Background
Plantar fasciitis (PF) is one of the most common problems associated with foot pain. The causes of PF include excessive physical activity, [
1] obesity, [
2] age, [
3] prolonged standing, [
2] altered biomechanics, [
4,
5] limited ankle dorsiflexion with foot postures such as pes cavus and pes planus, [
6] and hamstring tightness [
7]. Among these, limited ankle dorsiflexion is caused by tightness of the gastrocnemius (GCM), which may increase the stress on the plantar fascia because it affects the alignment of the calcaneal bones [
8]. Hamstring tightness may induce prolonged forefoot loading, that can result in increased repetitive stress on the plantar fascia [
7,
9]. Therefore, most therapists have focused on restoring the flexibility of the posterior muscles, such as the GCM and hamstring muscles in PF patients.
Weakness of the GCM [
5] and proximal muscles, [
5,
10] such as the gluteal and tensor fasciae latae muscles, in patients with PF have been reported, which may impact the plantar fascia load distribution. Recently, a systematic review reported that intrinsic muscle strength is associated with symptoms of PF [
11]. Furthermore, a recent study by Lee et al., [
5] reported that increased foot pressure in patients with PF may be associated with weakness of the GCM and hip muscles. Therefore, muscle weakness may be an important etiological factor in PF. To date, however, no study has investigated the strength and reaction time of proximal muscles, such as the hamstring and quadriceps muscles, in patients with PF. These muscles are known to play a vital role in the alteration of lower extremity biomechanics [
12‐
15] and may contribute to increased plantar fascia load.
The purpose of this study was to analyze the differences in the strength and reaction time of the quadriceps, hamstring, and GCM muscles as well as foot pressure and posture, between patients with PF and normal controls. We hypothesized that the quadriceps, hamstring, and GCM muscles of PF patients would show decreased strength and delayed reaction time, and these patients would have increased foot pressure compared to normal controls.
Discussion
The most important finding of this study was that quadriceps weakness, delayed reaction time of the hamstring and quadriceps muscles, and rapid reaction time of the GCM muscle can all be demonstrated in the affected ankles of PF patients. The reaction time of the GCM muscle also had a significant negative correlation with the strength and reaction time of the quadriceps muscle. Furthermore, foot pressure at the forefoot and hindfoot significantly increased, and the affected ankles of patients with PF had a higher incidence of hindfoot valgus than those of normal controls.
Weakness of the GCM in patients with PF has been reported [
5,
24]. However, these studies investigated PF patients with concurrent tightness of the GCM muscle. Therefore, previous studies were limited because muscle length directly affects muscle strength [
25]. However, In this study, PF patients without muscle tightness had weakness of the quadriceps muscle, with no significant difference in the strength of the hamstring and GCM muscles between the groups. Although the reason for these results is unclear, it may be explained by the use of compensatory movement strategies to reduce foot pain. During the gait cycle, [
26] foot posture changes from supination to pronation during the change in phase from heel strike to weight acceptance. In patients with PF, foot pain may be due to a stretched plantar fascia in the pronated foot [
26]. As a result, patients may use compensatory movement strategies, such as rapid hip flexion to reduce foot pain. In the weight acceptance phase, the quadriceps, hamstring, and GCM muscles, (especially the quadriceps), are highly active in stabilizing the hip and knee joints against gravity and weight [
13,
14,
25]. However, in PF patients, the function of the quadriceps may be gradually reduced by insufficient weight transfer due to such compensatory strategies, thereby resulting in quadriceps muscle weakness. Another possible explanation is overuse of the hip flexion movement performed to reduce foot pain caused by a stretched plantar fascia. The quadriceps muscle is a hip flexor, and weakness in this muscle may result from its overuse [
27,
28] in an effort to reduce foot pain. Previous studies have reported that decreased quadriceps strength can lead to increased plantar fascia load and decreased control of pronation of the foot, [
26,
29] thereby increasing foot pain. Further prospective studies are necessary to elucidate the results of PF patients in this study.
In the PF patients in this study, the reaction time of the hamstring and quadriceps muscles was delayed, whereas the reaction time of the GCM muscle was rapid compared to those of the control group. We believe that these results may be attributable to joint stabilization strategies in the lower extremity. Muscle reaction can be defined as the ability of the muscle to maintain joint stability while performing a functional task [
30,
31]; thus, rapid muscle reaction time is an important factor for increased joint stability [
31,
32]. The quadriceps, hamstring, and GCM muscles all contribute to the stability of the knee joint. Lloyd and Buchanan reported that the co-contraction of the quadriceps and hamstring muscles directly supports the valgus and varus moments at the knee joint [
33]. The valgus and varus moments of the knee joint can impact foot pronation and supination, respectively, [
34‐
36] which may increase plantar fascia stress owing to increased pressure in the forefoot and the hindfoot [
20]. In PF patients in this study, the hamstring and quadriceps muscles showed a delayed reaction time, with greater pressure in the forefoot and hindfoot, and hindfoot valgus on pedobarography, despite having normal feet on plain radiographs, compared to those in the control group. Hence, functional abnormalities of the hamstring and quadriceps muscles may contribute to increased pressure in the forefoot and hindfoot. In particular, the reaction time of the GCM muscle showed a significant negative correlation with the strength and reaction time of the quadriceps muscles in this study. Therefore, we believe that the GCM muscle may respond rapidly to support the valgus/varus moments in patients with PF whose hamstring and quadriceps muscles have a delayed reaction time. Previous studies have also reported that the GCM muscle plays an important role in supporting the frontal plane knee alignment (valgus/varus moments) at the knee joint [
33,
37]. Kvist and Gillquist, [
38] and Meunier et al. [
39] reported that the GCM muscle is neurologically connected to the quadriceps muscle. Consequently, we believe that the reaction time of the hamstring and quadriceps muscles should be assessed and improved, as necessary, in patients with PF.
There were several limitations to the present study. First, the strength of gluteal and hip muscles, such as the hip abductors, was not evaluated in this study, even though previous studies [
5,
10,
40] have reported that hip muscle strength is closely related to foot pain. Second, post rehabilitation results were not included in the correlation analysis. To confirm that the functional abnormalities of the hamstring and quadriceps muscles shown in our results represent a definite etiology of PF in patients without tightness of the GCM and hamstring muscles, further evaluations of the quadriceps and hamstring muscles should be done following rehabilitation. In addition, further studies on how the performance of the hamstrings, quadriceps, and GCM muscles of patients who were excluded from this study will contribute to PF will also improve our understanding of PF in various patients. Finally, intrinsic foot muscle function was not assessed. Intrinsic foot muscles play an important role in the stability of the normal foot and in lower extremity function; thus, impairment of these muscles may affect lower extremity biomechanics, which may result in changes in the function of the quadriceps, hamstring, and GCM muscles.
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