A Normative Movement Profile of Stair Ascent and Descent: an EMG to Muscle Torque Calibration
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Introduction . Ambulating up and down stairs is a common a functional task. Performing it efficiently is integral to a person's quality of life. Analysis of how one ascends and descends stairs may be more sensitive in identifying subtle functional differences not evident during level walking. These tasks place higher demand on balance, which necessitates increased muscular contribution, larger range of motion (ROM), forces, moments, and powers to transition the body to the next step. (3, 10, 19, 40) The relationship between EMG and force during dynamic activity however is non-linear and dependent upon numerous factors.(11) The force-length and force-velocity relationships must be considered, as well as force sharing and contraction dynamics that include angular velocity, distribution of mass and cross sectional area of the muscle.(11-13, 40) Recent efforts to derive an EMG-force model have been undertaken for clinical analysis although many parameters were not considered.(11-13, 58) Using parameters limited to angle, angular velocity, physiological cross sectional area (PCA) and a rough estimation of equal force sharing between muscles provided a reasonable estimate of individual muscle moments. (11) Methods . 9 subjects between the ages of 18 and 25 with a B.M.I. less than 25 kg/m 2 with no history of orthopedic, neurological, or balance problems underwent gait analysis as they walked up and down instrumented stairs. Stair set-up was comprised of three steps, two of which had a height of 18 cm, tread depth of 28 cm, and a width of 60 cm and the third step as an extended platform. Kinematic and kinetic data were compared between level walking (force plate 1), transition from level walking to stair ascent and from stair descent to level walking (force plate 2), and stair walking for ascent and descent (force plates 3 and 4). Independent t-tests (with appropriate correction for multiple comparisons) were used to assess differences between force plates (FP). Statistical significance was accepted at p<0.05 and trends defined as 0.05<p<0.10. EMG to force regression equations were developed for discrete knee flexion angles using isokinetic testing at fixed angular velocities. Results . Ground reaction (GRF) forces upon FP1 displayed similarity to typical level walking profiles but deviations occurred on the transition step, FP2, as anteroposterior shear braking increased significantly and shear propulsion was reduced.(p<0.008). Anteroposterior shear forces differed between level walking and stair ambulation.(p<0.008) Similar first and second peak GRF forces were observed for level walking and transition (FP1 and FP2) whereas the second peak amplitude during stair ascent was significantly larger than the first. (p<0.05) Discernible changes were observed in lower extremity kinematics and kinetics when transitioning from level walking to ascent (FP2) as hip and knee joint ROM was reduced (p<0.05). Peak hip flexor moments and angular impulses generated were also significantly reduced. (p<0.008) When ascending stairs, sagittal joint mechanics were similar between FP3 and FP4 but were significantly different when compared to level walking (FP1 and FP2) evidenced through greater joint ROM during stair ambulation and increased internal extensor moments during the first half of stance. (p<0.008) As subjects descended the stairs, the first vertical GRF peak was consistently higher when compared to the second peak (p<0.05) for FP4 and FP3 but similar during transition to and level walking. Anteroposterior shear braking was highest on FP4 (p<0.008) as subjects decelerated their momentum from level walking while braking forces on all other steps were similar. At IC and throughout stance on FP4 a larger (p<0.008) and prolonged hip extensor moment was evident with a concomitant knee extensor moment smaller than that seen on the second step of descent (p<0.05). Knee kinetics showed a biomodal profile for stance during stair ascent, but to different magnitudes. Knee kinetics for level walking demonstrated a flexor moment during midstance to terminal stance that was not evident for stair descent. Ankle Kinetics displayed a biomdoal profile generating two peak plantarflexor moments for all steps except FP1 where there was no peak during the first half of stance and likewise no power generation until the second half of stance. Muscle moment estimations using an EMG to force calibration demonstrated irregularity of amplitude but similar decreasing force generations as the knee extended in pull up phase when compared to joint moments calculated using inverse dynamics. Conclusions . The objective for this study was to quantify sagittal gait mechanics and estimate individual muscle moments upon the second step among young healthy adults as they ambulate up and down stairs. Our findings suggest joint mechanics are heavily dependent on the steps taken: level walking, transitioning from level walking to stair ascent and from stair descent to level walking, and stair negotiation. Increased neuromuscular and biomechanical demand was evident when comparing stair ambulation to level walking. Transitioning between stair ascent and level walking as well as level walking and stair descent displayed unique biomechanics manifested in altered joint moments and ROM. Muscular force as estimated using an EMG to muscle force calibration demonstrated altered force generation at different knee flexion angles and identified the vasuts medialis and vastus lateralis muscles as the primary force generators of knee extension during the pull up phase of ascent. Summed inferred muscle moments about the knee showed deviation when compared to knee moments calculated using inverse dynamics and is thought to be a result of varying angular velocity among subjects when compared to fixed velocity during isokinetic calibrations. *Please refer to dissertation for references/footnotes.