Abstract
Isometric strength training is gaining popularity within the performance environment of strength and conditioning, and specifically in soccer. Although physiotherapists are already familiar with and regularly use this type of strength training, most practitioners still find it to be somewhat obscure. Although this kind of training is quite approachable due to its technical simplicity, it is still unknown whether it can enhance professional soccer players' performance outcomes. The physiological adaptations and effects of isometric strength training will be examined in this article along with the possibility that it can enhance professional soccer players' linear sprinting abilities.
Introduction
The physical attributes necessary for effective soccer performance can vary greatly (Jaspers et al., 2017; Walker & Hawkins, 2017) and the rising physical demands of elite soccer (Thorpe et al., 2016; Arcos et al., 2017) indicate that the addition of strength and conditioning (S&C) training might be advantageous to assist players meet these demands. Strength training is frequently used as a part of physical preparation for a wide range of people, including elite strength and power athletes and injured general population. Increased muscle hypertrophy (Damas et al., 2015), improved tendon quality (Couppé et al., 2008; Arcos et al., 2017; Magnusson & Kjaer, 2019), greater strength, power and range of motion (Morton et al., 2011), postponed muscle fatigue (Aagaard et al., 2011; Tanaka & Swensen, 1998), and improved voluntary contraction are all effects of resistance training that have been extensively studied (Aagaard et al., 2002). The vast majority of resistance training programs consist of dynamic exercises using the stretch-shortening cycle (SSC) (Kraemer & Ratamess, 2004). Nevertheless, isolated concentric, eccentric, and isometric contractions are growing increasingly popular because they have particular benefits for enhancing musculoskeletal characteristics and neuromuscular performance (Burgess et al., 2007; Malliaras et al., 2013; Kubo et al., 2001). This essay focuses on isometric contractions, in which the muscle-tendon unit maintains a constant length, and its impact on linear sprinting in professional soccer as a training method.
Athletic development of lower limb strength
Lower extremity strength training is crucial for physical success because it has been demonstrated to have a positive impact on soccer-specific motions like sprint speed and jumping (Magnusson & Kjaer, 2019; Morton et al., 2011; Aagaard et al., 2011). Several sports’ fundamental requirements call for athletes to exert significant forces quickly for multiple locomotor type actions such as accelerating, decelerating, or changing direction (Tanaka & Swensen, 1998; Aagaard et al., 2002; Kraemer & Ratamess, 2004). Under these circumstances, more force is exerted on the ground and more velocity is produced (Burgess et al., 2007; Malliaras et al., 2013). Physical qualities like strength and speed have been demonstrated to have a profoundly positive ability to change soccer performance (Kubo et al., 2001). In order to improve athletic development and support both individual and team sport performance, lower limb strength must be developed (Aagaard et al., 2002). As a result, in order to enhance performance and minimize injuries, strength development is commonly prioritized by coaches in professional soccer. Several games are played each week as elite soccer players compete throughout extended competitive seasons (Krebs et al., 1983; Abbott & Wilkie, 1953). It is crucial that key physical characteristics are maintained, performance is high, and injuries are limited during the whole competitive season because games with bigger implications, such as play-offs, knockout stages of competitions, or cup finals, frequently take place near the end of the season. So, in order to ensure weekly performance remains high, practitioners must strike a though balance between training to increase performance capacity and giving enough rest and recovery.
Implications of isometric strength training
In contrast to general strength training and plyometric training (PT), isometric strength training (IST) is frequently utilized in injury rehabilitation because it does not have a large impact force on the lower limbs (Rhyu et al., 2015; Rio et al., 2015). Furthermore, it has been shown that the isometric mid-thigh pull (IMTP) peak force and sprint (Lum & Joseph, 2020) and endurance running performance are substantially associated. Additionally, IST has been shown to enhance musculotendinous stiffness (Burgess et al., 2007; Lum & Barbosa, 2021), muscular force production (Bimson et al., 2017; Kubo et al., 2017; Lum & Barbosa, 2021; Lum et al., 2021; Lum & Joseph, 2020), and even sports-related performance outcomes (Albracht & Arampatzis, 2013; Bimson et al., 2017; Lum et al., 2021). Intriguingly, according to Burgess et al. (2007), IST increased musculotendinous stiffness more than PT did. The findings of Kubo et al (2017) emphasized more tendon stiffness during a period of IST as compared to PT corroborated this conclusion. This implies that, in comparison to PT, IST might be just as effective to enhance sprinting performance.
