Resistance Exercise for Improving Running Economy and Running Biomechanics and Decreasing Running-Related Injury Risk: A Narrative Review

-accepted that at least a certain amount of resistance exercise (RE) is recommended for most endurance athletes. In this review, we aim to summarize the evidence regarding the effects of RE on running economy, running biomechanics, and running-related injury risk in endurance runners. The evidence robustly shows that lower limb RE is effective for improving running economy and performance, with a combination of strength and plyometric training being recommended to improve RE. Isometric training is also emerging as a possible alternative to implement during periods of high overall training load. Lower limb RE may change some aspects of joint kinematics during running; however, the evidence regarding the effects on kinetics is limited. Lower limb RE may help reduce running-related injury risk, but further evidence is needed. Keywords:runners; biomechanics; training; prevention; exercise 1. Introduction The utility of resistance exercise (RE) for endurance athletes has been a matter of debate among sport scientists for a long time. Today, it is well-accepted that at least a certain amount of RE is recommended for most endurance athletes. In this paper, we focus on the evidence regarding the

evidence is needed. Keywords:runners; biomechanics; training; prevention; exercise 1. Introduction The utility of resistance exercise (RE) for endurance athletes has been a matter of debate among sport scientists for a long time. Today, it is well-accepted that at least a certain amount of RE is recommended for most endurance athletes. In this paper, we focus on the evidence regarding the effects of RE for endurance runners. We address the effect of RE on running economy, running biomechanics, and injury risk. Highly trained endurance runners normally have similar and very high VO2max values (70–80 mL/kg/min with relatively small variations between athletes [1,2]); therefore, their performance depends in large part on their running economy [3]. Running economy is de ned as the amount of oxygen consumed ( VO2) at a given submaximal running speed and is expressed in mL/kg 1 /min 1 [4]. Simply put, at a given speed, a runner with a better running economy consumes less oxygen compared to a runner with a lower running economy [5]. Studies typically report a very strong correlation between running economy and endurance running performance (r = 0.8–0.9) [6]. A plethora of modi able determinants of running economy have been reported, including stride length, stride rates, contact times, lower vertical oscillation, greater leg stiffness, lower limb kinematics, alignment of the ground reaction force and lower limb axis during the propulsive phase, arm swing, lower coactivation, and rm footwear [7–9]. In addition, several anthropometric characteristics such as height, limb dimensions, and body composition have been suggested to in uence running economy [10]. In relation to this article, neuromuscular properties such as muscle strength, elastic energy utilization, eccentric power, and leg muscle stiffness are of primary interest [11]. The reutilization of elastic energy is a mechanism that can substantially reduce energy consumption during running [12]. The ability to reuse elastic energy during the propulsive phase of the running cycle is determined by reactive strength and leg muscle stiffness [7,13]. Reactive strength is the ability of the musculoskeletal system to produce a Sports2022,10, 98.

that can substantially reduce energy consumption during running [12]. The ability to reuse elastic energy during the propulsive phase of the running cycle is determined by reactive strength and leg muscle stiffness [7,13]. Reactive strength is the ability of the musculoskeletal system to produce a Sports2022,10, 98.

