Cardiovascular responses to carrying groceries in bags with and without handles.
Shopping for groceries may involve carrying heavy loads some distance. The inability of the individual to perform tasks such as carrying groceries diminishes independence and may be part of a downward spiral of physical activity as that individual becomes dependent in progressively more activities and becomes progressively more deconditioned. Upper extremity activities including sweeping, lifting pots, erasing chalk boards, and changing light bulbs can create a high relative oxygen demand in patients with COPD. (1) Although individuals with COPD have the same V[O.sub.2] for a given activity, the fraction of V[O.sub.2] max is greater and tasks are perceived as more strenuous for these individuals. (4)
Grocery carrying may be performed by holding bags that are large enough to hold two gallon-sized containers against the chest, or by carrying the same load at the sides of the body divided among bags equipped with handles. Simple kinesiologic analysis of the two types of carrying suggests that holding a bag against the chest with elbows flexed would be more demanding due to a longer resistance arm. This is suggested by studies investigating static upper extremity positioning in patients with chronic airway obstruction. Static holding of arms in an elevated position increases metabolic rate and ventilation. (2) Similar results were found comparing resting the upper extremities at the side, unsupported elevation of the upper extremities, and supported elevation. (3) However, individual variation in neuromuscular strategies for holding a load with the upper extremities is noted to occur. (5) A second factor to consider is the fraction of voluntary muscle contraction elicited in the small muscles in the wrists and fingers, which would seem to be greater when carrying groceries at the side in bags with handles. Thirdly, the combination of static activity such as holding grocery bags with dynamic exercise of walking is not easily quantified by heart rate increase or heart rate recovery cost. (6)
A better understanding of how positioning a load carried primarily by the upper extremities affects the cardiopulmonary system will enable us to develop strategies to decrease the metabolic demands placed upon the body and increase the efficiency when performing upper extremity ADLs. The purpose of this study was to determine whether physical demands differ between carrying in bags with handles compared with carrying in bags held against the chest. Heart rate, blood pressure, galvanic skin response, and electromyography from representative muscles were selected as measures of physical demands imposed by the two carrying techniques.
Healthy young adults were recruited from a sample of convenience. Subjects had no known pathologies of the musculoskeletal or cardiovascular and pulmonary systems. Informed consent was obtained from twelve subjects (three male, nine female) ranging in age from 21-30 (median age = 22) with a median body mass index of 24 who completed the study. This study was approved by the Institutional Review Board of the University of South Alabama. A repeated-measures design was used. Each subject walked at 2.5 mph on a motorized treadmill for three minutes carrying two one-gallon milk containers filled with tap water (16 pounds, 7.26 kg). This load was chosen based on pilot studies to produce a level of exertion between 12 and 13 (somewhat hard) on the Borg Scale of Perceived Exertion. During one trial, the load was carried in a grocery bag held against the chest. The other trial consisted of carrying a single gallon container held at each side in a grocery bag equipped with a handle. The order of trials was determined by randomized blocking. The odd numbered subjects were randomized into sequence and the even numbered subjects performed the two trials in the opposite sequence of the preceding subject to ensure equal numbers in the two sequences. Between trials, subjects were allowed to rest in standing until all measurements returned to the standing resting values obtained before the first trials.
Measurements included heart rate using a BCI 3301 pulse oximeter, systolic and diastolic blood pressure using standard sphygmomanometry, surface electromyography from the anterior deltoids and wrist/finger flexors of the dominant upper extremity, and galvanic skin response from the dominant hand. The EMG sites were chosen as they are expected to vary with the type of carrying technique employed.
For the EMG measurements, skin was prepared over the anterior deltoid and mid-forearm with 4x4" gauze sponges moistened with acetone. Disposable, pre-gelled pediatric EKG electrodes were placed on the following sites: the negative and positive electrodes of each deltoid lead were placed parallel to the direction of fibers of the anterior deltoid at equal distances between each other, the acromion, and deltoid tuberosity. The wrist/ finger flexor electrodes were placed parallel along the midline of the anterior surface of the forearm at equal distances between each other, from the medial condyle of the elbow, and flexor tendons crossing the wrist. A five-lead wire with a common ground for both leads was attached to an ADInstruments dual Bio Amp/ Stimulator, connected through an ADInstruments PowerLab 4SP data acquisition device and recorded using Chart 5.1 for Macintosh OS X. Root mean square values were determined by software. The common ground lead was placed over the seventh cervical vertebral process.
Galvanic skin response was recorded as an index of generalized sympathetic response. This response has been demonstrated to vary with the fraction of maximum voluntary contraction elicited by a load regardless of the size of the muscles involved. For example, galvanic skin response is similar during handgrip and knee extension at a given fraction of maximum voluntary contraction. (7) Galvanic skin response was recorded with standard GSR electrodes on an ADInstruments GSR amp connected to the same data acquisition device. Galvanic skin response was zeroed electronically and individually at the start of each session, but was not rezeroed between trials. Return to baseline GSR was required before advancing to the second trial.
