The Stamina Bar is generally implemented as a secondary or tertiary game mechanic to standard health and mana bars. It should be noted, however, that some games refer to what is typically considered the "health" or "life" bar, as the stamina bar. This is either due to poor localization or design choice.
The Stamina Bar in the correct sense, however, does not measure a players life power or magic reserves. Instead, it is a more malleable concept of power or endurance.
Sports games commonly use stamina bars to represent fatigue. This is a natural choice considering the nature of human physical abilities. Even within this subcategory the use can be broken down again in several ways.
Some games use a stamina bar to simulate human limits and provide an element of strategy in an otherwise straightforward, twitch based game. For example, in Track and Field games, the distance running events often have stamina bars. While sprinting to the front of the pack is effective in shorter races, longer ones require a strong steady pace, with a dedicated push towards the finish. The player must estimate how much stamina he or she needs to reserve throughout the race, and adjust his or her speed accordingly. This adds an additionally element to what otherwise might be a single button gameplay mechanic.
The second major way in which sports games implement stamina bars are implemented in sports games is as a balancing mechanic. In real life, players in team sports must take a rest during games. In the Madden Series of Football games, this is used to prevent a single player, such as a star running back, from dominating the game. After each play an inset graphic appears on the play-calling screen, indicating which offensive players are tired. If a player carries the ball on several plays in a row, his stamina bar will decrease, indicating a drop in stats. This forces the player to make strategic decisions on player management, and also prevents a player from repeatedly using one star player to win the game.
Other common examples are pitchers in baseball games. As their stamina decreases, so does the speed of their pitches. Hockey games also have stamina bars for each "line" of players, and as the stamina bar decreases, so does the players, until a line change is made.
Fighting games will also use stamina bars in addition to health bars. The first is, similar to the football example described above, as a way to limit players from "spamming" or repeatedly using, the same move. Performing attack moves will deplete a fighter's stamina bar, and only by dodging or blocking can the fighter replenish it. This prevents a player from "turtling" or hiding in the corner spamming an attack move, preventing the opponent from getting close. Typically, the more realistic a fighting game aims to be, the more likely it will have a stamina bar used in this way. Boxing games commonly limit a players offensive punches in this way.
The second way fighting games will use a stamina bar is more complex. A stamina bar will be used almost as a second health bar. A player can typically use offensive moves that damage either the health bar or stamina bar. As the stamina bar decreases, the player does not lose health, but will become more susceptible to health lose or submission. Each game handles this in a different way, but the following a few illustrative examples. Wrestling games will allow for win by submission or knock out. Punches and kicks will typically lower the health bar, but also the stamina bar. The player may attempt to lower the stamina bar, and then try to pin the opponent. Final Fantasy Dissida also uses a similar setup. Attacks damage either the health bar or the stamina bar, however lowering the stamina bar then causes health based attacks to do more damage. This means a player may find it better to spend time lowering the opponent's stamina, then going for one killing blow. The trade off is that in lowering the stamina bar, no actual damage is being done.
Role Playing Games and MMOs
Role Playing Games often use stamina bars. This is especially common for online games as well. Stamina bars are used as a way to limit players ability to do repeated actions. In online based games, this is typically a way to prevent macros, bots, or other devices from completing basic tasks while the user is not controlling the game. These stamina bars decrease with every action, and require the player to periodically rest. Resting may simply be kneeling on the ground for a period of time, however when a player's stamina bar empties, no action are possible.
This is so highly prevalent in online role playing games due to the repetitive grinding nature. For example, players may need to kill many thousands of mobs to gain a level, so some players may try to take advantage by parking their character in a low level area, and setting them up to simply attack anything. Since they are strong enough to not worry about dying, the user can leave the game on for many hours without interacting, and return to find their character has gained several levels in the time they were away. Typically, this comes into play with resource gathering, such as fishing. A stamina bar means a user will only be able to cast for a period of time before the game requires the user enter a rest command, then restart the process.
Newer role playing games have become increasingly sophisticated in this regard. For example, Final Fantasy XI set some tasks, such as fishing, to deplete stamina since they were prime for abuse, but left other tasks, such as traveling around the world, to not deplete stamina, as this was not prone to abuse and made it easier for the player to travel without interruption. Also, the stamina bar decreased with a measure of randomness, so it was impossible for a bot to predict how long it would need to rest each period.
First Person Shooters
First person shooters may use a stamina bar to add a level of realism. For example, it is common that players will only be able to sprint or run for a limited period of time. The game RTCW interestingly also uses a stamina meter for jumping as well as running, limiting the players ability to effectively perform these maneuvers.
FPS games also borrow a stamina related tactic from RPG games where inventory items each have a weight, which decreases the amount of stamina a player has.
STALKER SoC makes use of this, a player with a full inventory is only able to run for a few steps before becoming tired.
Strategy / stealth games
In these games the stamina bar is used when the character does great physical efforts like climbing a cord or a wall or doing moves that requires a lot of strength to be maintained during a long period of time, if the bar is depleted, then the character stop what he was doing, and in the case of where he was climbing a cord he can fall from a n high height and take severe damage, and in the case of a stealth game or a game with a stealth component make noise and take the risk to attract nearby foes.
Other Reasons for Use
Stamina Bars have other reasons for use. A fine example is later Metal Gear Solid games. In these games, it was noted that killing was not always the best option. This was at times a moral decision, as the player found himself confronting neutral third parties. Therefore, it was decided to make an option for knocking out, or non-lethally incapacitating, the enemy. To this end, bosses have both a health and stamina bar. Some attacks, such as guns and grenades, cause damage to the boss's health bar. Others, such as tranquilizer darts and stun grenades, cause damage to their stamina bar. Finally, kicks and punches may damage both bars. The player is given the choice of doing the "morally correct" action of simply stunning the boss, and preserving her life, or killing the boss, the "morally incorrect" choice. Typically, using the stunning option is also harder for the player to complete, as opposed to simply shooting and killing the boss.
Some game use stamina bars as a way to make the game less violent. For example, the Pokemon series uses stamina bars the same way most games use health bars. When the stamina bar is depleted, the Pokemon is said to have "fainted", although the effects are the same as death in most other games.
The poorly localized Monster Rancher for PSX used a type of stamina bar called "guts" or "wills." As the "Guts" or "will" meter increased, attacks did more damage. Some monsters had fast "guts" regeneration, enabling them to attack more often (but usually they had poor accuracy and hit less).