Driving range at lunch

Hit the driving range for a medium bucket at lunch to redeem myself after the round this morning. Just me and another guy. Ground was a little muddy but not bad. Some of the shots actually felt pretty good with the PW and even the 9i but I started to get tired with the 8i. Switched to the driver but gave that up. Hit a few with the 5W to the same result. Tried a few with the Niblicks and that was futile. My lower back just can't do that. Wonder how I can strengthen it? Maybe I just need to hit the small bucket?