Research has shown that walking with crutches requires up to twice as much energy as normal gait.
While this may be true of an individual using heavy underarm crutches and the "swing-through" technique (more on this hereafter), a lightweight forearm crutch user can be much more energy efficient.
Interestingly, few countries in the Western World still advocate underarm crutches, the U.S. being one of them. Europe as a whole shifted almost exclusively to forearm crutches for both short and long-term users several decades ago.
Physically-challenged individuals who rely on crutches on a day-to-day basis nearly always favor forearm crutches. Aside from their light weight and relative compactness, forearm crutches allow for greater mobility, conform more readily to a user's gait and most importantly, are more energy efficient.