Oxygen Consumption and Blood Pressure Are Not Influenced by Use of a Backpack Hip Strap

Introduction

Backpacks are designed to decrease the subjective and objective effort of moving while carrying moderate to heavy loads and are used by students, military personnel, and recreational hikers.^1,2 Previous research suggests that backpack loads exceeding 15% of body mass are likely to cause adverse effects on walking biomechanics. ³ In addition, the physiologic stress of carrying a backpack increases as the mass of the backpack increases.^4,5 To balance discomfort with the need to carry significant loads, a pack load of no greater than 30% of the wearer’s body mass has been recommended for recreational hikers. ⁶

Backpacks designed for heavier loads often have an attached hip strap, which helps secure the load to the body. This results in more mass being placed closer to the wearer’s center of mass. ⁷ In addition, by reducing the load on the wearer’s shoulders, a backpack hip strap also decreases the subjective discomfort of the wearer. ⁸ Despite the effects on comfort, relatively little research has been published examining the effect of a hip strap on energy expenditure while walking.

Several studies have considered the effects of walking with a backpack and the changes in physiologic response over the duration of a trial and at various different backpack loads. ^{9 -}11 Yet, only one study ¹² examined the effect of a hip strap on physiologic responses to walking with a backpack. That study found that the use of a backpack hip strap resulted in a lower oxygen consumption and rate of perceived exertion compared with no hip strap. The results of that study suggest that the use of a hip strap reduces energy cost during backpack carriage. ¹² However, that study employed a protocol that was only 10 min in duration, a self-selected light exercise intensity, and a standardized absolute backpack load of 24 kg for all participants. No research has been done to examine the effect of the use of a hip strap when performing a bout of exercise at a standardized relative moderate intensity, utilizing a recommended standardized relative pack load of 30% of body mass ⁶ and a duration in line with recommendations for cardiorespiratory exercise. ¹³ Therefore, the purpose of the current study was to investigate the effect of a backpack hip strap on energy expenditure and cardiovascular responses while walking for 30 min at a moderate intensity with a load equal to 30% of the wearer’s body mass.

Methods

All volunteers reported no known injuries to the feet, ankles, knees, hips, and or spine within the year prior to testing. Participants self-reported partaking in regular exercise at least 3 times per week for a minimum of 30 min each session and had experience carrying heavy backpacks. Lastly, participants reported being free from known cardiovascular, metabolic, or neurological diseases or disorders. Prior to data collection, the experimental procedures and risks of the study were explained to the participants, after which they provided written informed consent. The institutional review board of Montclair State University approved the consent form and experimental protocol prior to data collection.

Each participant reported to the laboratory for one screening visit and two testing visits. During the screening visit, the participants filled out questionnaires regarding health history. Height was measured using a stadiometer (Detecto, Webb City, MO) and body mass was measured using the bioelectrical impedance scale (Tanita MC-780U Segmental, Tokyo, Japan), which also predicted body composition. Resting blood pressure and heart rate (HR) were measured using an automated blood pressure cuff (BP785N, Omron, Kyoto, Japan). Resting HR was then used to calculate 40-50% of the participants’ heart rate reserve (HRR) using the Karvonen formula. ¹⁴ The target heart rate zone was 40-50% of HRR because it corresponds to the lower limit of moderate exercise intensity according to the American College of Sports Medicine. ¹³ Participants were then asked to perform a 10-min walk carrying a backpack (Commander + Pack Bag, ALPS OutdoorZ, New Haven, MO) containing 30% of their body mass. The backpack load of 30% of body mass was chosen because it has been identified as the recommended upper load limit for recreational hikers. ⁶ Throughout the initial 10-min treadmill test, a Polar E600/T31 (Polar, Kempele, Finland) HR monitor was used to monitor HR. Participants were positioned on a treadmill at a grade of 3%. The speed was slowly adjusted until the participant’s HR was within 40-50% of their HRR for three consecutive min. The corresponding speed was recorded and used for the following two testing visits.

