Hiking is one of the most popular recreational pursuits in the world. A bout of hiking activity can range from a simple day hike to an expedition hike lasting several months. In either case, the use of a backpack to carry supplies has become the preferred method of external load carriage in the Western world. Hiking and backpacking are typically characterized by a long continuous exercise of low intensity, showing positive effects on the cardiovascular system, cardiopulmonary system, and the active and passive structures of the locomotor system (16). The positive effects can be reduced by the occurrence of injury or pain (4). The carrying of an external load has been associated with an increased risk of musculoskeletal disorders in both recreational and working populations (12). Studies have examined the loading of the joints in the lower extremity while carrying external loads. These studies report that the larger the external load, the larger the forces experienced by the joints (9,15,17). Moreover, these forces seem to be exaggerated during downhill walking.
The forces experienced while walking downhill are similar to those of level running (10). Kuster et al. (10) found that while walking downhill there were much higher compressive forces at the patellofemoral and tibiofemoral joints compared with walking on level ground. These forces were reported to be as high as three to four times that of level walking. As a deterrent to injury, manufacturers claim that the use of hiking poles can reduce the amount of force felt at each of the joints by as much as 25%, allowing people to walk downhill with less postsession pain. Much of the theory behind this reduction is increased balance and support, which is believed to allow the hiker to transfer some of the forces from hiking downhill to the poles and to allow for less force to be transferred through the joints of the lower limbs.
Although manufacturers make claims of reducing forces through the use of trekking poles, they have not cited any research to support those claims. Research showing a reduction in net joint forces (NJF) and net joint moments (NJM) while using the poles has been shown independent of the manufacturers (14,16,19). Neureuther (14) noted that over the course of an hour of hiking, up to 25 tons of cumulative force can be reduced by the use of hiking poles. Schwameder et al. (16) noted a reduction in both vertical and shear forces (anterior/posterior) during downhill walking with the use of the hiking poles. They also found that vertical ground reaction forces were reduced by 4-21%, and the tibofemoral shear forces were reduced by 11-21% at the knee. Also, patellofemoral compressive forces were reduced by 12%. Similar reductions were noted for the joint moments. The combination of these force and moment reductions help illustrate the value of the hiking poles while walking downhill. In addition, it has been shown that hiking poles provide increased stability and more upright posture, which is believed to be related to improved breathing than when walking downhill without the use of the trekking poles (6,8).
Hiking is a way of life in many areas of the world and helps maintain a healthy lifestyle. In the last few years, the popularity of hiking poles has increased, and many technological advances have been made to help reduce the amount of injury experienced during hiking. For those who hike downhill for their livelihood, the reduction of injury becomes even more imperative. The use of poles has been claimed to reduce forces and shown to reduce moments at the joints of the lower extremities under controlled conditions (16). However, the question remains: will those claimed reductions hold true when the effects of an external load are added?
The purpose of this research was to examine the effect of using trekking poles on lower-extremity NJF and NJM while hiking downhill at slopes that closely resemble mountain paths while carrying an external load (backpack). It was hypothesized that the correct use of hiking poles would help reduce the amount of potentially damaging forces and moments at the joints of the lower extremity while hiking downhill with the addition of an external load by means of wearing a backpack, both expedition and day pack.
The present study tested the usefulness of hiking poles to help reduce peak NJF at the knee, peak NJM, about the mediolateral axis, and peak NJP at the joints of the lower extremity. This study used an experimental design. Data were collected while participants were walking downhill at a 20° gradient (36.4% slope). All participants repeated this condition while wearing a large expedition pack, a smaller day pack, and no backpack while using two hiking poles and no hiking poles. The dependent variables were the ankle, knee and hip peak NJM and power, and NJF at the knee. Ground reaction forces (normal, vertical, and anterior/posterior), joint angles, and stance times were used for descriptive purposes only and were not included in the statistical analysis.
Participants were selected on a volunteer basis from a self-reporting healthy, active, nondisabled adult population. The sample consisted of 15 male adult participants (ages 20-49, height ranging from 1.63 to 1.86 m, and weights ranging from 600 to 1063 N). The participants were required to have experience using hiking poles and hiking while wearing a backpack to participate in the study.
