Strength Training for Enhancing Performance and Reducing Injury Risk

Richard C. Blagrove and David R. Hooper

Highlights

• • Strength training activities (resistance training and plyometrics) can enhance middle- and long-distance running performance, maximal sprint speed, and running economy.
• • Overuse injuries are common in distance runners. High-intensity strength training performed frequently in short bouts may help reduce the risk of certain types of overuse injury.
• • Endurance athletes can be prone to low bone mineral density, and strength training may be a potent stimulus to combat this issue.
• • For runners new to strength training, developing movement competency across a wide range of exercises should be prioritised initially.
• • Resistance training exercises (e.g. back squat, deadlift, step-ups) should be performed twice per week in the preparation for competition phases or targeted events.
• • Plyometric exercises (e.g. skipping, hopping, jumping and bounding) develops neuromuscular qualities relating to the stretch-shortening cycle and offers a high level of transfer to running.

Introduction

In the winter and spring of 1957 I must have run 2500 miles in training and lifted thousands of pounds in weights.

—Herb Elliott, Olympic 1500 m Champion, 1960; World Record Holder 1500 m and 1 mile, 1958; quoted in Trengrove (2018)

The use of weight rooms and gymnasium-based exercises as an adjunct to a distance runner’s training programme is not novel. In the 1950s, Percy Cerutty (coach to Herb Elliott), controversially advocated the regular use of heavy' weight training for his group of highly successful Australasian distance runners. Peter Snell, the triple-Olympic gold medallist from the 1960s, is also known to have incorporated explosive bounding into his running sessions, and Sebastian Coe was a strong exponent of strength work and circuit training as part of his physical preparation in the late 1970s and 1980s. Although strength and conditioning exercises have been successfully used for decades by distance runners, it is only over the last 20 years that sport scientists have begun to understand the role of the neuromuscular system in endurance running events, and how strength training can be used to enhance performance. The aim of this chapter is to provide a scientific rationale for the inclusion of strength training in the programme of a distance runner and to outline practical recommendations for the implementation of appropriate strength training activities.

The Physiological Basis for Strength Training in Runners

Middle- (800 m-3 km events) and long-distance (5 km-marathon events) running performance is principally limited by the body’s ability to deliver oxygen to the working muscles and clear the metabolites that inhibit muscular contraction (Joyner, 1991; Brandon, 1995). From a specificity perspective, the use of extensive and intensive running sessions is logically the best stimulus to drive adaptations to the cardiovascular and metabolic systems to enhance these processes. It may therefore seem counter-intuitive for a distance runner to perform resistance training exercises, which sit at the opposite end of a specificity continuum and largely target neuromuscular-related qualities.

In addition to cardiovascular and metabolic factors, which are crucial to develop in distance runners, the amount of energy' that runners use to sustain a given running speed is also known to be an important determinant of performance (Ingham et al., 2008a; Blagrove et ah, 2019). The less energy that is required to maintain a given sub-maximal pace, the more ‘economical’ a runner is said to be, and considerable variability exists between runners (Daniels, 1985). In highly trained runners, economy distinguishes performance more accurately than other physiological determinants of performance such as maximal oxygen uptake (K0_2mlx) or running speed at a given blood lactate value (Conley and Krahenbuhl, 1980; Allen et ah, 1985; McLaughlin et ah, 2010). Moreover, running economy appears to become a more important factor in determining performance as race distance increases (DiMenna and Jones, 2016), therefore understanding how this quality can be enhanced is crucial.

Running economy is influenced by a multitude of intrinsic (physiological, morphological, neuromuscular and biomechanical) and extrinsic (technological and environmental) factors (Pate et ah, 1992; Barnes and Kilding, 2015a; Moore, 2016). Many of these factors are modifiable using appropriate training interventions, including progressive increases in the volume of running (Mayhew et ah, 1979; Morgan et ah, 1995; Jones, 2006), and potentially gait retraining strategies (see Chapter 13). Importantly, several neuromuscular (eccentric strength and rate of force development) and structural (tendon stiffness) qualities contribute towards running economy (Barnes et ah, 2014; Li et ah, 2019). Therefore, training exercises, which specifically target development of these capacities, potentially offer one means of enhancing economy.