On a different day, the participants reported to the laboratory to perform one of two backpack treadmill tests. One test employed the use of the hip strap (strapped) and the other was without the use of a hip strap (unstrapped). The 30-min treadmill trials were counterbalanced and took place on different days, with between 2 and 10 d separating the trials. The time of day for the trials was not standardized between all subjects, but each subject reported for both of their trials at a similar time of day. Prior to the start of each trial, the participants were asked to refrain from physical activity for 24 h, avoid caffeine and alcohol consumption as well as illicit and over-the-counter drug use for 12 h, and fast for at least 4 h. Participants sat quietly for a 5-min baseline period before beginning the 30-min treadmill walking trial.

Cardiovascular measurements were made using a human non-invasive blood pressure (NIBP) continuous monitor (ADInstruments, Colorado Springs, CO). The NIBP allows for beat-by-beat finger systolic blood pressure (SBP), diastolic blood pressure (DBP), mean arterial pressure (MAP), and HR ¹⁵ using ADInstruments PowerLab with LabChart 8 software (ADInstruments, Colorado Springs, CO). The NIBP monitor was placed on the middle finger of the right hand. The right arm was placed in a sling hanging from the neck and rested against the participant’s abdomen with the elbow at approximately a 90° angle to ensure that the arm and hand maintained the same position during both trials. Participants were then outfitted with an appropriately sized facemask, which was used to continuously monitor whole-body aerobic metabolism using the Vmax Encore metabolic cart (Yorba Linda, CA).

Muscle oxygen saturation (SmO₂) and total hemoglobin (THb) were measured on the participant’s left side in the muscle of the forearm (FM), quadriceps (QM), and calf muscle (CM) via near-infrared spectroscopy (NIRS) using Moxy sensors (Moxy Monitor System, Hutchinson, MN). The FM sensor was placed at the widest part of the anterior portion of the lower arm and was used as a control site for these measurements. The QM sensor was placed midway between the greater trochanter and patella and in the anterior mid-line of the thigh. The CM sensor was placed on the lower leg over the belly of the gastrocnemius muscle. The QM and CM were measured to more directly monitor oxygen handling of the prime movers between trials. All sensors were outfitted with light shields to prevent ambient light from influencing the signal and were gently wrapped to ensure stable positioning while still allowing for normal blood flow.

Once the participant completed the treadmill test, the backpack and equipment were removed and the subject was encouraged to perform a 5-min recovery walk. On another day, the subject reported back to the laboratory and followed the same protocol in the opposite hip strap condition.

During the three visits, each participant carried the same loaded backpack (Commander + Pack Bag, ALPS OutdoorZ) adjusted to weigh 30% of their body mass, including the mass of the backpack. Sand-filled sacks of predetermined mass were placed inside the backpack in order to achieve the proper mass load for each subject. The backpack was loaded such that the mass was positioned at approximately the level of the subject’s waist. During the strapped trial, the middle of the hip strap was placed over the iliac crest of the wearer and tightened. The shoulder straps were used during both conditions and were self-tightened by the subject to their level of comfort.

Data from baseline and exercise were averaged (5 min of baseline and exercise at 0-5, 6-10, 11-15, 16-20, 21-25, and 26-30 min) for the following variables: oxygen consumption (VO₂), respiratory exchange ratio (RER), minute ventilation (V_E), HR, FM SmO₂ and THb, QM SmO₂ and THb, and CM SmO₂ and THb. Change scores from the last 2 min of the seated baseline were calculated for SBP, DBP, and MAP. Change scores are reported for these variables because although the NIBP system allows for noninvasive continuous measurements with accurate change scores, absolute values are generally believed to be less accurate using this technique. ¹⁶ A two-way repeated measure analysis of variance (ANOVA) test was used to determine differences across time and conditions (strapped vs unstrapped). When the ANOVA was significant (P<0.05), a Bonferroni post hoc analysis was run to determine where the difference existed.

