Predictable Perturbations: An Innovative Clinical Perspective

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INTRODUCTION

Falls are the leading cause of non-fatal injuries and death by injury in the United States, accounting for approximately 48% to 75% of all unintentional injuries reported for adults 65 years and older1 with hip fracture incidence expected to risedue to the growth in the older population.2

Most falls occur in or just outside the home3 resulting in persistent strength and mobility deficits contributing to declines in balance, limiting capacity for independent function, and further increasing risk of recurrent injury.4

Fall risk depends on an interaction of many factors, and falls are particularly difficult to explain in older adults who live active, independent lives.5 Current tests are less successful in explaining falls in active aging adults than in those who are described as frail.5 Timed mobility performance measures may not adequately challenge systems to detect problems essential in fall risk situations in individuals not exhibiting existing balance deficits or outwardly displaying observable limitations.6, 7 The inability of certain balance and timed mobility performance measures to explain falls is due in part to intrinsic ceiling effects, compromised sensitivity associated with a lack of variability in maximum performance scores as well as their lack of responsiveness to falls in active community-dwelling aging adults.5, 8 ,9   New assessment tools are needed for an active aging population. Such tools should contain more challenging performance-based measures 5 including situations requiring reactive balance.10   Researchers need to test individuals as they react to externally imposed perturbations.10 Following a perturbation that might cause a fall, one must recover balance using a feet-in-place postural strategy response, or a protective compensatory step.11

Perturbation assessment and treatment paradigms have gained recent attention. 9,12-15  Designed with a reactive bias in an attempt to mimic “real life” circumstances, these perturbation studies do not provide objective or quantitative stepping performance measures or procedures which are clinically safe or of feasible practical use in multiple clinical settings.

The use of a predictable perturbation assessment paradigm is supported by studies concluding that predictability and prior knowledge of magnitude and direction of perturbation forces do not alter EMG latency of lower extremity motor responses and have no effect on automatic postural responses.16,17  A valid, reliable, practical, safe and clinically feasible predictable perturbation method  (ICC = 0.94, ROC AUC = 0.992, sensitivity = 93%, specificity = 96.6 %), has been described.18

Perturbation Neurophysiology

Predictable Scaling

Individuals without neurologic impairments proportionately scale to the magnitude of their automatic postural responses to the magnitude of their disequilibrium19. This scaling is based on both the direct sensory characteristics, such as the initial speed of the perturbation and anticipatory mechanisms based on prediction of displacement characteristics, such as the estimated displacement amplitude19. The nervous system must rely on predictive mechanisms based on prior experience.

Responses: Proactive and Reactive

Proactive and reactive adaptations each have an important role in fall prevention. Reactive adaptations can reduce the likelihood that a balance loss will lead to a fall, whereas proactive adaptations can eliminate the occurrence of a balance loss entirely.19 Proactive adaptations can be highly effective when the direction of a perturbation is foreseeable and can lead to desirable movement patterns that allow balance to be maintained under both perturbed and unperturbed conditions.19 When perturbations are less certain, reactive responses may play the dominant role in avoiding a fall. It may thus be argued that both proactive and reactive adaptations should be targeted in interventions to reduce fall incidence in older adults. Proactive adaptation to movement stability represents a first line of defense against falling, whereas reactive responses represent a second line of defense; both are important.20 Adaptive feed-forward control of stability is based on a continuously updated internal model of COG thus appears to be used by old and young alike.20 Both anticipatory and reactive mechanisms are regularly employed to control balance during gait, centrally organized patterns of muscle activity, and modulated based upon available sensory information, biomechanical constraints, support surface conditions and behavioral goals and learning.12 Anticipatory mechanisms are based on a feed-forward movement plan utilized in predictable, well- learned situations, whereas reactive mechanisms are generated by the use of sensorimotor feedback utilized in unpredictable situations.12 Reactive postural control can be used to modify movements already in progress and can be either automatic (reflexive) trip, or volitional in the case or a self-initiated correction of foot placement.12

