Muscle synergies in preparation to a step made with obstacle in elderly individuals

Abstract

Background: To evaluate if multi-muscle synergies are comprised of flexible combinations of a small number of postural muscles to stabilize the center of pressure (COP) shift during preparation to making a step in the elderly (self-paced level stepping vs. obstacle crossing stepping).

Methods: Electromyography (EMG) signals of leg and trunk muscles were recorded. Linear combination of integrated indices of muscle activity (M-modes) and their relationship to changes in the COP shift in the anterior-posterior (AP) direction were first determined. Uncontrolled manifold (UCM) analysis was performed to determine the extent to which variance of the M-modes acted to produce a consistent change in the COP displacement.

Results: The elderly were capable of stabilizing the COPAP coordinate based on co-varied involvement of the M-modes. The synergy index (∆V) changes in the elderly emerged later (100 ms prior to t0) and its magnitude was smaller as compared to that reported in younger persons.

Conclusions: Our study reveals that aging is associated with a preserved ability to explore the flexibility of the M-mode compositions but a decrease ability to use multi-M-mode synergies following a predictable perturbation.

Figure 1

The experimental setup. A. The subjects were required to step over an obstacle of 15% body height from quiet stance in a self-paced way. B. In the control trials, the subjects were required to hold a load (5.3 Kg) in front of the body or behind the body for 5 s, they held a handle which was connected to the load through a pulley system. Location of some of the EMG electrodes is also shown in panel A (GL lateral head of gastrocnemius, GM medial head of gastrocnemius, SOL soleus, ST semi-tendinosus, BF biceps femoris, GMED gluteus medius, ES erector spinae, TA tibialis anterior, VL vastus lateralis, RF rectus femoris, TF tensor fasciae latae, RA rectus abdominis).

Figure 2

Typical EMG patterns averaged across twenty trials by a representative subject for the ST _CS (dark lines) and ST _OS (gray lines) tasks. Time zero corresponds to the alignment time, the time of toe off. Note the early increase in the activity of dorsal muscles, accompanied sometimes by a decrease in the activity of ventral muscles. The EMGs were recorded in muscles of the right side of the body. The EMG scales are in arbitrary units and time is in ms.

Figure 3

Average COP _AP displacement is shown for a representative in the ST _CS and ST _OS tasks. Negative values correspond to backward displacements. Dark and gray lines indicate the ST_CS and ST_OS tasks, respectively.

Figure 4

Loading coefficients for the PCA of the ST _CS and ST _OS tasks for a representative subject. Loading magnitudes over | ± 0.5| are shown in black (significant loadings).

Figure 5

Mean across subjects ± standard deviation of ∆V (panel A) and ΔV _Z (panel B) indices for the control of the COP _AP shift. Adjacent pairs of bars represent the ST_CS (left, stripped bars) and ST_OS (right, gray filled bars) tasks. ∆V indices and ΔV_Z indices were averaged over five 100 ms time intervals starting 500 ms prior to t0 and ending up at t0.