Isometrics are a great way to strength train
Isometric means that there is no change in the length of the muscle.
Add a couple of these exercises into your workout to switch up your routine.
You will be strengthening the major muscle groups as well as all the smaller stabilizing muscles.
Plank with Glute Squeez
Isometric Lateral Raise
Isometric Wall Sit
Do each move for 60 seconds, rest for 30 seconds and repeat 2-3 more times
Then move onto the next exercise.
[for full descriptions of the exercises go here]