4. Discussion
This study revealed that during all of the functional tasks performed before feedback training, on average, the force put through the UE by the study participants exceeded that generally recommended with sternal precautions 10 lb (4.5 kg) or less. With only verbal instructions regarding UE weight bearing, patients may not be able to accurately estimate the amount of force put through their UE. Po-Chen and Cherng found that healthy older adults on average put 22-28 lb (10.0-12.7 kg) of UE force through a standard walker when given no instructions to limit weight which is similar to the mean value during ambulation with a standard walker in our study of 30.6 lb (13.9 kg). Ishikura found that when healthy subjects used a 4 wheeled walker, force put through the UE varied from 13 to 40% of body weight during a gait cycle which is similar in magnitude to our data for front wheeled walker ambulation (on average up to 25% of body weight).
This same trend has been observed in several previous studies that examined weight bearing through the lower extremities and, therefore, inversely through walker handles, during ambulation in patient populations. Fast and colleagues found that when patients used a standard walker to reduce weight bearing through the lower extremities, force through the walker was 20-49 lb (9.1-12.7 kg), but when used for balance, the force through the walker was only 2-28 lb (0.9-12.7). Because patients recovering from cardiac surgery with median sternotomy typically do not need to unweight the lower extremities, and use a walker primarily for balance stability, the latter values reported by Fast et al. are similar to the mean of 30.6 lb (13.9 kg) found in our current study. However, it should be noted that in the studies by Po-Chen and Cherng, Ishikura, and Fast et al., force was measured through a walker without instructions to limit UE weight bearing, and the studies were focused on limiting lower extremity weight bearing. In a cohort of young subjects, a previous study found that average UE weight bearing during ambulation with a walker was greater than 10 lb (4.5 kg).
Studies examining compliance with touch down weight bearing (defined as <25 lb) in patients with lower extremity trauma have found that placing too much weight through the involved side is common. Hustedt and colleagues instructed healthy subjects to use touch down weight bearing with axillary crutches and found that weight was often up 2.5 times greater than the prescribed restriction. Similarly, we found that subjects commonly applied too much UE weight bearing force through a walker when given no feedback, and they often exceeded the 10 lb precautionary limit by 2-3-folds.
Ambulation using a front wheeled walker produced less UE weight bearing force than ambulation using a standard walker, which is consistent with a previous study that found that when young subjects were asked to place less than 10 pounds of pressure through walker handles, greater UE weight bearing force occurred when ambulating with a standard walker (18.5 lb) compared to a front wheeled walker (11.7 lb). This finding supports current sternal precautions and general clinical consensus that if an assistive device is needed for ambulation, a front wheeled walker is preferable for patients after median sternotomy. A plausible explanation is that patients using a standard walker must lift it off the ground for a portion of the gait cycle and then place it back down, whereas when using a front wheeled walker it remains on the ground throughout the gait cycle. Lifting a walker may result in less consistent force through the walker, creating cyclical peaks above the 10 lb (4.5 kg) limit. Previous studies have shown that lower extremity weight bearing (and therefore inversely UE weight bearing) varied throughout the gait cycle during ambulation with a 4 wheeled walker and standard walker. Another possible explanation for our finding that less UE weight was placed through the front wheeled walker as compared to the standard walker relates to the type of gait pattern employed. When patients ambulate using a standard walker they use a "step-to" pattern, but they use a "step-through" pattern with a front wheeled walker, so UE force is distributed more evenly throughout the gait cycle and which helps reduce peak forces.
Results indicate that UE weight bearing force was lower during walker ambulation than sit-stand transfers at baseline and with a simulated lower extremity impairment (walking boot). Although studies have examined UE forces during activities of daily living, little information is available on UE force during transfers. In previous studies, Anglin and Wyss and LaPier and Cleary reported higher maximal UE loads during stand to sit than during sit to stand which is not consistent with the results of this study. This may be related to the older age of the cohort in our current study. Perhaps older adults need more UE assistance with the concentric sit to stand portion of transfers than younger adults.
The results of this study demonstrate that a brief feedback intervention can be effective at reducing UE force exerted during both ambulation with an assistive device and sit-stand transfers. Both visual and auditory feedback were utilized to provide participants information regarding force placed through their UE. A recent systematic review examining the efficacy of feedback for improving gait parameters found that visual (60%) and auditory (40%) feedback were most commonly employed. In the current study, a combination of both auditory (buzzer) and visual (force output on tablet screen) feedback was given to subjects. This feedback was provided concurrently (during practice of the skill) as opposed to terminally (after practice of the skill) to best facilitate acquisition of a novel skill. Study participants were first given prescriptive (information on exactly how much weight was being placed through the walker) visual feedback while statically practicing UE weight bearing in standing or sitting. Then, they were given descriptive (buzzer was sounded if force exceeded the 10 lb limit) auditory feedback while practicing the whole task. This method facilitated part practice first followed by whole practice, a commonly employed principle of motor learning. After feedback training, UE force was reduced during all functional tasks by 46-56%. But the average of each functional task was still greater than the 10 lb (4.5 kg) limit typically prescribed with sternal precautions. Although other studies have demonstrated that concurrent visual feedback training is effective at modulating force when weight bearing is limited, study results suggest that a more intensive feedback intervention (i.e., more repetitions of practice within a training session) and/or multiple training sessions would be ideal for older patients recovering from median sternotomy.