The physiological characteristics of muscles, tendon stiffness and health, joint angle-specific torque, and metabolic processes can all change as a result of IST (Alegre et al., 2014). Similar to resistance training in general, the stimulus can be changed by adjusting a number of factors. The most popular isometric training variations include using unique techniques such as blood flow restriction and contraction intent like ballistic training methods, vibration techniques and electrical stimulation (Balshaw et al., 2016; De Ruiter et al., 2005; Sjøgaard et al., 1988; Silva et al., 2008). Furthermore, recent studies have shown distinct neuromuscular differences between “pushing” and “holding” isometric contractions. “Pushing”, can be defined as exerting force against an immovable object. Whereas “holding” is defined as maintaining a joint position while resisting an external force (Garner et al., 2008; Rudroff et al., 2011; Schaefer & Bittmann, 2017). Practitioners would benefit from knowing the loading conditions that produce the desired adaptive response in muscle and tendon
Isometric strength training is said to provide a number of benefits. First of all, in rehabilitation settings, isometric training enables a precisely controlled application of force within pain-free joint angles (Hasler et al., 1994; Krebs et al., 1983). Second, as maximal isometric is greater than that of concentric contractions, isometric strength training offers a way to create force overload (Abbott & Wilkie, 1953). Third, a professional who is familiar with the physical demands of a sport may be able to use isometric training to concentrate on particular weak areas in a range of motion that can improve performance and injury prevention (Tsoukos et al., 2016; Van Beijsterveldt et al., 2013). By changing the excitatory and inhibitory functions in the corticomotor pathways, isometric contractions can also be employed to produce and immediate analgesic effect and enable pain-free dynamic loading (Rio et al., 2015; Goodwill et al., 2012). Moreover, isometric methods are extremely accurate for determining and monitoring changes in force generation (Murphy et al., 1995; Murphy & Wilson, 1996; Wilson & Murphy, 1996). Although multi-joint appraisals showing potential, the capacity of isometric assessments to predict dynamic performance is debatable (McGuigan et al., 2010; Drake et al., 2017; Oranchuk et al., 2017).
Implications of linear sprinting
One of the most effective training stimuli is sprinting due to the mechanical difficulties alone. A maximal run that lasts fewer than 15 seconds from start to finish is considered to be a sprint if the athlete’s center of mass (CoM) is moved horizontally as a result (Ross et al., 2001). Acceleration, transition, and top speed are the three phases that sprinting is typically classified into (Nagahara et al., 2017). Also, it should be highlighted that sprinting is a talent in and of itself that should be developed gradually in accordance with the principles of motor learning (Moir et al., 2017). The ability of a muscle to contract, which is controlled by the neuron innervating the activated fibers, will play a significant role in the high rates of force production required during sprinting. Sprinting causes both central and peripheral nervous system alterations that together increase neural drive (Ross et al., 2001). There is significant overlap between the brain adaptations to running and resistance training, with sprinting potentially allowing for more effective selection of a few specific adaptations. The first is that, compared to resistance training, sprinting requires relatively quick energy storage and return. As a result, sprinting may theoretically result in higher increases in muscle spindle excitation, voluntary activation, and firing frequency (Kyröläinen et al., 2005; Ross et al., 2001). In addition, sprinters’ H-reflex (or Hoffmann’s reflex), which measures the motor units triggered by an electric stimulation, is significantly lower than that of other athletes. This is likely because sprinting preferentially recruits fast fiber MU’s (motor units), which have a high activation threshold (Kumar et al., 2012). Due to the particular burden exerted on the nervous system during training by sprinting, it is believed that a program including both resistance training and sprinting is best practice in the majority of power sports.
There are a vast variety of physical and physiological adaptations that may arise as a result of exercise and training. These modifications could affect a person’s work capacity, sprint speed, cardiac output, and motor unit recruitment, among other things. It should be emphasized that the adaptations depend on the type of training a person has or is undergoing right now. Furthermore, it makes sense that sprint training will focus on the type II muscle fibers given the specific neurological demands of repeatedly performing sprints. This could be helpful for facilitating the switch to faster myosin heavy chain isoforms, increasing the size of the current type II fiber, or raising the ratio of type II to type I cross-sectional area, boosting the rate of force development potential (Andersen & Aagaard, 2006; Van Cutsem et al., 1998). Although sprinters tend to have longer muscle fascicles, which may provide them an advantage in shortening velocity due to the addition of sarcomeres in series, modifications in muscle architecture are also probably evident (Abe et al., 2001). An increase in sarcoplasmic reticulum volume may contribute to both the shortening velocity advantage and potentially even the architectural modifications itself. This promotes increased calcium ion release and improves the environment for quick contractions (Rtenblad et al., 1998; Ross & Leveritt, 2001). Due to sprinting during training, muscle tissue and the muscle-tendon complex may also change in stiffness (Kubo et al., 2017; Kubo et al., 2001). This can be helpful for running, but it may also help athletes in team sports perform other activities like jumping and changing directions. Despite this, there is still limited agreement on the best training recommendations for reaching particular results.
The impact of isometric strength training on linear sprinting
While most progressive resistance training techniques do promote muscle size, it is crucial to know how to change each training technique’s intensity, frequency, and duration for maximum effectiveness. It has been shown that IST causes noticeable hypertrophy (Bobbert et al., 1996; González-Alonso et al., 2008; Häkkinen et al., 1998; Kubo et al., 2017). Many patterns occurred when muscle volume changes between isometric training types were compared, supporting well-known dynamic training concepts. IST at long muscle lengths (LMLs) was superior to comparable quantities of training at short muscle lengths (SMLs) for developing muscular size, according to numerous studies (González-Alonso et al., 2008; Häkkinen et al., 1998).
