Knuckles the Echidna™Knuckles' trademark ability is his immense superhuman strength, so vast that he can shatter boulders, punch holes through thick steel and lift objects several times his own size and weight with ease. Knuckles' strength is said to be equal to Sonic's speed (Mach 1-5 on average), meaning he is capable of lifting and pressing anything between 100 to 500 metric tons, which makes him one of the physically strongest characters of the series. Knuckles' strength seems to lie in his upper body, particularly in his arms and fists, the latter of which are noted to be as hard as rock. 
Another testament to his brute strength is that he can punch or throw objects through air with such force that he can ignite fire, and create fissures, tremors and even small volcanic eruptions by punching the ground. He can even strike with such force that it causes the friction in the air to create explosions with enormous power. He can bury himself and attack his enemies from above.
Like most other character in the series, Knuckles is quite fast on his feet, being able to run at super speeds, even being able to keep up with Sonic and Tails' running speed. However, he is not quite as fast as Sonic or Shadow at their maximum speed and he runs fairly slow by comparison. Additionally, his high speed does not seem to extend to his upper body, and he has been noted to have slow movements in combat. Over time, Knuckles began to lose some of his speed, but also gain a lot more power.
Knuckles also possess near-maximized physical endurance to match his strength, and is capable of taking as much major punishment as he is able to deal out, even being able to survive a bomb exploding right in front of him. Knuckles has as well enough stamina to hold out against an army of robots without rest.
Unlike Sonic, Knuckles is a very experienced swimmer, due to his interest of being a treasure hunter, and can swim to incredible depths and with both great speed and skill. Knuckles is also a proficient burrower, able to dig into any wall or floor and excels at tunneling through subterranean regions with great speed.
Knuckles is also shown to have sharp reflexes and acrobatic skills, being able to do special tricks and movements while in mid-air, though he is not nearly as acrobatic as Sonic or Shadow are, nor can he jump very high. To make up for his lack of agility, however, Knuckles possesses a wide range of movement abilities. By stretching his arms and legs outwards while airborne, Knuckles can catch the wind under his dreadlock-like spines, allowing him to glide long distances through mid-air and can do so with great control over his movements. Also, by using the spikes on his fists, Knuckles can attach himself to walls and climb up any kind of surface with ease, no matter the material.
Knuckles has the uncanny ability to sense the chaos energy of the Chaos Emeralds and the Master Emerald, or similar objects like Sol Emeralds, and can track down their "signals" which makes him a living Emerald Detector. Like Sonic and Shadow, Knuckles can also harness the power of the Chaos Emeralds to empower himself.
Being the guardian of the Master Emerald, Knuckles is a very powerful fighter. He is a master in a number of martial arts, focusing on hand-to-hand combat, and is capable of taking on even the most powerful of opponents without rest, such as when he was able to take down several of the military trained Nocturnus Clan soldiers. Even more so, he has been able to fight Sonic the Hedgehog to a standstill on several occasions. Also, in Sonic Chronicles: The Dark Brotherhood, Tails noted that anyone to beat Knuckles in a fight must be very powerful, a testament of his fighting abilities.
When in battle, Knuckles makes most use of his powerful arms and fists to take on his foes, with his specialty lying in punching moves where he uses his spiked fists to their fullest (there are some cases where it resembles a form of boxing). In terms of fighting style, Knuckles' focuses specially on power, using strong and powerful punches, such as powerpacked jabs or piercing uppercuts, to deal damaging blows to his opponents. Once he catches an opponent in his combos, he keep them trapped by keeping up his strikes, not allowing his opponents a chance to counterattack. For stronger strikes, Knuckles tend to build up momentum with his fists by swinging them around, before hitting his opponent with full force. In his Major Eruption move in Sonic Battle, Knuckles takes advantage of his burrowing abilities, hiding underground and hitting opponents with an uppercut as he leaps out when his opponent least expects it.
Despite being a powerhouse in close combat, as stated by Sonic, most of Knuckles' fighting moves are slow and easy to read, making Knuckles more vulnerable to those who give themselves time to read his moves.