Sports2022,10, 98 2 of 16 large force with concentric contraction immediately after rapid eccentric contraction, also known as eccentric–concentric contraction or the stretch–shortening cycle (SSC) [14]. Muscle stiffness is de ned as the ability of a system to resist deformation and is calculated as the ratio of the force produced to the change in muscle length. It represents the integrative action of muscles, tendons, and ligaments that work together as a spring that stores and reuses elastic force [15]. The greater the muscle stiffness, the less muscle activation is required to transfer energy during running, thus reducing energy expenditure and improving running economy [16]. Stiffness on the muscle level is dif cult to assess during dynamic tasks, such as running. Consequently, studies have typically calculated quasi-stiffness, which is de ned as the ability of the human body to resist displacement [17]. For instance, vertical quasi-stiffness is de ned as the ratio between changes in the ground reaction force and vertical displacement of the center of mass, while joint quasi-stiffness describes the angular displacement within a joint in relation to joint moments [18,19]. RE can increase reactive strength and quasi-stiffness (although the results on directly measured muscle stiffness are inconclusive [20]). The increase in quasi-stiffness is one of the primary candidates for underlying the improvement in running economy after RE [21], which is also corroborated by simulation studies demonstrating that a decrease in stiffness results in increased energy expenditure during running [22]. Everything else being equal, stiffness is inversely related to exibility; thus, it is not surprising that studies have shown an inverse relationship between exibility and running economy [23,24]. On the other hand, it seems that if muscle stiffness is preserved after stretching intervention, running economy is not affected [25]. Given that suf cient range of motion (particularly at the hip and ankle joints) is needed for optimal running technique, performing some exibility exercises in addition to RE could be bene cial. The interested reader is referred to the review paper on the acute and chronic effects of stretching on running economy by Carter et al. [25]. The incidence of

Given that suf cient range of motion (particularly at the hip and ankle joints) is needed for optimal running technique, performing some exibility exercises in addition to RE could be bene cial. The interested reader is referred to the review paper on the acute and chronic effects of stretching on running economy by Carter et al. [25]. The incidence of running-related injuries ranges between 3% and 85% [26]. The ma- jority of these injuries are overuse/chronic [27], with the highest incidence reported for iliotibial band syndrome (ITBS), patellofemoral pain syndrome (PFPS), Achilles tendinitis, plantar fasciitis, and medial tibial stress syndrome [28]. According to Troy et al. [29], sub- optimal biomechanics results in an uneven distribution of forces between body segments. This is seen particularly in people with PFPS and ITBS, where unevenly distributed forces increase stress on a particular part of a structure [30,31]. One study demonstrated that PFPS is associated with weak muscles that control the hip and muscles that regulate pelvic movement and external knee rotation [32]. Additionally, abnormal biomechanics has also been observed in these individuals. A similar nding was found in runners with ITBS, in whom increased internal knee rotation and increased hip displacement were observed [33]. RE seems to be a promising tool to improve running economy and potentially for strengthening the weakened muscles that cause altered biomechanics and, by extension, increased injury risk. Therefore, the purpose of this narrative review is to summarize the evidence of the effect of RE on running economy, running biomechanics, and running- related injury risk. Systematic reviews to date have generally shown that RE improves running economy [34]; however, most of the recent reviews focused only on the effects of strength training in highly trained runners [35], such as concurrent heavy weight and explosive training [36]. Existing reviews on running biomechanics and injury-risk typically focus on multiple interventions or interventions other than RE [37–40]. The purpose of this paper is to summarize, in a narrative manner, the available evidence on effects of different RE modalities for different domains of application related to running. We believe that this paper will

concurrent heavy weight and explosive training [36]. Existing reviews on running biomechanics and injury-risk typically focus on multiple interventions or interventions other than RE [37–40]. The purpose of this paper is to summarize, in a narrative manner, the available evidence on effects of different RE modalities for different domains of application related to running. We believe that this paper will outline speci c gaps in the literature and also help coaches and clinicians who work with runners to design RE programs for speci c purposes and goals. A search of the literature was performed in January 2022. The following keywords were used in various combinations in PubMed and Scopus databases: running, runners, running- related, injury, biomechanics, performance, and economy. Upon retrieval of the relevant full texts, potentially useful references were retrieved from the reference lists of the included

