The phrase "a sharp back" is correct and usable in written English.
It can be used in contexts where you are describing someone's physical posture or the appearance of their back, possibly in relation to fitness or aesthetics.
Example: "After months of training, she finally developed a sharp back that showcased her dedication to strength training."
Alternatives: "defined back" or "toned back".
He provided a sharp back-heel to Anthony Martial, whose shot was blocked by Jack Butland, and became the dominant player in the second half.
Hawley's guitar moves from dusty twang to delicate chimes and back to the Byrdsian jangle with terse solos that never lose their purpose, all the while backed by a sharp backing band led by the concise, supple patterns of drummer Dean Beresford.
He did some songs with a sharp backing band (anchored by the Roots' drummer Questlove) and others with a D.J. Either way, Jay-Z's delivery was typically sharp, and except for one glitch (the speakers cut out for most of "Frontin'," a duet with Pharrell Williams from the Neptunes), this was about as smooth and as strong a hip-hop concert as anyone has ever managed.
70 min The superb Robben teases Cole and then plays a sharp ball back to Kroos on the edge of the area.
The moves amounted to a sharp scaling back of what had once been a gold-plated campaign.
Analysts caution, however, against expecting a sharp bounce back to the double-digit annual growth rates seen before the global financial crisis.
After weeks of the all-consuming debt debate in Washington, the political conversation is about to take a sharp turn back to the 2012 presidential election as Republicans gather in Iowa for the first real test of momentum and organization.
The comments – made by the company's CEO, Stephane Richard – have emerged amid a sharp push back by the Israeli government against growing calls for an international boycott of Israel over its continuing occupation of Palestinian territories.
Assuming that they are neither planning to raid Social Security nor counting on favorable revenue surprises to save them (unfavorable surprises are, of course, out of the question), they must have in mind a sharp scaling back of government programs.
