The impact of pack load and footwear on lower limb injuries
An in-depth research article

By Crossfire March 27, 2018

The impact of pack load and footwear on lower limb injuries
An in-depth research article

Posted in by Crossfire on March 27, 2018.

The following synopsis is from ongoing research being conducted and kindly shared with Crossfire Australia. The author has asked not to be identified at this stage as yet as the research remains ongoing.

I have done work regarding injuries and pack load and also the impact of footwear on lower limb injuries. As far as shoulder injuries are concerned we assessed male and female service personnel to identify the risk of a shoulder injury as the pack they were carrying at the time was believed to be the cause of shoulder injuries. We found 43% of males and 38% of females were at risk of injury due to lack of appropriate upper body conditioning before commencing with training. Our research also revealed the type of pack being used may be a contributing factor to shoulder injuries. The pack being used at the time made use of an internal frame, and when worn without the belt a force in 3 planes is produced – the expected drag down, a rearwards force as the pack levers off the lower back, and also a distracting force or rotational component (the strap attachments are more central on a soft frame). The difference with a framed pack is that it doesn’t lever off the lower back and straps are usually held wider on a fixed frame.

Shoulder straps of a pack usually cause a traction injury of the C5 and C6 nerve roots of the upper brachial plexus. This results in the entrapment of the long thoracic nerve causing numbness, paralysis, cramping and pain in the shoulder girdle, elbow flexors and wrist extensors. There are also numerous other factors which may cause shoulder injury to soldiers when carrying packs; these include the amount of load carried, duration of pack carriage, physical conditioning of the soldier and different pack types (badly designed packs not suited for military use). Studies have shown that soldiers carrying packs without a frame had higher incidences of brachial plexus palsy. Bessen et al (1987) reported an injury rate 7.4 times higher for soldiers wearing a pack without a frame compared to those using a frame.

A heavy load transferred through the shoulder strap of a pack to the underlying soft tissue may affect the neural performance of the upper limb due to trapped nerves or a reduction in blood supply. Load/pack carriage systems address this by transferring loads to the hip area through external rigid frames and waist belts, therefore the use of an external frame and hip belt will reduce the incidence of rucksack palsy by reducing pressure on the shoulders. Research has shown that framed packs with hip belts reduce the neuromuscular activation of the trapezius muscle whereas a frameless backpack increases activation of the trapezius muscle, subsequently, a framed pack will reduce the incidence of rucksack palsy.

The wearing of a chest strap also appears to be beneficial for reducing brachial plexus strains by positioning the shoulder straps in a more medial aspect. The more lateral the placing of the shoulder strap, the less protection the clavicle can provide to the subclavian artery and the brachial plexus.

This, in combination with inadequate upper body strength and stability is a significant risk for shoulder injuries.

Our results suggest a frameless pack being carried by cadets with unconditioned, functionally asymmetrical upper bodies are the causes of the high number of shoulder injuries. As a result of this, the pack was replaced with a pack having an external frame, this reduced the injury rate significantly.

As far as lower limb injuries are concerned we have found that continuous wearing of military boots, particularly boots with stiff upper shafts and thick inflexible soles are a cause of ankle and lower limb injuries. We measured the range of movement, electromyography, lower limb strength, balance and stability on military recruits before they were issued with military boots and re-tested them 12 months after being in the military and having worn military boots. This study documents the adaptations due to wearing the military boot for just one year.

It significantly reduces the strength and endurance of the gastrocnemius muscles in plantarflexion in addition to a reduced range of inversion and eversion motion.

  • In tandem, these changes cause a weakening of the ankle joint and increased reliance on the boot to support the ankle.
  • The stiff and high shaft of the military boot restricts movement and over time decreases normal range of motion in all planes of ankle motion so that the natural structures of the foot become weakened. This leads to the reliance of the footwear to support the foot and ankle.
  • When the boot is not worn there is no external support of the joint which would be a major contributor to injuries in activities of sport and running when wearing normal flexible and soft training shoes.
  • The ability of the muscles to respond quickly is decreasing.
  • The balance is significantly compromised, with a larger movement of the centre of pressure in the medial-lateral direction.
  • Medial and Lateral Gastrocnemius becoming less active.
  • It appears Tibialis Anterior remains or is more active.
  • The lack of medial-lateral muscle engagement likely contributes to the high rates of inversion injuries.
  • Tibialis Anterior seems to be overworking to maintain balance – can lead to medial tibial stress syndrome (MTSS) (shin splints).
  • The boot is making the muscles crossing the ankle joint ‘lazy’.