Isometric resistance exercise is a dynamic form of exercise that is carried out against a constant or variable load as a muscle lengthens or shortens through the available range of motion. Dynamic strength, muscular endurance, and power can be developed with isotonic exercise.
Concentric/Eccentric
- Although a maximal concentric contraction produces less force than a maximal eccentric contraction, adaptive strength gains after a concentric or eccentric exercise program appear to be similar.
- Concentric exercise has less mechanical efficiency than eccentric exercise.
- A limited degree of transfer of training causes eccentric strength gains when concentric training is performed.
- The velocity at which concentric or eccentric exercise is performed directly affects the force-generating capacity of the neuromuscular unit. At slow velocities, a maximum eccentric contraction generates greater force than a maximum concentric contraction, but as the velocity of exercise increases, concentric contraction forces rapidly decrease and eccentric contraction forces increase slightly and then generally level off or decrease. In a strength-training program, when heavy loads are lifted or lowered, isotonic exercises are usually performed at slow speeds to safely control momentum and minimize the possibility of injury.
Isokinetic exercise is a form of dynamic exercise in which the velocity of muscle shortening or lengthening is controlled by a rate-limiting device that controls (limits) the speed of movement of a body part.
- Isokinetic exercise can strengthen muscles more efficiently than isotonic exercise.
Static strength increases due to isometric training are joint angle specific (Gardner, 1963; Meyers, 1967; Williams & Stutzman, 1959). If isometric training is performed at a joint angle of 90 degrees, strength will be increased at this joint angle of 90 degrees, strength will be increased at this joint angle but not necessarily at other joint angles. This joint angle specificity appears to have a carryover of plus and minus 20 degrees of the training joint angle (Knapik, Mawdsley, & Ramos, 1983). In addition, twenty six-second isometric contractions cause a greater carryover or transfer to joint angles besides the training angle than sis six-second contractions (Meyers, 1967). This demonstrates that a greater number of contractions lead to a greater carryover of strength at all four angles other than the training angle. Isometric training of the elbow flexors at four different angles does increase static strength at all four angles and significantly increases dynamic power demonstrate that if isometric training is used to increase strength throughout a joint's range of motion, training must take place at several joint angles and a large number of training contractions should be used.
It is times possible to take advantage of isometric training?fs joint angle specificity. Each dynamic exercise has a sticking point. Sticking point refers to the joint position in a movement where the mechanical advantage is at its lowest point and therefore is the most difficult potion of the movement to perform. Performance of isometric training at the sticking point will increase strength at this joint angle and therefore aid in performance of the dynamic exercise. Motor Performance Isometric training does not increase motor performance ability (Clarke, 1973; Fleck & Shutt, 1985). This may be due in part to two factors. Isometric training can reduce the maximal speed , with little resistance to its movement, is not increased due to isometric training (DeKoning, Binkhorst, Vissers, & Vos, 1982). Motor performance tests involve movement at maximal speeds with little resistance. A decrease in a limb?fs maximal speed or a lack of improvement in maximal speed with small resistance will lead to no increase in motor performance ability. The aforementioned studies involved isometric training at only one joint angle. Dynamic power over a wide range of resistance can be increased due to isometric training at four different joint angles within the range of motion (Kanehisa & Hiyashita, 1983). The possibility exists therefore that increases in motor performance may occur if isometric training is performed at several joint angles.
