Alterations in evertor/invertor muscle activation and center of pressure trajectory in participants with functional ankle instability

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Abstract

Participants with ankle instability demonstrate more foot inversion during the stance phase of gait than able-bodied subjects. Invertor excitation, coupled with evertor inhibition may contribute to this potentially injurious position. The purpose of this experiment was to examine evertor/invertor muscle activation and foot COP trajectory during walking in participants with functional ankle instability (FI). Twelve subjects were identified with FI and matched to healthy controls. Tibialis anterior (TA) and peroneus longus (PL) electromyography (EMG), as well as COP, were recorded during walking. Functional analyses were used to detect differences between FI and control subjects with respect to normalized EMG and COP trajectory during walking. Relative to matched controls, COP trajectory was more laterally deviated in the FI group from 20% to 90% of the stance phase. TA activation was greater in the FI group from 15% to 30% and 45% to 70% of stance. PL activation was greater in the FI group at initial heel contact and toe off and trended lower from 20% to 40% of stance in the FI group. Altered motor strategies appear to contribute to COP deviations in FI participants and may increase the susceptibility to repeated ankle inversion injury.

Introduction

Ankle injury continues to be the most common injury in sports (Fernandez et al., 2007, Hootman et al., 2007). Ankle sprains also represent a large portion of orthopedic injuries among the general population (Waterman et al., 2010), with an associated cost of $2 billion annually (Soboroff et al., 1984). Some ankle sprains are treated with limited or no long-term consequence, however as many as 73% of athletes who suffer an ankle sprain experience repeated ankle injury, and 59% report significant long-term disability and residual symptoms (Yeung et al., 1994). These findings characterize chronic ankle instability (CAI), a firmly established orthopedic problem, which may also play a significant role in ankle osteoarthritis (Valderrabano et al., 2006).

CAI was recently characterized by Delahunt et al. (2010) as “an encompassing term used to classify a subject with both mechanical and functional instability.” Several factors contribute to the chronic nature of ankle instability. Mechanical factors, such as joint laxity, play a significant role in CAI (Hubbard and Hertel, 2006). Sensorimotor deficits (functional instability – FI) also play a primary role in perpetuating the chronic nature of ankle instability (Hertel, 2008). Reported sensorimotor deficits include muscle weakness and dysfunction (Hartsell and Spaulding, 1999, Hopkins et al., 2009, Kaminski and Hartsell, 2002, Palmieri-Smith et al., 2009), static and dynamic postural control alterations (Hertel and Olmsted-Kramer, 2007, McKeon and Hertel, 2008), altered integration of sensory information at the CNS (Hass et al., 2010, Wikstrom et al., 2010), and altered muscle spindle sensitivity (Hopkins et al., 2009, Mora et al., 2003). Many of these sensorimotor deficits have been linked to altered mechanics during various functional movements in patients with ankle instability (Monaghan et al., 2006, Spaulding et al., 2003).

The foot evertors, as the primary resistance to inversion stress, have received considerable attention for their potential contribution to FI (Hopkins et al., 2009, Konradsen and Ravn, 1991, Palmieri-Smith et al., 2009, Santilli et al., 2005). Indeed, peroneal dysfunction has been well documented in subjects with ankle instability (Hopkins et al., 2009, Konradsen and Ravn, 1991, Palmieri-Smith et al., 2009, Santilli et al., 2005). While the evertors likely do not provide a timely protective contraction to prevent injury during unanticipated foot inversion (Delahunt, 2007, Hopkins et al., 2007, Munn et al., 2010), they do help control foot position during functional movement (Delahunt, 2007, Louwerens et al., 1995). Invertor/evertor coupling during movement facilitates a neutral position, aids in balance, and controls loads during the stance phase of gait (Louwerens et al., 1995, Matsusaka, 1986). In addition to reported decreased evertor activation during functional movement (Hopkins et al., 2009, Santilli et al., 2005), there is limited evidence of increased invertor activation during functional movement in participants with ankle instability (Delahunt et al., 2007, Louwerens et al., 1995). Increased invertor activation coupled with decreased evertor activation could result in a more supinated foot position during the stance phase of movement in patients with ankle instability. This idea is consistent with reports of laterally-deviated plantar pressures during the stance phase of gait in patients with ankle instability (Nawata et al., 2005, Nyska et al., 2003).

The purpose of this study was to evaluate potential invertor/evertor imbalances and plantar pressure deviations in participants with FI during the stance phase of gait. We hypothesized that FI participants would demonstrate increased tibialis anterior (TA) and decreased peroneus longus (PL) activation, and laterally deviated center of pressure (COP) trajectories relative to matched controls.

