Biomechanics

Musculoskeletal ailments have multifactorial etiologies, including intrinsic and extrinsic factors. Biomechanically, these etiologies can be classified in three categories:

1. Training errors9

2. Muscle tightness and/or imbalance10

3. Malalignment and/or abnormal biomechanical movement8,11

Orthotic and footwear intervention will address the extrinsic etiologies of muscle imbalance, skeletal malalignment, and the associated abnormal biomechanics.

Muscles with their associated tendons control or produce movement. For the Achilles tendon, the movement producer is the gastrocnemius-soleus complex. This includes the independent gastrocnemius and soleus muscle origins with their combined Achilles tendon insertion distally. The anatomy of this muscle-tendon unit allows it to affect the knee joint, ankle joint, subtalar joint,

Figure 8.1. Clinical presentation of right Achilles tendinopathy.

and indirectly the midtarsal joints of the foot. (Fig. 8.2).

The ankle and subtalar joint axes allow triplanar range of motion. Practically, their predominant planes of motion with respect to the Achilles tendon are first in the sagittal plane and second in the frontal plane. The Achilles tendon muscle complex can act around these axes and planes of motion in three different modes of contraction:

1. Concentrically: the muscle tendon complex shortens as it develops tension.

2. Isometrically: the muscle tendon complex does not change in length while it develops tension.

3. Eccentrically: the muscle tendon complex lengthens or stretches while it develops tension.

Tendons, including the Achilles tendon, are subjected to more tension during an eccentric

Figure 8.2. Relationship of Achilles tendon to ankle joint and subtalar joint axes.

muscle contraction and are more likely to fail while undergoing this mode of contraction.12 Because of this, the discussion of foot orthotics and their effects on lower extremity biomechani-cal stress should focus primarily on the eccentric function of the gastrocnemius-soleus muscle tendon complex in the sagittal and frontal planes.

The primary collective function of the gastroc-nemius-soleus complex is to restrain the forward motion of the tibia and to moderate the extension of the knee that occurs while the body transits over the planted foot in walking and running. It therefore stabilizes the leg during the stance phase of gait.13,14

Individually, the gastrocnemius muscle acts during the swing and stance phases of gait at the knee, ankle, and subtalar joints. It works initially during the beginning of the swing phase at the knee joint, where it contracts concentrically to flex the knee and aid in foot clearance before leg extension. At the ankle joint, it works eccentrically during the early swing phase, while dorsi-flexion of the foot occurs for toe clearance. Later in the swing phase, before the upcoming contact phase, it again functions eccentrically, maintaining flexion at the knee before heel contact.

With stance-phase placement of the foot, the gastrocnemius functions to decelerate, stabilize, and then accelerate the leg during locomotion.15 At the beginning of the stance phase of gait, it has a short-lived, modest, concentric tension to oppose the eccentric activity of the dorsiflexors as they control foot descent. After initial ankle plantar flexion at early stance phase, the ankle joint dorsiflexes, and the muscle again functions eccentrically. In the second half of the stance phase, the angle of the tibia is unchanged in walking, and the gastrocnemius-soleus complex undergoes isometric contraction.16 As the shift from eccentric to isometric function occurs, the heel is lifted from the ground aided by forward momentum.

At forefoot loading, the gastrocnemius proxi-mally works at the knee to decelerate internal rotation of the femur. At late midstance, it works eccentrically as the knee begins to extend, and initiates external rotation of the femur. At the end of midstance, it flexes the knee, which lifts the heel to initiate propulsion, and contributes to concen-trical plantar flexion of the ankle. At the subtalar joint, its likely function is supination at late mid-stance, depending on the axis location.

The soleus muscle works with the gastrocne-mius to stabilize the lateral forefoot to the ground in midstance and to decelerate subtalar joint pronation and internal leg rotation.15 At this moment in midstance, orthotics assist the most in decreasing eccentric stresses imposed on the muscle-tendon complex. The soleus then extends the knee indirectly by decelerating the tibia, and contributes to heel lift during propulsion by stopping ankle joint dorsiflexion as it decelerates the forward momentum of the tibia.