Muscle Enlargement
Various athletic groups exhibit enlargement of muscles due to strength training. An increased cross-sectional area of existing muscle fibers accounts for enlargement of the whole muscle. Hyperplasia is also a process by which muscle growth occurs; Fibers split leading to an increase in the total number of muscle fibers present. In laboratory animals, muscle growth has occurred due to hypertrophy alone (Bass, Mackova, & Vitek, 1973; Coldberg, Eltinger, Goldspink, & Jablecki, 1975; Gollnick, Timson, Moore, & Riedy, 1981; Timson, Bowlin, Dudenhoeffer, & George, 1985). Increased muscle size in strength-trained athletes has also been attributed to hypertrophy of existing muscle fibers (Prince, Hikida, & Hagerman, 1976; Haggmark, Jansson, & Svane, 1978; Gollnick, Parsons, Riedy, & Moore, 1983). This increase in the cross-sectional area of existing muscle fibers is attributed to increased size and number of the actin and myosin filaments and addition of sarcomeres within existing muscle fibers (Gordon, 1967; McDougall et al., 1979). Hyperplasia has been implicated in contributing to muscle enlargement in laboratory animals (Gonyea, 1980; Ho et al., 1980). Criticism of these studies has claimed that damage to the muscle samples as well as degenerating muscle fibers account for the observed hyperplasia. Several studies comparing body builders and power lifters concluded that the cross-sectional area of the body builders' individual muscle fibers was not significantly larger than normal; yet these athletes possessed larger muscles than normal (MacDougall et al., 1982; Tesch & Larsson, 1982). This indicates that these athletes have a greater total number of muscle fibers than the control group; Hyperplasia may account for this increase. Yet another study using body builders concludes that, on the contrary, they possess the same number of muscle fibers as the control group but possess much larger muscles (MacDougall et al., 1984). This study suggests that the large muscle size of body builders is due to hypertrophy of existing muscle fibers rather than hyperplasia. Another study of hyperplasia in laboratory animals has indicated that in order for hyperplasia to occur in cats the exercise intensity must be sufficient to recruit FT fibers (Type 2B) (Gontea, 1980). It is possible that only high-intensity resistance training can effect hyperplasia. Though no concrete evidence supports the relationship, there are indications that hyperplasia occurs due to resistance training. Due to these conflicting results, this topic remains controversial; further research on elite competitive lifters may help to resolve the controversy.
Conventional weight training in humans (Gonyea & Sale, 1982) and animals (Edgerton, 1978) appears to hypertrophy, selectively, the Ft fibers to a greater degree than the ST fibers. Studies do, however, indicate that it may be possible to selectively hypertrophy either the FT or the ST fibers depending on the training regimen. Power lifters who train predominantly with high intensity(i.e., heavy resistances) and low volume (i.e., small number of sets and repetitions) have been shown to have Ft fibers with a mean fiber of 79 mm²x 100 in the vastus lateralis (Tesch, Thorsson, & Kaiser, 1984). Conversely, body builders who train predominantly with a lower intensity but a higher volume have been shown to have FT fibers with a mean fiber area of 62 mm²x 100 in the same muscle (Tesch et al., 1984). This study indicates that the high-intensity/low-volume training of Olympic and power lifters and the low-intensity/high-volume training of body builders may selectively hypertrophy the FT fibers and ST fibers, respectively. An increase in the number of capillaries in a muscle may also enlarge the entire muscle. As with the selectively hypertrophy of FT fibers, the increase of capillaries appears to be linked to the intensity and weight lifters exhibit no change in the number of capillaries per muscle fibers; due to hypertrophy of the fibers those same athletes showed a decrease in capillary density (i.e., the number of capillaries per cross-sectional area of tissue) when compared to non-athletic individuals (Tesch et al., 1984). It has been proposed that the training performed by body builders induces increased capillarization (Schantz, 1982). Thus, high-intensity/low-volume strength training actually decreases capillary density whereas low-intensity/high-volume strength training has the opposite effect of increasing capillary density. An increase in capillary density may facilitate the performance of low-intensity weight training by increasing the blood supply to the active muscle. The short rest periods used by body builders during their work-outs result in large increases in blood lactate levels (Noble et al., 1984); increased capillary density might aid in lactate clearance.
