Pack the portable hand strengthener of your choice in your gym bag or briefcase, and work those muscles in your spare time. Adjustable hand grips with selectable resistance levels help you stay challenged as you get stronger. Some hand grips come with metal coils that you can swap out, while others offer a dial-selectable resistance. Look for grips with comfortably cushioned, ergonomic handles to help keep your palms and fingers comfortable through each set of squeezes. Gel hand grips and stress balls are easily portable, and they offer a tactile element that many mechanical grip strengtheners lack. There are putty and gel hand strengtheners that mold to fit the shape of your hands for a more customized feel.