Sports2022,10, 98 3 of 16 articles. Both reviews and original research were considered. Studies were included if they analyzed runners, triathletes, or military personnel. Studies were excluded if the article was written in a language other than English. Although we primarily searched for randomized controlled trials and other experimental study types (i.e., assessing the effects of RE on running economy, running biomechanics, and injury-risk), we also included cross-sectional studies into the narrative where appropriate. 2. Resistance Exercise for Improving Running Economy and Performance Running performance on middle and long distances depends on anthropometric, biomechanical, and physiological characteristics [41]. In his endurance performance model, Joyner considered three key physiological parameters— VO2max, blood lactate concentra- tion, and running economy [42]. Paavolainen et al. suggested that among well-trained distance runners, running economy is a better predictor of performance than VO2max [43]. A recent systematic review showed improvements of running economy without changes in VO2max and blood lactate concentration in pre-trained athletes after RE intervention [41]. However, the results between studies differed considerably, which can be attributed to different running speeds being used for running economy assessments and other inconsis- tencies in the measurement protocols [41]. Di Prampero et al. demonstrated that runners improved running economy more while running at competitive speed [44]. After 8 weeks of concurrent explosive strength training and endurance training, young runners improved their anaerobic and neuromuscular properties without reducing their aerobic capacity [45]. An older systematic review supports these ndings with improved running economy (+8%) in elite long-distance runners after explosive strength training. However, in untrained run- ners, that was not the case, as they improved only in VO2max [46]. In competitive runners, even a small improvement in running economy can have a major impact on long-distance running performance, especially in marathons and ultramarathons [46]. Cross-sectional evidence shows that muscle strength and quasi-stiffness are positively related to running economy, which supports the use of RE to improve running economy. Li et al. [47] studied the relationship between the neuromuscular properties of the lower extremities and running economy at three different speeds (12, 14, and 16 km/h). Running economy at

especially in marathons and ultramarathons [46]. Cross-sectional evidence shows that muscle strength and quasi-stiffness are positively related to running economy, which supports the use of RE to improve running economy. Li et al. [47] studied the relationship between the neuromuscular properties of the lower extremities and running economy at three different speeds (12, 14, and 16 km/h). Running economy at 12, 14, and 16 km/h was signi cantly affected by eccentric strength in leg press (r = 0.52–0.63), as well as reactive strength (r = 0.41–0.57) and leg quasi-stiffness (r = 0.68–0.76), determined from drop jump testing. During eccentric contractions, the muscle–tendon unit is stretched, absorbing mechanical energy, which is then reused dur- ing concentric contractions [48]. As mentioned before, the action of stretching, which is immediately followed by muscle contraction, has been termed as the stretch–shortening cycle (SSC). During the SSC, the muscle works as a spring and is able to produce more force in the concentric action (compared to concentric movement alone), which is crucial for running economy [48]. Up to a 50% increase in force can occur due to energy reuse during SSC actions [49]. These research ndings describe potential underlying mechanisms of improvement in running economy and performance after RE in endurance runners. In the following sections, the effects of speci c RE modalities are discussed. Although some forms of RE may elicit hypertrophic effects and consequently lead to an increase in body mass, such an effect has not been shown in most studies conducted on runners. Docherty & Sporer [50] described the concept of the interference phenomenon between concurrent endurance training and RE. Based on their review of the literature, simultaneously training for strength and endurance compromises the development of muscle strength (as compared to using only RE). However, the development of endurance (speci cally aerobic power) appears to be unaffected by concurrent RE, which supports its use for endurance athletes. The fact that the body mass is unchanged in most RE intervention studies is important to point out, as the effect of RE could also be accompanied by an increased muscle mass and thus

using only RE). However, the development of endurance (speci cally aerobic power) appears to be unaffected by concurrent RE, which supports its use for endurance athletes. The fact that the body mass is unchanged in most RE intervention studies is important to point out, as the effect of RE could also be accompanied by an increased muscle mass and thus increased total body mass, which could negatively affect the relative physiological determinants of endurance, such as running economy and VO2max [51]. Li et al. [11] studied the impact of complex training, which included