The amount of energy required by the muscles of the lower limb to sustain a given running speed is largely dependent upon the magnitude and rate of muscle fibre length change (Fletcher and Macintosh, 2017). A muscle that remains static (isometric contraction) during the ground contact phase of the running stride will activate less muscle mass (Chow and Darling, 1999) and use less energy compared to a muscle that lengthens (eccentric contraction) upon initial ground contact and subsequently shortens (concentric contraction) during ‘push- off’ (Fletcher and Macintosh, 2017). If a muscle were able to remain in an isometric state during the propulsive phase of a stride, the change in muscle-tendon unit length and joint angles

(flexion at ankle, knee and hip) could theoretically be achieved using only the tendon. This is far more energy efficient as tendons are highly elastic structures that have a large capacity for storage and return of elastic strain energy.

Strength training typically results in an increase in recruitment of motor units (the muscle fibres that are innervated by a nerve), the rate at which motor units are activated, and tendon stiffness (Folland and Williams, 2007). Therefore, by improving these qualities, the lower limb is able to cope better with the magnitude and rate of force expression required during ground contact to enable muscle fibres to lengthen and shorten to a lesser extent; and for the tendons to accommodate any change in muscle-tendon unit length required (Fletcher and Macintosh, 2017). Furthermore, possessing a capability to express higher levels of absolute force means that relative force production for tasks requiring sub-maximal sustained levels of force production (such as endurance running) is also lower, which may also contribute to improving running economy. The energy savings that result from improved running economy are therefore likely to enable a faster speed to be achieved over the duration of an event (Beneke and Hiitler, 2005; Saunders et al., 2010; Hoogkamer et al., 2016), or a faster finish in the closing stages of a race (Damasceno et al., 2015).

Of the muscles in the lower limb, the ankle plantar flexors (gastrocnemius and soleus) contribute most to generating vertical support force during sub-maximal running and increases in stride length (Dorn et al., 2012). The energetic cost of running is also strongly related to force-length-velocity potential of the soleus complex (Bohm et al., 2019). However, at faster speeds (> 5 m.s^-1; < 3:20 min.km^-1), muscle fascicle shortening rates increase, which reduces the force the ankle plantar flexors can produce due to their force-velocity relationship (Lai et al., 2015). Beyond 7 m.s^-1 (< 2:23 min.km^-1), peak ankle plantar flexor force begins to decrease, and running speed is generated to a greater extent by the forces produced by the gluteus maximus, hip flexors and hamstrings (Dorn et al., 2012). This shift in muscle recruitment strategy can help inform choice and prescription of strengthening exercises for the middle- and long-distance runner.

Distance running performances are strongly influenced by aerobic factors. However, the middle-distance events, and competitive races involving surges in pace and sprint finishes, also rely heavily on the capacity of anaerobic energy processes (Thompson, 2017). For an 800 m runner, near-maximal velocities of running are reached during the first 200 m of the race (Reardon, 2013), which necessitates a high capacity of the neuromuscular and anaerobic system. Moreover, possessing greater maximal speed also increases a runner’s anaerobic speed reserve, which has been shown to be an important component of success in elite male 800 m running (Sandford et al., 2019a). Whereas strength training adaptations positively influence running economy by reducing the rate of energy turnover within active muscles, high-speed running is predominantly limited by the production of high vertical ground reaction force relative to body weight in a short ground contact time (Weyand et al., 2000). The ability to apply higher levels of force to the ground may also partly explain the improvements that have been observed in middle-distance running performance (1.5-3 km) following a period of strength training (Ramirez-Campillo et al., 2014; Skovgaard et al., 2014; Pellegrino et al., 2016).

Does Strength Training Improve Performance?

Several studies have noted strong relationships between proxy measures of anaerobic power or neuromuscular factors and distance running performance (Houmard et al., 1991;

Paavolainen et al., 1999b; Nummela et al., 2006; Hudgins et al., 2013; Bachero-Mena et al., 2017). This suggests that higher levels of strength are indicative of faster running performances. Correlation findings are useful in helping to explain how much variability in distance running performance can be explained by an anaerobic or neuromuscular factor, but this does not imply that greater strength capabilities can cause an improvement in distance running performance.

At least 25 training intervention studies have investigated whether strength training (heavy resistance training, explosive resistance training, plyometric training, or mixed modality training) can improve performance and/or the physiological factors that are important for distance running, compared to running training only (see the review by Blagrove, Howatson et al., 2018a). Strength training activities performed 2-3 times per week for a period of 6-14 weeks improve measures of strength and explosive power in middle- and long-distance runners (Trowell et al., 2020), and these improvements likely underpin the positive changes typically observed in running economy and performance compared to a programme that involves just running (Beattie et al., 2014; Berryman et al., 2017; Denadai et al., 2017; Blagrove, Howatson et al., 2018a). Importantly, most studies have not observed noticeable changes in body composition (i.e. muscle mass) following a period of strength training (Blagrove, Howatson et al., 2018a).

