1. Introduction
The popularity of running has been increasing during the last years due to its benefits for health, accessibility and low cost, becoming one of the most common ways to exercise [
1]. Despite its numerous benefits, an increase in the prevalence of running injuries can be observed. It has been suggested that around the 42.7% of runners will get injured each year [
2], being the majority of this injuries due to overuse [
3].
Repeated and accumulated exposure to impact accelerations during long-distance running can overload and fatigue the musculoskeletal system, reducing its ability to absorb them and increasing the risk of injury [
4]. As a result, impact acceleration analysis has received considerable scientific interest in order to reduce these accelerations during running and decrease the incidence of overuse injuries [
5].
Several factors can affect these impact accelerations during running, including stride parameters (length and frequency) [
6], fatigue [
7,
8], running mechanics [
9], foot strike pattern [
10], sports equipment (footwear, compression socks or plantar supports) [
11], running surface [
12,
13,
14] and running speed [
15,
16,
17,
18].
Among all the factors that can influence impact accelerations, running surfaces have been shown to influence acceleration impacts during: overground running vs. motorized treadmill [
7,
12,
13], concrete vs. grass [
14], woodchip trail vs. synthetic track/concrete [
15]. On the other hand, it has been shown that impact accelerations increase with higher velocities [
17,
18].
Runners, whenever they can and if the weather conditions allow it, train outdoors on different surfaces: asphalt, grass and even in the mountains. However, there are sports modalities in which it is necessary to train within a facility, and the treadmill is the only training alternative available due to its characteristics and the possibility of maintaining a certain speed and slope.
When comparing overground vs. treadmill, running on a treadmill results in higher stride frequencies and shorter stride lengths [
19], being this parameters closely related to running economy. According to Hunter & Smith [
20] novice runners select lower stride frequencies than the optimal one, while experienced runners choose unconsciously higher frequencies, optimizing energy expenditure and improving running economy. Moreover, running on treadmill produces lower tibial peak acceleration and impact rates compared to the overground running [
7,
13].
Nowadays, new treadmill designs, such as the curved non-motorized treadmill (cNMT), have demonstrated to be a valid and reliable tool for rehabilitation, training and laboratory based research [
21,
22,
23]. cNMT have been designed to evaluate the strength, maximum speeds, and power of the athlete, allowing a more specific running evaluation. cNMT have a curved non-motorized surface, which requires the runner to impact the surface and propel the band with each stride [
23,
24]. The main difference compared to motorized treadmills (MT) is that cNMT allows participants to self-select the speed and allows a more valid and ecological laboratory assessment of running performance [
25,
26]. While different studies have been analysing the cNMT on sprints [
27], endurance running [
26], cardiometabolic demands [
28,
29] and team-sport running [
30]; other studies have focused on physiological and perceptual variables comparing cNMT with MT [
21,
25], and overground running [
23].
However, only few studies have observed biomechanical changes during walking [
29,
31] or running on cNMT [
32], observing shorter stride length compared to MT [
33]. In terms of impact accelerations, just one research have analysed tibial impact acceleration during running on cNMT vs. MT [
34], but no study of the impact transmission from the tibia to the head was carried out.
To our knowledge, no previous research has analysed the effect on head and tibial accelerations during running on cNMT. Therefore, the aim of the present study was to analyse impact accelerations, spatio-temporal parameters and perceptual changes during running on cNMT vs. MT at different speeds in recreational runners. It was hypothesized that: (a) running on cNMT would reduce impact acceleration parameters, increasing stride frequency and reducing stride length in comparison with motorized treadmill running; and (b) the effect of running speed would affect impact acceleration, increasing when running at higher velocities.
4. Discussion
The main objective of the present study was to analyse the influence of the treadmill system and speed on spatio-temporal, impact accelerations and perceptual parameters while running. To date, no studies have analysed head and tibia accelerations during running at different speeds on curved non-motorized treadmill in comparison with motorized treadmill. Based on the results achieved, we partially reject the null hypothesis since running on cNMT reduces impact acceleration parameters compared with MT, specifically in the parameters of head rate, tibial peak and tibial magnitude. While we cannot reject it in the parameters of stride frequency and stride length, as an increase in frequency and a reduction in stride length were not observed. Likewise, we reject the null hypothesis regarding the effect of speed on acceleration impacts, since they were significantly higher at higher speeds (3.33 m/s) than at lower speeds (2.77 and self-select speed).
Results of this study shows statistically significant (
p < 0.05) reductions in head rate acceleration, tibia peak acceleration and tibia acceleration magnitude when participants ran on cNMT in comparison with MT, but no differences were found in other impact acceleration parameters. These reductions could be caused by the concave belt, which has a pronounced forward lean and favours forefoot striking instead of midfoot/heel striking [
34].