Feasible Stability Region

With repeated perturbations affecting posture, the CNS likely builds new, or updates existing internal representations to improve its feed-forward control, while decreasing a person’s reliance on feedback corrective mechanisms for recovery.14 The relationship between a standing person’s center of mass (COM) and base of support (BOS) defines the stability limits that, outlines a “stability region”. The BOS consists of the outline area of each foot in contact with the ground and the area between the feet in bipedal stance. An age- related increase in body sway often is cited as an indication of a decline in stability, and has been associated with falling among older adults. No conclusive evidence however, indicates that people who sway with greater amplitude are less likely to recover balance after perturbation.14A feasible stability region (FSR) exists between forward and backward loss of balance thresholds.14 Balance loss occurs when a large-scale perturbation displaces the COM state outside the FSR exceeding in place ankle and hip strategies resulting in a compensatory step and establishing a new BOS.14 Unperturbed locomotion is a series of controlled volitional forward falling constantly requiring forward stepping.14

Plasticity

Neuromuscular protective mechanisms against falls can be developed or enhanced with appropriate adaptive training. With repeated exposure to perturbations, a newly acquired, predominantly predictive form of adaptive control emerges, with a decreased reliance on feedback corrective mechanisms for recovery.14 The CNS builds, refines or updates an internal representation of the potential threats that may occur in the external environment.14

Retention

Retention within the CNS usually is considered a function of long-term changes that occur within the neural circuitry, a consequence of the process of consolidation or stabilization of long- term memory. This process accompanies the formation of new synapses, synthesis of new protein and increase in the strength of existing synapses in the cortical and sub-cortical structures (basal ganglia, cerebellum) for tasks involving voluntary movements.14 The retention of adaptive behavior may be conditioned by the penalties imposed upon an inappropriate response by the CNS and increased potential of injury. A highly threatening environment would be sufficient to induce long-term retention of acquired motor behavior.   Emerging evidence supports applying perturbations mimicking real-life situations as a form of motor training, with long-term effects on postural stability for prevention of loss of balance and falls.14 Older adults can rapidly develop adaptive skills for fall prevention in a similar manner as young adults.14

The primary benefit of perturbation-based training is a reduction in movement time, rather than time required to detect instability and initiate the response.15

Repeated Incremental Predictable Perturbations in Standing: RIPPS

Overview

Based upon a recent study18, The Spring Scale Test (SST): A Reliable and Valid Tool for Explaining Fall History, a clinically practical, predictable perturbation method exists. Predicated on repeated incremental predictable perturbations in standing (RIPPS), the RIPPS method is a first attempt, single- failure protocol clinically developed to quantify forward and rear direction stepping limits with applications as an assessment and an induced stepping treatment paradigm. Designed with a feed-forward bias, the RIPPS method is both reactive and anticipatory, consisting of repeated rounds of progressive sagittal plane loading and unloading forces to stress the limits of forward and rear stepping postural responses. Beginning at 1- pound waist pull force, each round of loading and unloading is increased by 1 additional pound to the limits of postural stability determined by RIPPS performance criteria.  Forward and rear directional stepping limits are quantified as percent of total body weight (TBW %) for the purposes of documenting fall risk assessment and responses to induced stepping limit training. The use of % TBW to quantify perturbation force is well established.10-15 ,18    

Instrumentation and Setup

Perturbation forces are quantified by a pocket-sized linear spring scale strain gauge calibrated in 1 pound increments, is affixed to a 5-inch wide padded waist belt secured around the client’s waist and connected to the examiner via a 4 foot length safety tether strap. Perturbations are administered with the examiner positioned in close proximity to the client, standing approximately 3 feet from a compliant support surface. Anterior direction limit testing (rear stepping) is performed with the examiner facing the client, while posterior direction limit testing (forward stepping) is performed with the client’s back toward the examiner.

RIPPS Perturbation Method

Loading waist pull forces are administered in a predictable, gradual, gentle, accommodative fashion. Clients are continuously instructed to resist loading forces to their maximum limit and are reminded of the RIPPS performance criteria.

Unloading occurs at each round of progressive 1-pound incremental accommodated loading force. Unloading is administered in a quasi-random fashion within a 5- count window, at the discretion of the examiner. Clients are continuously reminded of the RIPPS unloading performance criteria.