Ground reaction forces were measured using a Bertec force plate (Bertec Instruments), collected at 1000 Hz. Force data were analyzed using DataPac 2K2 Software (Run Technologies). Participants were videotaped at 60 Hz using a Panasonic PV-GS200 Mini-DV camcorder, from a sagittal plane. The video data were digitized using a Peak Performance System (Peak Performance Technologies). Kinematic data were also filtered using a 12-Hz low-pass Butterworth filter.
A wooden ramp constructed to simulate hiking at a 20° gradient was used. The downhill portion of the ramp was covered with a flooring carpet to help simulate pole plants on trails. The ramp was 1.22 m (4 ft) wide and consisted of 3.05-m-long (10 ft) ramps with a 1.22-m (4 ft) level section in the middle. The force plate was placed in the downhill portion of the ramp, 0.66 m (2 ft) from the bottom, mounted flush, but on a separate platform, with the wood surface. The force plate was covered with a nonskid tape to prevent any slipping on the change of surfaces between the wood and the metal plate.
All participants had the project explained to them and were asked to sign a written informed consent, and approval was obtained by the university institutional review board. Each participant was measured for anthropometric data, which included height, weight, leg length, and length of each segment. These measures were used for demographics and inertial estimations.
The participants were required to walk up the ramp to the level section and down the other side. All participants were given ample practice time to become accustomed to each condition on the ramp. Criteria for participants becoming accustomed to the procedures were defined as being able to obtain a steady state of motion while walking down the ramp, having consistent strides while striking the force plate with the predetermined foot, the left foot for the current study, and using proper technique with the poles. Practice sessions were recorded and analyzed to ensure that each participant maintained a steady state of motion while walking downhill. During data collection, pace was evaluated by the use of an infrared timing device placed on the ramp when crossing the force plate and ensuring that walking speed was within the acceptable range. Footfalls were not constrained mechanically; however, visual inspection of the hikers were used to eliminate all shortened strides, stutter steps, and reaching movements. All participants were given instruction on the correct use of the poles, as described by Neureuther (17), as well as given the opportunity to watch a video produced by Recreational Equipment, Incorporated (REI, Summer, WA) on the correct use of the poles.
All participants were required to walk down the ramp while carrying an external load in the form of a backpack. The backpacks consisted of a large extended expedition pack and a smaller day pack. Both backpacks were Arc'Teryx, the Bora 80 and Bora 40, for larger and smaller packs, respectively (Vancouver, Canada). The backpack loads were based on a percentage of body weight. According to Ghori and Luckwill (5), adjustments do not significantly change after 30% of body weight, so this weight was selected for the larger pack weight. Because the smaller day pack is approximately half the size of the larger pack, the weight of the smaller pack was 15% of participant's body weight. The backpacks were sized for different body sizes and were adjustable to the individual participants. To ensure the load was approximately in the same position, the pack fit was assessed in two places, as described by LaFiandra and Harman (11). The loads within the packs consisted of foam blocks placed around weights to completely fill the packs. The foam blocks were cut so that all loads were held in a constant position within the pack for each trial size.
The data collection consisted of 10 successful trials per participant at each condition (with and without hiking poles, with and without a pack, either wearing a day pack or a large expedition pack), for a total of six conditions and totaling 60 trials for each of the 10 participants. An average of 12 trials were needed to complete 10 successful trials within given constraints. The average of the 10 trials for each condition was used for analysis. For each participant, the order of conditions was selected randomly to counterbalance for any practice effect. Rest periods were provided as needed; however, no participants requested any additional breaks beyond that taken to change pack and pole conditions. A successful trial was defined as striking the force plate with a consistent stride with the predetermined foot at the specified velocity; unsuccessful trials were removed from the data pool. The participants were instructed to walk at 3 mph (1.34 m·s−1) of path distance ± 5%. This speed was selected on the basis of research on downhill walking that showed the optimal walking speed (13). Participants were instructed to wear the same shoes for each trial and to wear the shoes in which they would normally hike. Each participant had markers placed on the distal end of the fifth metatarsal, head, heel, lateral malleolus to mark the ankle, lateral epicondyle of the knee, greater trochanter of femur to mark the hip, and the acromioclavicular joint to mark the shoulder. These markers were used to create a link segment model to calculate the kinematic measures of the participant during the gait cycle.
