The underlying mechanisms of improved balance after one and ten sessions of balance training in older adults
Abstract: Training improves balance control in older adults, but the time course and neural mechanisms underlying these improvements are unclear. We studied balance robustness and performance, H-reflex gains, paired reflex depression, and co-contraction duration in ankle muscles after one and ten training sessions in 22 older adults (+65yrs). Mediolateral balance robustness, time to balance loss in unipedal standing on a platform with decreasing rotational stiffness, improved (33%) after one session, with no further improvement after ten sessions. Balance performance, absolute mediolateral center of mass velocity, improved (18.75%) after one session in perturbed unipedal standing and (18.18%) after ten sessions in unperturbed unipedal standing. Co-contraction duration of soleus/tibialis anterior increased (16%) after ten sessions. H-reflex gain and paired reflex depression excitability did not change. H-reflex gains were lower, and soleus/tibialis anterior co-contraction duration was higher in participants with more robust balance after ten sessions, and co-contraction duration was higher in participants with better balance performance at several time-points. Changes in robustness and performance were uncorrelated with changes in co-contraction duration, H-reflex gain, or paired reflex depression. In older adults, balance robustness improved over a single session, while performance improved gradually over multiple sessions. Changes in co-contraction and excitability of ankle muscles were not exclusive causes of improved balance.
Summary (2 min read)
- Training improves balance control in older adults, but the time course and neural mechanisms underlying these improvements are unclear.
- Previously a rapid improvement of balance control in young adults after one session of balance training has been shown8, while results of short-term training in older adults were inconsistent9,10 and most studies have focused on training over several sessions spread over multiple weeks4.
- Supraspinal mechanisms also affect the excitability of the alfa motoneuron pool and therefore the H-reflex gain.
- To do so, the authors assessed changes in balance robustness (as the duration that participants were able to keep their body balanced while surface stiffness was decreased) and balance performance (measured as the mean absolute value of the mediolateral center of mass velocity during unipedal balancing).
- The Pre-measurements, the 30-min training session, and the Post1 measurements were performed on the same day.
Instrumentation and data acquisition
- For all unipedal tasks, a custom-made balance platform controlled by a robot (HapticMaster, Motek, Amsterdam, the Netherlands) was used.
- The rotation of the platform can be controlled by the robot, simulating a tunable stiffness and damping or applying position-control.
- Surface EMG data were collected from three muscles on the preferred stance leg: m. tibialis anterior (TA), m. peroneus longus (PL) and m. soleus (SOL).
- A kinematic model of the participant was formed by relating the cluster positions to anatomical landmarks in an upright position, using a four-marker probe45.
- The optimal cathode position was determined in each subject by probing the popliteal fossa and delivering 5-10 mA stimulations to find the location that resulted in the largest SOL H-reflex amplitude ~25 ms after stimulation.
Unipedal balance tasks
- In the unperturbed task, the stiffness of the platform was set at a constant value.
- To normalize task difficulty to balance robustness, this value was set at 1.3 times the stiffness at which balance loss occurred during the assessment of balance robustness in the Pre-measurement.
- In the perturbed task, twelve perturbations were imposed by the platform in the form of mono-phasic sinusoidal rotations either in medial or lateral direction (amplitude of 8°, angular speed of 16°/s).
- The perturbation direction was randomized and the inter-perturbation duration was randomly selected between 3-5 s.
- This task was performed five times with two minutes rest in between trials.
H-reflexes and Paired Reflex Depression
- H-reflex gain and PRD were derived from the high-pass filtered (10 Hz, bidirectional, 2nd order Butterworth) EMG activity of the SOL.
- No correlations were observed between changes after one session or ten sessions of training (Table 5).
- In older adults, the mechanisms underlying improvements in balance performance and robustness after the training remain unclear.
- In the first session, the participants were trained individually.
- The nine sessions of the 3- week training program took place in a group setting (6-8 participants).
- All training sessions were supervised by a physical therapist who ensured that the sessions remained safe, yet sufficiently challenging for all the participants.
- Solely standing balance exercises, focusing on unipedal stance, were included in the training program49.
- Extra time was required to switch the devices between the training partners in the exercises with equipment.
- The trajectory of the center of mass (CoM) was estimated from a full body kinematic model50.
- Balance performance was expressed as the mean absolute center of mass velocity in the mediolateral direction (vCoM).
Co-contraction duration (CCD)
- Antagonistic co-contraction is the concurrent activation of antagonistic two muscles.
- Co-contraction was derived from three muscle pairs: SOL/TA, TA/PL and SOL/PL.
- EMG data were high-pass (35 Hz, bidirectional, 2nd order Butterworth) and notch filtered (50 Hz and its harmonics up to the Nyquist frequency, 1 Hz bandwidth, bidirectional, 1st order Butterworth).
- Finally, the authors determined the percentage of data points during the perturbed and unperturbed tasks at which both muscles in a pair exceeded the mean muscle activity of baseline unipedal stance.
- Since for Pre and Post1 time-points the measurements were performed on the same day, the same unipedal trial was used as a reference for these two time-points.
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The authors studied balance robustness and performance, H-reflex gains, paired reflex depression ( PRD ), and co-contraction duration ( CCD ) in ankle muscles after one and ten training sessions in 22 older adults ( +65yrs ). Mediolateral balance robustness, time to balance loss in unipedal standing on a platform with decreasing rotational stiffness, improved ( 33 % ) after one session, with no further improvement after ten sessions.