Results of the study also demonstrated that the immediate improvements in UE weight bearing force are retained in the short term. Force during all functional tasks was not significantly different immediately postfeedback training or/and with repeated testing 2 hours later. Hustedt and colleagues also found 2-4 hour retention of lower extremity touch down weight bearing compliance after feedback training. In addition, they also found no significant differences in lower extremity force immediately after feedback training, 6-8 hours later, and 22-24 hours later, suggesting that retention of a single bout of feedback training may last up to 24 hours.
Results of this study suggest that PM muscle activation is relatively small (<23% MVIC) during functional mobility and that feedback training can further minimize it. During this study, PM muscle activity was measured because it attaches to the lateral borders of the sternum, pulls horizontally from medial to lateral, and is the primary mover for shoulder horizontal adduction. To date, only a single study has examined PM muscle EMG activity during ambulation with a walker or during sit-stand transfers. A few studies have measured PM muscle EMG activity during similar activities. Pectoralis major muscle EMG activity was 13.0-14.4% MVIC while pushing a 4 kg weight forward on tracks and 7% MVIC in the "prayer" position (kneeling with hands on floor in front of knees) both of which are different shoulder positions than used in this study. A previous study found lower PM muscle activation in a cohort of young subjects which ranged from 3.0% to 9.2% as compared to our findings in older subjects which ranged from 13.6% to 22.1%.
This study revealed interesting relationships between function mobility and peak UE weight bearing force. Surprisingly, lower extremity strength (30 sec sit to stand test) was not correlated with the amount of UE weight bearing during the functional tasks, and handgrip strength was directly associated with UE weight bearing force during transfers and standard walker ambulation. Other studies have found that handgrip strength is an important prognostic indicator of frailty, which Graham and Brown defined as "a geriatric syndrome characterized by an excess vulnerability to stressors, with reduced ability to maintain or regain homeostasis after a destabilizing event" that is associated with "an increased risk of disability, morbidity, and mortality." There were moderate correlations between UE weight bearing force and balance and during walker ambulation (4 square step test) and sit-stand transfers (center of gravity anterior-posterior stability index). In addition, quality of life was also related to UE weight bearing force during functional mobility tasks, specifically the subdomains of role limitations—physical health, role limitations—emotional, and social functioning. When examining demographic characteristics (age, height, weight, and body mass index), only body mass index had a moderate relationship with peak UE weight bearing force during sit-stand transfers. In summary, 4 square step test, SF-36, and body mass index could be helpful clinically to identify patients susceptible to using UE weight bearing force greater than 10 lb (4.5 kg) during functional mobility tasks and, therefore, determine who would benefit most from postoperative feedback training with an instrumental walker.
Study results demonstrated that the relationship between PM muscle and UE weight bearing force is strong within subjects. In a previous study, we found strong intrasubject correlations between PM muscle EMG values and both peak and average UE force. Muscle activation in this study was monitored via EMG, which measures the degree of muscle activation (motor neuron recruitment) by quantifying the number of action potentials; as such, EMG is an indirect indicator of muscle force production. Intrasubject correlations were strong (most >75%) suggesting that estimating UE force using PM muscle EMG values for a given subject might be possible.
Overall, our findings suggest that use of an instrumented walker and feedback training would be beneficial in clinical practice, especially to help older patients more accurately follow weight bearing instructions. Most previous studies have used bathroom scales, force plates/pressure sensing mats, or foot pressure sensors to measure weight bearing through the lower extremities. Others have described use of an instrumented walker to measure force placed through the assistive device, but commonly, this was accomplished with sensors placed in the walker legs and not the handles. The instrumented walker used in this study had force transducers incorporated into the handles which Khodadadi et al. found result in easier installation and less error compared to installation on circular vertical walker legs. An inexpensive, lightweight walker without bulky add-ons for the measurement and display of force would have multiple clinical applications for providing feedback to patients who need to limit weight bearing through the upper or lower extremities. In addition to patients recovering from median sternotomy, an instrumented walker and feedback training would be useful for patients who need to limit weight through a lower extremity, for example, following a fracture. In this study, weight bearing was equal bilaterally through the UE, indicating that possibly only a single walker handle needs to be instrumented. Although, UE force asymmetry may occur in patients with significant unilateral extremity disorders.
Several limitations need to be considered when generalizing the results of this study. This prospective study only included 30 subjects, but based on effect size, this provided adequate statistical power (>80%). Older (60-85 years of age) healthy study participants were utilized in order to evaluate the safety of the instrumented walker and to reduce variability due to other factors, such as pain and impaired cognition that can occur after surgery. It would be expected that this functionally independent population should be better able to use only 10 lb (4.5 kg) or less of UE weight bearing force than populations with functional limitations and/or that use an assistive device for mobility.