Following instruction and baseline measurements, the motorized treadmill was set to 2.5 mph and zero grade. As soon as the subject was safely walking at the pace of the treadmill, the appropriate load was handed to the subject to carry for the next three minutes. Heart rate, GSR, and EMG were recorded continually.
Blood pressure was determined before each trial and during the last 30 seconds of each trial. Values obtained immediately prior to the end of three minutes were used for analysis. Bags were taken from the subject and the subject was allowed to gradually decrease walking speed before stopping. At the end of the first trial, heart rate and GSR were monitored constantly. When these values returned to baseline, blood pressure was measured again. When blood pressure was the same as the previous baseline, the second trial using the other carrying technique was begun with the subject increasing in walking speed to 2.5 mph and being handed the grocery bags. The pulse oximeter was placed on the index finger of the non-dominant hand. In cases in which a poor signal was obtained, the subject carried the load without the use of the non-dominant index finger.
Main effects of rest vs carrying and method of carrying were analyzed by 2 x 2 factor ANOVA. Although a main effect of carrying vs rest was shown (p < .01), a significant interaction between carrying vs rest and method of carrying was also found (p < .01). In order to account for individual differences in initial values for heart rate and blood pressure, t tests were performed for the normalized change in heart rate, systolic, and diastolic blood pressure between carrying techniques [100 x (3 minute value--resting value) / resting value]. Although all other data were recorded within the allotted three minutes, some blood pressure values were obtained after the three minutes due to technical difficulty and not used for statistical analysis. Because EMG and GSR data did not meet the assumptions of parametric statistical analysis, the percentage changes in EMG and GSR values from baseline to three minutes (calculated as noted for the cardiovascular parameters) were analyzed using the signed rank test, which is the nonparametric equivalent of the paired t test. Data analysis was performed with SPSS version 17.01 for Windows.
Resting heart rate and blood pressure values are shown in Table 1. As noted in methods, ANOVA demonstrated a greater heart rate during carrying compared with the resting value (p < .01). Systolic blood pressure was also increased during carrying (p < .01), but no change in diastolic pressure was found (p = .40). A greater percentage increase in heart rate from its resting value occurred when the load was against the chest than when held in handled bags at the sides (p = .02). No differences could be observed in percentage change from resting value in either systolic (p = .94) or diastolic blood pressure (p = .49) between carrying techniques (see Table 2). Galvanic skin response increased significantly more (p = .04) during trials in which the load was carried against the chest than during the trials in which the load was carried at the sides (Table 3). No significant differences were observed between the two trials in increase in wrist/finger flexor EMG (p = .40), but anterior deltoid EMG increased more when carrying the load at the sides (p = .04).
DISCUSSION AND CONCLUSIONS
Grocery carrying is a nearly universal task that is trivial for those without cardiopulmonary limitations. Carrying loads with the upper extremities is also a component of many other daily tasks. Physical rehabilitation may be useful in improving performance of such tasks. However, simple tasks may become excessively strenuous for patients with cardiopulmonary disease (1,4) or those deconditioned due to acute or chronic illness or injury and therefore, energy conservation may also become necessary.
Carrying loads such as grocery bags may be accomplished by one of two primary techniques--holding a load close to the body across the chest, or holding the load close to the sides of the body. The literature does not provide a clear answer of which technique produces less cardiovascular stress. Elevation of unsupported upper extremities can be very stressful for persons with cardiopulmonary dysfunction. (2,3) Moreover, a superficial kinesiologic analysis would predict that holding a grocery bag against the chest creating a long resistance arm at the elbow is likely to be a greater burden than holding the same weight close to the body with the shoulders and elbows in straight positions. However, individual differences in muscle activation while holding a load make prediction of the contribution of individual muscles more complex and motor strategies may vary over the course of holding a load. (5) Analysis is further complicated by the use of small muscles to stabilize the shoulders and grasp bags when holding them against the side of the body. The use of small muscles to hold a load may lead to a greater percentage of maximum voluntary contraction in these muscles than would presumably occur with use of larger muscles that are likely to be involved in holding a bag against the chest. Because cardiovascular stress is directly related to the fraction of maximum voluntary contraction elicited, the possibility exists that carrying the same mass within bags carried by handles at the sides of the body may create greater cardiovascular stress.
The stress of carrying the load was monitored in this study in three ways--heart rate, blood pressure, and skin galvanic response. Additionally, the demand placed on muscles common to both carrying techniques was measured to give insight about assumptions underlying choices of carrying techniques.
