Key takeaways:

• The Achilles tendon's unique structure and function: The Achilles tendon is a powerful energy-storing structure, capable of storing and returning nearly 90% of the energy required to stretch it. This elastic recoil is crucial for efficient locomotion.
• "The free energy supplied by this elastic recoil significantly reduces the metabolic cost of locomotion and has played a huge role in our success as bipeds."
• Non-insertional Achilles tendinopathies are prevalent: These injuries, occurring at the midportion of the tendon, are common among runners due to impaired circulation in this area.
• Limitations of traditional treatment: The widely used Alfredson protocol, while helpful, shows limited long-term effectiveness. Nearly 60% of patients still experience pain five years later.
• New insights into tendon healing: Recent research suggests that stimulating intratendinous fluid flow and interfascicular sliding is critical for effective tendon remodeling.
• Heavy-load, long-duration isometric contractions: These exercises increase fluid outflow from the tendon core, promoting healing.
• "The applied load must be heavy and sustained for long-duration."
• Bent knee heel raises: These specifically target the soleus muscle, enhancing the non-uniform sliding of tendon fibers against each other, which stimulates tenocyte remodeling.
• "Individuals with damaged Achilles tendons show a more uniform pattern with limited variation in interfascicular sliding."
• Importance of synergist strengthening: Engaging muscles like the flexor hallucis longus helps offload the Achilles tendon during recovery.
• "By exercising the toe muscles in a lengthened position, strength gains are increased nearly fourfold compared to conventional exercises."
• Proposed exercise protocol: The document suggests a protocol incorporating bent knee heel drops, heavy-load isometric contractions, and exercises that engage the toe muscles. The frequency can be adjusted based on individual fitness levels.
• "I’ve been using this exercise protocol for almost 2 years now, and in my experience, it definitely produces better outcomes than the Alfredson protocol, especially in elite athletes."

Further research:

• Optimal exercise frequency for different patient demographics.
• Long-term effectiveness of the proposed protocol compared to other interventions.

Conclusion:
This document presents a compelling case for incorporating new concepts into Achilles tendinopathy rehabilitation. By understanding the importance of intratendinous fluid flow, interfascicular sliding, and synergist strengthening, clinicians can potentially achieve better and faster recovery for their patients.

The understanding of ageing on muscle loss (called sarcopenia) has undergone rapid development over the last 50 years. Although we don't know all the reasons why people lose muscle mass as they age, most of the reasons are probably now known.1 As we age, the muscle loss is typically worse in the hands and feet, the latter causing issues with balance and tripping. 

Diabetes is known to accelerate the ageing process and the more poorly controlled the diabetes typically the worse the muscle loss.2 To quantify the amount of muscle loss a group of researchers looked at MRIs of the foot and ankle in images that were taken of the same people 10 years apart.3 To further investigate the results they divided the patients up broadly into those that had neuropathy, those that didn't have neuropathy and a healthy control group without any disease.
​
The results can be seen in the following graph:

Annual loss of foot and ankle muscle in people aged around 35 - 60 years. Comparing MRIs of patients feet, looking at the same muscles with an approximate 10 year gap in between.
​
Leg muscles:

Neuropathic patients: About 5% of muscle lost each year.
Non-neuropathic patients: About 2% loss of muscle each year. 
Controls: About 1.5% loss of muscle each year.

Foot muscles:

Neuropathic patients: About 3% of muscle lost per year.
Non-neuropathic patients: About 1% lost per year.
Controls: About .2%  lost per year.

Conclusions/interpretation: Muscular atrophy in long-term diabetic neuropathy occurs early in the feet, progresses steadily in the lower legs, relates to severity of neuropathy and leads to weakness at the ankle.

Reference

1. McCormick, R., & Vasilaki, A. (2018). Age-related changes in skeletal muscle: changes to life-style as a therapy. Biogerontology, 19(6), 519-536.
2. Park SW, Goodpaster BH, Strotmeyer ES, De Rekeneire N, Harris TB, Schwartz AV, Tylavsky FA, Newman AB: Decreased muscle strength and quality in older adults with type 2 diabetes: The Health, Aging and Body Composition Study. Diabetes 55:1813-1818, 2006
3. Andreassen, C. S., Jakobsen, J., Ringgaard, S., Ejskjaer, N., & Andersen, H. (2009). Accelerated atrophy of lower leg and foot muscles—a follow-up study of long-term diabetic polyneuropathy using magnetic resonance imaging (MRI). Diabetologia, 52(6), 1182-1191.