Physical activity also increases the size and strength of ligaments and tendons (Fahey, Akka, & Rolph, 1975). The increased size of these structures may cause in part the apparent enlargement of a muscle due to weight training. As the skeletal muscles become able to develop more tension through training, damage to ligaments and tendons are more likely to occur. Increased strength of the ligaments and tendons is a necessary adaptation to aid in preventing possible damage to these structures caused by the muscle?fs ability to lift heavier weights and develop more tension..
The initial quick gains in strength during the first few weeks of a weight training program along with no noticeable increase in muscle mass suggests that other factors besides muscle mass are involved in muscular force production. Following a resistance-training program there are weak relationships between increases in strength and changes in limb circumference (Martian & De Vries, 1979, 1980), muscle fiber cross-sectional area (Costill, Coyle, Fink, Lesmes, & Witzmann, 1979), and muscle fiber type (MacDougall et al., 1980), indicating other factors are responsible for gains in strength. In one study isometric training produced a 92% increase in maximal static strength but only a 23% increase in muscle cross-sectional area (Ikai & Fukunaga, 1970). On the basis of this kind of evidence researchers have concluded that some neural factors have a profound influence on muscular force production. Such neural factors are related to the following processes; increased neural drive to the muscle, increased synchronization of the motor units, increased activation of the contractile apparatus, and inhibition of the protective mechanisms of the muscle (i.e., golgi tendon organs). In young males (20-30 years) it is believed that neural factors are the predominant cause of strength increase during the first three to five weeks of training (Moritani & DeVries, 1980). After this period hypertrophy of the muscle becomes the predominant factor in strength increases.
Researchers have investigated neural drive to a muscle using integrated electromygram (EMG) techniques (Hallinen & Komi, 1983; Kamen, Kroll, & Zigon, 1984; Moritani & DeVries, 1980; Sale, McDougall, Upton, & McComas, 1983; Thorstensson, Karlsson, Viitasalso, Luhtanen, & Komi, 1976). EMG techniques measure the electrical activity within the muscle and nerves and indicate the amount of neural drive to a muscle. In one of these studies, 8-weeks of dynamic constant resistance weight training shifted the EMG activity to muscular force ratio to a lower level (Moritani & DeVries, 1980). Because the muscle produced more force with a lower amount of EMG activity, more force production was realized with less neural drive. Calculations predicted a 9% strength increase due to training-induced hypertrophy; actuality, however, strength increased 30%. It is believed that this increase in strength beyond that expected from hypertrophy resulted from the combination of the shift in the EMG-to-force ratio and the 12% increase in maximal EMG activity. This and other researches support the idea that an increase in maximal neural drive to a muscle increases strength. The studies reveal that less neural drive is required to produce any particular sub-maximal force after training; consequently there is either an improved activation of the muscle or a more efficient recruitment pattern of the muscle fibers. Because it has been demonstrated the improved activation of the muscle does not occur after training (McDonagh et al., 1983) it follows that a more efficient recruitment orders responsible for the increased force produced. A second neural factor that could cause increased force production is increased synchronization of motor unit firing. The greater the synchronization the greater the number of motor units firing at any one time. Increased synchronization of motor units has been observed after strength training (Milner-Brown, Stein, & Yemin, 1973). During sub-maximal force production, however, increased synchronization of motor units is actually less effective in producing force than asynchronous activation of motor units (Lind & Petrofsky, 1978; Rack & Westbury, 1969). Thus, it is unclear whether greater synchronization of motor units produces greater force or not.
Training has been shown to increase the period of time that all motor units can be tonically active from several to 20 seconds (Grimby, Hannerz, & Hedman, 1981). An adaptation of this type may not cause an increase in maximal force but does aid in maintaining it for a longer period of time. It has also been observed that during maximal voluntary contractions the high threshold FT motor units normally do not reach stimulation rates required for complete tetany to occur (DeLuca, LeFever, McCue, & Xenakis, 1982). If the stimulation rate to these high threshold motor units were increased, so also would be the actual force production. Sale et al. (1983) have investigated the possibility that following strength training additional motor units can be recruited. As a mechanism to increase force production, this process assumes that an individual is not able to simultaneously activate all motor units in a muscle prior to training. Belander and McComas (1981) found that this is true adaptation to strength training require further research before concrete conclusions can be reached.