The video of each trial, defined as stance phase of gait cycle (from heel strike to toe off), was digitized on an automatic digitizing system. The two-dimensional kinematic estimations included linear velocities, displacements, accelerations, and angular velocities, displacements, and accelerations. Sagittal plane joint angles included a plantar foot-shank segment angle, a shank-thigh, and thigh-trunk angle for the ankle, knee, and hip, respectively.
Kinetic data were collected with the ground reaction forces being at a 20° angle from the vertical, and the kinematic data were collected with the traditional coordinates of absolute vertical and absolute horizontal. To combine the two sets of data in the process of inverse dynamics, the kinetic data were rotated 20° to the absolute vertical. This was done through the use of the trigonometry functions. That is, on the basis of the ground reaction force measured (due to force plate orientation) and the angle of the ramp, in radians, the absolute vertical and horizontal forces were calculated. These values were then combined with the kinematic data to calculate the joint moments and power using an inverse dynamics approach.
All moment-of-inertia estimates were calculated using the multiple regression formulas by Zatsiorsky (21). The kinematic and kinetic data were combined to obtain NJF and NJM, and data were graphed at 1% intervals over the stance phase of the gait pattern to obtain peak NJF and NJM, using Newtonian mechanics and inverse dynamics as described by Winter (20). All kinetic measures, NJM, NJF, and power were normalized to percentage of stance time and in magnitude to the participant's body mass, or body mass and backpack mass combined. The process of inverse dynamics was used to estimate joint power and moments. All joint moments were calculated about the frontal or transverse axis at the joint center position for each joint.
Tests for main effects of the dependent variables were conducted using a repeated-measures analysis of variance (ANOVA). The dependent variables were the peak (maximum and minimum) NJM and NJP at the ankle, knee, and hip, and NJF at the knee (averaged across each participant's 10 trials per condition). The familywise alpha (α) level for the repeated-measures ANOVA was set at P < 0.05, and a Bonferonni adjustment was then used to adjust the α level to account for the multiple tests. All data were analyzed using a 2 × 3 (pole × pack) design. A total of 13 statistical tests were run with a familywise alpha level of 0.05 and a resulting test wise alpha level of 0.00385.
If statistical significance was found a between conditions, paired t-tests were used to examine where the differences existed within the conditions, where applicable. The effect sizes were examined for any treatment effect and practical significance. Effect sizes were calculated using standardized mean differences. An effect size of greater than 0.8 was considered a strong effect, a result of 0.5-0.7 was considered a moderate effect, and any result below 0.5 was considered a weak effect. Statistical power was calculated to be 0.69 for 15 subjects, an alpha of 0.05 and an effect size of 0.8.
A significant decrease was noted in the peak plantarflexor moment (F(1,14) = 20.055, P < 0.00385, ES = 0.40) with the use of the trekking poles (Table 1). Poles had no significant effect on the peak dorsiflexor moment during the stance phase. No significant effect of packs were found for either plantarflexor or dorsiflexor moments at the ankle. No significant interaction effect between poles and packs were noted. The use of the poles are effective at reducing the primary moment, the plantarflexor moment, at the ankle regardless of whether the hiker is carrying an external load.
The knee extensor moment was statistically lower (F(1,14,) = 14.578, P < 0.00385, ES = 0.40) with the use of poles (Figs. 1 and 2). The knee flexor moment, usually small in downhill walking, showed no difference between pole conditions. No significant differences were found between pack conditions (effect sizes ranging from 0.01 to 0.04) and no interaction effect between poles and pack were observed. Results show a reduction in the predominant knee moment when using the poles across all pack conditions. The use of poles significantly reduced the flexor moment at the hip. (F(1,14) = 32.469, P < 0.00385, ES = 0.63) (Table 1). There was not a significant effect on the flexor moment attributable to the use of the backpacks (ES = 0.09) or an interaction between the poles and packs. However, the packs did have an effect on the extensor moment early in the stance phase. (F(2,28) = 12.336, P < 0.001, ES = 0.83). The packs increased the extensor moment during the initial stages of the stance phase. Pairwise comparisons showed a significant reduction in the extensor NJM between the no-pack and expedition pack conditions (Table 2). The pack conditions showed a general trend toward a decrease in the flexor moment while packs were being worn. There was not a significant difference in pole conditions for the extensor moment (ES= 0.24) or any interaction between poles and packs for the extensor moment.
