Functional Task and Balance Performance in Bed Rest Subjects and Astronauts.

INTRODUCTION: The purpose of this study was to determine how short- and long-duration spaceflight affects astronauts' performance on functional tests that challenge the balance control system (Seated Egress and Walk; Object Translation; Recovery from Fall/Stand; and Jump Down) and on clinical tests of balance function (Computerized Dynamic Posturography and Tandem Walk). In addition, we examined how exercise affects functional performance after long-term axial body unloading during 70 d of bed rest at 6° head-down tilt. METHODS: Data were collected twice during the 2-mo period before spaceflight or during the 2-wk period before bed rest, and four times after flight or bed rest: on the day of landing or the day bed rest ended, 1 d and 6 d later, and a final session 12 d after bed rest or 30 d after spaceflight. RESULTS: For bed rest subjects, long-term axial unloading alone caused functional performance deficits immediately after bed rest. However, the addition of an exercise regimen did not significantly improve median functional performance immediately after this axial unloading. For spaceflight subjects, the length of the space mission was directly related to the severity of functional performance deficits within 1 d of landing and during the subsequent recovery period after flight. DISCUSSION: The performance data suggest that an additional sensorimotor-based countermeasure may be necessary to maintain functional performance at preflight levels immediately after spaceflight.Miller CA, Kofman IS, Brady RR, May-Phillips TR, Batson CD, Lawrence EL, Taylor LC, Peters BT, Mulavara AP, Feiveson AH, Reschke MF, Bloomberg JJ. Functional task and balance performance in bed rest subjects and astronauts. Aerosp Med Hum Perform. 2018; 89(9):805-815.

Physiological and Functional Alterations after Spaceflight and Bed Rest.

INTRODUCTION: Exposure to microgravity causes alterations in multiple physiological systems, potentially impacting the ability of astronauts to perform critical mission tasks. The goal of this study was to determine the effects of spaceflight on functional task performance and to identify the key physiological factors contributing to their deficits. METHODS: A test battery comprised of seven functional tests and 15 physiological measures was used to investigate the sensorimotor, cardiovascular, and neuromuscular adaptations to spaceflight. Astronauts were tested before and after 6-month spaceflights. Subjects were also tested before and after 70 d of 6° head-down bed rest, a spaceflight analog, to examine the role of axial body unloading on the spaceflight results. These subjects included control and exercise groups to examine the effects of exercise during bed rest. RESULTS: Spaceflight subjects showed the greatest decrement in performance during functional tasks that required the greatest demand for dynamic control of postural equilibrium which was paralleled by similar decrements in sensorimotor tests that assessed postural and dynamic gait control. Other changes included reduced lower limb muscle performance and increased HR to maintain blood pressure. Exercise performed during bed rest prevented detrimental change in neuromuscular and cardiovascular function; however, both bed rest groups experienced functional and balance deficits similar to spaceflight subjects. CONCLUSION: Bed rest data indicate that body support unloading experienced during spaceflight contributes to postflight postural control dysfunction. Further, the bed rest results in the exercise group of subjects confirm that resistance and aerobic exercises performed during spaceflight can play an integral role in maintaining neuromuscular and cardiovascular functions, which can help in reducing decrements in functional performance. These results indicate that a countermeasure to mitigate postflight postural control dysfunction is required to maintain functional performance.

Sex differences in leg dexterity are not present in elite athletes.