Neither age nor training status appear to be moderating factors in the degree to which running economy and performance can improve following a period of strength training (Denadai et al., 2017; Blagrove, Howatson et al., 2018a). Indeed, ‘possibly beneficial’ (—3.5%) changes in running economy were observed in post-pubertal adolescents following 10 weeks of multimodal strength training compared to a running-only group (Blagrove et al., 2018a). Moreover, masters age (>40 years) runners significantly improved (—6.2%) their running economy at marathon pace after 6 weeks of concurrent strength and endurance training compared to a control group (Piacentini et al., 2013). Similarly, several studies that have used recreational- level distance runners have reported benefits to running economy following a period of strength training (Johnston et al., 1997; Turner et al., 2003; Albracht and Arampatzis, 2013; Festa et al., 2019). These improvements are of a similar magnitude to those observed in highly trained and elite distance runners who were exposed to comparable volumes of strength training (Paavolainen et al., 1999a; Millet et al., 2002; Saunders et al., 2006).

Several reviews have also been conducted summarising the literature that has investigated the effect of strength training interventions on acceleration and maximal speed, concluding that plyometrics (de Villarreal et al., 2012) and resistance-based training (Seitz et al., 2014; Bolger et al., 2015) have a positive effect on sprint performance. The same conclusion was also provided in a recent paper summarising the effects of strength training on maximal sprint speed, specifically in distance runners (Blagrove, Howatson et al., 2018a).

Does Strength Training Reduce Injury Risk?

Overuse type injuries are common amongst runners, with between 18% and 92% of runners per year suffering an injury, and prevalence rates of 7 to 59 injuries per 1,000 hours of running (Hreljac, 2005; Saragiotto et al., 2014). Common overuse injuries include bone-stress injuries, iliotibial band syndrome, shin splints, patellofemoral pain, plantar fasciitis and Achilles tendinopathy (Hreljac, 2005; Francis et al., 2019). Overuse injuries generally require a long time for recovery and are often the reason for stalled progress or runners quitting the sport.

Although a reasonable body of research exists showing strength training provides boosts to running economy and performance, the evidence indicating that long-term strength training lowers the risk of developing an overuse injury is far less convincing. It is therefore surprising that the most popular reason for runners to engage with strength and conditioning activities is the belief that it lowers their risk of getting injured (Blagrove, Brown et al., 2020b).

The underlying reasons that a runner develops an overuse injury are multifaceted and complex. Other than previous injury, very few other risk factors have been consistently identified (Saragiotto et al., 2014). Runners should therefore prioritise strengthening tissues that have previously been injured and address specific movement limitations that may have contributed towards the injury. The risk factors for each type of running injury differ slightly; however, errors in training prescription (i.e. rapid changes in running distance, weekly volume and/or running intensity) increase the likelihood of overuse injury (Hreljac, 2005). It is therefore advisable that runners monitor their training closely and aim to progress training gradually. This also applies to the introduction and progression of strength and conditioning training.

Research investigating risk factors associated with a runner’s anthropometry and biomechanics have shown inconsistent results (Ceyssens et al., 2019; Vannatta et al., 2020). Previous work has shown that female runners who sustained overuse injury exhibit larger hip adduction angles during the stance phase of running (Ferber et al., 2010; Milner et al., 2010; Noehren et al., 2012) and greater contralateral hip drop is also associated with common injuries (Bramah et al., 2018). It is therefore advisable that runners (especially women) utilise and practice the practical screening tests highlighted in Chapter 7 to evaluate movement abnormalities and improve neuromuscular control.

Weakness in specific muscle groups has also been linked to certain overuse injuries in runners. Compared to non-injured runners, weaker gluteal muscles have been shown in runners suffering from patellofemoral pain (Cichanowski et al., 2007; Ferber et al., 2011; Finnoff et al., 2011; Luedke et al., 2015), iliotibial band syndrome (Fredericson et al., 2000), shin splints (Becker et al., 2018) and Achilles tendinopathy (Niemuth et al., 2005; Franettovich et al., 2014). Similarly, lower strength in the calf muscle group is associated with a higher likelihood of Achilles tendinopathy (Mahieu et al., 2006; O’Neill et al., 2019). Although this body of work indicates that low levels of strength may predispose runners to overuse injury, several of these studies were not prospective, and therefore reduced strength may be a consequence of an injury, rather than the cause.

