Treadmill Running - A Review

There is much discussion in literary and viral circles regarding the differences between treadmill running and running outdoors across hard surfaces or "over ground." Unfortunately, information is often taken out of context and passed from one web site to another, changing slightly each time, much like a message in a game of ‘broken telephone’. This article will review and compare Treadmill Running (TMR) against Hard Surface Running (HSR), look at a few considerations for treadmill utilization, and provide some practical implementation guidelines.

Biomechanical and Physiological Differences Between TMR and HSR

Spatial Parameters

Running speed is made up by stride length (the length between strides on the same foot in the direction of locomotion) and stride frequency (how often the strides occur) (Grimshaw, 2006; Nordin & Frankel, 2001). To maintain a given speed, a change in one stride parameter must be offset by a change in the corresponding parameter. For example, if stride length is reduced then stride frequency must increase to maintain a set speed and visa versa. A study by Riley et al., (2008) comparing HSR and TMR at the same speed (approximately 13.8 km/h) found that stride frequency was significantly higher and stride length shorter on a treadmill when compared to HSR. These results confirmed those of Schache et al., (2001) and Frishburg (1983) who likewise found increases in stride frequency and decreases in stride length when TMR. Furthermore, both of these latter studies found a decreased swing phase and an increased stance phase when TMR versus HSR (Frishberg, 1983; Schache et al., 2001) indicating an increased period where the foot is in contact with the treadmill belt.

Biomechanical - Muscular Interactions and Differences

As the tread belt rolls backward it drags the foot backwards, eliminating the need for the Gluteals and Hamstrings to pull the upper body forward thus making the movement easier; hence its potential lower energy cost (Frishberg, 1983). The Hip Flexors however, now have to work harder to bring the limb, which is being dragged backward, forward. Furthermore, the Frishburg (1983) study found that the lower limb was required to move through a greater range of motion in the stance phase when TMR (X=60.6 vs 54.5 deg). Similarly, Schache et al., (2001) found TMR to have a lower angle of hip flexion at initial contact and considered the increased hip extension position to be caused by the treadmill belt dragging the foot rear-ward. With this in mind, if the Hip Flexor muscles are tight, the increased hip range of motion requirement on a treadmill may cause the pelvis to rotate or the lumbar spine to hyperextend to release the tension on the hip as the limb is dragged backwards. This increased pelvic rotation and/or lumbar hyperextension, which typically occurs with fatigue (which increases stance phase duration) and at high speeds, places strain on the hips and lowerback and has the potential to cause a number of injuries. It should be noted however that a study by Riley et al., (2008) found no significant differences in hip flexion/extension ranges or pelvic rotation between HSR and TMR. The subjects in this study (n=20) who were regular runners and ran at least 15 mi (24 km) per week, were, however, only assessed over a short period (no more than 15 consecutive gait cycles). Hence these runners would not have been subject to the impact of fatigue.

With the belt moving backwards, the ability of the gastroc-soleus complex (calf muscles) to push off is reduced (Riley et al., 2008; Baur, Hirschm, ller, M¸ller, Gollhofer, & Mayer, 2007) and these muscles must work harder in order to achieve an effective push off. This is one of the reasons why you tend to power walk when you transition from the treadmill back onto hard surface – your calf muscles have been activated more than usual for hard surface locomotion. Furthermore, research has shown that running on the flatter, predictable surface of a treadmill requires less ankle stabilization than that required for running across land, with muscles like the peroneals found to be less active when TMR (Baur et al., 2007).

Skeletal Impact Differences

The skeletal impact can be considerably less when TMR as there is a flexible striding surface located underneath most treadmill belts to absorb the impact on foot strike. Furthermore, a kinematic study by Nigg et al., (1995) found that treadmill runners tended to plant their feet in a flatter position (less heel strike) when compared to HSR. In combination, the flexible striding surface, together with a flatter foot strike, may contribute to a decreased Ground Reaction Forces (GRF) (GRF being an equal and oppositely directed force from the ground (Gambetta, 2007; McLester & St. Pierre, 2007)). This hypothesis is supported by Riely et al., (2008) who found TMR to have a lower GRF, hence skeletal impact, compared to HSR.

