By Hyunwook Lee, MS, LAT, ATC*; S. Jun Son, PhD, ATC†; Hyunsoo Kim, PhD, ATC‡; Seunguk Han, MS, ATC*; Matthew Seeley, PhD, ATC*; and J. Ty Hopkins, PhD, ATC*
Lateral ankle sprains (LASs) often result in damage to the lateral ligaments of the ankle, reducing both static and dynamic ankle stability and resulting in a continuum of disability. Indeed, up to 80% of individuals who experience an initial LAS will sustain recurring injury, often developing chronic ankle instability (CAI), a condition characterized by pathomechanical, sensory-perceptual, and motor-behavioral impairments.1 Further, up to 78% of those with CAI will go on to develop posttraumatic ankle osteoarthritis, which can interfere with daily activities of living and reduce quality of life.2
Submaximal force steadiness and accuracy refer to the ability of a muscle to produce a steady and accurate contraction during a static or dynamic task.3 Rice et at3 demonstrated that measuring the regulation of submaximal muscle force was more relevant for daily activities (eg, walking, driving a car, stepping over obstacles, ascending and descending stairs) and sport-related activities (eg, squatting, sprinting, jumping, landing, and cutting) than earlier measures (eg, joint position sense, static force sense).
During daily activities, maximal voluntary activation was used for only 56 seconds.4 In addition, moderately active college students used 17% of their maximal quadriceps and hamstrings force in daily activities. Submaximal force steadiness has been studied in various populations, including those with anterior cruciate ligament reconstruction,5 knee and hip osteoarthritis,6,7 a history of falling,8 and subacute stroke.9 Previous researchers observed that people with these conditions had less force steadiness and accuracy than their uninjured counterparts.
However, no investigators have examined force steadiness and accuracy in patients with CAI. The recently updated model of CAI10 proposed that 6 factors contribute to motor-behavioral impairments: altered reflex, neuromuscular inhibition, muscle weakness, balance deficits, altered movement patterns, and reduced physical activity. Rice et al3 reported that reduced force steadiness was associated with neuromuscular inhibition and muscle weakness in patients with knee pain. If those with CAI show impairment during force-steadiness measurement, this could be a key characteristic of CAI.
Therefore, the purpose of recent research, “Submaximal Force Steadiness and Accuracy in Patients With Chronic Ankle Instability”, published in the Journal of Athletic Training, was to examine the effect of CAI on the submaximal force steadiness and accuracy of the ankle evertors and invertors and the hip abductors. We hypothesized that patients with CAI would show less force steadiness and accuracy in all 3 muscle groups compared with healthy control participants.
The experimental procedures are illustrated in Figure 1. A Biodex dynamometer and Advantage Software (model 3 dynamometer; Biodex Medical Systems, Inc.) were used to measure the maximal voluntary isometric contraction (MVIC) and force steadiness and accuracy of the ankle evertors and invertors and the hip abductors (Figure 2).
We measured the MVIC to permit comparisons of the maximal force of those muscles between groups. Participants (N=42; 19 male, 23 female) were provided an opportunity to become familiar with the isokinetic dynamometer and testing procedure and to perform as many warm-up repetitions as desired (at least 5). During the practice session for the MVIC measures, they were instructed to perform the task at various force outputs (25%, 50%, 75%, and 100% of MVIC).
After a 3-minute rest, participants performed 3 MVIC trials by contracting the muscles (ankle evertors and invertors and hip abductors) as hard as possible for 3 seconds while minimizing other movements. A 1-minute rest was allowed between trials. Previous authors6-9 examined various force increments (10%, 20%, 25%, 30%, 40%, and 50% of MVIC) to measure force steadiness in different joints. Because no earlier investigators measured force steadiness in patients with CAI, we chose to use 10% and 30% of each person’s MVIC based on a study11 in which the authors measured force sense for the same motions.
