The hip flexor muscles are major contributors to lumbar spine stability. Tight hip flexors can lead to pain in the lumbar spine, and hence to an impairment in performance. Moreover, sedentary behavior is a common problem and a major contributor to restricted hip extension flexibility. Stretching can be a tool to reduce muscle tightness and to overcome the aforementioned problems. Therefore, the purpose of this systematic review with meta-analysis was to determine the effects of a single hip flexor stretching exercise on performance parameters. The online search was performed in the following three databases: PubMed, Scopus, and Web of Science. Eight studies were included in this review with a total of 165 subjects (male: 111; female 54). In contrast to other muscle groups (e.g., plantar flexors), where 120 s of stretching likely decreases force production, it seems that isolated hip flexor stretching of up to 120 s has no effect or even a positive impact on performance-related parameters. A comparison of the effects on performance between the three defined stretch durations (30–90 s; 120 s; 270–480 s) revealed a significantly different change in performance (p = 0.02) between the studies with the lowest hip flexor stretch duration (30–90 s; weighted mean performance change: −0.12%; CI (95%): −0.49 to 0.41) and the studies with the highest hip flexor stretch duration (270–480 s; performance change: −3.59%; CI (95%): −5.92 to −2.04). Meta-analysis revealed a significant (but trivial) impairment in the highest hip flexor stretch duration of 270–480 s (SMD effect size = −0.19; CI (95%) −0.379 to 0.000; Z = −1.959; p = 0.05; I2 = 0.62%), but not in the lowest stretch duration (30–90 s). This indicates a dose-response relationship in the hip flexor muscles. Although the evidence is based on a small number of studies, this information will be of great importance for both athletes and coaches. 876 ijerph International Journal of Environmental Research and Public Health Int J Environ Res Public Health Multidisciplinary Digital Publishing Institute (MDPI) PMC7922112 7922112 7922112 33671271 10.3390/ijerph18041936 The Influence of Stretching the Hip Flexor Muscles on Performance Parameters. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license ( http://creativecommons.org/licenses/by/4.0/ ). Abstract The hip flexor muscles are major contributors to lumbar spine stability. Tight hip flexors can lead to pain in the lumbar spine, and hence to an impairment in performance. Moreover, sedentary behavior is a common problem and a major contributor to restricted hip extension flexibility. Stretching can be a tool to reduce muscle tightness and to overcome the aforementioned problems. [ 2 ] reported mean performance impairments of 3.7% and 4.4% immediately after static stretching and proprioceptive neuromuscular facilitation (PNF) stretching, respectively, but an increase in performance of 1.3% after dynamic stretching. Both muscle tightness and muscle stiffness can be reduced by single stretching exercises [ 3 , 4 ]. However, whilst muscle tightness is defined as a limited range of motion, muscle stiffness is An average of ≥8 h mean sedentary time has been reported in a youth population [ 14 ], and also in an elderly population [ 15 ]. It is therefore likely that most of the sedentary population have tight hip flexors. This is underlined by the findings of Mettler et al. [ 16 ], who reported that two-thirds of the investigated population had limited hip extension flexibility, and hence tight hip flexors. Similar to the other muscles of the lower leg (i.e., plantar flexors [ 17 ]), hip flexor stretching decreases tightness (and hence increases hip extension ROM) following an acute bout of stretching [ 12 ]. This will likely counteract the aforementioned problems when applied repeatedly [ 16 ]. Studies were excluded if: (1) the intervention was performed on children or an elderly population; (2) muscles other than the hip flexors were stretched (except for leg extensors in combination with the rectus femoris); and (3) the study only focused on the effects of stretching on flexibility or ROM. We then categorized the analyzed parameters into the following three groups. [ 27 ] m 15 25 ± 1.5 Static Unassisted: 4 × 15 s [15 s rest] 1 min/leg <POD Unassisted: 4 × 15 s [15 s rest] + assisted: 28 × 15 s [15 s rest] 8 min/leg <POD POD = point of discomfort, POMD = point of mild discomfort. Table 3 Summary of the Results of the Studies which Investigated the Acute Effects of Stretching of the Hip Flexor Muscles. Study Stretching Type Results (Performance) Results (Range of Motion [°]) Outcome (Change in %) Outcome (Δ-Values) Hip Knee Aslan et al. This has also been observed in similar studies of the plantar flexors, where static stretching of the calf muscles for 1 min [ 37 ] and 3 min [ 36 ] did not induce changes in maximum voluntary isometric contraction (MVC), while 5 min of static stretching caused a decrease [ 41 ]. 4.3.2. Sport-Specific Parameters The sport-specific parameters such as sprint time , countermovement jump height , and foot speed investigated in three studies showed either an improvement or no change in performance, which resulted in an average improvement of 0.16% (median: 0.34%; CI (95%): −0.45 to 1.11). Additionally, hip flexor stretching can be a preventive approach against injuries. Especially soccer players with tight hip flexors might benefit since tight hip flexors lead to greater activation of the synergistic rectus femoris which likely leads to overloads and/or fatigue of the muscle [ 43 ]. However, the limited amount of studies about the acute effects of PNF stretching (n = 2) and dynamic stretching (n = 2) on performance does not allow a clear conclusion to be made as to which stretching technique should be preferably applied to avoid performance impairment. 6.