As it happens, to decide what features were important in determining a minimalist shoe an expert panel of 42 shoe boffins got together, did some number crunching and came up with a list of key features. Then to make it more useful they created a minimalist index system so shoes could be rated and then their respected result can be reported when it is used in a research paper e.g. this research used a minimalist shoe with an MI of 58 for example. 

To come up with the Minimalist Index for a shoe, you assign 0 to 5 points for each of the following categories:
1. Weight. 5 points for when the shoe weighs less than 125 grams (4.4 oz); 0 points for shoes weighing more than 325 grams (11.5 oz).
2. Stack height. This is the thickness of the sole at the center of the heel. Shoes less than 8mm are given maximum points and shoes over 32mm are given 0.
3. Heel to toe drop. Less than 1 millimeter is considered most minimalist and over 13 millimeters is considered a zero score.
4. Motion control and stability technologies.  You start with five points, and lose one for every technology included in the shoe e.g. heel counter equals 1 point, mid-sole shank 1 point, board lasting 1 point, etc. 
5. Flexibility. Half of the points (i.e. 2.5) are awarded for longitudinal flexibility, which is how much the shoe folds up when you press from the heel and toe. The other half is awarded to torsional flexibility, which is how much you can twist the shoe when you rotate the toe in one direction and the heel in the other.

Add up all these points, and you get a score out of 25; multiply that score by four, and you get the Minimalist Index, ranging from 0 to 100.

Following on from the previous great work by Dominic Farris et al. we have a new article which gives us insights into the contribution of 2 of the largest intrinsic muscles of the foot, abductor hallucis and flexor digitorum brevis. Specifically, this research attempts to unravel how much of the foots function is passive and how much contributes by muscular function.

When humans walk it is well known that a significant amount of the energy for forward motion occurs through the use of passive (built-in) springs of the body. Essentially these springs, such as the achilles tendon and plantar fascia, can “catch” some of the energy of the body landing down on the ground, “stretches” like a rubber band and then recoils to return a lot of the energy collected to rise the body back up again. The achilles tendon of course is one of the most significant human springs during walking and especially running and provides a sort of “free” energy to these motions rather than meaning all energy for walking comes only from the “push” of muscles.

It is said that both Turkey’s and Kangaroos can reach a locomotion pace where the spring in their legs returns most of the energy for forward movement and therefore they can continue moving at this pace very efficiently with very little oxygen consumed relative to their mass. The kangaroo of course does this with enormously long feet with an enormously long achilles tendon to capture the bounce energy and return it. 

​Riddick, Ryan, Dominic J. Farris, and Luke A. Kelly. "The foot is more than a spring: human foot muscles perform work to adapt to the energetic requirements of locomotion." Journal of the Royal Society Interface 16.150 (2019): 20180680.

Seven percent of athletes who score less than 20% body weight will tear their ACLs each season.1

Recent research confirms that when piriformis strength is less than 20% body weight, the individual is significantly more likely to be injured. ACL tears are especially common when a weak piriformis is present.

The ToePro/Hip Strength Dynamometer makes it easy to measure hip strength. An ankle strap attachment allows you to create a ratio between hip strength and body weight. Simply multiply the patients weight in kg by .2 to work out what minimum strength is and perform the test e.g. 80kg x .2 = 16kg.

1. Khayambashi K,  Ghoddosi N, Straub R, et al. Hip Muscle Strength Predicts Noncontact Anterior Cruciate Ligament Injury in Male and Female Athletes: A Prospective Study. Am J Sports Med. 2015;(44):355-361.

Read the full article by Dr Thomas Michaud on hip strength and knee function here.

Relatively new article in the National Academy of Sciences journal. Farris, Kelly, Cresswell and Lichtwark I believe have written a wonderful paper clarifying the role of the intrinsic muscles of the foot.

Farris, D. J. et al. (2019) ‘The functional importance of human foot muscles for bipedal locomotion’, Proceedings of the National Academy of Sciences, p. 201812820. doi: 10.1073/pnas.1812820116.

The conclusion has an especially important comment: "In conclusion, we have shown that the PIMs (Plantar Intrinsic Muscles) actively contribute to stiffening of the MTP joint in late stance during walking and running, to assist propulsive push-off, and that the windlass mechanism cannot support this function without them."

It now appears likely then that strengthening the intrinsic muscles of the foot would indeed improve the power and stability of the MTPJs at propulsion. Not only that, it would also appear likely that their strengthening will reduce loads on the plantar fascia during propulsion.

