Who extends the hip in late stance?
When we watch a dog trot, we usually think about which muscles are working. I've always been just as curious about when they switch on and off.
We know the gluteals are primary hip extensors. The older needle EMG studies told us that the hip extensors do most of their work in the first half of stance. The hip keeps extending after that, though, all the way to liftoff. So what is happening in the second half of stance? Which muscles are still working, and how does their timing line up with what the joints are doing?
This is one of those questions that sounds basic but turns out to be surprisingly difficult to answer. A new study in AJVR by Kong, Harada and Hara (2026) gave me a nice way to think it through.
The study in brief
The authors recorded surface EMG from four hind limb muscles in four clinically normal Beagles trotting off-leash over ground. At the same time, they captured the dogs' movement with 3-D motion analysis. The muscles were the middle gluteal, cranial biceps femoris, vastus lateralis and lateral gastrocnemius.
Recording EMG and kinematics together matters. It lets us see muscle activation timing against what the hip, stifle and tarsus are actually doing in the same stride. We have surprisingly little canine data like this.
The authors also tried a different way of describing timing. Rather than relying only on "percentage of stance", which can only be calculated once the stride is finished, they used two visible kinematic events. The swing vertical moment is the instant when the limb is vertical during swing. The stance vertical moment is the instant when the limb is vertical during stance.
What I found interesting
The gluteal switches off early, as expected
The middle gluteal tended to stop firing earlier in stance than the other three muscles. Its activity was mostly finished soon after the limb passed vertical. This fits what we already believed: at the trot, the gluteal does its main job around touchdown and early stance, when the limb accepts load and starts moving the body forward.
The vastus lateralis stays on later
The vastus lateralis was the late one. In 5 of 8 limbs, it switched on later than at least two of the other muscles. In 4 of 8 limbs, it also switched off later.
This makes sense when we look at what the stifle is doing. The stifle extends through stance, and the vastus lateralis is a stifle extensor. The authors propose that, once the gluteals have done their part, the stifle and tarsal extensors keep the limb extended. Ground reaction force is then transmitted up that extended limb to the hip. The hip can therefore continue to extend in late stance without the gluteal actively driving it.
To me, this is the key idea from the paper. Hip extension in late stance is not only a hip muscle job. It is a whole-limb job, and the quadriceps play a part in it.
The biceps femoris and gastrocnemius sit in between
The other two muscles fell somewhere between the early-finishing gluteal and the late-finishing vastus lateralis.
The cranial biceps femoris is another hip extensor. In some limbs, it was the first muscle to switch on, activating before the other three in late swing. Its offset timing usually came between the gluteal and the vastus lateralis. So, alongside the gluteal, it helps prepare the limb for touchdown, but it tends to stay active a little longer into stance.
The gastrocnemius extends and stabilises the tarsus during stance. Its timing was more variable, and in one limb it switched off later than the biceps femoris. That fits the wider literature, where reported gastrocnemius offset has ranged from roughly a third to nearly three-quarters of stance. The values in this study fell within that range. This is a good reminder that no single "normal" timing value should be treated as definitive.
Put together, the sequence makes biomechanical sense. The hip extensors start the job, while the stifle and tarsal extensors carry it further into stance and keep the limb extended.
The left and right limbs didn't behave the same
This was the part I enjoyed. Even in sound dogs, muscle timing differed between the left and right limbs of the same dog. One dog showed clear timing differences between muscles in the left hind limb and none in the right.
The task was the same: trotting in a straight line. Each limb, however, seemed to organise its muscles slightly differently to do it. This was different enough that the authors analysed each limb separately rather than pooling them.
Muscle timing has a "clock"
Most muscle onsets (24 of 32) occurred in late swing, between the limb passing vertical and the paw landing. In other words, the extensors were already switching on to prepare for load before touchdown. Every offset occurred after the limb passed vertical in stance.
Exact timing could not be predicted from other gait variables. Even so, these two vertical-limb events gave a useful and observable frame of reference for when muscles turn on and off.
Keeping it in proportion
This is four Beagles, one gait, and four muscles. Surface electrodes cannot reach the deeper hip extensors, such as the semimembranosus, semitendinosus and adductor magnus. Those muscles may also contribute to late-stance hip extension.
The study also measured when muscles were active, not how much force they produced. It supports the late-stance stifle-extensor explanation, but it does not prove it. Kinetic data will be needed for that.
What this means for rehabilitation
At the trot, think of the gluteals as early-stance muscles. Their main contribution comes around touchdown and the first part of stance. If we use trotting as part of gluteal work, that is the phase we are mainly loading.
For example, if the goal is active hip extension through a larger range, tasks such as incline walking to extend from a more flexed position under load, which flat-ground trotting may not provide.
Hip extension involves the stifle extensors too. When a dog shows weak or incomplete limb extension in late stance, the gluteals are not the only muscles to consider. The quadriceps contribute to late-stance limb extension, which means our assessment and our exercise choices should include them.
Don't expect perfect symmetry. If sound dogs show left–right differences in muscle timing, a small asymmetry is not a problem in itself. It becomes meaningful when it changes over time, matches the history, or appears alongside pain or reduced function.
Closing thought
So, who extends the hip in late stance? Based on this study, not only the gluteals. Once the gluteal has done its early-stance job, the stifle and tarsal extensors appear to help carry hip extension through to liftoff, with each limb doing this in its own way.
Next time we prescribe a "hip extension" exercise, it is worth asking a slightly different question: which muscles do we want working, and in which part of the stride?
Reference
Kong L, Harada Y, Hara Y. Hind limb extensor muscle activation timing during trotting in Beagles shows limb-specific variability and can be referenced to kinematic events swing vertical moment and stance vertical moment. Am J Vet Res. 2026. doi:10.2460/ajvr.26.02.0046
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