An overuse injury occurs when the repetitive and frequent loading placed on a tissue exceeds the capacity of that tissue, which causes pain. Therefore, one logical way of lowering injury risk is to increase tissue capacity, so it can withstand higher volumes of loading. Gradual increases in running volume will progressively expose tissues to more load, and therefore cause the tissue to adapt; however, strength training exercises may provide a more potent stimulus to build structural resilience. Indeed, frequent and brief exposures to high magnitudes of loading appear to be best for driving positive changes in bone mineral density (BMD; Turner, 1998), tendon structure (Malliaras et al., 2013) and ligamentous tissue (Baar, 2017). Moreover, experts have suggested that muscle weakness is an important modifiable risk factor for athletic tendinopathies (O’Neill et al., 2016). Therefore, strength training, which exposes vulnerable tissues to higher magnitude and/or rates of loading compared to running, potentially offers a time-efficient and effective way of increasing strength and lowering the likelihood of an injury.

Bone Stress Injuries

Endurance athletes often exhibit low BMD due to factors related to the female athlete triad in women (see Chapter 18), and similar factors in men. The American College of Sports Medicine suggests that a criteria for low BMD is a Z-score of less than -1, meaning that the BMD is one standard deviation below the average for that person’s age, sex and race (De Souza et al., 2014). Prior research has demonstrated that 21.5% of women athletes exhibited a Z-score between —1 and -2, and a further 5.9% less than -2 (Gibbs et al., 2013). A more recent study broke down women collegiate athletes by sport, and noted that 19% of cross-country runners had a BMD of less than -1 (Tenforde et al., 2018). While there is considerably less research in this area in men, a similar percentage was noted in adolescent male runners, of 23.5% of runners with a Z-score ofless than -1 (Barrack et al., 2017). These low BMDs are a concern as they have been shown to lead to a heightened risk of stress reactions and/or fractures in women (De Souza et al., 2014) and an analogous process appears to occur in men (Tenforde et al., 2016).

The typical recommendation to stimulate adaptations to BMD is weight-bearing exercise with high impact (Turner, 1998). Thus, it appears paradoxical that individuals participating in high volumes of running; a weight bearing, impact exercise, could exhibit low BMD. This can be explained, at least in part, by low sex hormone concentrations associated with the female athlete triad (De Souza et al., 2014), exercise hypogonadal male condition (Hackney, 2008) or relative energy deficiency in sport (Mountjoy et al., 2014). In these cases, clearly, despite the weight bearing and impact nature of running, the stimulus is insufficient to develop, or even maintain, BMD.

Although low BMD alone is not used to diagnose the presence of osteoporosis - it must be accompanied by a history of stress fractures (De Souza et al., 2014) - the osteoporosis literature is able to provide guidance on how to enhance BMD. While aerobic activity is suggested to be capable of limiting the reduction of BMD, it is stated that progressive strength training is superior to aerobic activity for developing BMD (Benedetti et al., 2018). It is recommended that resistance training should involve the use of heavy loads (>85% of one repetition maximum), as this appears optimal to enhance BMD (Turner, 1998; Watson et al., 2018). Other recommendations related to strength training include loading the hip and spine, as these areas are particularly prone to low BMD and bone development is highly site specific (Benedetti et al., 2018). Thus, exercises such as the barbell back or front squat, lunges or deadlifts would be particularly beneficial.

Training to Reduce Overuse Injury Risk

It has previously been shown that increasing the volume and intensity of strength training is associated with a reduced risk of sports overuse injury (Lauersen et al., 2018), even compared to other types of conditioning such as stretching, proprioception training or a mixed exercise approach (Lauersen et al., 2014). However, these conclusions were largely based upon research in soccer players and documented overuse injuries not typically observed in distance runners. Studies specifically using distance runners have reported no differences in overall injury rates between a group exposed to a strength-related training intervention compared to a placebo-intervention or running only control group (Bredeweg et al., 2012; Baltich et al., 2017; Toresdahl et al., 2020). These investigations used large cohorts of recreational runners over relatively short periods (3—6 months) and unsupervised low-intensity strength or neuromuscular training interventions <4 times per week. It may be the case that higher-intensity loading regimens over longer durations are required for the benefits of strength training to manifest as reductions in overuse injury occurrence. One study noted that eight strengthening and stretching exercises incorporated into the daily warm-up routine of military recruits during 14 weeks of intensive training resulted in a 75% reduction in overuse knee injuries (Coppack et al., 2011). Similarly, a three-week daily hip abductor-strengthening protocol was shown to be effective in decreasing pain and stride variability in runners with patellofemoral pain (Ferber et al., 2011). Strengthening exercises for the gluteal muscles (Earl and Hoch, 2011) and knee extensors (Eapen et al., 2011; Chiu et al., 2012) have also been shown to reduce pain and improve function in physically active non-runners with patellofemoral pain syndrome. The most common site of injury in runners is the knee; therefore, these findings are useful.

