Baseball and softball players come in all shapes and sizes. From 5' 5" Freddie Patek (a 3-time All Star) to 6'11" Randy Johnson, Babe Ruth to Albert Pujols, or Crystl Bustos to Cat Osterman, you get all body types. As I've stressed repeatedly, the metabolic demands of these games are minimal. High level players do not have to be in top physical condition to perform well.
Yet this "distinction" is pretty unique in the world of sports. Most basketball players are tall and fast. Most football players are big and muscular. Most gymnasts are small and muscular. Some pretty specific body types are required for participation in these sports.
In baseball and softball, however, most players are of pretty normal size. The average MLB player (all positions) is 6' tall and weighs about 190 lbs. Larger than the average human being, but not very big in the world of professional sports.
For the most part, at any level of play, there is not a great deal of difference between the skill level of the teams and individual players involved. There are a few superstars, a LOT of "average" players, and a few below average players. The typical Bell Curve distribution.
What can help separate the performance of individual players and teams is appropriate strength and conditioning. How? Take two players of approximately the same skill-level, with one of them doing appropriate in-season training and the other not. Which one will most likely perform better on-field? Obviously the player (or team) that has maintained his/her strength and stamina levels.
Being better conditioned allows a player to more consistently, and for a longer timeframe, properly repeat the physical mechanics required for their position
Pitching is the most active position in both baseball and softball, with catching closely behind. While the act of pitching does not require a high level of conditioning for high level performance (see Bartolo Colon and David Wells, amongst others), sport-specific conditioning does enhance performance and prevent injury.
No one with any knowledge of physical training principles would debate that a strong and flexible muscle/tendon is more injury-resistant than an untrained one, and will usually out-perform an untrained one.
Interestingly, pitchers who are heavy and apparently not well conditioned are fairly commonplace, but you rarely see players at other positions similarly out of shape. And while pitchers work much harder during the course of a game than any infielder or outfielder, it is not uncommon for one or more players in the IF or OF to go through an entire game without having a single ball hit to them. Their greatest exertion occurs when they jog in and out to their position on the field! Regardless of these anomalies, there's no reason for any ball player to be less conditioned than a pitcher! And you'll find that most ball players are in good physical condition.
STRENGTH & CONDITIONING:
Numerous games and practices leave little time in the average player's in-season schedule for conditioning, and conditioning work is usually not much fun. Yet its importance cannot be dismissed. When is the most important time of the competitive season to be at your best?
At the END of the season, when you and your team are fighting for a playoff spot, and when you actually make the playoffs!
This is where the payoff is, be it money, prestige or other rewards!
Given the time constraints of the regular season schedule, the first thing many players and teams drop is any conditioning work they may have been doing. So any strength and conditioning gains that were made in the off-season slowly diminish over the course of the competitive season, leaving all such players and teams similarly worn out. We can assume their relative skill levels remain unchanged.
MLB PLAYERS AND DRUGS
Baseball seasons can be lengthy. Regardless of age or ability level, the grind of a long season takes its toll, physically and mentally. This is among the reasons some MLB players have resorted to pharmaceutical assistance via steroids and, to a far greater extent, amphetamines.
Known as "greenies" or "beans" to ball players and "speed" to the general public, they're illegal for use without a doctor's prescription by the Controlled Substance Act of 1970. Some estimates of amphetamine use by MLB players run as high as 80%. Curiously, this widely acknowledged use of greenies isn't talked about much.
In 1970, Jim Bouton in his classic book, "Ball Four," first exposed greenie use, much as Jose Canseco's "Juiced" has done with steroids. Back then, Bouton's account of greenie use was taken seriously enough to merit a trip to the commissioner's office.
Amphetamine use is now so widespread that there is even a term for those players who choose not to "bean up" - it's called "playing naked." Peer pressure is reportedly so great to take greenies that some players who choose not to partake are accused of "letting the team down."
Players tak these "ergogenic aids" in the hope of maintaining energy and boosting performance all season long. One MLB manager has suggested that if greenies are banned (as recently proposed by MLB Commissioner Bud Selig) the season would have to shortened. It is much easier, of course, to "bean up" than it is to put in the hard work of daily conditioning. But even with steroids, you still have to spend some time in the weight room.
The future of professional baseball?