Electromyographic studies indicate increased activity of the gastrocnemius-soleus muscle complex from 5% to 45% of the total gait cycle.15 The primary stress and the primary eccentric function is therefore from just before heel contact to late midstance, which is consistent with the clinical timing of failure and injury (Fig. 8.3).

Heel lift is ultimately a combination of the forward momentum of the trunk, deceleration of the forward momentum of the tibia, and active concentric knee flexion by the gastrocnemius. Because of the progressive movement of the center of gravity and the concentric contraction, at heel lift there is less strain on the Achilles tendon complex. Despite the decreased strain during this phase of gait, forefoot orthotic modifications can help facilitate smooth sagittal plane motion and decrease strain.

The gastro-soleus complex works only slightly differently during running.16 With a heel-contact to toe-off pattern, it initially undergoes a stronger eccentric contraction before heel strike to counterbalance the sudden pull of the tibialis anterior. During stance phase, it maintains a more exertive and longer eccentric-versus-isometric contraction to stabilize the lower extremity. It also acts as a more integral part of the shock-absorbing mechanism by mediating the rate of dorsiflexion of the ankle and flexion of the knee during the stance phase of gait. Propulsion in running is aided even more than in walking with forward momentum and the use of muscles of the low back and hip.

Runners with different running styles who display different ground contact patterns, and especially midfoot to forefoot strikers, can be more predisposed to Achilles tendon injuries.

(A) Heel contact —achilles tendon preloaded as eccentric resistance to dorsiflexors.

(B) Heel contact to late midstance—achilles tendon works eccentrically to stabilize tibia as it moves forward; to stabilize lateral forefoot; to decelerate subtalar joint pronation and internal leg rotation. Orthotics have their greatest effect during this phase.

(C) Late midstance to early heel-off—contraction moves from eccentric to isometric.

(D) Heel-off—momentum pulls heel off the ground over forefoot rocker.

(A) Heel contact —achilles tendon preloaded as eccentric resistance to dorsiflexors.

(B) Heel contact to late midstance—achilles tendon works eccentrically to stabilize tibia as it moves forward; to stabilize lateral forefoot; to decelerate subtalar joint pronation and internal leg rotation. Orthotics have their greatest effect during this phase.

(C) Late midstance to early heel-off—contraction moves from eccentric to isometric.

(D) Heel-off—momentum pulls heel off the ground over forefoot rocker.

Figure 8.3. Eccentric function of Achilles tendon in heel-to-toe gait.

This change in strike pattern increases the moment arm of the forefoot with respect to the ankle joint axis, and increases the eccentric torque on the Achilles tendon. Forefoot strikers would have comparatively more Achilles tendon strain than rearfoot strikers, and will require more attention to forefoot control in their orthotic devices.

Furthermore, increasing stride length and speed can also increase Achilles tendon injuries. With increased stride length, the eccentric force on the Achilles increases from contact to forefoot loading to control forward momentum. Over-striding does not allow for a smooth gait, and there is a transient braking action at the beginning of each contact phase. Increasing speed decreases the base and angle of gait, and increases the varus striking position of the foot. This foot plant position increases the vectors of force that cause the foot to pronate more rapidly, putting more twist on the Achilles tendon, causing the gastro-soleus complex to fire eccentrically against a stronger pronation moment around the subtalar joint axis. This eccentric load can be lessened with orthotic positive cast modifications that will help to invert the end product orthotic with respect to the foot plant position. Forefoot strikers training at increased speed with an excessive stride length are predisposed to Achilles tendon injury that can be lessened with orthotic intervention.

Where malalignment and associated abnormal movement patterns are considered part of the etiology of Achilles tendinopathy, custom foot orthotics can be helpful. These devices work primarily through the midstance phase of gait to stabilize the foot, ankle, and knee and decrease the eccentric load placed on the muscle-tendon complex around these joint axes.

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