This study investigated the effects of a 60-minute, full-body MT treatment on balance, neurological, and cardiovascular measures in healthy, older individuals. Surprisingly, there was no immediate post-treatment effect on the balance measures. Clinically, we often see patients exhibiting some instability after MT. Our study result is encouraging in that it suggests that MT does not increase postural instability and that current precautions are adequate. However, influence of disease condition, medications, and individual patient differences must always be kept in mind, and proper precautions taken.
Postural control changes over the post-treatment time points (immediate, 20-and 60-minutes post-treatment) were determined using individual growth curve modeling. Significant group differences over the 60 minutes following treatment were revealed for the rectangular displacement area in both double-legged stance conditions (), and in average displacement velocity over time, in both eyes-open, single-legged and double-legged stances. These results indicate increasing stability over time in the MT group compared to the control group. Older individuals have been shown to have increased center-of-pressure variables (decreased stability) while maintaining an upright stance.(46)
While a certain amount of movement (displacement) is required as the body adjusts to maintain posture, after the MT individuals required less movement in order to maintain upright stance. Moreover, the movement that did occur was slower (lower velocity), suggesting more control. The dynamic balance (single-legged) measures yielded fewer significant effects on balance. Many of the participants experienced difficulty maintaining the single-legged stance, especially in the eyes-closed condition, which simply may have been too challenging for this population.
Maintenance of balance is a complex process involving neural, muscular, skeletal, visual, vestibular, and other components. Clinical evidence suggests more research is required to identify specific mechanisms of action of MT on balance and postural control. The postural control literature does suggest some areas for future study. Specifically, work in the area of aging suggests that any alterations in the ‘normal’ way in which postural control is maintained increases the likelihood of accidental falls and resulting injuries. (47)
These alterations may include increased levels of muscular coactivation in the lower extremity in response to either perturbations,(48)
or during quiet standing.(23)
Electromyography studies suggest that older individuals often assume a more rigid, flexed position during quiet standing compared to younger persons. Maintenance of this flexed position requires activation of the hamstring muscles to avoid further forward movement of the center of gravity. Coactivation of the biceps femoris and vastus lateralis is required to sustain an upright posture in this flexed position. The increased muscle activation is thought to be an accommodation for weaker muscles or decreased proprioception.(49)
Increases in lower extremity muscle activity, such as muscle coactivation, has also been shown to increase postural sway through noise-like fluctuations which may be detrimental to the maintenance of postural control.(50)
Thus, the literature suggests that optimal postural stability occurs within a specific range of muscle activity, with any increase or decrease in activity levels requiring a corrective action or coactivation, which may increase instability.(51)
Postural stability could be improved through reduction of muscular coactivation, compensation, or optimization of muscle activity. Previous work indicates that MT may reduce muscle activity and increase range of motion in treated muscles.(31)
The MT intervention in the current study may have reduced muscle activity in the muscles of the lower extremity, resulting in a reduced tendency to assume a rigid, forward flexed posture. While the lack of change in the control group indicates that simple relaxation did not produce the balance improvements, studies indicate stretching and mental relaxation (both of which occur during an MT treatment) may positively influence balance and may be contributing factors in the observed effects.(52)
Cognitive effects are also an important part of balance. Studies indicate that fear of falling and other cognitive processes influence balance and create changes in postural control strategies in older individuals.(53)
As MT has been shown to decrease stress and anxiety and improve mental state, these factors may also have a positive influence on balance.(26)
Clearly more work needs to be done in these areas.
One way MT may influence muscle activity (and, thus, ability to maintain balance) is through the modulation of motoneuron pool excitability.(54–55)
ratio is an indicator of the proportion of the motoneuron pool that is being recruited during testing. The Mmax
represents full recruitment of the motoneuron pool, while Hmax
indicates how much of the motoneuron pool is being recruited during testing. This can be influenced by injury, treatments, training, fatigue, illness, aging, or other factors. The current study found a decrease in the Hmax
ratio after treatment in the MT group compared to the control group. This suggests that the treatment had a physiological effect on the neuromuscular response and, at 60 minutes post-treatment, fewer of the motoneurons were recruited during a contraction in the treatment group than in the control group. A similar study also found decreased H-reflex measures, along with decreased muscle electrical activity and increased range-of-motion after MT to the neck and shoulders.(56)
While electromyography (EMG) was not recorded in the current study, increased EMG activity is known to occur as a muscle fatigues, which results in an increased recruitment of additional motor units in order to complete a task. Decreases in EMG may signify reduced muscle fatigue or spasm. Changes in the H-reflex suggest that MT is also influencing the neuromuscular activity which may, in turn, produce an effect on how the body maintains postural control.
Interestingly, the conditioning protocols evaluating pre- and postsynaptic inhibition were not significantly changed. However, looking at the average means (), there do seem to be large changes pre- and post-treatment in these variables, with moderate effect sizes for the postsynaptic inhibition (RI) variables. The H-reflex variability was larger than usual in the older participants, which may have masked any significant changes in these measures. Additional work is required in this area to identify the mechanism of change in the H-reflex and any subsequent effects on the maintenance of postural control. Significant correlations were found between H-reflex measures and postural control variables in both the control and treatment groups, suggesting a relationship between H-reflex levels and ability to balance. Moreover, the number of significant correlations increased in the post-treatment measures compared to the baseline measures, even when collapsed across group. The correlations provide evidence of a relationship between motoneuron pool excitability and postural control, and suggest the MT treatment influences these measures, although how this occurs is yet to be determined.
There was no immediate change in the cardiovascular measures in either group, which was also unexpected. However, the single treatment did produce a significant decrease in systolic and diastolic blood pressure over time. This decreasing trend began post- MT and was continuing downward at the 60 minute time point. Unexpectedly, heart rate showed a slight increasing trend over time in the MT condition, although this change would not be considered clinically significant. Previous studies indicate decreases in heart rate and blood pressure after MT.(25)
However, these studies did not always indicate the amount of time that passed between the treatment and the assessment, were usually completed on a younger age group, and utilized different inclusion criteria. Cardiovascular changes in the current study may have occurred more slowly in the older population, or it may be simply that we measured changes over time where other studies identified the response at a single time point. This finding reinforces the need to assess measures over time, as well as immediately posttreatment, in order to understand the physiological and clinical effects of therapeutic interventions.
The current study included only healthy, fairly active older persons with controlled blood pressure and limited medications. Results on a larger cross section of the population and those with medical issues commonly affecting this population may produce different results. The difficulty of the single-legged stance may have hidden any dynamic influences of the treatment. Future work should include a functional balance test more suited to the study population. This study assessed only acute effects of one 60-minute MT treatment; work on repeated treatments is reported in a separate study. Finally, future studies should use an alternative treatment (ie. heat packs) control group rather than simple relaxation in order to provide a comparison to other common therapies.