RIPPS Performance Criteria

RIPPS loading forces must be accompanied by a foot- flat or heel – sole contact with floor postural response, defined as accommodation. RIPPS unloading postural responses must not exceed a 3- step response.

RIPPS End Points

A RIPPS end point occurs when either a loading or unloading RIPPS performance criteria is not achieved at a given round of waist pull force value.

RIPPS Directional Limit Score

A RIPPS TBW % directional limit score is obtained for both the anterior and posterior directions. A directional limit force value is derived from the round previous to the directional end point force (failure) round. The directional limit TBW % score is calculated by dividing the spring scale measured force in pounds by client’s body weight.

RIPPS TBW % Performance Measure

The lower of the 2 directional limit TBW % scores is the RIPPS TBW % performance measure of clinical significance.

RIPPS Clinical Applications

Assessment

The RIPPS method is domain specific for in place and stepping postural responses.

The RIPPS 10 % TBW performance value is highly discrimiant to fall status providing clinicians with a highly sensitive and specific fall risk screening tool capable of ID deficits that otherwise would be missed in the active community living older adult. The RIPPS  % TBW performance measure should guide functional locomotion recommendations, goals and treatment interventions where induced stepping limit deficits are identified.  The RIPPS 10 % TBW value should be considered a minimal threshold performance value consistent with known non-fallers over the age of 65 with a mean 12.3 %TBW18, suggesting a functional stepping ‘reserve’ exists and could be attainable and should be a clinical treatment outcome particularly since the 80 to 89 age group represented the largest sample sub group in the SST study.

Despite the predictable, anticipatory design of the RIPPS method, reactive postural responses are typical, dominating anticipatory postural responses in those individuals with compromised balance evidenced by apprehension, excessive loading hip strategy, multiple steps in response to unloading and excessive upper extremity responses. Ceiling effects rarely occur utilizing the RIPPS method of assessment.

Induced Stepping Treatment Paradigm

Induced stepping has been associated with greater skill retention .14

The RIPPS method offers a safe option for induced step training for individuals 65 years of age and over, requiring intervention having attained a RIPPS % TBW score at or below 10 %, addressing the lower RIPPS directional limit % TBW score. Once a RIPPS TBW % directional step limit deficit has been identified, RIPPS induced step training would involve blocks of repeated rounds of loading and unloading at progressive waist pull forces. Anecdotal evidence suggests that sustained  % TBW values equal to or greater than 10 % over a 2- week period for 3 consecutive treatment sessions could suggest retention of newly acquired stepping skill. Further study is warranted to examine training protocols and long- term skill acquisition and fall status. Manipulation of feed-forward and reactive responses would be possible by variations in perturbation loading/unloading force intervals.  Non-stepping training options are possible employing sustained, continuous loading to limits of foot-flat accommodation to augment scaling or ankle and hip stabilization strategies. Lateral perturbation methods, (feet in place and induced stepping) may also provide further clinical assessment measures and training options.

Summary

The goal of RIPPS is to introduce percent of total body weight (TBW %) as a practical clinical balance measure for fall risk assessment and treatment purposes. Research supports the reliability and discriminant validity of the RIPPS 10 % TBW performance value for explaining fall history in active independent community living older adults.

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Louis DePasquale PT, MA

Master of Arts Kinesiology, New York University Physical Therapy Certificate, Columbia University B.S. Physical Education, Manhattan College Affiliations: Bon Secours Health System, Francis Schervier Long Term Home Health Program Hebrew Home at Riverdale, Long Term Home Health Program Practice: 30+ years geriatric home care setting Publications: • DePasquale L, Toscano L. ”The Spring Scale Test (SST): A Reliable and valid Tool for Explaining Fall History.” JGPT 2009; 32:(4). • Bohannon R, DePasquale L. ”Physical Functioning Scale of the Short-Form (SF) 36: Internal Consistency and Validity with Older Adults.” JGPT 2010; 33(1). • DePasquale L, ”Perturbation Neurophysiology.” Advance: for Physical Therapy and Rehab Medicine 2011: October 17. • DePasquale L, ”Safety in the Balance.” Physical Therapy Products. November 2011.

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