In those with PD, muscle activation patterns appear to be intact, but impairments are noted in the timing and magnitude of contraction of agonist and antagonist muscle groups.
“On” Versus “Off” States
Postural responses differ depending on the effects of L-dopa medication alone or in combination with other anti-Parkinson’s medications. An “on” state refers to a period when dopamine levels are optimal, PD symptoms decrease, and mobility improves. An “off” state occurs when dopamine levels are less than optimal, increased PD symptoms are apparent, and mobility may be more difficult. See chapter 2, “Medical and Surgical Interventions,” page 16.
Muscle activation while “off”
Dimitrova and colleagues (2004) compared the postural responses of people with PD in their “off” mode with healthy age-matched controls in response to low-amplitude multi-
directional surface translations in two stance positions—narrow-based stance and wide-based stance. The researchers found the muscle activation synergies to be similar between the subjects with PD and controls, but the magnitude of muscle activation of the agonist muscles in those with PD was below those of the controls except for the trunk musculature, which exhibited higher than normal muscle activation. For example, to control backward sway (to prevent a loss of balance backward, the muscles attempt to pull the body forward), the group with PD activated the anterior tibialis first, the quadriceps second, and the abdominal muscles third, as would be expected in norms, but the magnitude of activation varied in comparison with controls. In addition, the muscle activation of the antagonist muscle groups was of a larger magnitude and with earlier onset when compared with controls. Therefore, during a backward sway, the antagonists (the gastrocnemius, hamstrings, and especially the erector spinae) were activated sooner and with relatively larger magnitudes, resulting in cocontraction.
Although evidence is limited, is it possible that the increased activation of the erector spinae can contribute to the propensity of backward postural instability in people with PD? When changing from a wide base of support to a narrow base, both groups exhibited increased magnitude of muscle activation, but the group with PD remained below normal levels. Ultimately, the relative activation imbalance of the agonistic muscles and the antagonist muscles results in cocontraction and reduced adaptability to postural changes.
Muscle activation while “on”
Another study evaluated postural responses in people with PD and compared the differences in postural responses in the “on” state versus the “off” state. Subjects were asked to rise on their toes while postural responses were measured. The results showed that L-dopa can improve the magnitude and relative timing of postural muscles, although not to the level of normal values (Frank et al., 2000).
Impairment of Postural Reflexes
Postural reflex impairments are resistant to dopamine replacement medications (Bloem, 1992). Difficulties begin with the inability of the postural muscles to respond rapidly and with enough magnitude to destabilizing forces. In fact, L-dopa has been shown to worsen postural responses by decreasing tone. Tone would otherwise act passively to reduce the amount of sway (Horak et al., 1996).
Depending on disease progression, various synergistic stepping strategies (or lack thereof) in response to perturbations can be observed. These responses can range from taking a few steps with the ability to self-correct to having no response when the person starts falling like a tree and needs to be caught. In addition, stepping strategies tend to be delayed and underscaled (Beckley et al., 1993). L-dopa medication improves step length and accuracy in people with moderate and severe PD, but these improvements are not consistent (Jacobs & Horak, 2006). A person with PD may be categorized in different Hoehn and Yahr stages depending on improvements in stepping from L-dopa. A person may be in Hoehn and Yahr stage 3 during an “off” state and in Hoehn and Yahr stage 2 during an “on” state.
Proprioception and Anticipatory Postural Adjustments
People with PD tend to lack accurately scaled movements because of inability to integrate proprioceptive inputs centrally. Therefore, they must rely more heavily on visual input to optimize movements. This reliance increases with disease severity (Keijsers et al., 2005). The underscaled movements found in stepping strategies are caused partly by impairments of central proprioceptive integration (Jacobs & Horak, 2006).
When performing volitional activities, people with PD demonstrate impairment in the preparatory postural set preceding an activity (Bazalgette et al., 1987; Frank et al., 2000; Kaneoke et al., 1989; Rogers et al., 1987). A study on lateral stepping strategies found that people with PD did not activate anticipatory postural adjustments (APAs) 56% of the time before stepping. The lack of APAs or trunk-righting ability contributes to reduced ability in balance recovery (King & Horak, 2008).
