The ability to use superhuman strength is one of the characteristics that set vampires apart. Edward’s physical strength is seen in stark contrast to the fragility of human Bella: "You are so soft, so fragile. I have to mind my actions every moment that we’re together so that I don’t hurt you. I could kill you quite easily, Bella, simply by accident… I could reach out, meaning to touch your face, and crush your skull by mistake." (TW14)
His strength leads to one of the tensions in his relationship with Bella, namely that he can never let down his guard for fear he will either accidentally hurt her, or let his vampiric nature seize the blood he so desires. He can certainly move her effortlessly, as we see when he carries her to the school nurse, "as easily as if I weighed ten pounds instead of a hundred and ten." (TW5) Edward’s strength can be most clearly seen in the incident where he saves Bella from Tyler’s truck. Not only is he not injured himself but he even forces the metal out of the way by the pressure he exerts on it. "I had seen the" very distinct dent that fit the contours of Edward’s shoulders…as if he had braced himself against the car with enough force to damage the metal frame." (TW3)
Yet, Edward’s "iron strength" (TW13) is nothing compared with what it would have been after Carlisle first changed him. Meyer states that: "New vampires are also immensely strong for their first year of life, also a product of the excess of blood left in the body." (PC1) This would imply that a vampire’s strength dips before a feed, and their energy/strength levels are refuelled by hunting. When asked about this, Stephenie Meyer explained that hunting makes a "minimal difference" (PC5) to the physical strength levels of a vampire, as even a thirsty vampire is capable of great feats of physical strength. Stephenie Meyer is clear that vampire strength is very much dependent on the individual. Emmett was colossally strong as a human, and even more so as a vampire, but each individual vampire has their own personal range. However, it’s important to remember that, "skill is going to count more than strength," (PC5) so an experienced vampire could still outwit a newborn one in a fight.
A vampire’s diet does not have a massive impact on the levels of strength. "Human blood does make them stronger than animal blood, but only fractionally. Big game (bears, wildcats, predators) makes them stronger than the "weaker" blood of herd animals." (PC5) Ultimately, physical strength is always going to come down to the individual vampire, rather than secondary factors like their diet. Strength, of course, isn’t always physical. It is impossible for a vampire to die through physical starvation (TW15), but what the lack of blood intake does alter is a vampire’s mental state. After Carlisle realised that he had become a vampire, he tried to destroy himself. "He grew very hungry and very weak. He strayed as far as he could from the human populace, recognising that his willpower was weakening too." (TW15)
Vampires are less capable of acting with rational thought, and therefore more likely to act through their natural animal instincts when they are thirsty. This explains why Bella is in so much danger when Edward hasn’t been hunting. Edward does show an inner strength in resisting the lure of Bella’s blood. Edward admits that at times he wondered if he was "strong enough" (TW14) to resist his natural urges. Finally, his inner strength comes to the fore as he admits "I couldn’t live with myself if I ever hurt you." (TW13)