Force Deficits and Sleep Deficits in Dancers

I’ve written before about how we structure our workouts for dancers by leading with a power or plyometric movement before leading into a primary strength movement. The most recent Journal of Dance Medicine & Science includes what I think is going to become a really important study called “Analysis of the Force-Velocity Profile in Female Ballet Dancers” that gives some quantitative weight to what we’ve been doing based on experience (and the strength and conditioning literature).

“Previous investigations have reported that female dancers have insufficient resistance strength training in their weekly routine and a high amount of training hours devoted mainly to the repetiton of technical skills,” the authors write, before going on to conclude:

“This study indicates that professional ballet dancers at all company ranks are velocity oriented and suggests that a high or low force deficit may negatively affect their jumping ability and therefore the quality of their performance.”

To understand the study’s conclusion, we should revisit briefly the idea of the force-velocity curve. What the study is saying, if you look at the graphic below, is that dancers spend most of their training time at the far reaches of the velocity end of the spectrum, on the “x” axis, and not enough time further up the curve on the force end of the spectrum, or “y” axis. This leaves them with a force deficit that prevents them from jumping as powerfully (or landing as safely) as they otherwise could.

The reason I think this study is so important is that it starts to add quantitative depth to what strength coaches (and other dance medicine professionals) could clearly see just by evaluating a typical dancer’s weekly schedule. There’s simply not enough time devoted to having the dancer work under any sort of a heavy load, which is known to strengthen connective tissue and yield efficient, durable, athletic results.

I continue to be amazed at how far we have to go in the field of dance medicine when it comes to caring for dancers. Ask any young, ambitious dancer about their schedule and you’ll find hours upon hours of physical work specific to dance, and then hours more of random “cross training.” Young dancers are left with little time to socialize, study, or sleep, and because so little of their cross-training is targeted to them individually, the worst part of it is that they’re doing a lot of work that isn’t actually helping them.

The reason we’re such advocates for strength training as a mechanism for supporting dancers is that it is an efficient delivery system for force and velocity—and because we know it doesn’t interfere with the necessary technical work demanded of dancers who are honing their craft. Wasted movement outside of the dance studio is harmful not just because it creates unnecessary physical stress but because it crowds out the recovery process.

Here’s how a 2014 study described the effects of a chronic lack of sleep on adolescents:

“Our study shows that the amount of sleep per night is associated with the risk of injuries in adolescent ath- letes. This association was observed even after accounting for other factors that might affect injury rates such as grade in school or amount of time spent participating in sports. This finding is not surprising in view of previous research that shows that even modest sleep loss is associated with impairment of psychomotor performance. Sleep deprivation can affect motor function, mood, and cognitive functions, all of which could affect a young student athlete’s performance and injury risk.”

In other words, what we’re doing when we ask dancers to take random cross-training classes outside of the dance studio is we’re not only not providing them with the force they need to perform well, but we’re also steering them into injury by cutting into their free time. It’s the worst of all worlds: ineffective and potentially injurious. This is true for young dancers, but it’s also true in the professional ranks.

Another study specifically looked at psychological factors contributing to dance injuries and found that sleep was a critical factor.

Percent of time injured was negatively associated with global sleep quality (r = -.45, p < .001), habitual hours of sleep per night (r = -.37, p < .01), subjective sleep quality (r = -.32, p < .01), sleep efficiency (r = -.39, p < .01), and social support (r = -.29, p < .02), while percent of time injured was significantly positively correlated with delayed sleep onset (r = .44, p < .001) and daytime sleepiness (r = .50, p < .001).

This is why we keep coming back to this idea of having an efficient system for strength training dancers. It’s not just that there are some exercises (like a reverse lunge) that are more effective for an athlete than others (like a seated leg extension on a machine). It’s that the dancer has to pay for any inefficiencies with more time spent exercising, which in turn diminishes the amount of time they have for other areas of their life that are critical for peak performance and general wellbeing.