Orbital stability of muscle activations during step climbing un young subjects

INTRODUCTION Falls in the elderly represent a well known problem, having large clinical and economic consequences [1]. The effects of muscle activation patterns on the kinematics and the change of these relationships with aging are still not well known. Many human tasks, like step climbing, are structurally cyclic, and are result in several biomechanical time-varying variables (kinematic, dynamic, electromyographic). These variables are all the manifestation of the same motion pattern. In particular, EMG signals coming from different muscles should not be analyzed independently when studying the muscular pattern during the temporal evolution of the task. Orbital stability allows to study the stability of motor tasks considering all these variables together. This approach has recently been applied to human walking [2] showing encouraging results; it seems to be a powerful tool for studying periodic-like systems that involves many variables like human motion. A better comprehension of the stability of muscular patterns during motor tasks could give a significative contribution in assessing the risk of fall. The aim of the present study was to investigate orbital stability of muscle activation during climbing of a single step in young subjects. METHODS Three participants [25±0y, 1.78±0.08m, 72±9Kg] performed 2 repetitions of a step climbing test. During the tests force plate (FP4060-07-1000, Bertec, USA) data was aquired at a sampling frequence of 800Hz and EMG (PocketEMG, BTS, Italy) signals of 8 muscles of the right leg (biceps femoris, rectus femoris, vastus medialis, gluteus maximus, tibialis anterior, soleus, gastrocnemius medialis, erector spinae) were acquired at a sampling frequency of 1000Hz. EMG signals were bandstop filtered at 50Hz, high-pass filtered at 30Hz and demeaned (MATLAB 7.7.0, Mathworks, Natick, NA). Then, signals were rectified, normalized with maximum voluntary contraction (MVC) technique and low-pass filtered at 3Hz. One smoothed, the signals were down-sampled to 200Hz to determine linear envelopes. Force plate data were also down-sampled at 200Hz. EMG signals were also conveniently stretched or shrinked on the temporal axis by resampling intervals between main motor events, so that the most important phases of the task were coincident on the samples vector. EMG data were defined a 16-dimensional state space where is the normalized linear envelope of EMG voltage and is the corresponding time-derivative. The sensitivity of EMG patterns to small perturbations that naturally occur during step climbing was quantified through orbital stability analysis by calculating maximum Floquet multipliers (FM) using an established technique [3]. Orbital stability is defined using Floquet multipliers, that quantify how periodic systems respond to small perturbations discretely from one cycle to the next. If FM have magnitude < 1, perturbations tend to shrink by the next repetition, and the system remains stable. The magnitudes of maximum FM was computed at each sample of the task considering the different phases of the task (leading heel off, leading toe off, leading heel strike, trailing toe off, trailing heel strike). The first limb to climb over the step is defined as leading limb, while the other limb is defined as trailing limb. All the subjects used the right leg as leading limb. The different phases of the task were identified from force plate data. RESULTS AND DISCUSSION Preliminary results obtained in investigating orbital stability of muscle activation during step climbing give interesting cues, and seem to confirm that the technique is adequate for studying human motor tasks. Values of maximum Floquet multipliers are very low (〈maxFM〉 = 0.0081); the subjects show very strong orbital stability in all the task phases (Table 1, Fig 1). This result was somehow expected: stability of young subjects in performing a basic motor task such as step climbing is expected to be very hi...