee extension force production.9This can occur throughopen, closed, and composite-kinetic chain exercises. Composite-chain activity occurs when there is a weak link in the kinetic chainthat creates abnormal motor synergy patterns.10Composite-chainactivity is present when the loaded limb’s contact surface is ableto move freely, such as in three dimensions. Consequently it iscritical for individuals with a knee injury to initiate early quadri-ceps activation11and preferably with minimal perceived voluntaryactivation.The use of active exercises is one of the simplest andmost efficient techniques to achieve active selective quadricepsactivation.12,13It would be even more advantageous if these exer-cises were easy to perform, enabled a rapid gaining of specificquadriceps activity and activation but without significant pain orperceived exertion levels. It has been suggested that to gain acute changes in quadriceps torque and activation levels there are gen-erally two options: the use of external dis-inhibitory interventions,such an example is cryotherapy8; or external facilitators such asmuscle stimulation.14An alternative method to consider is the useof exercises that specifically induce a higher level of automaticor innate activation of the required muscle groups.13,15However,there is debate as to whether preferential strengthening or acti-vation of the quadriceps can be affected by altering lower limbjoint orientation or muscular co-contraction.16If the quadricepsare selectively activated by the innate process of simply perform-ing the activity, then subsequent recruitment and control will befacilitated. It is advocated that the action of innate or involuntarymuscle activation focuses the individual’s ability to the requiredactivation and facilitates subsequent control of the required musclein the voluntary and active settings.13,15This aspect of knee exer-cise and quadriceps activation is an area where further research hasbeen recommended.16The activity of slacklining is defined as ‘the action of retain-ing balance while standing or moving on a tightened band’.17Ithas been demonstrated to activate or provide training-inducedimprovements in the rate of force development in lower limbactivity,18,19prevention of injury in elite athletes,20posturalstability21and postural control22as a more challenging exercisefor the knee and hip joints than multi-functional rocker boards andair cushions.23Slacklining has also and been reported to reducepatella subluxation within a physiotherapy supervised exerciseprogramme24and enhance functional knee joint stability throughpreparatory activation of the rectus femoris muscle.22This actionis coupled with the ability to provide learned motor control, suchas down regulation of the Hoffman reflex, as well as a trainingeffect and mechanisms that are pre- rather than post-synaptic.17Given that balance and strength promotion are important for injuryprevention, the use of an activity such as slacklining should beinvestigated in the early rehabilitation phase.18,19,21This studyaimed to investigate the effectiveness of slacklining as a rehabil-itation exercise in spontaneously inducing quadriceps activationin the clinical rehabilitation environment.

2. Methods

   Design: An observational cross-sectional cohort with a single-session measurement set was used. This cohort was used toinvestigate and compare slacklining and the level of quadricepsactivation (dependent variable) achieved during three differentknee exercises (independent variable) of (i) traditional clinicalbased open kinetic chain, (ii) closed kinetic chain, and (iii) com-posite kinetic chain.

   Participants: Participants (n = 49) with an age range of 13–72years (41.8 ± 16.8, 57% female) were recruited from a physiother-apy outpatients setting. A total of 51 participants were recruitedwith three cases eliminated that were three standard deviationsoutside the mean or equivalent to 0.12% chance of being part ofthe intended population of interest.

   Inclusion criterion were a kneeinjury sustained within the preceding two weeks. Exclusion criteriawere red flag signs, including fracture, age <13 years and inabilityto understand spoken English. A total of 18 different conditionspresented including soft tissue injury, pre- and post-operative,degenerative and overuse (Table 1).

Written informed consent wasgained and all participant information was de-identified to ensureanonymity and compliance with ethical standards of the Universityof the Sunshine Coast Human Ethics Committee as per the approvalnumber A/12/434.

Procedures: Participants completed five separate quadricepsexercises in the clinical setting at a self-perceived effort (PE) ofa ‘strong to maximal contraction’. This method was chosen as it replicated the clinical rehabilitation situation, rather than a con-trolled laboratory environment.

These exercises included fourstandard quadriceps rehabilitation exercises: two open-chainexercises13of an inner range quadriceps (IRQ) contraction throughthe final 30 degrees of knee extension over a foam roller, and; astraight leg raise (SLR), a contraction of the extended knee thenlifting the limb from the resting surface.

