Your core muscles are a complex group of muscles. Excepting your arms and legs, everything else is called the core. They are composed of glutes, hips, abdominal muscles, inner abdominal muscles, pelvic floor, and scapula. They are responsible for your movement, transfer of strength and stabilisation. Having a strong core is essential to physical function. Unlike your arms (biceps?) they are not easily visible. Despite this, you want to work to build a strong core especially as you age.
Trainers have long advocated a plank for building your core. There are other interesting exercises that also help.
Bird Dog: In this form of exercise, you get on your hands and knees and then stretch one hand and leg. With your arm and leg outstretched, you hold the position for up to 10 seconds and then repeat this 10 times for a total set of 3.
Band Anti Rotation: in this exercise, you kneel on one leg with the other leg outstretched. Using a cable with weights attached, you pull into your chest. Breathe out while you pull in and hold for 10 seconds. Repeat 10 times for a total set of 3.
Dead Bugs: In this exercise you lie on your back. Start with folding your legs and arms into a locked position and then alternate kicking one leg in while you stretch your hand. Alternate with the other hand and leg for a total of 10 reps and a set of 3.
Tummy Vacuums: In this exercise you get on your hands and knees. Make sure that your spine is in a neutral position. While exhaling, draw your stomach in. Hold your stomach in for as long as you can and then release. Repeat 10 times for a total set of 3.
These exercises will help you improve posture, reduce back aches and improve your gait.