Based upon the preceding discussion, the frequency and intensity with which musclestrengthening exercises are performed seems to be important for reducing the likelihood of overuse injury. Studies have also shown that short-frequent bouts (10-15 min) of neuromuscular training have the largest preventative effect for lower extremity injury in youth game sport athletes (Steib et al., 2017). It is therefore recommended that short bouts (<20 min) of intense strength training exercise are performed on most days of the week to minimise the risk of overuse injury. Changes to BMD also take several months, therefore regular and consistent long-term exposure to strength training is required to observe a reduced risk of bone stress-related injury.

Understanding 'Strength' and How Can It Be Developed

Strength can be defined as the ability to voluntarily apply force under a specified set of movement constraints to achieve a specific task outcome (Goodwin and Cleather, 2016). In the context of distance running, task-specific strength describes the way in which force is applied to the ground during each stride. For a runner to become more economical at a given running speed, lower peak forces produce less muscle activation and therefore a lower energy turnover (Fletcher and Macintosh, 2017). To improve economy, runners therefore need to develop the ability to manage force appropriately at ground contact whilst still producing enough force to overcome gravity and maintain running speed. As the foot remains in contact with the ground for 0.15-0.25 sec during running, the rate at which force needs to be developed is high. To develop higher competitive running speeds and maximal sprint speed, greater ground reaction forces need to be produced within shorter periods of time (Weyand et al., 2000). Therefore, selecting exercises that activate larger amounts of muscle mass in the lower limb (to increase motor unit recruitment) and switch muscles on quickly (to improve firing frequency) will help control and produce high forces, and decrease the relative proportion of muscle mass required to produce force.

Resistance training exercises are ideal for achieving these outcomes, with moderate-heavy loads and low repetition ranges (3-8 repetitions) required to enhance motor unit recruitment. Light load ballistic exercises using low repetition ranges (3-6 repetitions) are most appropriate for enhancing firing frequency. Plyometric training exercises also expose the body to higher forces than the running stride due to the increase in vertical drop height or the horizontal distance covered with each step. Maintaining a short (~0.2 s) ground contact time during plyometric exercises therefore enhances similar neuromuscular qualities to resistance training with the added benefit of developing stretch-shortening cycle capabilities that include storage and return of elastic energy from tendons.

There has been speculation that performing solely resistance training exercises provides little benefit to performance in runners (Dankel et al., 2017). Three studies, two in recreational- level runners (Damasceno et al., 2015; Karsten et al., 2016) and one in well-trained duathletes (Vikmoen et al., 2016), have, however, observed improvements in time trial performance following a period of resistance training, and several others have shown benefits to running economy (Johnston et al., 1997; Storen et al., 2008; Albracht and Arampatzis, 2013; Piacen- tini et al., 2013). Given the lack of biomechanical specificity between the force expression during running and traditional resistance training exercises, it is perhaps unsurprising that plyometric training or a multi-activity approach to strength training provides greater benefits to performance, at least in the short to medium term. Possessing greater maximal strength may, however, provide a greater magnitude of improvement in explosive strength capabilities (Cormie et al., 2010; James et al., 2018), which has important implications for the long-term periodisation of training.

Training Specificity

Training sessions and exercises can be represented on ‘specificity pyramid’ as shown in Figure 14.1. Based upon the principle of specificity, the most effective training to prepare the body for a given race would be to simulate the actual event as closely as possible. Given the lengthy recovery time and tedium associated with this approach, a better training strategy' would be to take a longer-term view and use training sessions to develop specific physiological capabilities that underpin performance in the target event. This is also a less exhausting approach than simply mimicking a maximal race effort on a frequent basis; therefore, a greater

FIGURE 14.1 The training activity specificity pyramid volume of work can be performed over a given period, which is likely to lead to greater long-term improvements. Running training (interval training, tempo runs, fartlek, long-slow distance running) obviously represent the most specific and important sessions in a runner’s programme and generates cardiovascular and metabolic adaptations that enhance the bioener- getic pathways relevant to performance.

Strength training activities (plyometric and resistance training) sit below running training in the specificity pyramid. Unilateral plyometrics (hopping and bounding) have a high level of movement similarity to running; however, the force demands are far greater. Bilateral jumping exercises provide less of a stability challenge compared to unilateral plyometrics; however, the eccentric control requirements and vertical forces are typically higher. Therefore, jumping offers a strong stimulus for neuromuscular development and enhancement of stretchshortening cycle qualities that are important during running.

