# Effects of Isometric and Isotonic Training on Health-Related Fitness Components in Young Adults
## 1 Introduction  
Isotonic training is very popular among athletes as well as recreationally active individuals. Isotonic exercise includes a shortening/concentric contraction of the agonist while simultaneously displaying lengthening/eccentric of the antagonist (or vice versa). Isotonic training allows utilizing the stretch-shortening cycle, i.e., muscle lengthening immediately followed by muscle shortening [1]. Benefits of isotonic resistance training are as follows: isotonic exercises can be performed with or without gym equipment, they improve the size and muscle strength, increase the cardiovascular fitness, support performing daily activities without fatigue, improve bone density, and reduce the risk of sports injuries.  
On the other hand, isometric exercise is a static exercise, so when the force is applied, the length of the muscle remains the same. Isometric muscle function has subjectivelytwo different modes of performance; one can either hold isometrically, and thus resist an impacting force, or push isometrically and work against a stable resistance [2]. The benefits of the isometric resistance training are as follows: isometric exercises (hold for 10 to 30 s) can be performed with or without gym equipment, they improve the flexibility of joints, manage reducing blood pressure, and can be used for rehabilitation [3].  
Resistance training has a positive effect on lower limb power (i.e., vertical jump height) and strength (i.e., 1 RM squats and bench press) [4], flexibility [5], and muscular endurance [6], improving dynamic balance, maintaining healthy body weight, increasing the range of motion and overall muscular fitness [7], and improving the quality of life [8]. Following the initial stage of the program with low weights (<60% of 1 RM), the intensity, complexity, and movement speed gradually increased on a weekly basis. Free weights have a naturally stirring, constantly changing external moment arm with single joint exercises and multiple changing external moment arms in multiple joint exercises [9]. Multi machines use a mechanically designed and engineered eccentrically shaped pulley known as a cam that varies the external moment arm [10].  
Incorporating both isotonic and isometric resistance exercises in regular training programs may increase muscle strength and the range of motion, and may have many health-related benefits improving quality of life [11]. Participation in resistance training has been associated with reduced central adiposity and reduced unhealthy fat gain, It can also lead to weight loss [12]. The aquatic form of these exercises may stimulate improvements in measures of fatigue [13]. However, resistance training protocols need to be age-adjusted [14] with appropriate volume for improving muscle strength and hypertrophy [15].  
Overall, both isotonic and isometric trainings are beneficial for athletes as well as the general population [16] and are recommended for the enhancement of their performance. However, less in known about their effects on fitness and health in recreationally physically active adults participating exclusively in university physical education classes. Therefore, the aim of this study was to compare the effects of isometric and isotonic training protocols on health-related fitness components in young university students. We hypothesized that isotonic exercises will be more effective in enhancing health-related fitness components compared to isometric exercises.