We studied whether the time-varying forces that control unstable foot-ground interactions provide insight into the neural control of dynamic leg function. Twenty elite (10F, 26.4±3.5yrs) and 20 recreational (10F, 24.8±2.4yrs) athletes used an isolated leg to maximally compress a slender spring designed to buckle at low forces while seated. The foot forces during the compression at the edge of instability quantify the maximal sensorimotor ability to control dynamic foot-ground interactions. Using the nonlinear analysis technique of attractor reconstruction, we characterized the spatial (interquartile range IQR) and geometric (trajectory length TL, volume V, and sum of edge lengths SE) features of the dynamical behavior of those force time series. ANOVA confirmed the already published effect of sex, and a new effect of athletic ability, respectively, in TL (p=0.014 and p<0.001), IQR (p=0.008 and p<0.001), V (p=0.034 and p=0.002), and SE (p=0.033 and p<0.001). Further analysis revealed that, for recreational athletes, females exhibited weaker corrective actions and greater stochasticity than males as per their greater mean values of TL (p=0.003), IQR (p=0.018), V (p=0.017), and SE (p=0.025). Importantly, sex differences disappeared in elite athletes. These results provide an empirical link between sex, athletic ability, and nonlinear dynamical control. This is a first step in understanding the sensorimotor mechanisms for control of unstable foot-ground interactions. Given that females suffer a greater incidence of non-contact knee ligament injuries, these non-invasive and practical metrics of leg dexterity may be both indicators of athletic ability, and predictors of risk of injury.

Characterization of the disruption of neural control strategies for dynamic fingertip forces from attractor reconstruction.

The Strength-Dexterity (SD) test measures the ability of the pulps of the thumb and index finger to compress a compliant and slender spring prone to buckling at low forces (<3N). We know that factors such as aging and neurodegenerative conditions bring deteriorating physiological changes (e.g., at the level of motor cortex, cerebellum, and basal ganglia), which lead to an overall loss of dexterous ability. However, little is known about how these changes reflect upon the dynamics of the underlying biological system. The spring-hand system exhibits nonlinear dynamical behavior and here we characterize the dynamical behavior of the phase portraits using attractor reconstruction. Thirty participants performed the SD test: 10 young adults, 10 older adults, and 10 older adults with Parkinson's disease (PD). We used delayed embedding of the applied force to reconstruct its attractor. We characterized the distribution of points of the phase portraits by their density (number of distant points and interquartile range) and geometric features (trajectory length and size). We find phase portraits from older adults exhibit more distant points (p = 0.028) than young adults and participants with PD have larger interquartile ranges (p = 0.001), trajectory lengths (p = 0.005), and size (p = 0.003) than their healthy counterparts. The increased size of the phase portraits with healthy aging suggests a change in the dynamical properties of the system, which may represent a weakening of the neural control strategy. In contrast, the distortion of the attractor in PD suggests a fundamental change in the underlying biological system, and disruption of the neural control strategy. This ability to detect differences in the biological mechanisms of dexterity in healthy and pathological aging provides a simple means to assess their disruption in neurodegenerative conditions and justifies further studies to understand the link with the physiological changes.

Strength, Multijoint Coordination, and Sensorimotor Processing Are Independent Contributors to Overall Balance Ability.

For young adults, balance is essential for participation in physical activities but is often disrupted following lower extremity injury. Clinical outcome measures such as single limb balance (SLB), Y-balance (YBT), and the single limb hop and balance (SLHB) tests are commonly used to quantify balance ability following injury. Given the varying demands across tasks, it is likely that such outcome measures provide useful, although task-specific, information. But the extent to which they are independent and contribute to understanding the multiple contributors to balance is not clear. Therefore, the purpose of this study was to investigate the associations among these measures as they relate to the different contributors to balance. Thirty-seven recreationally active young adults completed measures including Vertical Jump, YBT, SLB, SLHB, and the new Lower Extremity Dexterity test. Principal components analysis revealed that these outcome measures could be thought of as quantifying the strength, multijoint coordination, and sensorimotor processing contributors to balance. Our results challenge the practice of using a single outcome measure to quantify the naturally multidimensional mechanisms for everyday functions such as balance. This multidimensional approach to, and interpretation of, multiple contributors to balance may lead to more effective, specialized training and rehabilitation regimens.

Outcome measures for hand function naturally reveal three latent domains in older adults: strength, coordinated upper extremity function, and sensorimotor processing.