An a priori power calculation was performed using an alpha of 0.05, a power of 0.80, and an estimated 10% difference in oxygen consumption between the strap and unstrapped conditions. The results revealed that 17 participants were needed for this study. All statistical analyses were performed using SPSS (IBM, version 25, Armonk, NY). All results are reported as mean±SD unless otherwise noted, and the α-level was set at P<0.05.

Results

Twenty-three participants (12 male, 11 female; age 24±4 y) completed the study. Anthropometric and resting measures from the participants were: body mass (72.7±5.4 kg), height (1.70±0.10 m), body fat (23±8%), resting blood pressure (SBP 113±10 mm Hg; DBP 68±9 mm Hg), and resting HR (67±16 beats·min^-1). The target HRR was 118±12-130±10 beats·min^-1. The backpack mass was 21.6±4.6 kg, treadmill speed was 1.2±0.2 m·s^-1, and grade for all subjects was 3%. Laboratory conditions were stable throughout the data collection trials (temperature 18.5-20°C; relative humidity 30-40%).

No main effect for strapped vs unstrapped conditions was found for any of the variables (P>0.05). There was a difference in most variables from baseline to the first 5 min of exercise, which was maintained throughout the study.

Metabolic data are presented in Table 1. A main effect for time was found (P<0.001) in the VO₂ response. All exercising VO₂ values were elevated compared with baseline (P<0.001), but there were no differences in VO₂ between any exercising time points (P>0.05). There was no main effect for condition (P=0.842) and no interaction (P=0.467). There was a main effect for time on RER. The first 5 min of exercise resulted in a decrease in RER compared with baseline and all other time points (P<0.05). No differences existed between conditions (P=0.886). There was an interaction between condition and time on RER (P=0.048). A main effect for time was observed for V_E such that all exercising V_E values were increased during exercise compared with baseline (P<0.000). There was no main effect of condition on V_E (P=0.926), but there was an interaction (P=0.043) between condition and time.

Cardiovascular data are presented in Table 2. There was a main effect of time on HR such that there was an increase in HR from baseline to 5 min (P<0.000), but HR was not different between conditions at any time point (P=0.194), with no interaction (P=0.143). There was a main effect of time such that ΔSBP was different between 5 and 25 min (P=0.033) and 5 and 30 min (P=0.018). There was no main effect for time on ΔDBP nor ΔMAP (P=0.142 and 0.128, respectively). Also, the ΔSBP, ΔDBP, and ΔMAP were not different between conditions at any time point (P=0.708, 0.374, and 0.389, respectively), with no interaction (P=0.954, 0.302, and 0.521, respectively).

Table 3 contains SmO₂ and THb data. There was a main effect for time with FM SmO₂, which was increased from baseline to all exercise time points (P=0.030), but there was no difference in FM SmO₂ between conditions (P=0.243), with no interaction (P=0.139). Similarly, QM SmO₂ was elevated during exercise compared with baseline (P=0.007), but no changes were observed between conditions (P=0.359), and there was no interaction (P=0.734). Calf muscle SmO₂ also increased from baseline (P<0.000) but was not different between conditions (P=0.888), with no interaction (P=0.130). FM THb was similar at all time points (P=0.924) and was not different between conditions (P=0.426) nor was there any interaction (P=0.892). QM THb was increased from baseline at all exercise time points (P=0.007) but was not different between conditions (P=0.087) with no interaction (P=0.734). Similarly, CM THb was greater at all exercise time points compared with baseline (P=0.301) but was not different between conditions (P=0.563), with no interaction (P=0.595).

Discussion

The key findings of this study are that the use of a backpack hip strap did not have an impact on any of the measured physiologic variables when participants walked for 30 min at a constant moderate intensity, carrying a load of 30% body mass.

