Presenting complaint:

This is the case of a healthy pro-footballer presenting with a left subacute 5th metatarsal stress fracture that was picked up on MRI. The injury site was asymptomatic in footwear, except when the player was wearing his Nike Air Zoom Mercurial Superfly 9 Elite football boots. The injury developed after a period of intensive training.

Pathomechanics & Causative factors:

This athlete was in great shape with a high level of fitness and conducted regular strength & conditioning training, with no obvious areas of weakness. He has a fairly ‘neutral’ foot i.e. no obvious excessive pronation nor supination motion of the feet. The patient presented with a moderate structural leg length inequality estimated to be circa 5-7mm – left being the longer, and a forefoot equinus foot-type – common in athletes. The player was performing at a high level with no previous history of over-use injury.

He came in with two pairs of football boots that he currently wears, but only one pair created pain over the fracture site during use. The boots were of the correct fitting for the foot size and were:
1. Nike Air Zoom Mercurial Superfly 9 football = these elicited pains while wearing (walking, running).
2. Nike Hypervenom = No pain while wearing (walking, running).

Nothing too remarkable came up during the assessment apart from evidence of asymmetry in leg length and increased load over the left heel on the pressure plate. On inspection of the boots there was a difference between the two in terms of the construction of the longitudinally hard plastic outsole. The Nike Air Zoom Mercurial had raised hard plastic ridges on the medial and lateral borders joining the studs (x8 on the forefoot) for grip. The construction of the ridges run parallel towards the medial tubercle of the calcaneum – along the path of the plantar aponeurosis. This type of design allows the boots to grip the grassy playing surface under medio-lateral movements and also speed changes typical of those during training and competition. It also acts as a sort of ‘torsion-plate’ to prevent unwanted flex in the outsole to protect the plantar fascia. The Nike Hypervenom in contrast has 6 studs under the forefoot, and a more simple flat outsole with a gentle curve side-to-side, and longitudinally. The Hypervenom outsole was also slighter wider than the Mercurial.

While wearing the Nike Air Zoom Mercurial the player instantly experienced pain in the fracture site that appeared to align over the lateral ridge on the outsole. The too-flexible upper allowed the 5th metatarsal to ‘bulge’ over the edge almost suspending the lateral margin of the foot over the side of the outsole. The role of an outsole plate is to protect the plantar surface of the foot during the multi-directional nature of the sport. An outsole that is too narrow will allow the foot to drop over the edge in extreme lateral movements, potentially causing harm.

Who is driving this new trend towards lighter, more flexible and narrower outsole profiles? The players or the designers? Are they even talking with each other? In my opinion running shoe and football boot design at the moment is not on-point for the vast majority of users! I have particularly identified a couple of problem boots including the Nike Elite series i.e. the Nike Air Zoom Mercurial et al, which is a £260 boot; and the Adidas F50 adiZero series which are too light and flexible in my opinion. Of note, sports practitioners are seeing more metatarsal stress fractures in their clinics.

I have identify three main mechanical reasons for this:
1. The outsole plate is too narrow and does not support all five metatarsals, coupled with thickened plastic ridges uncharacteristic of traditional boots but characteristic of the new designs. This places unnecessary upwards pressure on the 5th metatarsal in players with feet that supinate; have a lateral shift of weight; and that at risk of ankle inversion sprains e.g. lateral shift of the CoM on the shorter limb side.
2. The boot is too light and flexible and offer no resistance or support. This boot has no torsion plate under the outsole longitudinally – flexes under load from above resulting in too much forces being placed on the plantar aponeurosis increasing risk of delayed heel lift (in the opposite way to a carbon plate in the Nike Alphafly) and increased tissue stress on the plantar plate. The flexible upper is an issue because when the player kicks through there is less protection from the toes box and the forces are transfers backwards through the metatarsals into the midtarsal join, increasing the risk of stress responses.
3. Trauma from other players. Historically football boot uppers were made from padded leather which offered at least some protection from the opponent’s studs. The absolute worst boot I have ever seen for lack of upper protection and unnecessary flexibility was the Puma v1.08, and I can proudly remember confiscating a pair from Australia international footballer Mark Viduka when he played for Newcastle United. I still have his boots!

I like a traditional football boot with a balance between flexibility and protection but still allows for the speed and pace of the modern game. For example, the Adidas Copa Mudial which is still very popular amongst professional footballers, and half the price of the Nike Elite series. It does have a low stack between the heel and forefoot, but this can be overcome by adding a full-length sole of EVA inside the boot with a 5mm heel raise gradient – to – 1mm thick under the toes.

Complex movements of the feet and legs don’t necessarily require complex boot designs. Maybe a less-is-more approach should be adopted in sports shoe design. What do you think?

Keep it simple! And only get complex when there is no alternative to a basic solution. The same is true of running shoes. They have become much too complex and beyond the needs of most runners. Just because new design software and production technique are exponential innovations doesn’t mean that our evolved morphology over millions of years has to keep pace. Yes, I know! boot design has been too rudimentary in the past and due for an update, but maybe it’s time to take a look at shoe design through the last three decades and take a more simplistic approach.

As featured in the @BIOMECHANICS:ACADEMY