The poles help reduce the peak power absorption about the ankle (F(1,14) = 16.623, P < 0.00385, ES = 0.46) (Table 3). Peak power absorption occurs at 87% of the stance phase. For power absorption, the addition of backpacks did not have a significant effect, and there was no significant interaction effect between the poles and packs. There was no significant difference observed for power generation for any conditions at the ankle. Thus, the addition of the packs did not show a significant increase in power generation from the musculature, and the poles were effective in helping reduce the amount of energy that needed to be absorbed around the ankle.
For power absorption at the knee, there were significant results for packs (F(2,28) = 20.698, P < 0.00385, ES = −0.40); however, although it is often believed that carrying the packs will add to the power absorption at the knee, it was just the opposite (Figs. 3 and 4). With the addition of the packs and the external load, there is a reduction in the power absorption. Pairwise comparisons showed evidence of a decrease in the power absorption between the expedition pack condition and both the day pack and the no-pack condition. All other comparisons were statistically nonsignificant. The power generation at the knee showed no significant differences for any condition.
At the hip, only the power generation when adding the backpack showed a significant difference (F(2,28) = 8.323, P = 0.001, ES = 0.58). A pairwise post hoc comparison showed that the expedition pack condition was significantly lower in power generation compared with both the day pack and the no-pack conditions (Table 4). All other comparisons for power generation conditions and power absorption were found to be nonsignificant.
NJF at the knee.
Vertical NJF at the knee were found to be significant for the main effects of poles (F(1,14) = 22.913, P < 0.00385, ES = 0.25) and for packs (F(1.257, 17.597) = 23.998, P < 0.00385, ES = 0.50), which were examined using a Greenhouse-Geisser adjustment because of a violation in sphericity for this condition. There was not a significant interaction effect. The use of the poles showed a significant reduction in the vertical NJF at the knee (poles: mean = 13.106 N·kg−1, SD = 1.09 N·kg−1; no poles: mean = 13.673 N·kg−1, SD = 1.01 N·kg−1). Pairwise comparisons of the backpack condition revealed the no-pack condition (mean = 13.751 N·kg−1, SD = 0.32 N·kg−1) had significantly higher knee-joint forces than the expedition pack condition (mean = 12.969 N·kg−1, SD = 0.53 N·kg−1, ES = −1.39), there were no significant reduction in knee forces between the day pack (mean = 13.449 N·kg−1, SD = 0.34 N·kg−1) and the no-pack condition or the expedition pack condition.
There were no significant effects for any condition involving poles or packs for the horizontal NJF. There was also a nonsignificant interaction effect between the two factors.
The results of the current study demonstrated that the use of poles is effective in reducing the peak NJM of each of the joints in the lower extremity, whether an external load in the form of a backpack or no external load is present. These results are consistent with those found in previous studies involving the use of hiking poles while descending a hill (16,19).
Although reductions per impact may seem rather small, a practical examination reveals that these reductions may be magnified in real-world situations. It has been reported that the average stride length while walking downhill has been reported between 1.25 and 1.5 m (3,9). A brief view of the mapped trails in four different national parks in the western United States (Bryce National Park, Rocky Mountain National Park, Glacier National Park, and Zion National Park) revealed an average day-hike length of 7.16 km (4.45 miles). If one used the average of the two extreme stride lengths noted for this study, 1.375 m, this would equal approximately 5207 impacts on a simple day hike. In the current study, a reduction of the peak primary joint moment of the ankle, knee, and hip of 16.4, 10.61, and 9.65%, respectively, was observed. Although the reductions were statistically significant, it becomes much more important to examine these reductions from a practical standpoint. As noted, the reductions in the joint moments involve adjustments being made in terms of the force production around the joint, because the joint angles are not significantly different and, thus, not changing the moment arm during the stance phase.
Even during an average hike, muscles in the lower limbs would work significantly less with the use of the hiking poles. The joint moments would decrease by 0.13, 0.15, and 0.14 N·m·kg−1 for the ankle, knee, and hip, respectively, for every step. Thus, over the course of an average day hike, an overall reduction of 676.91, 781.05, and 728.98 N·m·kg−1 for each of the joints in the lower extremity would be experienced (the reductions reported are for a relatively short hike, bearing in mind that many long hikes with the heaviest pack often go upwards of 20 miles per day or more; approximately five times the distance reported and, thus, five times the number of impacts). This reduction indicates that the muscles around each joint are not required to produce as much torque, or they are not working as hard.