Heart rate is a widely used index of exercise stress. In this study, heart rate increased significantly more when the load was carried against the chest, suggesting that the distance that the load is held away from the body using this technique has greater influence on cardiovascular demand than the cost associated with stabilizing the shoulder and grasping bags with the fingers. This conclusion is further supported by the greater increase in galvanic skin response, an index of general sympathetic nervous system activation. Static muscular effort such as handgrip produces a graded response in skin sympathetic activity to the percentage of maximum voluntary contraction (8) and is similar with smaller muscles used for handgrip and larger muscles used in knee extension, whereas heart rate and blood pressure demonstrate a greater response to the larger muscle activity. (7) The EMG activity of the anterior deltoid was greater when carrying the load in bags held by handles at the side of the body, presumably due to the need to stabilize the glenohumeral joints. Contrary to expectations, the wrist/ finger flexor EMG activity did not differ between the two carrying techniques. Given that these small muscles are significantly involved in carrying grocery bags at the sides of the body, the concern that this technique would cause greater cardiovascular stress due to greater involvement of small muscles was not realized. Therefore, the primary cardiovascular difference between the two techniques appears to be caused by the length of the resistance arm created by holding the upper extremities in flexion to hold the grocery bag against the chest.
This study examined the responses of young healthy subjects carrying a load of approximately 16 pounds while walking at a speed of 2.5 mph for three minutes. This load was chosen to create an intensity of exercise between light and somewhat hard for this young population, based the assumption that the average person would be willing to experience this range of exertion during grocery carrying. This absolute load is likely to be more stressful than a deconditioned person would be expected to carry and we do not know what level of exertion a given person with cardiovascular deconditioning would be willing to experience. The results of this study can only suggest differences in cardiovascular stresses related to kinesiologic principles that may apply to different methods of carrying. A study using subjects with cardiopulmonary dysfunction or deconditioning and carrying a load representative of actual grocery purchases by them is needed to confidently promote one type of carrying technique as a means of energy conservation for those with diminished aerobic capacity or muscle strength. Another consideration that could not be tested with this sample of young adults was whether carrying a load against the chest might further compromise breathing in those with chronic lung diseases.
Determining means of more efficiently performing everyday activities may become very important in maintaining activity and cardiovascular conditioning for many individuals. To become more functionally independent, individuals with cardiopulmonary disease must make many changes in what tasks they perform and how they perform them. Many of these changes in lifestyle are unappealing, which leads to poor compliance. However, these patients can often benefit from many simple and non-threatening changes in their performance of activities of daily living, such as arm position when carrying loads. Success in one task by implementing a simple change may motivate patients to find solutions to accomplish other tasks within their cardiopulmonary limitations and promote a more independent lifestyle.
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Glenn Irion, PhD, PT, CWS Associate Professor, Physical Therapy Department, University of South Alabama, Mobile, AL. email@example.com
Heather Hillhouse Melancon, MSPT Physical Therapist Therapeutic Concepts Bogalusa, LA
Nick Nuchereno, MSPT
Brian Strawbridge, MSPT DCH Outpatient Physical Therapy Tuscaloosa, Al
Jessica Young, MSPT Physical Therapist Riverside Medical Center Franklinton, LA
Table 1. Cardiovascular Values at Rest Bags held at side Bags held against chest HR (bpm) 81 (76-86) 82 (77-87) SBP (mmHg) 120 (114-126) 120 (115-125) DBP (mmHg) 81 (78-85) 84 (81-87) Values are means with 95% confidence interval shown in parentheses Table 2. Percentage Changes in Cardiovascular Values from Resting to 3 Minutes of Carrying. Bags held at side Bags held against chest p HR 35% (22-48) 49% (42-56) .02 SBP 9% (6-13) 9% (6-13) .94 DBP 4% (0-7) 5% (1-9) .49 Values are means of the percentage changes from resting values as: 100 x (value at 3 minutes of carrying the load--resting value) / resting value. 95% confidence interval shown in parentheses. p value from paired t tests of percentage change from resting. Table 3. Percentage Changes in EMG and GSR from Resting to 3 Minutes of Carrying. Bags held at side Bags held against p chest Deltoid EMG 94% (16-172) 10% (0-35) .04 Wrist flexor EMG 48% (0-112) 41% (0-122) .40 GSR 7% (0-30) 62% (14-111) .04 Values are means of the percentage changes from resting values as: 100 x (value at 3 minutes of carrying the load--resting value) / resting value. 95% confidence interval shown in parentheses. p values from signed rank tests on percentage change from resting.
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|Title Annotation:||RESEARCH REPORT|
|Author:||Irion, Glenn; Melancon, Heather Hillhouse; Nuchereno, Nicholas; Strawbridge, Brian; Young, Jessica|
|Publication:||Journal of Acute Care Physical Therapy|
|Date:||Dec 22, 2010|
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