There was a general trend towards a reduced aEMGpush of TB and AD and lower aEMGrec of AD at high frequency (see Table 2). For lower limb muscles, the only significant difference was a higher aEMGpush of Glut for NF+20 compared to NF−20 (P = 0.025, d = 0.76).
The mass or mass distribution of the poles did not significantly affect V˙O2, aEMGtot or aEMGlower. In addition, ANOVA did not show any cross-interaction of mass-frequency on V˙O2, aEMGtot and aEMGlower. More surprisingly, the same results were found on global upper limb muscle activation. Looking more specifically at each muscle, it was found that only two upper limb muscles were influenced by mass or mass distribution when considering aEMGcycle. BB activation was significantly higher with MP and HP than with LP (P = 0.003, d = 0.23), and AD activation was significantly higher for LCoG than either MCoG (P = 0.005, d = 0.51) or HCoG (P = 0.015, d = 0.42). Moreover, when focusing on the recovery phase, TB and BB activation was significantly higher with heavy poles (see Table 3) and exhibited a slightly higher aEMGrec with HP and MP than with LP (P = 0.037 for TB, P = 0.01 and 0.007 for BB, d between 0.08 and 0.19).
The present study provides additional evidence to support the assumption that the use of hiking poles during uphill walking does not lead to an increased energy cost and redistributes muscular activity from lower to upper limbs. It also gives a precise description of muscular activity patterns during pole walking. More importantly, three main results were obtained: (i) hand-carrying loads or using poles of up to 360 g required more muscular activity of upper limb muscles but did not result in a significantly higher energy consumption, (ii) walking with poles at high frequency increased energy cost compared to preferred frequency, whereas low frequency did not affect the global energy cost despite changes in muscle recruitment, and (iii) no interaction between pole mass and walking frequency was observed on either EMG or V˙O2.
Influence of using poles.
Hiking with poles or carrying loads of up to 360 g in the hands does not significantly change energy costs during uphill walking (mean values around 0.54 mL O2·kg−1·m−1). However, for rapid level walking and exaggerated arm movements with poles having a similar mass, opposite results have been found in the literature because the metabolic response increased by between 12% and 23% (19,20). Besides the exaggerated arm movements in these two studies, another possible explanation for these increases lies in the fact that vertical forces exerted by the poles on the ground are probably much greater in the vertical than in the horizontal direction, as shown by Komi (11) in cross-country skiing. Thus, it can be assumed that lower propulsive forces can be exerted with hiking poles on level ground (i.e., horizontal forces) than in uphill walking where the vertical component plays a more important role. Cross-country skiers, for whom propulsive arm forces are essential, exhibit low cycle frequencies (0.7 to 1.1 Hz), longer poles, and trunk flexion to produce a more effective force at the end of the propulsive phase (17). In spite of these characteristics, horizontal forces were shown to be three times lower than vertical ones on level ground in cross-country skiing (11).
EMG profiles of the lower limb muscles for the NP condition are consistent with those previously observed (10,12,13). In addition, aEMG values displayed in the present study provide additional evidence of the efficient redistribution of muscular activity from lower to upper limbs. When the subjects walked with poles, there was a significant lowering in aEMGlower by 15% through increased activation of the upper limb muscles. The reduced aEMGlower concerned only the muscles implicated in knee extension and plantarflexion and activated during the stance phase. Indeed, the aEMG of VL, GL, and Sol decreased respectively by 14%, 26%, and 22% when using poles, which was similar to the results of Knight and Caldwell (10), who showed decreases of 12% to 16% for the same muscles.
Neither Glut nor BF were influenced by the use of the poles. This can partly be explained by the fact that one important role of hamstring muscles and Glut was shown to be to decelerate the leg prior to heel contact (12,25,27). Hamstring muscles actually counteract the quadriceps action during knee movements in order to protect the anterior cruciate ligament (4,12). This deceleration is dependant on velocity, which in the present study was identical with or without poles. There is also more Glut activation for steep grades of more than 15% than on level ground after mid-stance and until heel-off, in order to elevate the body's center of mass, as described by Tokuhiro et al. (25). Again, the elevation was similar with or without poles. Yet hamstring muscles act synergistically with the quadriceps during the stance phase. Because VL activity was reduced with poles, one might expect that BF would exhibit the same tendency, as found by Knight and Caldwell (10). This was not the case in the present study. A complementary explanation could be that another role of these two muscles is to counterbalance gravitational forces during weight acceptation (27). Unlike Knight and Caldwell's study, no backpack was used, so the gravitational forces found were slightly lower, which could explain the lower modifications of EMG activity with poles observed for these muscles. Hiking poles therefore seem to significantly relieve the muscles implicated in propulsion but have a lesser influence on the muscles involved in leg swing or to face gravitational forces.