Several studies have examined the effect of backpacks on energy cost while walking. ^{9 -}11,17,18 However, those studies did not systematically examine the effect of hip strap use on energy expenditure. One previous study ¹² examined the effect of a backpack hip strap and found that exercise VO₂ was greater when no hip strap was worn as compared with when a hip strap was used. In that study, exercise intensity was self-selected, the exercise interval lasted only 10 min, and the absolute carrying load was standardized at 24 kg for all subjects. When compared with that study, the present study’s findings are more applicable to recreational hikers.

The exercise interval in the present investigation was 30 min versus 10 min in previous work. ¹² The 30-min interval better reflects recommendations for daily cardiorespiratory exercise. ¹³ The present investigation also employed a standardized relative backpack load of 30% of the participant’s body mass, which is the recommended upper limit for the backpack load in recreational hikers. ⁶ Future research should compare the effect of hip strap use during different durations, intensities, and loads on physiological variables.

The researchers in the previous study ¹² used an all-purpose lightweight individual carrying equipment military backpack with a load distribution over the mid-thoracic region. The present study used an ALPS Commander backpack with the load distribution at the waist. The difference in load distribution may have influenced the differing results because external load has been shown to affect energy costs and loaded carriage capacity. ¹⁶ Loads that are closer to the wearer’s center of mass, as was the case in the present investigation, have shown to decrease postural instability, which leads to a reduction in energy costs. ¹⁹ Other research ²⁰ found that carrying a load closer to the lower back, as was the case in the current study, reduced the amount of energy expended when compared with having the load placed closer to the upper back. Future research should incorporate the effect of load placement while also considering the effect of a hip strap on energy expenditure.

It has been established that SBP increases as workload increases. ^{21 -}24 Blood pressure recovery rise was also shown to be greater following exercise when a backpack loaded with 20% versus 10% body mass was carried. ² The present study did not find a difference in SBP between the strapped and unstrapped conditions when carrying 30% of body mass. This may have occurred because the physical mass of the backpack remained unchanged despite the fact that the hip strap reduced the vertical load from the shoulders to the hips. Blood pressure may have remained the same between the 2 conditions because the mass of the backpack did not change. Also, cardiovascular response may also have remained similar because the mass remained the same.

Previous studies have examined muscle oxygenation and backpack carriage, specifically shoulder muscle oxygen.^25,26 Those studies found that backpacks reduce muscle oxygenation in the shoulder and upper extremities. The present study measured the SmO₂ of the prime movers while walking, with the forearm serving as a control. No difference was found at any of the sites between the strapped and unstrapped conditions. There was no difference in total VO₂ or THb between the 2 conditions, indicating that neither was influenced by use of a hip strap. We did not assign a non-backpack trial for comparison. Our results suggest a hip strap does not influence muscle oxygenation compared with carrying a backpack without using a hip strap.

Limitations

The limitations of this study include the use of a single backpack load, exercise duration, and exercise intensity. Testing multiple backpack loads, durations, and exercise intensities might better determine the effect that the use of a hip strap exerts on a variety of physiologic variables. Another limitation is placing the subject’s arm in a sling to collect accurate blood pressure data while walking. This created a fixed and unnatural position that likely influenced walking biomechanics. Lastly, only college-age, physically active participants were examined in this study, making it difficult to generalize our findings to other populations.

Conclusions

Many backpacks used for hiking or carrying moderate to heavy loads have been designed with hip straps, which may lead people to believe that this design feature is essential. Our findings indicate that a backpack hip strap may not have an effect on the energy cost or several other measures of physiological stress when walking at a moderate intensity carrying a moderate to heavy load. Thus, people may choose a backpack with a hip strap based upon perception of a benefit that may not exist. To further clarify a hip strap’s efficacy, future studies should compare the metabolic cost and cardiovascular strain when a hip strap is used under different loads, exercise intensities, durations, and load placements.