Speedy interval sessions require rest between repetitions–and especially when you’re pushing your limits, the natural instinct may be to stop and put your hands on your knees while you catch your breath. But experience teaches us a counterintuitive lesson: Gentle jogging during those precious snippets of recovery sometimes makes it easier to run fast on the next rep. That’s because jogging keeps more blood flowing through your legs, clearing away the metabolic waste products that build up during hard running and contribute to muscle fatigue.
This doesn’t mean jogging is always best. Standing or walking allows your muscles’ supply of phosphocreatine–the energy that fuels short bursts of intense effort–to recharge most effectively. Make your choice depending on what you hope to accomplish during the session.
When to jog: British researchers recently tracked the lactate levels of cyclists during “active” and “passive” interval recovery. (While lactate is no longer thought to cause muscle fatigue, it rises and falls in sync with properties that do.) Active recovery–the equivalent of jogging–caused lactate to drop after about 90 seconds; for shorter recoveries, the active recovery offered no advantage.
Jogging is best between reps of VO2 max workouts, which involve repetitions lasting three to five minutes. Workouts like 4 × 1200 meters at 5-K pace with 3:00 to 4:00 rest will leave you heavy-legged after each rep, and jogging easily will flush your legs out to get ready for the next one.
When to walk: Given that lactate clearance is enhanced only after about 90 seconds, it’s tempting to walk during all shorter recoveries. This would be right if the goal were to run the workout as fast as possible, but the real goal is to run faster in races. Jogging short recoveries ramps up the aerobic demands of the workout, so it’s the right call sometimes (see “Seasonal Variety,” right).
The best time to walk recoveries is when you’re doing short intervals to work on your top speed, in which case starting with full phosphocreatine stores gives you an edge. For a workout like 6 × 200 meters with 2:00 rest, walk the whole recovery.
When to push it: Walking and jogging aren’t the only options. Marathoners and half-marathoners can try intervals just faster than threshold pace (between half-marathon and 10-K pace), alternating with recoveries slightly slower than threshold. These “float” recoveries teach your body to quickly move lactate out of the muscles and into the bloodstream, where it can be reused as fuel.
Try 4 × 5:00 at 10 seconds per mile faster than half-marathon pace, recovering with 5:00 at 10 to 15 seconds slower than marathon pace. Do this during a marathon buildup, and as you get fitter, speed up the recovery to marathon pace.