There was significantly more activation of all four upper limb muscles with poles than without, and the increases amounted to roughly +50% for BB, +70% for AD, +100% for LD, and +150% for TB. This result is in agreement with and completes the data of Knight and Caldwell (10) who noticed a similar increase in EMG for TB with poles when uphill walking, but with a 9% grade at a speed corresponding to 55-65% of their theoretical maximal heart rate, an intensity comparable to that of the present study.
For BB activation, the difference between HW and NP was similar to the difference between HP and NP (about +50%) which suggests that the mass of the poles was solely responsible for the higher activation of BB. For AD, the increase of 70% for HP when compared with either NP or HW indicates that this muscle was only affected by the use of poles and not their mass. In addition, when comparing HP and HW, LD and TB exhibited a much higher activation when the poles were used, which is merely due to a stronger contraction during the pushing phase of the cycle (Figs. 3 and 7) where elbow and shoulder extensions are performed.
Overall, these results suggest that walking with poles using the alternate stride technique, by relieving knee extensor muscles, could alleviate knee joint forces and, during long walks, lessen fatigue of the lower limb muscles. This should also lead to increased stability and a lower risk of injury. However, complementary studies need to be performed to confirm this hypothesis.
Effect of inertia and frequency.
In the range of masses studied in the present study, the mass of the poles was shown to have no direct influence on V˙O2 during uphill walking. Similarly, the position of the center of gravity of the pole had no significant influence on global V˙O2. Other studies have reported opposing results regarding the mass but using much heavier loads, e.g., 1.36 kg for Graves et al. (6). For lower limbs, no author has shown any significant influence of masses of less than 500 g (3,15) while running. It could therefore be assumed that equipment weighing less than 500 g does not lead to significant increases in energy cost.
Regarding the EMG activity, BB was the only muscle affected by the additional mass of the poles (+9% between both MP and HP compared to LP). This could indicate that a 360-g pole induces fatigue in this muscle more rapidly than a 240-g one, even though it is probably not a limiting factor during hiking. No upper limb muscles other than BB are influenced by pole mass.
Similarly, the position of the center of gravity of the pole has no major influence on muscle activation. The only difference was found for AD which showed higher activation during the recovery phase for low load distribution than with either medium or high load distributions. This also seems to indicate that a distal mass could be more strenuous for some shoulder flexion muscles. This result is in accordance with those of Royer et al. (2) who studied walking with masses at different proximo-distal positions on lower limbs but with much higher loads (2 kg).
The present study shows that uphill walking with poles at high frequency is less efficient than uphill walking with poles at preferred or low frequency. At the beginning of the cycle, with the pole in an upright position, propulsive forces were shown to be lower (16). Therefore, it can be hypothesized that for high frequency, as cycle time becomes shorter, propulsive forces exerted on the poles, mainly at the end of the cycle, are lower. On the contrary, a longer cycle time during low frequency allows the walker to exert a longer and more effective force with the arms. This was illustrated by the higher activation of TB during the pushing phase and of AD during the whole cycle for NF−20. This therefore allowed a significant reduction of VL and Glut activation at NF−20 compared to NF+20. Yet, it resulted in a 15% higher activation of GL. These opposing results did not lead to a modification of global metabolic cost, probably because of the relatively lower mass of AD, TB, and GL compared to either VL or Glut. Nevertheless, local muscular fatigue could arise from this situation. Further studies on the propulsive forces exerted by the upper limbs during pole walking at different gradients should be performed to characterize the kinetic contributions of hiking poles.
The actual mass of hiking poles does not influence energy cost even though differences of mass of about 100 g or of 20 cm in load distribution on hiking poles can be detected through EMG analysis of the BB and AD. Within this mass or COM distribution range, the low alterations observed suggest that poles could contain additional concepts (adjustments, shock absorbers, additional features, etc.) without significantly increasing neuromuscular fatigue. This result was reinforced by the fact that no interaction between mass and frequency was found for any parameter.
At preferred frequency, the use of poles reduced the activation of lower limb muscles by 15% and increased upper limb muscle activation by 95%. High frequency was physiologically inefficient, whereas walking at low frequency redistributed the muscular work from thigh muscles to calf and upper limbs muscles, but this did not lead to an increased V˙O2. Thus, during uphill walking, repeatedly changing the frequency from preferred to low frequency could be a useful strategy to lessen fatigue. Reducing the frequency of arm movements separately from legs (i.e., one pole planted every two steps) could also provide physiological and biomechanical benefits. Complementary studies on walking techniques would be necessary to investigate this hypothesis.
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Keywords:©2008The American College of Sports Medicine
OXYGEN COST; EMG; MASS; LOAD DISTRIBUTION; WALKING FREQUENCY