Often, with the increased muscle mass and statistics comes injury and time on the disabled list. Players wrongly assume that all they have to do is lift and get big, and thereby their numbers will improve. Unfortunately, they often find that their improperly structured, steroid based workouts produce muscles that quickly grow and become stronger than their tendons, and problems result.
This can, of course, happen without the use of steroids. And most players can get appropriately stronger without resorting to illegal substance use. So why take the risk to health and reputation? There are a number of reasons, most of which I don't want to get into here. Here are two that I will mention:
1) While everyone is familiar with steroids' anabolic (muscle building) effect, they generate another lesser known but highly significant benefit: anti-catabolism. Catabolism is the breakdown of muscle tissue in the body. During physical activity, muscle fibers are damaged at a micro level, putting them in a position to either be re-built both bigger and stronger (with proper rest and nutrition), or subject to the effects of catabolism.
Steroids allow one to work out longer and harder while mitigating this anti-catabolic effect. In other words, steroids can help keep a player's body from breaking down over the course of a long season. Users stay stronger, longer, than non-users - a significant competitive advantage during the playoffs. Non-users are therefore more subject to the ravages of catabolism which is the natural consequence of any type of physical exertion.
Greenies similarly aid ball players by delaying the onset of fatigue by stimulating the central nervous system. Similar to caffeine, but far more powerful. Oddly, MLB banned another stimulant, ephedra, after the 2003 death of Baltimore Orioles pitcher Steve Bechler. who was using it during spring training to help with weight loss.
Comparatively, ephedra is a much safer compound than amphetamines, yet MLB "looks the other way" rather than ban them and test for them.
2) MLB produces a LOT of money for a lot of people, communities, and industries. For this reason the season will never be shortened. This simple fact, coupled with the lame drug testing system currently in place, means the temptation to cheat will remain high, regardless of who gets caught.
Among the lessons we learn from all of this is that
Players resort to drugs to enhance performance because a long season of games and practice wears the body down, or de-conditions them!
De-conditioned players do not perform well, which costs them money come contract-renewal time. As opposed to appropriate strength and conditioning work, however, amphetamine and steroid use are a significant detriment to health. Until recently, the incentives to cheat have been greater than the penalties for getting caught. For that matter, it wasn't even possible to get caught!
Perhaps the steroid-related suspension of Rafael Palmeiro will change things, but I doubt it. Despite his numbers, his Hall of Fame candidacy is now seriously in question. He will likely become better known for his steroid use than for his on-field accomplishments. And, given what we know of the the side-effects of steroid use, we may now have a bit more insight into why Palmerieo endorsed the drug Viagra...
WHAT HAPPENS IN A GAME: ANECDOTAL OBSERVATION
Let's examine what the most active player, the pitcher, does during a game. First, s/he winds up and delivers a pitch, which takes about 2 seconds. The ball is thrown back, and then the pitcher does various things to get ready to throw his/her next pitch. This can involve checking the scoreboard, checking any runners, checking the position of fielders, looking at coaches, getting the sign from the catcher, etc.
Let's say all of this takes about 20 seconds. If a pitcher throws an average of 15 pitches/inning, an inning can last 5 minutes or so. This is obviously oversimplified, as there are a number of things that can take place to change this time frame. But for our discussion here, these numbers will suffice.
So, the pitcher performs an explosive activity for a very short period of time, then rests for a time frame that is about 10 times longer. S/he is, in effect, performing a type of activity known as interval training, with a work:rest ratio of 1:10.
Then, this pitcher sits down between innings for the same 5 minutes (or more) while the other pitcher performs. Pitchers will, of course, expend some additional energy fielding their position, and may, depending on the level of play, also hit and run the bases. Ultimately, though, not much work and a lot of rest.
Clearly, baseball and softball are games of intermittent, not continuous, activity. This is why aerobic training is of little value to a pitcher, or any ball player, for that matter.
Let's face it - while it may be true that hitting a baseball is the single hardest thing to do in all of sports, you don't have to be in very good physical condition to do it. The reason is that the metabolic demands of baseball and softball are fairly minimal.
WHAT HAPPENS IN A GAME: CLINICAL RESEARCH
An interesting study was performed by Potteiger, et al  in which they measured several physiological and bio-chemical responses experienced by pitchers during the course of pitching a game. Six college-age men with varsity pitching experience at the collegiate level participated in the study. They pitched a simulated game of seven innings, with 14 pitches per inning.