Understanding the mapping between individual outcome measures and the latent functional domains of interest is critical to a quantitative evaluation and rehabilitation of hand function. We examined whether and how the associations among six hand-specific outcome measures reveal latent functional domains in elderly individuals. We asked 66 healthy older adult participants (38F, 28M, 66.1 ± 11.6 years, range: 45-88 years) and 33 older adults (65.8 ± 9.7 years, 44-81 years, 51 hands) diagnosed with osteoarthritis (OA) of the carpometacarpal (CMC) joint, to complete six functional assessments: hand strength (Grip, Key and Precision Pinch), Box and Block, Nine Hole Pegboard, and Strength-Dexterity tests. The first three principal components suffice to explain 86% of variance among the six outcome measures in healthy older adults, and 84% of variance in older adults with CMC OA. The composition of these dominant associations revealed three distinct latent functional domains: strength, coordinated upper extremity function, and sensorimotor processing. Furthermore, in participants with thumb CMC OA we found a blurring of the associations between the latent functional domains of strength and coordinated upper extremity function. This motivates future work to understand how the physiological effects of thumb CMC OA lead upper extremity coordination to become strongly associated with strength, while dynamic sensorimotor ability remains an independent functional domain. Thus, when assessing the level of hand function in our growing older adult populations, it is particularly important to acknowledge its multidimensional nature-and explicitly consider how each outcome measure maps to these three latent and fundamental domains of function. Moreover, this ability to distinguish among latent functional domains may facilitate the design of treatment modalities to target the rehabilitation of each of them.

Long term functional outcomes after early childhood pollicization.

STUDY DESIGN: Retrospective Cohort INTRODUCTION: Important outcomes of polliciation to treat thumb hypoplasia/aplasia include strength, function, dexterity, and quality of life. PURPOSE OF THE STUDY: To evaluate outcomes and examine predictors of outcome after early childhood pollicization. METHODS: 8 children (10 hands) were evaluated 3-15 years after surgery. Physical examination, questionnaires, grip and pinch strength, Box and Blocks, 9-hole pegboard, and strength-dexterity (S-D) tests were performed. RESULTS: Pollicized hands had poor strength and performance on functional tests. Six of 10 pollicized hands had normal dexterity scores but less stability in maintaining a steady-state force. Predictors of poorer outcomes included older age at surgery, reduced metacarpophalangeal and interphalangeal range of motion, and radial absence. DISCUSSION: Pollicization resulted in poor strength and overall function, but normal dexterity was often achieved using altered control strategies. CONCLUSIONS: Most children should obtain adequate dexterity despite weakness after pollicization except older or severely involved children. LEVEL OF EVIDENCE: IV.

Quantitative assessment of dynamic control of fingertip forces after pollicization.

Dexterity after finger pollicization (reconstruction to thumb) is critical to functional outcomes. While most tests of hand function evaluate a combination of strength, coordination, and motor control, the Strength-Dexterity (S-D) paradigm focuses on the dynamic control of fingertip forces. We evaluated 10 pollicized and 5 non-pollicized hands from 8 participants ages 4-17 years (2 female, 6 male; 10.6 ± 4.5 years). Participants partially compressed and held an instrumented spring prone to buckling between the thumb and first finger to quantify dynamic control over the direction and magnitude of fingertip forces. They also completed traditional functional tests including grip, lateral pinch, and tripod pinch strength, Box and Blocks, and 9-hole peg test. Six of 10 pollicized hands and all non-pollicized hands had S-D scores comparable to typically developing children. However, dynamical analysis showed that pollicized hands exhibit greater variability in compression force, indicating poorer corrective action. Almost all pollicized hands scored below the normal range for the traditional functional tests. The S-D test Z-scores correlated moderately with Z-scores from the other functional tests (r = 0.54-0.61; p = 0.02-0.04) but more weakly than amongst the other functional measures (r = 0.58-0.83; p = 0.0002-0.02), suggesting that the S-D test captures a different domain of function. A higher incidence of radial absence in the hands with poor S-D scores (3/4 vs. 0/6 in hands with normal S-D scores, p = 0.03) was the only clinical characteristic associated with S-D outcome. Overall, these results suggest that while most pollicized hands can control fingertip forces, the nature of that control is altered.