Although resistance training exercises should bear some movement resemblance to the running action, it is important to remember that the purpose of these exercises is to overload the neuromuscular system: to increase maximal force production and rate of force development. Therefore, it is important to select the exercises and appropriate stimulus to achieve these outcomes. Similarly, specific strengthening for joints or structures that are vulnerable to injury (e.g. Achilles tendon) requires exercises that isolate and load the area directly. Often these exercises may have no real likeness to the running action but are necessary to build greater resilience and integrity in the tissue. In this regard, it is crucial that exercise selection and programming towards the bottom of the specificity pyramid is adaptation-led, as the priority is development of strength and structural capacity, rather than enhancing bioenergetic processes or the skill of running.

Programming Considerations Long-Term Planning

It is important that runners address programming of strength training with the same attention to detail as their running training. The manipulation of training variables in a cyclical, non-linear manner over a long period of time (6-12 months) is a well-recognised approach to enhancing strength qualities, compared to a non-periodised approach (Williams et al., 2017). Although a relationship appears to exist between the duration of a strength training intervention and the magnitude of change in running economy (Denadai et ah, 2017), there is currently a lack of research that has investigated whether further improvements are possible over periods of longer than a few months. Providing evidence-based guidelines around how strength training should be varied across a long-term period of preparation for an event is therefore problematic.

Figure 14.2 shows a generalised annual plan for a runner who is targeting the Northern Hemisphere track season or an early-autumn road event. Following the completion of an initial battery of movement and strength assessments, it is advisable that runners initially prioritise improving their weaknesses (see Chapter 7). For a runner new to strength training, this will usually be developing movement competency on basic exercises (e.g. squat, hip hinge, step-up, lunge, hop and stick) with their body weight or a light load (<20 kg) (Bla- grove, 2015). Alongside this, runners should focus on specific (isolation/single-joint) exercises or movement patterns than strengthen tissues that have previously been injured and/or areas identified as lacking capacity. Although high volumes of plyometric exercises should be

FIGURE 14.2 A generalised annual plan for a runner targeted a competitive track season in the Northern Hemisphere. The plan could also apply to a runner aiming for an early autumn road event, although the competitive phase could be shortened or removed. Darker shading indicates a higher relative emphasis on the type of training or activity category

avoided initially, low-level plyometrics (e.g. skipping, mini-hops, multi-directional jumps) can also be used concurrently alongside resistance training, which has been shown to be effective for enhancing running economy (Ache-Dias et al., 2017).

Although it is clear that significant gains in strength capabilities are possible when distance runners embark upon a strength training programme for the first time (Trowell et al., 2020), it is well-recognised that high volumes of aerobic running training attenuate strength-adaptation (Blagrove, 2013). Specifically, endurance training appears to compromise gains in explosive strength (or power) and may also interfere with the cellular signalling pathways that underpin an increase in muscle mass (Wilson et al., 2012). Despite this blunted adaptation, in the medium term (2-6 months), improvements in running economy, maximal speed, and performance are certainly possible by adding 2-3 sessions per week of strength training exercise (Blagrove, Howatson et al., 2018a). It is likely that higher volumes of running training (>5 hours per week) are associated with interference in explosive force-related adaptations, but less so maximal strength (Vikmoen et al., 2020). As runners increase their training volume over long periods of time (>6 months), interference with strength-adaptation therefore becomes more likely (Coffey and Hawley, 2017). Similarly, in terms of organisation of training, an emphasis on explosive resistance training and plyometrics during periods of high running mileage seems unwise. Instead, heavier resistance training exercises should be prioritised when running volumes are high, with emphasis switched to explosive strength training when running volume decreases and intensity rises (Blagrove, 2014).

Short-Term Planning

The positioning of strength training within a training week is an important consideration for a runner. It is vital that key sessions each week are not compromised by high levels of fatigue and that strength training can be integrated effectively around a runner’s other lifestyle commitments. As discussed, high volumes of running can negatively affect the quality of strength work and interfere with adaptive processes; however, the reverse is also true. The residual fatigue from a bout of resistance or plyometric training impairs performance during subsequent running sessions if recover)' is inadequate (Doma et al., 2017). Therefore, it is recommended that at least 24 hours separates a strength training session from a high-intensity running session.