The subjects were given a six minute rest period before beginning their next inning, and were encouraged to exert their normal level of effort with each pitch. The following indices were measured, pre and post performance:
In addition, 24 hours later, serum creatine kinase (CK)* and lactate dehydrogenase (LDH)* were analyzed as an indicator of skeletal muscle damage. For our purposes in this report, the items marked with the (*) are the ones I want to discuss further.
1) Lactic acid levels were unchanged between pre-exercise values and post game measurements.
2) VO2 was equal to 45% of maximum.
3) A significant difference for both CK and LDH existed between the pre-exercise and 24 hours-post-exercise values.
Lactic acid levels did not change through the course of the game from their
pre-game values. This makes sense, because pitching is not the type of
activity that generates lactic acid. The burning feeling one gets in the
muscles during intense weight lifting or sprinting is never felt by a pitcher.
At a minimum, any lactic acid that may be produced during the act of pitching is easily cleared by the body, so its production is not a limiting factor in pitching performance.
The phosphagen energy system provides energy for sports activities that last several seconds and is involved in all forms of exercise regardless of intensity . This includes baseball and softball pitching. Phosphagen replenishment occurs within 20 to 30 seconds. This means that after 20 seconds of rest following a pitch, the main energy system used for pitching is re-charged and ready to go again. This allows for the many and repeated pitches thrown by baseball and softball pitchers.
2) VO2, expressed as a percentage of maximum uptake, means that the pitchers were working at a level equal to 45% of their capacity - not very high. According to the authors, it is believed that this level of oxygen uptake during the work period (pitching) is for the purpose of resynthesizing ATP and PC (phosphagen) stores.
What this means in a practical sense is that oxidative (aerobic) activities contribute little to conditioning for the act of pitching. Excessive aerobic training, such as long, slow distance (LSD) running or lots of time on an exercise bike will allow a pitcher to perform at an even lower percentage of VO2 max. This will help the pitcher who decides to enter a 10K or marathon, but will do nothing for their pitching performance. Aerobic capacity is certainly not a limiting factor in pitching performance.
Aerobic training can be useful as the foundation of a ballplayer's conditioning regimen, but spending more than 10-15 minutes, 1-2 days/week for this purpose is a waste of valuable training time. One exception to this rule would be after an extended pitching appearance.
An easy "flush" run of about 15 minutes duration (no more) can help in recovery, generating increased blood flow to deliver needed nutrients to the damaged muscles and tendons that have been strenuously exerted, as well as helping to "wash out" any waste metabolites generated during this physical activity.
A better choice is sprint-intervals, which help strengthen a pitcher's legs while causing increased blood flow for this "flushing" purpose.
Long slow distance running is also a very poor way of developing leg strength, something that is valuable for a pitcher to have. The act of pitching occurs in a very short timeframe, not continuous and long lasting as occurs with LSD running. Pitching is more analogous to a single rep of a lower body plyometric exercise - quick and explosive.
Consider the muscular development and strength of the legs of marathon runners versus 100 meter sprinters. Baseball & softball players are aiming for something in between these two extremes. Finally, LSD training can detract from strength and power development, so keep this activity to a minimum as discussed above.
3) CK and LDH values were significantly elevated 24 hours after exercise. According to other investigators, this may be an indicator of skeletal muscle damage. In fact, LDH levels in the blood are used to make a diagnosis of heart attack, which is damaged heart muscle. Eccentric muscle contractions (muscle contraction while the muscle is lengthening) that commonly occur during pitching cause greater mechanical stress leading to an increased release of CK and LDH enzymes into the blood.
Eccentric contractions occur during pitching in two main muscle groups: A) The muscles of the legs contract eccentrically after pushing off the pitching rubber and completing the stride. B) The muscles of the arm contract eccentrically (particularly the shoulder - external rotators) during the deceleration (after ball release) phase of the pitch.
The pitchers in this study each threw 98 pitches, a not excessive number. Yet this workload was rigorous enough to elicit elevated readings of indicators of muscle damage. Clearly some rest is warranted between extended pitching appearances.
Continuing to work at maximum or near-maximum intensity not only puts the body into a catabolic state, it also does not allow time for rest and proper nutrition to make their contributions to the recovery process. The body actually develops and adapts to the stimulus of a hard workout (like pitching) during the REST period between bouts of strenuous activity.