Quantification of dexterity as the dynamical regulation of instabilities: comparisons across gender, age, and disease.

Dexterous manipulation depends on using the fingertips to stabilize unstable objects. The Strength-Dexterity paradigm consists of asking subjects to compress a slender and compliant spring prone to buckling. The maximal level of compression [requiring low fingertip forces <300 grams force (gf)] quantifies the neural control capability to dynamically regulate fingertip force vectors and motions for a dynamic manipulation task. We found that finger dexterity is significantly affected by age (p = 0.017) and gender (p = 0.021) in 147 healthy individuals (66F, 81M, 20-88 years). We then measured finger dexterity in 42 hands of patients following treatment for osteoarthritis of the base of the thumb (CMC OA, 33F, 65.8 ± 9.7 years), and 31 hands from patients being treated for Parkinson's disease (PD, 6F, 10M, 67.68 ± 8.5 years). Importantly, we found no differences in finger compression force among patients or controls. However, we did find stronger age-related declines in performance in the patients with PD (slope -2.7 gf/year, p = 0.002) than in those with CMC OA (slope -1.4 gf/year, p = 0.015), than in controls (slope -0.86 gf/year). In addition, the temporal variability of forces during spring compression shows clearly different dynamics in the clinical populations compared to the controls (p < 0.001). Lastly, we compared dexterity across extremities. We found stronger age (p = 0.005) and gender (p = 0.002) effects of leg compression force in 188 healthy subjects who compressed a larger spring with the foot of an isolated leg (73F, 115M, 14-92 years). In 81 subjects who performed the tests with all four limbs separately, we found finger and leg compression force to be significantly correlated (females ρ = 0.529, p = 0.004; males ρ = 0.403, p = 0.003; 28F, 53M, 20-85 years), but surprisingly found no differences between dominant and non-dominant limbs. These results have important clinical implications, and suggest the existence - and compel the investigation - of systemic versus limb-specific mechanisms for dexterity.

Test battery designed to quickly and safely assess diverse indices of neuromuscular function after unweighting.

Adequately describing the functional consequences of unweighting (e.g., bed rest, immobilization, spaceflight) requires assessing diverse indices of neuromuscular function (i.e., strength, power, endurance, central activation, force steadiness). Additionally, because unweighting increases the susceptibility of muscle to damage, testing should consider supplementary safety features. The purpose of this study was to develop a test battery for quickly assessing diverse indices of neuromuscular function. Commercially available exercise equipment was modified to include data acquisition hardware (e.g., force plates, position transducers) and auxiliary safety hardware (e.g., magnetic brakes). Ten healthy, ambulatory subjects (31 ± 5 years, 173 ± 11 cm, 73 ± 14 kg) completed a battery of lower- and upper-body neuromuscular function tests on 3 occasions separated by at least 48 hours. The battery consisted of the following tests, in order: (1) knee extension central activation, (2) knee extension force steadiness, (3) leg press maximal strength, (4) leg press maximal power, (5) leg press power endurance, (6) bench press maximal strength, (7) bench press force steadiness, (8) bench press maximal power, and (9) bench press power endurance. Central activation, strength, rate of force development, maximal power, and power endurance (total work) demonstrated good-to-excellent measurement reliability (SEM = 3-14%; intraclass correlation coefficient [ICC] = 0.87-0.99). The SEM of the force steadiness variables was 20-35% (ICC = 0.20-0.60). After familiarization, the test battery required 49 ± 6 minutes to complete. In conclusion, we successfully developed a test battery that could be used to quickly and reliably assess diverse indices of neuromuscular function. Because the test battery involves minimal eccentric muscle actions and impact forces, the potential for muscle injury has likely been reduced.