We are creatures of habit. Once we become good at something (especially in the fitness arena) we tend to continue doing the same thing. No one wants to be "bad" at something. The problem with this is that it might be what's holding us back from improving in our sport/s.
Here is a list of things to AVOID when strength training. By avoiding these mistakes, you will be able to improve your performance and take your fitness to new heights.
Only training upper body
Don't get too focused on how you look in the mirror. Having big arms and a broad chest may impress the girls and look good on the beach, but it won't do anything for your athletic performance. Your largest muscles are in your legs (including the glutes and hamstrings), and the more you strengthen them the more hormones you release to help build strength and size in your entire body. Your legs are what provides the foundation to any sport-specific movement.
Always working on your strengths
It's easy to work on things you are good at, but that won't make you a better athlete. In order to improve you have to eliminate any weaknesses, and that means probably doing things you aren't familiar with or good at. Most elite athletes don't have weaknesses, and if they do, I'm sure they are working on them right now. When was the last time you did deadlifts? They are one of the best compound moves (targeting several muscle groups and multiple joints) to help build strength.
Lifting weights too slowly in the gym
Do you always lift weights at the same speed? If so, you are missing a key component to improving your strength gains. According to Mike Boyle, of Mike Boyle Strength & Conditioning, "lifting weights with explosive speeds activates more fast-twitch muscle fibers, which have the greatest growth potential". You don't always have to lift weights with an explosive speed but it should be incorporated in your periodization training. For instance when lifting heavy weights (85%+ of 1RM) you want to lift slow and controlled. When performing certain movements with explosive speeds, do the lifting phase of each exercise as fast as you can.
Doing too much slow and steady running
Running too many slow miles is a repetitive movement and your body will adapt quickly and stop making progress. According to Boyle, "running doesn't activate fast-twitch muscle fibers, which are great fat burners and body builders". Start incorporating some intervals (short bursts of intense activity) in place of your slow runs, or run steep hills instead of always running on flat terrain. You can sequence your interval work many ways, for instance using a 1:1, 1:2, 1:4 or even a 2:1 ratio. Using the 1:2 ratio, this means that you would run hard for a length of time (say 30 sec.) and follow that with twice the amount of recovery time (which in this case would be 1 min.). According to a recent study in the Journal of Strength and Conditioning Research, interval training leads to gains in aerobic and anaerobic performance that are significantly greater than with steady state cardio.
Not incorporating rest into your weekly workouts
It’s easy to push ourselves hard in the gym, or spend hours doing cardio to improve endurance, but the most under-rated area of improving performance is NOT allowing for enough rest each week. The training you do breaks the body down and makes micro-tears in muscle tissue; it’s when you rest that you build back up and with proper nutrition get stronger. The goal of “rest” is to help the body regenerate and bounce back the next day. Rest doesn't have to be lying on the couch and doing nothing (aka: passive rest), it can be something that still makes you feel as though you are still exercising but without the stress to the muscles or mind (aka: active rest). Simple “active” rest ideas include yoga classes, foam rolling and stretching, going for a walk with your dog, or doing an activity different than the particular sport you are training for at a light intensity.
The best type of rest is sleep. Aiming for 7-8 hours of quality sleep each night will help speed recovery of sore muscles from hard training days and allow you to perform at your best day after day. Taking a 20-30 min. nap during the day is also a good option if you are not able to get 7-8 hours of sleep at night.
Aim for at least 1-2 rest days during your weekly training schedule.