Sports2022,10, 98 4 of 16 maximum strength training and plyometric training, on running economy and came to the same conclusion. Namely, body composition did not change with training, which indicated that the adjustment to resistance training was mainly due to adaptations on the level of the nervous system (e.g., increased frequency of motor unit ring and increased recruitment of motor units) [52]. 2.1. Strength Training Five studies included in a recent systematic review [35] reported a positive effect of strength training on running economy in highly trained endurance runners. The duration of the interventions ranged from 8 to 12 weeks, with one study using high loads (85% 1RM) and the other four using moderate loads (40–70% 1RM). The addition of RE to the endurance training of well-trained endurance runners improved running economy (+4%) after 10 weeks without changes in body mass and VO2[53]. In addition, an 8-week intervention of heavy weight training on well-trained endurance runners improved running economy (+5%) and postponed the time to exhaustion, while no changes in body mass were reported [54]. A study by Beattie et al. [51] performed on endurance runners also found that RE performed in addition to endurance training improved running economy and VO2max without signi cant changes in body composition, which was also replicated in other research [41,55]. A recent study [55] found similar results in triathletes, who improved their maximal strength, cycling economy, and running economy in a 26-week concurrent endurance and RE program. Additionally, Alcaraz-Ibañez and Rodr½guez-P²rez [41] found no signs of overtraining when RE was added onto the training schedule of previously trained endurance athletes. However, there were inconsistencies in improvements among athletes, which might be a consequence of different characteristics of the applied RE. 2.2. Plyometric Training Several studies have also investigated the effects of plyometric training on running economy. A 9-week intervention of plyometric training in highly trained distance runners improved running economy at 18 km/h 1 (+4.1%;p= 0.02), accompanied by trends for improved 5-jump plyometric test performance (+14.7%;p= 0.11) and a lower VO2max– speed slope (14%;p= 0.120) [56]. A 6-week plyometric training intervention improved running economy,

studies have also investigated the effects of plyometric training on running economy. A 9-week intervention of plyometric training in highly trained distance runners improved running economy at 18 km/h 1 (+4.1%;p= 0.02), accompanied by trends for improved 5-jump plyometric test performance (+14.7%;p= 0.11) and a lower VO2max– speed slope (14%;p= 0.120) [56]. A 6-week plyometric training intervention improved running economy, CMJ height, 5-jump plyometric test performance, and lower leg quasi- stiffness [57]. Importantly, the increase in quasi-stiffness was correlated with improvements in running economy. Increasing lower leg quasi-stiffness and reactive strength through ply- ometric training enables better utilization of the SSC [11,57]. Interestingly, Turner et al. [58] reported that moderately trained endurance runners improved running economy after 6 weeks of plyometric training; however, there was no improvement in vertical jump performance. This could have been due to the shorter duration of the plyometric training (10–15 min), which was not primarily focused on improving vertical jumping performance. In a systematic review of ve articles [59], it was reported that both plyometric training and heavy weight RE improved running economy by ~5%. In some studies, a short-term (4-week) heavy weight training intervention improved running economy (+6.3%; 44.3 4.9 to 47.3 6.8 mlkg 1 min 1 ), but plyometric training did not [60]. 2.3. Combination of Different RE Modialities Skovgaard et al. [61] studied the effect of a combination of strength training and speed endurance training on the athletic performance of runners. They concluded that this type of training improves running economy without increasing the risk of overtraining, which is consistent with other research ndings [54,62]. Li et al. [11] found that 6 weeks of complex training, which combines heavy weight training and plyometric exercises with endurance training, improved maximum muscle strength and power, running economy, and velocity at VO2max in well-trained endurance runners, which was not the case in resistance training, which consisted of only strength endurance exercises. A study by Barnes et al. [63] exam- ined the impact of heavy weight training and plyometric training on endurance runners

power, running economy, and velocity at VO2max in well-trained endurance runners, which was not the case in resistance training, which consisted of only strength endurance exercises. A study by Barnes et al. [63] exam- ined the impact of heavy weight training and plyometric training on endurance runners

This review summarizes the effects of resistance exercise on running economy and injury risk in endurance runners.