For runners who are limited for time due to work, family and/or social commitments, it may only be possible to combine running training and strength work within the same session. In this scenario, as improvements in running are the priority, this should come first. It is also recommended that the bout of running is followed by a high carbohydrate and protein snack before any strength exercises are performed. There is also evidence in physically active individuals that adaptations to aerobic-based training are enhanced when resistance exercise is performed immediately after (Wang et al., 2011). Wherever possible, at least 3 hours should separate a running and a strength training session to allow fatigue to dissipate and minimise interference with strength adaptations (Baar, 2014). If running and strength training can remain as separate sessions on a given day, for recreational-level runners it is recommended that easy running is performed in the morning and strength training in the afternoon. For well-trained runners accustomed to strength training, performing strength work in the morning when less fatigued may be preferable, as they are likely to be sufficiently recovered for an afternoon/evening run. In highly trained runners, it is common for strength training sessions (involving mainly resistance training exercises) to be positioned on the same day, and within a few hours, of a hard interval training session. This polarised design to a training week means all high-intensity physical work (running and strength training) takes place on the same day, allowing days in between to be dedicated (active) recovery days.

One option to avoid the fatigue associated with sessions (~1 hour) of strength training is to divide traditional training sessions into smaller ‘bite-size’ training units (Blagrove, Flowe et al., 2020). This approach, known as ‘micro-dosing’, may also appeal to those runners with busy lifestyles who struggle to find additional time in their schedules to add strength training sessions. Each training unit takes <20 minutes to complete, thus making it easy to integrate some purposeful strength training before or after running sessions. As previously highlighted, there may also be merit in using this strategy to reduce the risk of overuse injury long-term, although the efficacy for improving running economy and performance is currently uninvestigated. An example of how micro-dosing could be used across a training week is shown in Table 14.1, and an exercise routine is shown in Figure 14.3a. A common issue for nonelite runners is the lack of access to a suitable facility to perform resistance training. To some extent, the mini strength and conditioning routine shown in Figure 14.3a is also designed to address this issue.

A ‘micro-dose’ (1-6 sets at <10 sec per set) of high-intensity strength exercise (high load resistance or plyometrics) following a low-intensity run warm-up, and performed 5-10 min prior to a running session, may also be effective for enhancing subsequent performance in well-trained distance runners (Blagrove, Howatson et al., 2019c). A high-load exercise can elicit several physiological mechanisms that acutely augment neuromuscular performance in activities requiring sub-maximal or explosive force production, such as fast running (Blazevich and Babault, 2019). Indeed, a single set (5-6 repetitions) of a plyometric exercise (e.g. depth jumps or repeated maximal jumps) appears to acutely enhance running economy (Blagrove, Flolding

TABLE 14.1 Example of a training week for a non-elite distance runner and organisation of strength training using a ‘micro-dosing’ approach, which includes two short resistance training sessions

Note: S&C = strength and conditioning. Alternatively, a lower frequency of strength training could be used, with 2-3 main sessions spread through the training week. Ideally, running training sessions and strength training should be separated by > 3 hours.

et al., 2019c; Wei et al., 2020), and band-resisted squat jumps (4 sets x 5 repetitions) have been shown to improve performance in a session of 5 x 1 km runs (Low et al., 2019). Moreover, a series of pre-session ‘strides’ (6 x Ю sec at 1500 m pace) wearing a weighted vest (20% of body mass) has also elicited an acute benefit to running economy and time to exhaustion in well-trained distance runners (Barnes et al., 2015).

Tapering and Peaking

As the competitive racing season begins or a targeted event approaches, running training will tend to reduce in volume to accommodate an increase in intensity at race pace or faster (see Figure 14.1 and Chapter 9). During this period, there may be a temptation to exclude strength training activities, especially if they are deemed too fatiguing to enable adequate freshness for races or key training sessions. It is likely that cessation of strength training will lead to noticeable deteriorations in maximal strength, submaximal strength and power after ~15 days (Bosquet et al., 2013). Excluding strength training following a successful intervention period has also been shown to result in a detraining effect, which is likely to cause improvements in performance to return to baseline levels within 6 weeks (Karsten et al., 2016). Reducing strength training frequency from two sessions per week during preparatory phases, to one session per week (and maintaining or slightly increasing intensity) during competitive or peaking phases seems enough to maintain previous strength and physiological improvements (Beattie et al., 2017). A single session of explosive squat training or plyometrics (3-6 sets x 8 repetitions) may also be sufficient to achieve improvements in running economy within 8 weeks (Berryman et al., 2010). Alternatively, the micro-dosing approach (described previously) to organising strength training through a training week offers an alternative strategy to preserve strength adaptations whilst minimising fatigue.