Section snippets

Methods

Subjects (n = 12, 5 males and 7 females; age = 23 ± 4 yr; Height = 1.74 ± .14 m, Mass = 71.6 ± 17.6 kg) were identified with FI via the Functional Ankle Ability Measure (FAAM), the Modified Ankle Instability Index (MAII), and a physical examination. FI inclusion criteria were an ADL score of 90% or less and a sport score of 80% or less on the FAAM and 2 “yes” answers on questions 4–8 on the MAII. The physical exam included a talar tilt and anterior drawer performed by an experienced, licensed physical

Results

Fig. 2, Fig. 3, Fig. 4 summarize the functional analysis results. The red line represents a separation between groups with positive deviations representing greater FI values and negative deviations representing greater control group values. The shaded bands surrounding the fit line provide 95% confidence intervals for the population mean effect size (difference between control and treatment) throughout the stance phase. Therefore, where the shaded area crosses the zero line, statistically and

Discussion

Our data confirm the hypothesis that FI participants exhibit laterally-deviated COP trajectories during the stance phase of gait relative to matched controls. The lateral deviation was present at initial heel contact and continued from early midstance to toe off. Additionally, the TA was more active following heel strike and during midstance in participants with FI, while the PL was facilitated at initial heel contact and toe off and trended toward inhibition following heel strike during early

Conclusion

Participants with FI exhibit a laterally-deviated COP trajectory during the stance phase of gait. These participants also have higher activation of the TA during the early (15–30%) and mid-stance (45–70%) phases of gait. PL activation is also increased at initial heel contact and toe off in FI participants. These altered motor strategies and COP deviations could contribute to the increased susceptibility of repeated ankle inversion injury in this patient population.

Acknowledgements

We would like to acknowledge and thank Travis Dunn for his assistance in data collection and data reduction. There are no conflicts of interest to disclose for any of the authors of this manuscript. Funding for this project was provided by the Ira and Mary Lou Fulton Endowment.

J. Ty Hopkins is currently an associate professor at Brigham Young University, where he coordinates the graduate programs in athletic training and physical medicine and rehabilitation. He graduated from Brigham Young University with a BS in athletic training. His graduate work was completed at Indiana State University where he earned an MS and a PhD. The focus of Dr. Hopkins’ research centers on prevention and rehabilitation of lower extremity joint injury. Specifically, he is interested in how

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    J. Ty Hopkins is currently an associate professor at Brigham Young University, where he coordinates the graduate programs in athletic training and physical medicine and rehabilitation. He graduated from Brigham Young University with a BS in athletic training. His graduate work was completed at Indiana State University where he earned an MS and a PhD. The focus of Dr. Hopkins’ research centers on prevention and rehabilitation of lower extremity joint injury. Specifically, he is interested in how muscles provide stability to joints, how joint injury effects this stability, and strategies to provide better muscle contraction patterns for joint stability. He enjoys working with students in the laboratory and regularly uses his work from the laboratory in the classroom. He serves as a section editor for the Journal of Athletic Training, and he serves on the editorial board and as a reviewer with many other journals and granting agencies. He is also a Fellow with the National Athletic Trainers’ Association and the American College of Sports Medicine.

    Mark Coglianese graduated from BYU in Exercise Physiology in 1995. Graduated from Shenandoah University with a Master’s of Physical Therapy degree in 1999. Practiced full-time in an outpatient orthopedic setting from 1999 to 2008. 2008-present doctoral candidate at BYU in Exercise Science. Fall of 2011 became full-time faculty in Health, Recreation & Human Performance department at BYU-Idaho.

    Philip Glasgow is head of Sports Medicine at the Sports Institute Northern Ireland where he manages elite athletes during training and competition from across a wide range of sports including rugby union, hockey, athletics, boxing, skiing, sailing, cycling, swimming, squash and football. A graduate of the University of Ulster, his doctoral studies investigated factors influencing exercise induced muscle damage and its management. He has worked at a number of major international competitions including various World and European Championships as well as working as a HQ physiotherapist for the Irish team at the Beijing Olympic Games. His particular interests are in the field of functional rehabilitation and in the development of robust clinical assessment for sports specific skills.

    C. Shane Reese is Professor and Associate Chair in the Department of Statistics at Brigham Young University. He received his BS and MS in Statistics from Brigham Young University and PhD in Statistics from Texas A&M University. His research interests are Bayesian hierarchical models with applications to environmental applications, reliability, computer experiments, and sports. In addition, Dr. Reese has served on multiple National Academy of Science Committees. He is the two time winner of the Journal of the American Statistical Association Case Studies and Applications Paper of the Year. He is also a two time Chapter President (Utah and Albuquerque, NM Chapters) of the American Statistical Association.

    Matthew K. Seeley received his PhD in Exercise Science from the University of Kentucky (USA) in 2006. He is currently an assistant professor in the Department of Exercise Sciences at Brigham Young University in Provo, Utah, USA. His research involves various aspects of the mechanics of human locomotion.

    Institutional Review Board Approval for human subject study was granted for this study. All subjects provided written informed consent.

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