Participants were informed of how force steadiness and accuracy would be measured. During the test, they were instructed to attempt to stay as close as possible to the target force (10% or 30% of MVIC) for 15 seconds. Five trials were performed for each muscle; the first 2 were considered practice trials. Participants were able to adjust their force using a monitor (1m away) that showed the actual force.
While participants performed the force steadiness trials, 1 examiner indicated the target force line, so that each person easily recognized the target forces. The examiner randomly assigned a target force, 10% or 30% of MVIC, to each person. Force steadiness was defined as the standard deviation across the 10 seconds of data. Force accuracy was defined as the root mean square of the difference between the data and the target force (Figure 3).
Results and Discussion
The primary finding of this study is that our participants with CAI (n= 21; 9 male, 12 female) maintained less force steadiness in their ankle invertors during isometric contractions. In addition, they were less accurate in ankle eversion and inversion. The increased variability in force steadiness and accuracy represents altered motor-unit recruitment and firing rates, impaired proprioceptive information, increased activation of synergist and antagonist muscles, and altered spinal interneuron modulation of motor-neuron firing.3 We propose that the observed alterations in force steadiness and accuracy may be due to impaired proprioceptive function in these patients. Proprioceptive sensory inputs from muscles, tendons, and ligaments are transmitted from the peripheral nervous system to the central nervous system; this process is necessary for appropriate neuromuscular control.12 Neuromuscular control can be affected by (1) the collection of less peripheral information because of damaged proprioception, (2) an inability to integrate the peripheral information in the central nervous system, or (3) an inability to send out the centrally mediated information to the motor units.13 Thus, our results may be attributed to one or more of the aforementioned factors. Moreover, Chung-Hoon et al14 explained that less steadiness in force output may be caused by presynaptic inhibition of Group Ia afferents. Because of the depolarization of primary afferent fibers by interneurons, the input from Ia afferents to the active motor-neuron pool may be inhibited and consequently affect motor-unit activation in maintaining a certain force.15 Accordingly, patients with CAI may be unable to regulate presynaptic inhibition compared with healthy control individuals.15 Furthermore, Docherty and Arnold16 suggested a significant relationship between ankle instability and force sense. They showed that patients with functional ankle instability had deficits in precise force sense and joint position sense.16 Because CAI is associated with impaired proprioception, strength, and postural control as a result of repeated LASs,10 reduced force accuracy could be a consequence of the injury.
Our findings of impaired ankle and hip force steadiness and accuracy in patients with CAI provide useful insights for clinicians developing rehabilitation protocols. An indirect indication was that the CAI group had proprioceptive deficits in the force steadiness and accuracy of ankle eversion and inversion. This impaired proprioception might lead these patients to be more susceptible to injury positions as they have difficulty integrating the peripheral information and correcting their movement in relation to visual information. Restoring proprioceptive function of the ankle and hip plus visual training may be key to improving clinical outcomes for this population.17 Our data suggests that clinicians should continue to focus on restoring proprioceptive function in both the distal and proximal joints in conjunction with visual information to improve force control in various movements. Moreover, movement related functional rehabilitation exercises are necessary for adjusting and correcting movement errors and increasing the ability to produce appropriate force during a given task.
Hyunwook Lee, MS, LAT, ATC, is a PhD student in the Department of Exercise Sciences at Brigham Young University in Provo, UT.
Jun Son, PhD, ATC, is an Assistant Professor in the Graduate School of Sports Medicine at CHA University in Seongnam-si, South Korea.
Hyunsoo Kim, PhD, ATC, is an Assistant Professor in the Department of Kinesiology at West Chester University in West Chester, PA.
Seunguk Han, MS, ATC, is a PhD student in the Department of Exercise Sciences at Brigham Young University in Provo, UT.
Matthew Seeley, PhD, ATC, is an Assistant Professor in the Department of Exercise Sciences at Brigham Young University in Provo, UT.
Ty Hopkins, PhD, ATC, FNATA, FACSM, is a Professor in the Department of Exercise Sciences at Brigham Young University in Provo, UT.
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