Does Leg Exercise Make Your Arms Tired?

A new study from Israel Halperin and his colleagues at Memorial University of Newfoundland, just published in the European Journal of Sport Science, looks at the effect of repeated knee extensions on repeated elbow flexions. It’s part of a larger effort to understand how fatigue works, and how to distinguish between local fatigue (my arm muscles are tired and therefore can’t lift anymore) and non-local fatigue (I’ve exerted so much effort with my legs that I can’t lift my arms anymore).

The study design was fairly straightforward. To start, the volunteers either did nothing (the control) or did five sets of knee extensions to exhaustion. Then they had one minute of rest and did a single maximum voluntary contraction (MVC) with the elbow flexors (i.e. a biceps curl); then two more minutes of rest, then a sequence of 12 elbow flexor MVCs (contract for 5 seconds, rest for 10 seconds).

With the single MVC, there was no difference between the control and fatigued group. In the sequence of 12 MVCs, the two groups were evenly matched until the eighth repetition, at which point the fatigued group was no longer able to maintain force quite as well. Not that surprising, but the question is why? What mechanism saps their arm strength?

Previous studies have found that leg exercise doesn’t affect levels of fuel (glycogen, phosphocreatine, ATP) in the arm muscles. And with a weight-lifting protocol, it’s not about oxygen debt or heart rate. Interestingly, the researchers in this study found that lactate levels measured in the fingertips were 18% higher after the leg-fatigue protocol. Some researchers argue that the accumulation of metabolites like lactate, hydrogen and potassium ions in the blood directly affects the function of muscles throughout the body. The authors of the current study focus more on two centrally mediated possibilities:

  • Metabolic signals like elevated lactate act as signals that tell the central nervous system to automatically reduce neural drive to muscles. In other words, you’re still trying just as hard, but the signal from your brain to your arm muscles is reduced because incoming signals from the body are telling the CNS that you’re already tired.
  • Mental fatigue from the leg exercise means you’re unable/unwilling to push quite as hard with the arms – or, more precisely, that you’re unable to resist fatigue for as long. This is an important distinction, because in the initial MVCs, the arms are just as strong. It’s only after having to repeat the MVCs over and over that the diminished fatigue resistance becomes apparent.

This study can’t tell which of the three suggestions is correct, but it points the way to some interesting unanswered questions in the study of fatigue. And I guess it’s also a reminder of why weightlifters do arms and legs on separate days!

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