Take two groups of guys and have them do back squats. Both groups will use the same weight, bar position, foot position, and do the same number of reps. Now, how can you make one group get much greater activation in the core musculature?
Answer: Change how the weight is loaded.
This leads to drastically different training stimuli. Unstable loading is getting more popular. It's usually accomplished by hanging weight on the bar with bands (see video below with Joel Seedman) or by using a specialized "wobbly" bar.
Let's take a look at a new study on unstable loading.
Fifteen resistance-trained men performed 3 sets of 10 back squats at 60% of their 1RM under two conditions: stable and unstable load. The stable load was a just a normal barbell load. The unstable load had the weights suspended by elastic bands from the barbell.
Subjects were able to freely choose their stance. EMG of the biceps femoris, rectus abdominis, erector spinae, external oblique, rectus femoris, soleus, vastus lateralis, and vastus medialis were recorded. (EMG records the activity produced by muscles.) Peak ground reaction forces were also recorded.
EMG of the biceps femoris, erector spinae, rectus femoris, vastus lateralis, vastus medialis were not significantly different between stable and unstable loading. So your back and quads won't see any activation differences.
But the EMG of the rectus abdominis, external oblique, and soleus were significantly greater in the unstable group. The peak ground reaction forces generated were significantly lower in the unstable load compared to the stable load.
Unstable loading produced far greater core activation at the same level of submaximal loading. An increase of 85.7% in the rectus abdominis and 13.1% in the external oblique was seen. Also, a 72.2% increase in muscle activation in the soleus (the calf muscle used for balance and stability along with plantarflexion) was observed in the unstable loading trial.
If you want to increase core activation, try loading the weights on the bar with bands, but be very conservative when choosing a starting weight. The unstable loading adds a significant level of difficulty.
Note: Because of the mild decrease in peak ground reaction force, unstable loading shouldn't be used for explosive work because it decreases peak force generated. On top of that, common sense tells us that rapid movements of heavy weights swinging from elastic bands is an injury waiting to happen.
- Lawrence MA et al. Effects of an unstable load on force and muscle activation during a parallel back squat. J Strength Cond Res. 2015 Oct;29(10):2949-53. PubMed.