1. Introduction
Recreational running is becoming an increasingly popular pastime [
1], with approximately 15% and 70% of amateur athletes currently engaging in this activity in the United Kingdom and the United States, respectively [
2,
3]. Various studies have shown that 50% of runners suffer an injury each year [
4], although there are discrepancies in the literature, due to incidence values that vary from 18.2% to 92.4% [
5] and reported prevalence ranging from 46% to 90% among amateur runners [
6,
7].
In recent years, an increasing number of studies have been published in relation to the biomechanics of running, including factors that could help prevent and treat injuries in runners [
8,
9,
10,
11]. Running is a popular recreational activity, but a lack of adequate training in correct running techniques may account for the reported increase in injuries among these athletes [
12]. Thus, in this work, we aimed to provide an overview of the different normative patterns of lower limb muscle activation and pelvic joint ranges during running at self-selected speeds in men and women. We analyzed the biomechanics of running by measuring the activation of the main muscles involved in this activity, as well as the dynamic ranges of joint movement, especially in the pelvis [
13].
The choice of a preferred speed could be affected by the level of performance and the intensity of the training habits [
14]. It is reasonable to expect that amateur runners, with a higher level of performance, will train at higher intensities and, therefore, select a higher running speed for pleasure and metabolic cost [
14,
15].
Portable dynamic surface electromyography (sEMG) measurement devices, together with inertial sensor units (IMUs), are currently used for this type of analysis [
16,
17]. These systems provide information about muscle use intensity and activation time, and reflect the different contraction strategies, neuromuscular control systems, and three-dimensional (3D) pelvic kinematics used during running [
18,
19,
20]. The use of wearable systems for these biomechanical measurements allow the data to be captured under more realistic conditions [
21].
Given the intrinsic variability of these biomechanical values, the field still lacks a set of reliable reference values for use when assessing both the status and evolution of injured individuals. Some studies have determined these values based on the dynamic range of the pelvis and level of muscle activation by using sEMG for the main muscles involved in running [
21,
22,
23,
24,
25,
26,
27]. One study noted increased hamstring and hip flexor tension in runners caused by excess anteroposterior pelvic movement or tilt [
23], while in another, back pain was correlated with limited lower knee range [
24].
Many studies, exploring the differences in muscle activation in the stance and swing phases of running, are now available in the literature [
19,
28,
29,
30]. However, none have systematically categorized the values for muscle function and pelvic kinematics during the different phases of running. Moreover, running mechanics also differ between the sexes, but the differences in the normative patterns of muscle activation, in different phases of running between male and female amateur runners, has not yet been determined [
31].
Measuring and characterizing human movements during activity to evaluate athlete performance, improve technique, and prevent injuries is a crucial part of modern training programs [
32]. Collecting these data will increase scientific knowledge of kinematic patterns and the degree of muscle activation in runners. Therefore, the purpose of this study was to establish the differences between the sexes in terms of lower limb sEMG activity and three-dimensional (3D) kinematics of the pelvis during running.
4. Discussion
The main objective of this study was to establish whether there were differences between the sexes in muscular activation or in the 3D kinematics of the pelvis during the entire running cycle and in each of the running phases. We started with the hypothesis that the sex factor could determine the level of muscle activation throughout the running cycle and its component phases. Our results show that there were differences between the sexes, in terms of the total percentage of muscle activation during the entire running cycle in the vastus medialis. In addition, there were differences in the use of this muscle and the gluteus maximus between the sexes in the individual running phases. Moreover, there were sex differences in the rotation of the pelvis. The differences between the sexes in terms of the speed and distance traveled were similar to those previously described in the literature [
21].
Similar to the cohorts used in other studies [
19,
38,
39,
40], the participants in this work were recruited through random sampling, following established inclusion criteria, from among a population of amateur runners of different ages. We allowed the participants to self-select the speed at which they ran because, in addition to the effects of the age and body mass and body composition factors [
17], running speed is directly related to cardiovascular factors, such as individual aerobic threshold and performance [
16], and with biomechanical factors, such as stability, flight time, and leg contact time [
41]. In this same sense, work by Zamprano et al. [
14] and Lussiana et al. [
41] indicated that the speed chosen by each participant is related to their energy saving strategy. Thus, imposing a set speed upon runners, rather than allowing them to select the speed at which they are comfortable running, caused lower limb biomechanical changes and produced alterations in the muscle activation pattern and pelvis dynamics. These data are reinforced by those published by Kong et al. [
42], which concluded that self-selected velocities would eliminate abnormal kinematic patterns.
In this work, we placed the inertial sensor at the S1 level as a reference to quantify the kinematics of the pelvis. However, we are unable to compare our data with other methodologies, because no previously published work contrasted the kinematic data of the pelvis during running at self-selected speeds, except for the work by Perpiñá et al. [
24], who also placed the sensor at level S1. There were no significant differences between the sexes for the tilt range or pelvic obliquity kinematic values obtained. These values coincided with the expected normal values and were not novel. However, we did find that the mean lower pelvic rotation range for women (17.04° ± 5.72°) was significantly higher than the values found for men (12.53° ± 3.2°). Furthermore, the rotational ranges in men were lower than the reference values of 16°–18° provided in studies that dynamically measured the pelvis during running, perhaps because of differences in the speed used [
28,
43].