IN-SEASON STRENGTH & CONDITIONING: WHAT KIND AND HOW MUCH?
Some points to keep in mind in designing your conditioning program:
1) Any kind of conditioning will diminish over time if it is discontinued.
2) Skill work (drills, mechanics, etc.) is the priority during the season, in practices and games.
3) To improve performance in a particular sport, the training must be specific for that sport.
According to Baker  there are three types of resistance training:
General - exercises that increase the overall maximal strength of the muscles.
Special - exercises used to train for power development, also known as explosive isotonic exercises. Examples of these lifts include power cleans, snatches, pulls, and push presses. The training effect of these exercises is to convert general muscular strength to the special quality of power that is relevant to throwing.
Specific - exercises designed to follow the concept of specificity by providing a training stimulus that is very similar to actual motion in competition. Examples of these exercises for baseball-throwing motion are weighted baseballs, surgical tube exercises, and the Exer-genie cord. These exercises attempt to mimic the high-velocity ballistic throwing motion.
And I'll add one other:
Cross Training - defined as two or more different types of exercise performed in one workout or used alternately in successive workouts to achieve a specific training goal. One example would be training for the development of leg strength by running sprint intervals in one workout, and lower body weight lifting in another. Or, as in the case of Philadelphia Eagles quarterback Donovan McNabb, throwing a 5 oz. lacrosse ball instead of a football as part of a specialized footwork drill.
I list all of these seemingly obvious ways to condition (and there are others) to make a simple point: there are a number of legitimate ways for ball players to train so as to achieve a particular outcome, be it injury prevention or performance enhancement.
Why perform strength training? Because adequate levels of strength should be developed before performing more rigorous training such as power (plyometrics) work. Strength is the foundation of power - but these two attributes, while related, are not the same thing. Power development is trained for differently than strength development.
POWER = STRENGTH X SPEED
More specifically, Potteiger suggests that pitchers train primarily for power development:
such as sprint and interval training, resistance training and plyometrics
Finally, Gambetta makes a key distinction regarding the activities that can be used in training (emphasis mine):
"Another important rule to understand is the difference between similar movements and same movements. For example, it is common to see pitchers and quarterbacks throwing from their knees with the stated goal of improving arm strength. But from a biomechanical perspective this may be counterproductive.
Throwing involves the whole kinetic chain, and taking large segments out of the action will interfere with timing and could affect coordination during the complete throwing motion. Throwing from the knees is similar, but not the same, as throwing with the legs in the equation.
"How about the use of underweight and overweight balls for a pitcher? In this case, we found through biomechanical analysis that there was virtually no difference in biomechanics between throwing an underweight or an overweight ball, as long as they were not too heavy or too light. Therefore, this is a viable training activity that is bio-mechanically the same for the pitcher." 
As for how much in-season conditioning, players will have to determine this individually. Doing so will require some experimentation and guidance from coaches and strength professionals. Appropriate workouts will include:
Factor in the number of games and practices in a week, a player's conditioning needs, energy level, position played, nutritional status, school, work, etc. More information on appropriate training for ball players can be found here.
Separate yourself from the crowd - develop and diligently follow a baseball or softball specific strength and conditioning program. You'll reduce your chance of injury and improve your on-field performance!
 Potteiger, J.A., D. L. Blessing, and G. D. Wilson. The Physiological Responses to a Single Game of Baseball Pitching. Exercise Physiology Laboratory, Department of Health and Human Performance, Auburn University, Auburn University, Auburn, AL. Journal of Applied Sport Science Research Vol 6, Number 1, pp. 11-18. 1992
 Brooks, G. A., K. J. Hittleman, J. A. Faulkner, and R. E. Beyer. Temperature, skeletal muscle mitochondrial functions and oxygen debt. American Journal of Physiology, 220: 1053-1068. 1971.
 Baker, D. Improving vertical jump performance through general, special, and specific strength training: A brief review. Journal of Strength & Conditioning Research 10:131-136. 1996
 Gambetta, V. Connecting the Dots. Training & Conditioning p. 26. July/August 2005
2004, Baseball Fit, LLC. All Rights Reserved. Quotations with
attribution permitted. Cite source as Steve Zawrotny's BASEBALL FIT Hitting
& Pitching Academy - www.BaseballFit.com