We now see that we should not ignore the small stabilisers of all joints in assisting the prime movers, whether that be the shoulder, back, hip or foot. ​

Your toes retract upwards through increased activity of the extensor muscles (extensor digitorum/hallucis longus/brevis) in response to the restriction in ankle range (most likely due to a short calf). Luckily these extensor muscles do work harder otherwise people with reduced ankle range would trip over their toes during the swing phase of gait. Ground clearance during swing phase is not very large (less than 2 cms for most people depending on height) (Winter, 1992).

The downside of the increased extensor activity is it causes greater and greater hyperextension of the MTPJs and makes it harder for the intrinsics to keep the toes straight. During propulsion the lumbricals and interossei become ineffective in stabilising the toes and all we have left are the extrinsic flexors (Flexor digitorum longus which causes the toes to claw without the work of the intrinsics).

Therefore, to stop this process we can’t just focus on intrinsic muscle strengthening. The fault in many cases can be the reduced range at the ankle joint (especially in diabetes). For the most part, patients with this limitation primarily have calf weakness and subsequent shortening (they go hand in hand e.g. if you immobilize the calf, it weakens and shortens itself to maintain a healthy length-tension relationship) (DiGiovanni et al., 2002). In diabetes the restriction is in combination with AGE’s (Advanced Glycation End products) which essentially decreased the flexibility of collagen (Rao et al., 2006).

To get the best and most effective result for our patients we need to ask them to strengthen their calf muscles by performing an eccentric calf loading program. Eccentric strengthening of muscles in a lengthened position promotes greater strength and length by promoting the physiological process of sarcomerogenesis (Lynn and Morgan, 1994), essentially a process by which the muscle makes itself longer by adding on sarcomeres in series. Once you have done exercise in this way for a few months you will end up with an increased muscle resting length which will return some if not all the lost range in the ankle joint. In doing so you have reduced the need for the extensor muscles to work so hard to clear the ground. In return the patient will have improved range and power in propulsion which will have added benefits for hip and knee function. This is also by far a more effective program than stretching the calf muscles which will only offer short term benefits.

Essentially using the ToePro to perform calf eccentric exercises combined with the intrinsic strengthening aspect will give patients a very effective approach to maximising their potential to improve range and improve toe alignment and strength.

When do we call it quits?

The problem that will raise its head very quickly for patients is that if they have walked in this way for a long period of time the flexion deformity they have developed in their toes may have become fixed. The calf exercises will still be effective in improving the ankle range if the equinus is due to the tightened calf muscles. It will also reduce the risk of further progression of the deformity and should therefore be recommended and most likely will benefit more proximal joints.

It would not be unreasonable to recommend stretching of the shortened extensors passively (by say the hands, or by sitting on top of your feet with your feet maximally plantarflexed beneath you) or even to recommend strengthening the extensor tendons in a lengthened position also. The practitioner can also perform mobilisation and manipulation of both the ankle and toes to improve their range.


 
References
Cheuy, V. A. et al. (2015) ‘Muscle and Joint Factors Associated with Forefoot Deformity in the Diabetic Neuropathic Foot’, Foot and Ankle International, 37(5), pp. 514–521. doi: 10.1177/1071100715621544.
DiGiovanni, C. W. et al. (2002) ‘Isolated gastrocnemius tightness’, Journal of Bone and Joint Surgery - Series A, 84(6), pp. 962–970. doi: 10.2106/00004623-200206000-00010.
Lynn, R. and Morgan, D. L. (1994) ‘Decline running produces more sarcomeres in rat vastus intermedius muscle fibers than does incline running.’, Journal of applied physiology (Bethesda, Md. : 1985), 77(3), pp. 1439–1444. doi: 10.1152/jappl.1994.77.3.1439.
Rao, S. R. et al. (2006) ‘Increased passive ankle stiffness and reduced dorsiflexion range of motion in individuals with diabetes mellitus’, Foot and Ankle International, 27(8), pp. 617–622. doi: 10.1177/107110070602700809.
Winter, D. A. (1992) ‘Foot Trajectory in Human Gait: A Precise and Multifactorial Motor Control Task’, Physical Therapy. Oxford University Press, 72(1), pp. 45–53. doi: 10.1093/ptj/72.1.45.

The creator of the ToePro Dr Tom Michaud has released his own paper on the management of heel pain (plantar fasciitis). 

"Each year, nearly 2 million Americans seek medical attention for a painful heel. In the US alone, the annual economic cost for managing this common condition is in excess of $300 million. Until recently, it was believed the most common cause of heel pain was an over-pronated foot...."

Read the full article here.