Exercise Selection

In studies that have shown a benefit of resistance training on running economy and/or performance, exercises with free weights have tended to be utilised (Johnston et al., 1997; Millet et al., 2002; Piacentini et al., 2013; Skovgaard et al., 2014; Beattie et al., 2017; Giovanelli et al., 2017). Studies that have used only resistance machines (Ferrauti et al., 2010; Vikmoen

FIGURE 14.3a An example of a mini (home-based) strength and conditioning training routine that could be performed on most days of the week around running training

FIGURE 14.3b An example of a basic resistance training session for a runner et al., 2016) or single-joint exercises (Fletcher et al., 2010) have failed to show a positive effect of resistance training in runners. Multi-joint exercises using free weights are likely to provide a superior neuromuscular stimulus compared to machine-based or single-joint exercises as they demand greater levels of co-ordination, multi-planar control and activation of synergistic muscle groups (Schwanbeck et al., 2009) and they usually require force to be produced from closed-kinetic chain positions. These types of exercise also have a greater biomechanical similarity to the running action and so are therefore likely to provide a greater level of specificity and hence transfer of training effect. An example of a resistance training session is shown in Figure 14.3b.

It is advisable to include a mixture of bilateral and unilateral lower limb exercises in a strength training programme. Bilateral exercises require high levels of absolute force production and place a high demand on the central nervous system to recruit muscle fibres to produce force. Structurally loaded bilateral exercises (e.g. barbell back squat and deadlift) also cause sheering forces to the shaft of bones, resulting in improvements in strength of cortical bone (Lambert et al., 2020), which may lower the risk of bone-stress injury. Due to a phenomenon known as the ‘bilateral deficit’, most individuals are capable of generating more force, per leg (or the sum of forces on each leg), during single-leg exercises compared to a bilateral equivalent exercise (Diet^;n et al., 2003). Unilateral exercises therefore provide significant benefits to improving strength via this mechanism (Howe et al., 2014). Depending upon the exercise, single-leg dominant movements also tend to provide a higher level of movement specificity' for runners (e.g. step-up, hopping, bounding), which makes them appealing. Specifically', greater activation of key stabilising muscles is required to resist unwanted movements in the frontal and transverse planes (McCurdy et al., 2010; Lubahn et al., 2011), which may provide a greater carryover to running.

An intuitive reason to use unilateral strength exercises is to correct an inter-limb asymmetry that may exist (Howe et al., 2014). Although a large strength imbalance (>15%) between legs has been associated with a higher risk of injury in female games players (Knapik et al., 1991), there is currently no evidence that strength asymmetry is a risk factor for overuse injury in runners. Recent data shows that inter-limb hip abduction (gluteal) strength differences of >9% in adolescent female distance runners are associated with poorer running economy (Blagrove, Bishop et al., 2020). To correct pronounced strength asymmetry, the approach is unlikely to be as simple as using a unilateral dominant programme of strength training exercises. It is possible that an inter-limb asymmetry is caused by mobility or motor control issues, rather than being a neuromuscular recruitment deficiency (Howe et al., 2014); therefore, these should also be assessed so compensatory movement strategies don’t exacerbate the issue. Improving maximal strength on two legs has also been shown to reduce inter-limb strength asymmetry (Bazyler et al., 2014), so it is uncertain whether a pure unilateral programme would provide greater benefits in reducing the imbalance.

Summary

Distance running performance is principally limited by the cardiovascular and metabolic systems; however, neuromuscular factors contribute to running economy and maximal sprint speed, which can be enhanced using strength training activities. Overuse injuries in runners are common and can be the reason for quitting the sport. Of concern, low BMD is relatively common in runners, and running alone can be insufficient to stimulate improvements in

BMD. There is currently a lack of direct evidence that a long-term strength training intervention can reduce the likelihood of runners suffering an overuse injury. Despite this, it seems plausible that short bouts of frequent high-intensity strength exercise may lower the risk of developing bony, tendon- or ligament-related overuse injury. Furthermore, stronger gluteal muscles appear to be important for decreasing the chances of some types of overuse injuries, particularly in women. Initially, it is important that runners develop competency in a wide range of fundamental movement skills that can then be progressively loaded. Resistance training using multi-joint exercises should be performed at least twice per week and prioritised over plyometrics and explosive resistance training when the volume of running training is relatively high. Both bilateral and single-leg exercises offer benefits to runners; therefore, a combination of both should be incorporated into strength sessions. As part of a concurrent approach to the prescription of strength training, plyometric exercises should also be performed alongside resistance training, but initially in low volumes. As the competitive season or a targeted event approaches, resistance training volumes can be reduced and more explosive work prioritised on at least one occasion per week. The timing of strength training around running sessions is dependent upon the training status of the runner and their other lifestyle commitments. Wherever possible, runners should separate running and strength training by >3 hours within the same day, and leave > 24 hours after a strength training session before a high-intensity running session is attempted.

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