The decrease observed in the moments about the joints in the current study combined with no significant change in the joint positions while walking with and without poles help form the hypothesis that the body is in a better position mechanically to absorb some of the potentially damaging forces during the stance phase. Reductions in NJM observed with the use of trekking poles could lead to fewer overuse injuries at the muscular level. One adjustment that the combination of the poles and packs seems to make is that hikers using poles have been shown to have a more upright posture while walking (6,7). This adjustment causes an increased extension at the hip, which would increase the length of the hip flexors, possibly leading to an increase in eccentric activity, to remain upright. This upright posture not only alters the mechanical loads on the lower extremity but, possibly more importantly, allows for more efficient respiratory functions by allowing for a greater volume of oxygen to be taken into the lungs.
The addition of backpacks shows a statistically significant reduction in the power absorption during the knee extensor moment. Moving from no pack to the day pack showed a decrease of 19.63%, and when moving to the expedition pack, there was a 29.72% decrease compared with the no-pack condition. These results are in agreement with previous research that has shown that with the addition of the external weight, the body employs a protective mechanism to reduce the amount of joint forces experienced while walking downhill (18). These results lead to the assumption that the body is making an adjustment at the neuromuscular level to account for the additional load.
The only significant increase in power generation at any joint is at the hip. The expedition pack led to a significantly lower peak power generation than either of the other two conditions. The mean power generation for the other two pack conditions showed a 20.76 and 5.34% decrease in the power generation for the no-pack and day pack conditions, respectively. This leads to the assumption that as the weight of the pack increases, the adjustment made at the neuromuscular level becomes more apparent at the hip. It is believed that a reduction in the rate of work would allow for less fatigue in skeletal muscle, thus leading to an increased ability of skeletal muscle to create tension and help stabilize the joints, if needed. As the external load increased to the expedition pack, at 30% of body weight, the power generation decreased significantly, regardless of pole conditions. These adjustments could help reduce the amount of eccentric action for the hip flexors, which accounts for approximately 63% of the stance phase (Fig. 5).
The current study showed a reduction of vertical forces, with poles, at the knee per step of approximately 0.56 N·kg−1, offering a significant difference that is magnified over the length of a normal hike. Over the same 7.16-km hike and 5207 impacts, the cumulative vertical knee joint force will be reduced by 2915.92 N·kg−1. This reduction, if forces are applied to the joint, will help reduce the overuse injuries at the knee. The lesser knee-joint forces seen in the expedition pack condition in comparison with the no-pack condition are thought to be caused by a shifting of the load when using poles with the expedition pack, in that the upper extremities serve to take on some of the pack weight. However, upper extremities were not examined in the current study, so this remains a topic for future studies.
The original question for the current study was to examine whether the use of hiking poles while walking downhill continued to exhibit the same reduction in loading characteristics as have been previously reported when the addition of backpack is placed on the participant (1,2,8,16,19).
In conclusion, the evidence shows that the use of hiking poles is effective in reducing many of the contributing factors to pain and overuse injuries during downhill hiking. The evidence suggests that the use of the poles will help reduce the amount of muscle activity around the ankle and knee and limit the potentially damaging loading at the hip. The peak power results show evidence that there is a reduction in the eccentric muscle actions for the plantar flexors at the ankle and the extensors at the knee. This reduction will reduce the potential for damage at the cellular level within the muscles, allow for muscles to maintain contractility, and reduce the potential for pain and injury after exercise.
The strategy for using the poles seems to be twofold in that there is evidence that participants will use the poles to both reduce the loading by absorbing some of the potentially damaging forces and to help increase stability. The increase in stability would allow for participants to reduce the cocontraction of musculature around the joints in the lower extremity. These two adjustments combine to reduce the risk of injury and may help provide some information that would allow for a greater portion of population to engage in and maintain an active lifestyle. A reduction in soreness and potential for injury would allow increased adherence to the active lifestyle and possibly allow for those in an at-risk population to become more active and healthy.
International Society of Biomechanics Graduate Student Dissertation Award and an Atkinsons Faculty Grant of Willamette University, provided funding for this project.
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