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We know that muscle recovery from isotonic exercise occurs more slowly than recovery from isometric exercise, but the recovery curves have similarities. -Therapeutic Exercise for Musculoskeletal Injuries^ 6....

We know that muscle recovery from isotonic exercise occurs more slowly than recovery from isometric exercise, but the recovery curves have similarities. -Therapeutic Exercise for Musculoskeletal Injuries^ 6. Isometric exercise can be performed more frequently than dynamic exercise. You can train in isometrics more often, and make more rapid progress. Isometric training causes less microtrauma to the muscles than regular dynamic forms of resistance training.^ The result? You are less sore following the training—the kind of post-training inflammation and irritation called delayed onset muscle soreness (DOMS). You are also less functionally impaired.^ DOMS isn't just annoying; it's associated with significant loss of strength and range-of-motion following training. After a hard weights session, this period of lower function can last for days —despite therapeutic modalities (massage, TENS, etc).4 Obviously, minimizing DOMS has immediate benefits to the athlete—they are back to peak functioning extremely rapidly. They also suffer less pain as a result of their training; and this in turn means that motivation for hard training isn't impaired. (There is strong evidence that DOMS interferes with training motivation.^ It's difficult to keep returning to any activity which hurts a lot.) There is a much more important benefit, however. Since athletes recover much more rapidly from isometrics than dynamic resistance training, they are able to train much more frequently. The importance of frequency in training There are four fundamental variables to any kind of training program:^ Mode is what you do;  Volume is how much you do; Intensity is how hard you do it; and Frequency is how often you do it. MODE VOLUME INTENSITY “ The Square of Programming For more information on using these variables, see The PCC Instructor's Manual, chapters 25-26. In modern fitness culture, there is a huge amount of publicity given to the idea of intensity in training—particularly intensity of effort. The harder you work, the more likely you are to wind up a winner, right? In fact—probably not. Current research indicates that it is the frequency of training or practice—not the intensity—that turns also-rans into champions.^ Higher frequency equals more progress In terms of resistance training, simple math tells us why this is. Let's say, for example, that you perform a maximally intense training session, and make 3% progress—your strength improves 3% as a result of that single session. Let's also say that the session leaves you so drained and sore, that you can't work the same muscles again for four days. Compare this to a less intense session where you train hard, but don't push to the absolute limit. In this more moderate session, you only make 2% progress—2/3rds the progress of the first one—however you are able to train the same muscles again in two days. Which training session is best? Well the first session is more intense and results in more short-term progress. However, if you keep working out this way for 40 days, you will accumulate 30% progress (3% every 4 days). With the more moderate sessions, you would accumulate 40% of progress over forty days (2% every 2 days). The practical message—if you can find a method which allows you to train more frequently, you will end up making more progress in the long run. In terms of resistance training, isometrics is the ultimate exemplar of this kind of method. Isometrics and fatigue You can perform isometrics sessions more often than is the case with traditional dynamic training methods. This is because dynamic resistance training takes much longer to recover from than isometric training. There is a general (although, by no means universal) consensus in modern training that a muscle should be worked by regular, dynamic exercises no more than every 48 hours. In fact, even this may not be long enough to fully recover.^ Many elite powerlifters train each lift hard only once per week.2 There may be some science behind their programming: some studies indicate that muscle recovery still hasn't taken place 96 hours after hard training.^ Contrast this with isometric strength training. The most famous comprehensive scientific analysis of isometrics was undertaken by the German physiologist Theodore Hettinger M.D. After exhaustive experimental scrutiny, Hettinger concluded that not only could subjects easily recover from optimal levels of isometric training in under 24 hours, but that daily training was ideal in terms of strength-building.^ In fact, not only did the subjects in the German studies recover adequately from daily isometrics, their strength levels improved at a seemingly superhuman rate of 5% per day!12 Follow-up studies have corroborated Hettinger and Muller's findings regarding isometric training frequency.^ DOMS, sliding filaments and popping sarcomeres There are several factors involved in inter-session recovery. Joint pain (see chapter 4) and psychology^ are two examples, however the phenomena of DOMS discussed above is perhaps the most significant; the reduced-strength effect, as well as the associated pain, can significantly hamper training efforts, thereby necessitating extended periods between training sessions.  DOMS is the result of microtrauma—damage to the soft tissues on a cellular levels It used to be believed that lactic acid buildup—the kind that makes you feel the "burning" in your muscles during hard exercise—was to blame for DOMS. We now know that this is not the case One popular current model which explains DOMS is the popping sarcomere hypothesis.^ Sarcomeres are the little machines in your muscles which allow them to contract. They are much, much smaller than muscle fibers. Your muscles are made up of tubular fibers (called myocytes), which in turn are made up of protein-based chains called myofibrils. Myofibrils are made up of tiny sarcomeres. (A single muscle fiber may contain 100,000 sarcomeres). When you contract your muscles, filaments of protein in the sarcomeres shorten, sliding over each other. This is called the sliding filament theory of muscle contraction.^  SARCOMERE (a) svva 11 Z-band M-band Z-band (b) Titin Thick filament Thin filament Sliding filaments are what cause muscle contraction^  If the muscles begin moving under a heavy enough load, the filaments in the sarcomeres are unable to maintain their integrity and they "pop". This mechanical damage disrupts the calcium homeostasis inside the muscles^ and results in inflammation.