In the strength training world, this is called unilateral training (training one leg at a time, e.g. split squat, single leg box squat, step-up) or bilateral training (training two legs at a time, e.g. back-squat, deadlift). It’s a topic of hot debate on a regular basis, but what are the differences between training methods? Is one better than the other? Quick answer, no. There are those that vehemently defend the need to back-squat and deadlift, whereas others believe you can train effectively without needing to perform these lifts. Let’s look at the benefits and limitations of both methods when training endurance athletes. I emphasise endurance athletes and not team-sport athletes as we require different training adaptations for both these populations.
Unilateral – For example the Bulgarian Split Squat (BSS)
Benefits:
Specificity -
We run, cycle, and jump off one leg. The use of unilateral resistance training is as specific as you can get to replicating the sporting movement when in a gym environment. It makes sense to include this as either your core lift or assistance lifts. Unilateral exercises have shown to develop better strength in unilateral movements (11), and that’s what you need to run and cycle faster.
Balance and proprioception -
Unilateral exercises challenge your balance and proprioceptive abilities. Doing this requires more neural activity and more muscle activation, so you’re working harder, working more muscle fibres and recruiting more motor neurons, creating more opportunity for training adaptations (8, 9).
Use of ‘stabiliser muscles’ -
When using unilateral exercises you place more emphasis on muscles such as gluteus medius. These muscles must stabilise the pelvis when you perform these types of exercises, so you are training these muscles that will be required to stabilise you during running, cycling, jumping, etc. Evidence consistently reports greater activation of these muscles in unilateral exercises when compared to bilateral exercises (2, 4).
Equal work on each leg -
Most people have 1 leg stronger than the other. By using single leg training, you can address your weaknesses. When using bilateral exercises, your stronger leg may take more of the weight, meaning you’ll continue to have this imbalance and not really know about it.
You lift more weight -
A very simple example: let’s suppose you can back-squat 100kg for 10 reps, and let's assume you're also your bodyweight of 75kgs. So that equals 175kg. If you equate the distribution as equal, then you’re lifting 87.5kg on each leg. In a unilateral exercise, the Bulgarian Split Squat (BSS), you can lift 40kg on one leg, plus your bodyweight of 75kgs = 115kg applied to ONE leg. This is a bit of an oversimplification, because during the BSS, the front foot takes around 85% of the load, with 15% going to the rear foot (3). BUT, that still works out at 97.75kgs going to the front foot, 10kgs more than the Squat. That’s 11% more weight. 11% more opportunity to adapt.
You get the idea, it highlights a significant difference between Unilateral and Bilateral exercises...if you can balance!
Limitations
Greater reliance on balance -
While it is good to challenge your balance, it can limit the amount of weight you can lift. You must be safe and controlled to lift heavier weights. If you cannot balance during the unilateral exercises, then the load needs to be reduced, potentially resulting in less of a training stimulus.
Grip Strength -
Somehow, you need to lift the weight, be it on your shoulders or in your hands. Unilateral exercises can often require holding weight in your hand for exercises such as step ups or split squats. As you get stronger, you’ll need to hold progressively more weight. Unless you're training grip strength, your grip can give up before your legs. Placing a bar across the shoulders can be used but this must be done with caution due to the stress placed on the lower back in compromising positions.
Bilateral – Back-Squat.
Benefits:
Stability -
You don’t have to worry about balance as 2 feet are on the floor. You’re more stable, so more force can be applied to the lift, all but guaranteeing a high level of muscle recruitment. Most evidence does show the more weight you can lift, the more training adaptations occur. Maximum weight matters.
Sprint Times -
Research shows stronger athletes in the bilateral back-squat and hip thrust correlates with improved sprint times (12). If you need to move quickly at the end of the race, this is a key factor. Other research does report no difference between modalities (5, 13). Either way, unilateral training never seems to come out on top for increased sprint performance. Back-squat is king.
Improved change of direction speed/ability -
Research is consistent that the back-squat provides better improvements in change of direction speed compared to unilateral exercises (7, 10). This is due to a greater ability to decelerate from speed, i.e. slow yourself down then shift your weight the other way. Now this sounds quite irrelevant for cyclists, and it is, but stronger lateral stabilisers are never a bad thing for riding as it will stabilise the pelvis when you get out of the saddle. But if you are a runner, more specifically, a fell runner, ultra-runner, cross-country runner, then being strong and resilient when changing direction under fatigue is crucial to maintain balance and coordination on downhills and unstable, muddy, gravelly surfaces towards the end of a race. You’ll be quicker over this terrain and be less likely to lose your footing or get injured.
Greater velocity can be used -
This links to stability. The more stable you are, the more speed you can produce, as you recruit a greater number of muscle fibres to perform the task. If you are training to improve speed, then bilateral training can allow you to shift a lot of weight very quickly in a safe way.
Limitations:
Ego lifting -
We’ve all seen it, someone attempting to squat with a massive load of weight on the bar then only managing a few quarter reps. The back-squat is the king of ego lifting, nobody likes being seen with 20kg on the bar honing their technique for 3 months. The back-squat just lends itself to progressing yourself too quickly and trying to impress people. It's hard to resist, but it's a recipe for injury.
Risk of lower back pain -
Lower back pain is reported to be much more prevalent in people who back-squat with up to 46% of lifters experiencing lower back pain (6). However, this could simply be due to the fact there is much less research on those who train using solely unilateral exercises. Either way, it is a risk factor that must be considered. In my experience, those that suffer lower back pain usually squat with poor technique, or too much weight for their ability....usually both!!
Requires very good ankle mobility -
This is the most common limiter of squatting technique. Ankle mobility is essential to ensure the rest of the squat movement is efficient. Without adequate mobility, you will compensate for this with errors elsewhere in the exercise such as excessive forward lean or a ‘butt wink’ (a posterior tilt of the pelvis placing excessive load on the lumbar spine) and significantly increase injury risk.
Conclusion:
As usual, there are no absolutes, all training decisions must be made on an individual basis depending on many factors for each athlete. For me, I sway towards unilateral training for endurance athletes for a few reasons which I explain below. When I work with field sport athletes, I will mix the use of bilateral and unilateral a bit more.
Firstly, you run less risk of injury using unilateral exercises as the back-squat requires loading up a bar and placing a huge amount of weight on the lower back. The first rule of S&C is to not injure the athlete. Most endurance athletes do not have adequate trunk strength, mobility, or technical skill to perform the back-squat safely. It takes time to coach and develop the technique. During that time, you will struggle to load the athlete enough to make progress. So, you’re trading training adaptation time for technical coaching time in an area that isn’t even the athlete’s main sport, all for adaptations you can get through other means. Simply put, it takes too long working on trunk strength, mobility and technique to really see the benefits of back-squatting.
Secondly, unilateral exercise more specific to performance. We ride, run, jump, turn and twist using one leg at a time, therefore training in this manner adapts the body to those stresses. If training for a sport, you need to focus on the transference of strength/power to the performance. It’s all about sport specificity over gains in the gym.
Thirdly, most endurance athletes have terrible ankle mobility due to thousands of strides or revolutions at short muscle lengths. This compromises back-squat technique, it takes a long time to lengthen the gastrocnemius, soleus and achilles tendon and cannot be done over 1 pre-season. This will have to be worked on over a LONG period, which will then allow us to consider using the squat further down the line.
Ultimately, a combination of both methods is probably optimal IF athletes have the correct technical ability and mobility to perform the back-squat correctly. It’s just a case of how long you want to spend getting your athlete there.
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