The two closed-chainexercises13of both a step-up and a step-down off a standard heighthouse-step of 200 mm.25An example of the slackline activity, Stage1 – Step 1 ‘Stepping up’ is shown in Fig. 1 which26is consideredFig as the first and simplest activity involved in slacklining.26,27TheSlackline employed in this study was a ‘Fun Line’ manufacturedby Gibbon, anchored at a length of 3 m and a height of 25 cm.

The perceived effort was recorded after each category of open-chain, closed-chain and slackline (composite-chain) on an 11-pointnumerical rating scale (NRS) anchored at 0 = no perceived effort and10 = maximum perceived effort.Participants were permitted two familiarisation test trials foreach exercise and then repeated each exercise three times.

The fiveexercises were performed in the same order by all participants. Themedian score was recorded from the digital output on a ‘NeurotracMyoplus’ (Verity Medical UK) using skin-mounted electromyog-raphy electrodes in a longitudinal configuration.28This systemprovided both audio and visual biofeedback to the participant andease of recording of the digital output. Active encouragement fromthe therapist was not provided during the exercise, only throughthe initial instructions described and the EMG dual outputs. A one-minute rest was taken between each different exercise.

Statistical analyses: A simple repeated measures analysis of vari-ance (RMANOVA) was performed with each type of activity (innerrange quadriceps, straight leg raise, step-up, step-down, and slack-line) as the within-subjects variable. Standardised residuals wereexamined to eliminate within-cell outliers.

There were 49 cases inthe final sample to be analysed. Because the hypothesis was testingwhether slackline activity produced different results from the otherfour traditional activities, a priori simple contrasts were requestedcomparing that outcome to each of the other four. Mauchley’stest of sphericity was significant (indicating this assumption wasviolated) and thus, Greenhouse–Geisser corrected statistics arereported.For the analyses of perceived effort, similar analyses were per-formed.

The first perceived effort measure was taken after innerrange quadriceps and straight leg raise, the second after step-upand step-down, and the third after the slackline activity. Thus, thelast was compared to the first two via a priori simple comparisons.

Results

The population demographics are detailed in Table 1.Results of the RMANOVA indicated a significant F for thewithin-subjects analysis (F(2.52, 121.00)= 21.53, p < 0.0001). Impor-tantly, there was an impressive eta squared of 0.31, indicating that31% of the variance in quadriceps activation was accounted for byactivity (supplementary Table 1).

The findings shown in Table 2 demonstrate that quadriceps acti-vation for slackline activity was much higher than any of the otherfour groups, which were largely equivalent. The 95% confidenceintervals (CIs) for the first four groups, for example, overlap almostcompletely, whereas the 95%CI for slackline does not overlap at allwith any of the other four. This leads to highly significant contrastsbetween slackline activity and the other four groups (supplemen-tary Table 2). Consequently, we can conclude that quadricepsactivation via slackline activity is significantly (and substantially).3164.0485.319different to any of the other four, which are largely indistinguish-able from each other.When perceived effort was examined, the opposite effectwas observed. Again, the assumption of sphericity wasviolated, so Greenhouse–Geisser corrected statistics arereported. There is a highly significant difference across groups(F(1.62, 77.70)= 26.88, p < 0.0001) with a large effect size or etasquared = 0.36(supplementary Table 3). Importantly, as Table 3shows, the first two perceived effort scores during the open andclosed kinetic chain activity are largely equivalent, while theperceived effort during slackline activity was significantly lowerthan either of the first two measures (supplementary Table 4).

Discussion

The main findings of this study demonstrated that slacklining,as an exercise for rehabilitation of the knee in the acute injuryphase, enabled a statistically higher level of quadriceps activationto be achieved than traditional open- and closed-chain exercises.Furthermore, this level of activation is achieved by the injured indi-vidual at a lower level of perceived effort.

Consequently, we have anactivity that provides substantially enhanced quadriceps activationwith substantially reduced perceived effort.

Previous research has found that closed-chain exercises, suchas step-ups and unilateral leg press exercises, had the greatest lev-els of quadriceps activation and that the straight leg raise had thehighest level for open-chain exercises.13The use of slacklining pro-vides a composite-chain exercise that appears to garner the optimalaspects of both the open- and closed-chain activities.