It is important to address the fact that a large proportion of the studies cited in this essay involved untrained subjects. Active or recreationally trained volunteers were used in a small portion of the research. Competent athletes or highly trained volunteers were not assessed in any of the acceptable studies. Recent studies have shown that single-joint contractions are the primary exercises used in the isometric strength testing settings. A handful of the research studies subject athletes to training interventions including closed-chain movements. The IST domains of interest can range from training variations outside of joint position of contraction intensity to the chronic effects of IST at different joint angles (Alegre et al., 2014; Noorkõiv et al., 2014; Noorkõiv et al., 2015; Rasch & Pierson, 1964; Meyers, 1967; (Lindh M., 1981) and examining the effect of contraction intensity (Arampatzis et al., 2007; Arampatzis et al., 2010; Kanehisa et al., 2002; Khouw & Herbert, 1998; Szeto et al., 1989). The differences include the following: periodization, total volume (Meyers, 1967), rest period duration (Waugh et al., 2018) and contraction duration (Kubo et al., 2001; Schott et al., 1995). The purpose of contraction included “sustained” vs. “explosive”, and “fast” vs. “progressive” contractions (Balshaw et al., 2016; Tillin & Folland, 2014; Maffiuletti & Martin, 2001; Williams, 2011). Also, we can learn from the research literature that training interventions including joint angles >70° of flexion lead to a higher increase in muscle size than training involving joint angles ≤70°. When we examine how training intensity affects muscle hypertrophy, we can see that training intensities that are less than 70% of the MVIC (maximum voluntary isometric contraction) lead to a higher increase in muscle size than training intensities that are greater than 70% of the MVIC (Kubo et al., 2001; Kubo et al., 2006; Alegre et al., 2014; Noorkõiv et al., 2014; Schott et al., 1995; Ullrich et al., 2015; Balshaw et al., 2016).
Like other forms of resistance training, isometric training should be carried out in a manner that is most directly related to the main outcome objective. Evidence suggests that there is little difference between contractions completed with a ballistic or a progressive ramp to the appropriate intensity when muscle hypertrophy or maximal force output is the priority (Balshaw et al., 2016; Williams, 2011; Maffiuletti & Martin, 2001; Tillin & Folland, 2014). Nonetheless, isometric contractions should be carried out as such if rapid force production takes priority, as it does in a number of sports as well in linear sprinting (Balshaw et al., 2016; Williams, 2011; Tillin & Folland, 2014). Conversely, despite the potential to result in distinctive morphological tendon modifications, ballistic contractions may be inappropriate or excessively painful in rehabilitative or special groups (Rio et al., 2015; Massey et al., 2018). Hence, while ballistic contractions nevertheless offer unique neuromuscular benefits, prolonged isometric contractions often provide equivalent or higher morphological changes that are likely to be of interest to players participating in team sports like soccer (Balshaw et al., 2016; Maffiuletti & Martin, 2001; Williams, 2011).
Conclusion
The specific adaptations we can observe as a result of IST, such as increasing the size of the current type II fiber, increasing the sarcoplasmic reticulum volume, increasing the ratio of type II to type I cross-sectional area, increasing the rate of force development potential, and increasing the rate of immediate analgesic effect, tend to have a positive impact on linear sprinting performances. Despite the fact that sprinters' muscular fascicles are typically longer than those of other professional athletes, which may provide them an advantage in shortening velocity due to the addition of sarcomeres in series (Abe et al., 2001). It is a highly efficient method for increasing lower limb strength due to its low technical demands, muscle architectural modifications, and low risk of injury. The most advantageous training impact appears to be produced when IST is applied utilizing long muscle lengths. It is still necessary to conduct further research to determine the precise IST angles that will have the greatest impact on professional soccer players' abilities to improve their running.
Discussion
Because of the numerous advantages of isometric strength training, new techniques are constantly being developed as a result of the rising popularity of this type of strength training. The usage of “Run Specfic Isometrics” (RSIST), of which Alex Natera is the founder, is a significant and fascinating breakthrough that is very well-liked on social media The functioning and adaptations of RSIST are still the subject of debate in study, despite the fact that this can be a promising training technique for enhancing linear sprinting performance. Natera lists the following as some of the benefits of RSIST, in his online course “Isometric strength training with Alex Natera”, provided in cooperation with Sportsmith: movements are muscle action specific, specific joint angles and muscle lengths can be used, magnitude of force, force output in a specific time frame, low technical requirement and easy to learn, low coordinative demand and more effective at highlighting asymmetries. Shorter ground contact time, increased VO2max (maximum rate of oxygen uptake), improved running efficiency, increased top speed, rapid force production with high-speed muscle movements, beneficial on MD+2 (two days after the game is played) for sore players, overload forces HSR (high speed running) and sprinting, are some performance advantages that could result from RSIST.
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