In summary, postural instability in people who have PD is multifactorial and is related to disease severity. Several neurological impairments influence postural responses:
- Lower-extremity cocontractions cause reduced adaptability to postural changes.
- Reduced magnitude of agonistic muscle activation patterns in the lower extremities produces an underscaled stepping strategy.
- Postural reflex impairments result in a delayed stepping strategy or no stepping strategy.
- Proprioception integration impairments centrally can result in underscaled or inaccurate stepping strategies.
- Impairments in anticipatory postural adjustments result in reduced trunk-righting ability and lower extremity responses, causing greater difficulty in maintaining the COM over the base of support for balance.
To permit evaluation of postural stability, the balance challenge must be of sufficient magnitude to cause instability, thereby forcing the person to react in some way to regain balance. Because the reactions of destabilizing forces are unpredictable, the use of a gait belt is recommended during the evaluation and interventions. The ability to maintain postural stability is important for fall prevention regardless of direction, but the emphasis here is on posterior stability and lateral stability. Anterior stability can be evaluated with the functional reach test or with various components of the Berg balance test. See chapter 8, “Balance Evaluation,” pages 105–106.
Evaluating postural stability in people with PD requires creating a loss of balance in the posterior direction and observing recovery capability. Three tests can be used to evaluate posterior stability. The expected retropulsive test and the Nutt unexpected retropulsive test are both referred to as the pull test. The difference between the two pull tests is that in one of them the patient is forewarned and is expecting to be pulled off balance and in the other one there is no forewarning. The third test is the push and release test. These tests are designed to determine whether the person with PD is at Hoehn and Yahr stage 2 (bilateral disease, balance intact) or stage 3 (mild to moderate bilateral disease, some postural instability, physically independent). The execution of these tests and the position of the patient and therapist are important to obtaining accurate results.
Expected Retropulsive Test
This test procedure is required in the Unified Parkinson’s Disease Rating Scale (UPDRS) and is typically performed by a neurologist. Because patients are aware that they are going to be pulled backward during the test, they can prepare their postural set or alter their base of support so that they can pass the test. Inaccurate results can occur because in a home environment people are usually not warned in advance when they are going to lose their balance. The unexpected nature of falls is what makes them dangerous. Thus, I do not recommend using the expected retropulsive test.
Nutt Unexpected Retropulsive Test
When executed once, this test has a higher overall predictive accuracy when compared with the expected test (Visser et al., 2003), and it yields the most accurate results for appropriate therapeutic intervention. If executed more than once, the element of surprise (or unexpectedness) is gone and the results will be less definitive (Visser et al., 2003). Technically, the protocols for retropulsive tests are performed by pulling backward on the shoulders (see figure 7.3, a and b). Because of the wide variety of recovery abilities and for safety concerns, using a gait belt and pulling backward on the gait belt using the following method is recommended:
1. The therapist stands behind the patient.
2. The patient’s feet are positioned comfortably apart and parallel (not one ahead of the other).
3. Without warning, the therapist pulls the patient briskly backward using sufficient force to create a loss of balance that requires a stepping response.
Common errors in the patient’s position are
- bracing forward or
- standing with an increased base of support.
Common errors in the therapist’s position or execution are
- pulling continuously and steadily (which does not create the element of surprise needed or the velocity that would facilitate a stepping strategy sooner),
- not pulling with sufficient force to elicit a stepping strategy, or
- standing too close to the patient, limiting the patient’s space and ability to react (Munhoz et al., 2004).
0 = Normal; may take two steps to recover
1 = Takes three or more steps and recovers unaided
2 = Would fall if not caught
3 = Spontaneous tendency to fall or unable to stand unaided (test not executable)
This scale indicates the patient’s response to regain an unexpected loss of balance. Normally, a person is expected to take a sufficiently sized step or two to regain balance. Any response outside the norm is cause for therapeutic interventions.