The findings of this study support the recent arguments thatperceived effort during dynamic whole body exercise is indepen-dent of the physiological attributes of afferent feedback, such asthat from small-diameter muscle of the heart, and lungs.29It alsosupports the arguments that perceived effort for isolated and/orisometric exercises, such as those found in the basic stance actionof slacklining as tested in this study, are independent of the sensoryinputs from muscle spindles and Golgi tendon organs.30 These 30These results also support earlier work that suggests that slack-lining can be beneficial in providing both a prophylactic20,24and arehabilitation action.18,19,21The results of this work when viewedwithin the context of earlier slackline research findings and those ofperceived effort from both the afferent and sensory input perspec-tives, support the concept of innate activation of specific musclesduring dynamic whole body activities.13,15The findings of this study indicated support for the adaptiveplasticity in humans of the Hoffman reflex31as participants wererapidly able to reduce the level of wobble or co-contraction thatis initially present on first commencing a single leg stand on aslackline.26Furthermore, this finding was consistent among par-ticipants supporting previous research that this adaptation is areliable indicator of changes in the neuromuscular system.32Thisstudy supported the contention that the use of the skin mountedEMG is a simple, inexpensive and valuable adjunct to conventionaltherapeutic modalities. The procedure with EMG bio-feedback inthe early phase of rehabilitation enhances knee control and facili-tates innervation of the quadriceps, a key criterion in the regainingof knee extension and postural control.33There may be a central nervous system effect that provides a bilateral effect and such aneurophysiological change would be an asset to clinical rehabilita-tion for the lower limb.The study used a small population of consecutive participantsfrom a physiotherapy setting and consequently cannot be extrap-olated to other more diverse situations.

The use of surface EMGin a longitudinal array is a clinical tool but argued as less accu-rate than other methodological techniques as such as laboratorybased fine wire insertion. This study did not differentiate betweenthe different injury presentations but considered the participantsas a homogenous group. However, it is well documented that dif-ferent conditions will have different levels of inhibition based onthe degree of trauma and swelling, factors that were not taken intoaccount during this study.The strengths are that the study replicates the clinical situationand has direct application to the daily practice of individual kneeinjury rehabilitation. The activity is easy to perform, easy to learnand low risk. Consideration of the knee injuries as a homogenousgroup indicated that the application of slacklining in rehabilitationto overcome quadriceps inhibition is a realistic and clinically viableexercise. Future research will be able to investigate the specifics of quadri-ceps activation during slacklining on different individual injurycategories such as ACL as opposed to patella-femoral joint dysfunc-tion or osteoarthritis. The implications are that slackline exercisescan also be transitioned with equal effectiveness into the reha-bilitation and management of ankle and hip as well as core andwhole body stabilisation work. Laboratory based studies will berequired and also the determination of whether there is an age orgender based bias. Additional whole body balance approach activi-ties such as rocker board and air cushions also facilitate quadricepsfunction and may be an area of further consideration.23Similarly,the use of outcome tools such as the Balance Error Scoring Systemor Star Excursion Balance Test could be used bilaterally to deter-mine whether a central change had occurred due to the use ofslacklining.

Conclusions

The use of slackline rehabilitation training in the clinical set-ting can provide significant increases in activation and recruitmentof the quadriceps muscles. This activation occurs spontaneously atsignificantly lower levels of perceived exertion in a manner thatappears to be selective and simple to achieve. This suggests thatslacklining could be a valuable adjunct exercise for lower limb reha-bilitation programmes, particularly in the outpatient setting and incircumstances where the quadriceps are inhibited and activation isrequired.

Practical implications

?This method is complementary for use in the outpatients reha-bilitation setting.

?The techniques can be used with external electromyography(EMG) biofeedback in a practical setting with ease and simplicity.

?The exercises are rapidly transferable to a home programme forindividual rehabilitation and training.

?The exercise provides a practical approach that enables rapid acti-vation of the medial quadriceps for teaching, rehabilitation andre-education.

Acknowledgements

There was no financial assistance with this project.

Appendix A. Supplementary dataSupplementary material related to this article can be found,in the online version, at

http://dx.doi.org/10.1016/j.jsams.2013.11.007.

Keywords:Rehabilitation Exercise Injury Kinetic chain