When the toes take off during the propulsion phase in running, the pelvis presents its maximum anterior tilt level, slight ipsilateral obliquity to support, and slight external rotation [
27,
42]. This limits hip flexion and makes rotation the most advantageous mechanism to lengthen the stride. This increased pelvic rotation in women seems to be related to a genetic predisposition towards greater flexibility [
44] and a lower capacity for elastic energy storage [
45]. All of this is associated with a decrease in the peak vertical forces used by female runners [
46]; thus, requiring rotational compensation at every speed. Therefore, women must increase their dynamic range of rotation to increase their hip extension without altering the other kinematic variables and muscle activation factors. This would lead to greater stability and running economy in women due to structural differences in the female pelvis and hips compared to males [
22].
We also found different muscle activation responses in the different running phases according to the muscles studied. The gluteus medius is activated in women because they have increased pelvis–hip joint movement and the main function of this muscle is to stabilize these joints. Thus, when the ground reaction force is absorbed in the first part of the first or second stance, the gluteus medius performs more eccentric work in women than in men [
39]. In contrast, this muscle causes hip abduction in the first and second take-off phases [
47,
48]. Therefore, women require increased gluteus medius activation to meet the biomechanical requirements of running, particularly in the second stance. This can lead to the appearance of injuries, either because of a lack of activation or because of fatigue, which are both of primary clinical importance because these factors strongly correlate with the appearance of injuries [
49,
50,
51,
52,
53].
The gluteus maximus is activated when the foot first contacts the ground and stops hip and trunk flexion in this phase [
51]. This muscle also performs trunk extension and strengthens the knee when it is fully extended by acting through the iliotibial tract [
54]. Gluteus maximus activity increases during the flight and swing phases because, together with the hamstrings and psoas, it behaves as a hip and knee accelerator during this phase [
55]. In agreement with the data from this current study, several other authors also believe that contraction of this muscle at the midpoint of the oscillation phase (between the first double float and second stance) is involved in leg deceleration [
56] and may also be related to passive extension of the knee.
We obtained a mean gluteus maximus activation of less than 30.95 μV for men in the first double float phase in this study, which may correspond to a gluteus maximus activation deficit. In contrast, activation of this muscle in female runners in the second stance was below 75.24 μV. Furthermore, the hamstring muscles in this study showed increased activity to control hip flexion when the trunk was flexed, which was causally related to pelvis stabilization [
57,
58,
59]. Maximal medial and lateral hamstring activity during running occurs through eccentric contraction in the middle of the swing phase in order to decelerate the leg just before maximal hip flexion, and immediately after the start of the knee extension [
60,
61].
The increase in vastus medialis muscle activity we observed in women compared to men (as a percentage of the overall running cycle), as well as during the swing in the first double float and second stance phases, may be because women tend to be less stiff than men. This would reduce their energy storage capacity in the transverse and frontal planes of the trunk and hip muscles [
45], thus, decreasing the stability of passive structures and increasing their range of motion, in turn leading to greater stabilization at the muscular level [
62,
63,
64].
Another function attributed to the vastus medialis is stabilization of the patella within the trochlear groove [
63,
64,
65], thus, generating a medializing force vector upon the patella, which would cause its rotation when in extension [
66,
67,
68]. The quadriceps are also active during the swing phase of running, in preparation to receive the weight load [
69]. Interestingly, women seem to have increased quadriceps activation when performing sports activities [
70], which can substantially contribute to physiologically significant [
71,
72,
73] changes in muscle strength between the sexes [
71].
Our data also agreed with previous work showing that vastus medialis activation for hip muscle recruitment differs in women when in positions that are neutral or with a slight medial hip rotation [
74,
75]. Indeed, Montgomery et al. concluded that contraction of this muscle is required in the swing phase to provide knee extension, thus, stabilizing the patella before the heel strike [
73]. In addition, compared to men, we found structural and anatomical differences in the lower limbs of women during running. This reduced normative pattern of vastus medialis activation in women may help them cope with external forces. This is important because it would generate a neuromuscular imbalance between the vastus of the quadriceps, thus, producing a greater risk of injuries, such as patellofemoral pain in female runners [
72,
76].
To the best of our knowledge, this is the first time normative patterns for the running kinematics of the major muscles and range of motion of the pelvis have been specifically established for each sex. Our results support the stabilizing role the gluteus medius has on the pelvis and knee, as well as the role of the vastus medialis in balancing the patella and controlling the knee valgus during running. The co-contraction of these muscles, together with that of the gluteus maximus and hamstrings, produces adequate motor control. These data could prove useful in clinical settings to prevent the injuries most frequently found in female amateur runners.
One of the limitations of this work may be its sample size (although it was similar to the cohort sizes used in other studies) because it could limit statistical interpretation with the aim of establishing normative data. Furthermore, we did not consider the influence of age, which could have affected the choice of our participants’ running speeds, as well as their running economies [
40]. Finally, this study was novel, so the lack of publications about normative muscle activation levels and normative pelvic kinematic patterns limited our ability to compare these data with other work; this makes it harder to understand the true causes of the differences we found between the sexes. Future studies should analyze the differences between healthy individuals and those with certain running injuries in order to analyze their possible origins. This could allow personalized training and prevention plans to be established, and could increase the recovery speed in individuals who already have an injury.