Impact on tibial acceleration is related to lower limb fatigue injuries in runners and the risk for tibial stress fracture [
13,
15,
40], and it has been studied at different running surfaces [
12,
13,
14,
15,
41,
42]. In light of this, Montgomery et al. [
34] suggested that walking, jogging and running on the cNMT produces large reductions in tibial accelerations in comparison with overground and MT running. Nevertheless, other studies have not find any differences in tibial impact between a wide range of surfaces, as synthetic track, concrete, natural grass, MT or EVA treadmill [
43].
Shock attenuation in the present study did not differ significantly between running surfaces (MT or cNMT). Conversely, Dufek et al. [
16] showed significant differences in shock attenuation when it was compared between gender, speed (preferred and 10% slower) and surface (soft, medium and hard). It has been suggested that a reduction in shock attenuation caused by the running surface, fatigue or injuries can be harmful for the musculoskeletal system and increase the risk of injury [
44].
According toBruseghini et al. [
21], stride length and stride frequency were expected to differ between treadmills due to the belt friction, curvature and dimensions, but no significant changes (
p > 0.05) were found in spatio-temporal parameters when both treadmills were compared. In accordance with this outcomes, Seneli et al. [
45] neither found any differences in step length while walking, jogging or running on cNMT and MT. Different studies have analysed the influence of stride length and stride frequency on impact accelerations [
46], where peak impact acceleration showed a negative linear trend as stride length increased [
6,
44]. Other investigations showed differences between treadmills in stride length and stride frequency while walking at preferred speed [
21], and shorter stride length when participants ran on cNMT [
32,
34]. Moreover, it has been shown that stride frequency decreases after a 30 min fatiguing run [
44].
Rating of perceived effort and heart rate were significantly (
p < 0.05) higher while running on cNMT in comparison with MT. This fact could be because this type of treadmills require energy not only to drive the body itself, but also to drive the belt in every single step, to which friction and slope are attributed [
21,
25]. Different studies ensure that running on cNMT can produce greater perceived fatigue [
22,
28,
29], allowing participants to obtain greater physiological benefits associated with moderate and vigorous exercise without any substantial increase in effort compared to MT [
29].
Observing the differences between speeds, significantly (
p < 0.05) higher head impact acceleration, tibial peak acceleration and tibial acceleration magnitude when running at 3.33 m/s, increasing while running speed increased either on MT or cNMT. These results are in line with those observed by Sheerin et al. [
42], who found an increase 38% (3.8 g) in tibial acceleration from the slowest to the fastest velocities. However, only few studies have studied impact accelerations in the head and tibia during running at different speeds [
47].
Head accelerations remained constants for every treadmill and speed condition, being lower than tibia acceleration. It has been suggested as a protective behavior to prevent a possible disruption of the visual and vestibular system that could occur due to the excessive head acceleration [
7,
11,
45]. Moreover, differences in stride length have been observed between speed conditions, increasing linearly with speed, in agreement with previous studies [
47].
RPE and HR were higher when running at 3.33 m/s, condition which was found to be more exhausting by the participants and produced higher intensity, being one point lower in Borg’s scale on MT. Furthermore, running on cNMT not only increase physiological demands due to the increment in intensity but also required regular adjustments to keep uniform velocity, either through speed and/or stride length; thus, cNMT probably also requires greater neuromuscular control than MT [
29].
In summary, the results observed in the present study allow practical implications, showing that acceleration data were different when running on cNMT vs. MT. In addition, impact acceleration was influenced by treadmill and speed. Regarding the effort perception, the present study has shown important changes between cNMT and MT, with higher rating of perceived effort and heart rate while running in cNMT compared to MT.
5. Conclusions
In conclusion, running on cNMT reduces impact accelerations and produces higher heart rate and rating of perceived effort in comparison with MT, but no differences in spatio-temporal parameters were found between treadmills.
On the other hand, in relation to the speed effect on biomechanical variables, a logical increase was observed in impact accelerations, stride length, RPE and HR when running at 3.33 m/s compared to self-selected speed and 2.77 m/s.
Therefore, as a practical application, running on a cNMT could become an interesting training tool for athletes, trainers, physiotherapist and researchers due to the loading reduction and the increased physiological and perceptual response. Curved non-motorized treadmill could be a good strategy inside return-to-play rehabilitation protocols, high intensity training sessions or simply for loading reduction on long distance training athletes.
6. Limitations
The present study is not without limitations. In the present study, only the dominant leg was analysed; the analysis of the two legs could provide information on the symmetry of the running cycle in both extremities.
Another possible limitation would be related to the characteristics of the sample and the adaptation time to the treadmill, since there are few runners with previous experience using a curved non-motorized treadmill. In our study, we have tried to minimize this bias by using a protocol in which the participants had enough time to adapt to the new condition.
Therefore, with the results obtained in our study, we believe that future lines of research should aim to analyse running on both types of treadmill (cNMT vs. MT) in both lower extremities, as well as analysing with larger samples from groups with differing levels of sports experience. Finally, it would be interesting to evaluate the effect of cNMT training on running technique.