^ As the hours pass, a chemical cascade follows which sees prostaglandins released, which sensitize the nociceptors—pain receptors—which results in the soreness. The soreness tells us to "back off" as the damage heals. Movement under load is the cause of sarcomere damage It's clear from modern studies that contraction—mechanical tension—is what builds strength and muscle. However, the kind of microtrauma which causes delayed onset muscle soreness and hampers recovery is not caused by contraction or tension—it is caused by movement under tension. In particular, the science seems to indicate that eccentric movements should take the majority of the blame here. It's been well-established for decades that the lengthening phase of exercise does the bulk of the damage, literally tearing sarcomeres apart.22 In fact, heavy eccentric (negative) contractions can cause so much damage that muscle function is reduced for over two weeks following training.^ By contrast, isometric holds cause minimal muscle damage or post-exercise soreness.^  Damage of muscle following eccentric exercise (via electron microscope).^ The lengthening movement literally shreds muscle fibers up—is it any wonder that it takes so much time to recover from heavy weight-training? Concentric movements—although they can cause soreness by themselves^—do not seem to cause nearly as much damage. Unfortunately, it's almost impossible to use only concentrics during a workout, since we typically have to lower weights (eccentric movements) every time we lift them. (The only exceptions would be if we had training partners lower our weights on every rep; or if we could literally drop the weights after every rep—not too sensible during a bench press.) Even if we could only use concentrics during heavy training, some researchers suggest that eccentrics would inevitably begin slipping into our movements, particularly as we tire.^ During isometric training, there is no need for high-load, long-stroke eccentric motions—after a contraction, the athlete simply relaxes his or her muscles. No heavy weight needs to be lowered. As a result, isometrics is an incredibly pure method of training, and the lack of movement is what results in such a low level of damage and soreness.^ With isometrics an athlete has access to maximum intensity muscle contractions—the optimal trigger for strength and development—with minimum microtrauma or post-exercise recovery. The best of both worlds. The Fenn effect and recovery Another reason why isometric training doesn't leave athletes so fatigued following a training session is down to a little- known but well-established quirk of muscle energetics called the Fenn effect. This effect was discovered in the 1920's by a brilliant biologist, Wallace Osgood Fenn, who, as well as being chairman of the University of Rochester Physiology Department, was head of the American Physiological Society. Fenn's work was grounded in complex thermodynamics applied to muscle contractions; in brief, Fenn proved that concentric muscle contractions require more energy (up to triple the energy) and produce significantly more internal heat than equivalent isometric muscle contractions.^ Over time, physiologists came to realize that this effect has drastic consequences to muscles on a cellular level; for example, the energy utilization of dynamically-trained muscles means that they not only burn up more precious intracellular resources, they also produce far more waste buildup than statically-trained muscles. For example, the rate of ATP (adenosine triphosphate) breakdown in moving muscle contractions is up to three times greater than that for isometric contractions.^ This in turn results in greatly increased production of metabolites such as lactic acid, ADP, and chloride, among others.^ These waste products must all be gradually removed by the system, to keep the muscles functioning.  The Fenn effect essentially means that dynamic contractions are more costly to the cellular environment of the muscles, from a viewpoint of chemical energy, recovery of resources, and detoxification of waste buildup. This explains why an isometric training session—even one exploiting very heavy loads—typically leaves athletes feeling fresh and ready to work out again, compared to similar sessions with conventional dynamic lifts, which tend to induce immediate fatigue. Science and experience > subjective prejudice As with anything in any literature, there's no need to believe any of the studies quoted here. You can easily explore this issue yourself. Perform an isometric workout, and see how you feel in 24 hours. Certainly, once you have accommodated to isometrics, you will feel virtually no soreness, and experience zero functional impairment. If you are well conditioned, you actually feel fully recovered ten minutes after training—in fact, isometrics usually leave you energized and stronger than before you performed them.^2 This is a far cry from the exhausted, beaten-up feeling many athletes experience after a hard session with the weights. Unfortunately, the absence of the "beat-up" feeling traditional dynamics confers may be one of the reasons so many trainees are prejudiced against isometrics. If the training doesn't leave me exhausted and sore, it can't be effective, they reason. This old-fashioned notion—no pain, no gain—is a fallacy based on subjective psychological bias rather than science. Research has established that soreness and exhaustion after a workout are not related to productive results, either in strength gains or hypertrophy....it is commonly believed that muscle damage is necessary for size and strength gain. However, it seems unlikely that muscle damage and/or muscle soreness are essential for muscle adaptation. -Nosaka, K. et al. (2003) Muscle Damage in Resistance Training—is muscle damage necessary for strength gain and hypertrophy?^ Soreness following a workout is the result of microtrauma: damage, not stimulation. If this idea seems initially counterintuitive—I'm sore, it must have been a good workout—take a moment to reflect on the following: after running a marathon or performing three hours of flexibility exercises, an athlete will likely become terribly sore, and feel drained for days. They will not, however, gain any strength or muscle. Soreness and fatigue may make us feel like we've worked hard, but in reality, they are not good indicators of productive resistance training. The only reliable indicator is the ability of the athlete to handle progressively increased load—and all the studies prove that isometrics is unbeatable in this regard.