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‘Strength is the Glass’ is a term that I’ve heard thrown around the fitness industry for a couple of years now. Beautiful in its simplicity, it refers to the capability of the individual, based on their strength. Simply put, if you’re not able to perform a particular exercise or movement because you’re just not strong enough, then you possess a smaller glass. The stronger you get, the larger your glass grows and the more, for lack of a better term, ‘stuff’ you can fill it with. As you progress in your development of strength, so does your ability to perform more complex and challenging exercises. In addition to the ability to perform more complex and challenging exercises, you can then expand the application of these exercises. For instance, if your best deadlift is 100lbs, the use of high rep power cleans for a conditioning effect is probably not a very viable option. Strength is the glass.
Looking at the CrossFit hierarchy of development, (excluding nutrition) it is anchored by Metabolic Conditioning. Metabolic Conditioning is just a fancy term for your ability to perform a task. Metabolic Capacity is your strength combined with the number of times you can demonstrate that strength. Metabolic Conditioning is broad-spectrum, ranging from getting off the couch and walking to the kitchen to carrying a 100lb sandbag over a 10k ruck with 300m in elevation changes. If we imagine Grandma sitting in her chair, she wants to get up but must work very hard to accomplish this. If that same Grandma is now able to perform 25 air squats, standing up from her chair doesn’t seem so challenging. Applying this to CrossFit, we have a workout that is 21-15-9 reps of Deadlifts at 315lbs and 30” box jumps. If your best deadlift is 315lbs then you’re not even going to be able to get through the first round. If we add more strength, this workout becomes easier. You require a certain amount of strength to do anything. So when we say Strength is the Glass, we mean that with a greater level of strength, the possibility to complete more tasks is available.

Everything we do in CrossFit rests on the foundation of Metabolic Conditioning.
Now, if strength is the glass, then aerobic fitness is the structural integrity of the glass. A strong person is capable of many amazing feats, but his or her fitness determines how well they can combine those feats, how many combinations can create and how often they can perform before their glass falls apart. I can’t fill a 10-litre paper cup all the way to the top no matter what I fill it with. Therefore, I can possess exceptional strength but without aerobic fitness, my capacity to apply that strength would be limited.
Now the question is, ‘HOW is aerobic fitness the structural integrity of the glass?’ Without getting too far into the science of human performance, let’s re-term Metabolic Conditioning as simply Work Capacity. Going into a CrossFit workout you need a high level of work capacity. A 400lbs deadlift is fantastic, but what is even better is being able to perform that 400lb deadlift for multiple or consecutive reps. Even athletes competing in pure strength sports like powerlifting or throwing will perform 45-60 minutes of light aerobic work up to twice per week, throwing medicine balls, dragging sleds, or swinging kettlebells just to increase their work capacity… this is where aerobic fitness enters the equation. Imagine that if a powerlifter needs 45-60 minutes to aerobic work 1-2 times per week just for 9 lifts then what does a CrossFitter need?
The aerobic engine is the preferred and predominant system in almost all activities. Aerobic energy production is even responsible for regenerating ATP during high effort activities like weightlifting and plays a major role in the function of your autonomic nervous system (the part of your central nervous system responsible for subconscious activities like heart rate, digestion, respiratory rate, organ function, hormone production, salivation, perspiration, pupillary dilation, sexual arousal, breathing and swallowing). All of these will, in turn, help to play a major role in the production and gain of your strength.
Aerobic fitness allows you to use strength to create Work Capacity. You combine muscle-ups, power cleans, dead-lifts, running, jumping, throwing, and a host of other unrelated exercises concurrently to form the greatest workout that ever was – CrossFit. When training for life, strength is important but aerobic fitness is equally important. You can’t have a big glass without both.
In the next installment, I will discuss: What is Strength?