Inhibitory Mechanisms
Inhibition of muscle contraction by reflex protective mechanisms such as the golgi tendon organs has been hypothesized to limit muscular force production (Caiozzo et al., 1981; Wickiewicz et al., 1984). The effect of these inhibitory mechanisms can be partially removed by hypnosis. Ikai and Steinhaus (1961) found that force developed during forearm flexion by non-resistance-trained individuals increased 17% under hypnosis. In the same study force developed by a highly resistance trained individual under hypnosis was not significantly different from force developed in the normal conscious state. The researchers concluded that resistance training might cause voluntary inhibition of these protective mechanisms. These protective mechanisms appear to be especially active when large amounts of force are developed, such as maximal contractions at slow speeds of movement (Caiozzo et al., 1981; Wickiewicz et al., 1984).
Several practical applications derive from information concerning protective mechanisms that limit muscular contraction. Many resistance training exercises involve contraction of the same muscle groups of both limbs simultaneously or of bilateral contractions. The force developed during bilateral contractions is less than the sum of the force developed by each limb independently (Ohsuki, 1981; Secher, Rorsgaard, & Secher, 1978). The difference between the force developed during bilateral contraction and the sum of the force developed by each limb independently is called bilateral deficit. This bilateral deficit is associated with reduced motor unit stimulation (Vanderwoot, Sale, & Moroz, 1984). The reduced motor units could be due to inhibition of contraction by the protective mechanisms and subsequently less force production. Training with bilateral contractions reduces bilateral deficit (Secher, 1975) by bringing bilateral force production closer to the sum of unilateral force production. Although bilateral exercises might be important to maintain the deficit in, for example, sports where force production of one limb independently is required. Unilateral exercises can be performed using dumbbells and some types of weight training equipment. Knowledge of the neural protective mechanisms is also useful in understanding the expression of maximal strength. Neural protective mechanisms appear to have their greatest effect in slow-velocity/high-resistance movements (Caiozzo et al., 1981; Wickiewicz et al., 1984). A resistance training program in which the antagonists are contracted immediately prior to performance of the exercise is more effective in increasing strength at low velocities than a program in which pre-contraction of the antagonists is not performed (Caiozzo, Laird, Chow, Prietto, & McMater, 1983). The pre-contraction in some way partially inhibits the neural self-protective mechanisms, thus allowing a more forceful contraction. Pre-contraction of the antagonists can be used as a method both to enhance the training effect and to inhibit the neural protective mechanisms during a maximal lift. For example, immediately prior to a maximal bench press attempt, forceful contractions of the arm flexors and muscles that adduct the scapula (e.g., pull the scapula toward the spine) should make possible a heavier maximal bench press than no pre-contraction of the antagonists.
Strength Gain through ROM
Time per Training Session
Expense
Ease of Progress
Ease of Progress Assessment
Adaptability to Specific Movement
Probability of Soreness
Probability of Musculo-skeletal Injury
Cardiac Risk
Skill Improvement
Good
3
2
3
Excellent
2
Much Sore
Moderate
Slight
Slight
Poor
1
1
1
Dynamometer
Required
3
Little sore
Slight
Moderate
None
Excellent
2
2-3
2
Expensive
Equipemnt
Require
1
Little sore
Slight
Some
Some
Here is diagram which shows how the muscles (nerve system) are adapted by weight training. As you see, it is worthless to lift less than 50% resistance of your maximal strength. You have to work out with more than 50% resistance of your maximal strength. Easy job does not promote your strength!!
- 20-40% --- No improvement
- 40-50% --- Little improvement
- 50-60% --- Hypertrophy (gain muscle strength)
60-70
80-90
100
6-10
4-6
2-3