Metabolic Differences

Research on energy cost differences between HSR and TMR have provided divided conclusions. One research paper, which has become the sole justification for many web based articles, has found the need to increase treadmill incline grade by 1% in order to simulate the energy cost of HSR (Jones & Doust, 1996). Conversely there is other research that suggests there are no metabolic differences between HSR and TMR during flat running, incline running, or incremental running (Moss et al., 2007; Crouter, Foster, Esten, Brice, & Porcari, 2001; Mc Ardle, Katch, & Katch, 1996; Bassett Jr et al., 1985). A testimonial statement by Porcari that appears on several viral sites claims that if you are running at speeds under 14.5 km/h or 9 mph, treadmill running burns about the same number of calories as running outdoors. If, however, you run faster than this speed you will burn up to eight percent fewer calories on the treadmill. Unfortunately, none of these sites have referenced actual research to support these claims and only one relevant, academically published article produced by Porcari is available—the paper already sighted under the Crouter reference (Crouter et al, 2001). This paper found no metabolic differences between HSR and TMR. With this in mind, Porcari may have been referring to the earlier findings of Jones and Doust (1996) discussed above.

Neurological and Mental Differences

Unlike HSR, TMR pace is unchanging relative to indirect factors such as headwind, terrain changes, etc. Conversely, HSR requires the runner to alter pace and movement patterns to contend with environmental conditions. With this in mind, the machine mentally keeps the pace making it much easier to maintain a fixed pace. HSR, however, requires dedicated attention to adjust running stride to negotiate obstacles in order to maintain a set running pace. In contrast, many may find TMR monotonous and boring compared to HSR, which may not only be more mentally ‘active’ but also have more sensory stimulation. Thus TMR is considered by some as a useful means of increasing mental fortitude for distance running.

When it comes to proprioception, the unchanging treadmill surface means that the neuromuscular system does not have to work as hard to stabilize the body as every footfall is predictable. With every step on a HSR route the foot lands in a slightly different position as the surface changes subtly over terrain. Therefore, as discussed above, TMR requires less ankle stabilization when compared to HSR and important stabilizing muscles like the peroneals are less active (Baur et al., 2007).

Treadmill Specific Considerations

Belt Lag

When the foot strikes the treadmill belt, the force through the limb is transferred to the belt, which is traveling in the opposite direction. This impact force causes the belt to stop momentarily (even reverse direction slightly) before the motor reinforces belt speed. The amount of belt lag varies with the power of the motor (why the ‘horse power’ of the engine is important), looseness of the belt (why maintenance is important), belt speed (why machines have a maximum speed which should correlate to motor size) and the weight of the individual (why many machines have a weight limit). The effect of belt lag on the body is yet to be scientifically studied; however, the sharp, reverse-direction acceleration, for a high repetitive duration, suggests that belt lag may not be complimentary to the body.

Vibration

In general, the smaller and lighter the treadmill, the more the vibration (especially at higher speeds) there will be. This can be clearly felt when running on a cheaper "home use" treadmill compared to a more expensive industrial model. I found no research regarding a treadmill specific effect; however, whole body vibration (albeit over a long period, like truck driving) has been linked to lower back problems (Hill, Desmoulin, & Hunter, 2009; Paschold & Sergeev, 2009).

Treadmill Specific Uses

Speed Assisted Training

Speed assisted (or overspeed) training is known as an effective means of improving movement speed, in swimming (Girold, 2006), cycling (Foran, 2001) and running (Brukner & Khan, 2001). One of the methods of applying speed assisted training for running is through high speed treadmill training (Chandler & Brown, 2007; Foran, 2001; Sharkey & Gaskill, 2006). It should be noted that using speed assisted training may impact sprint running technique when excessive speeds (above 90% of maximal running velocity) or insufficient speeds (below 80% of maximal running velocity) are selected (Chandler & Brown, 2007). Finally, if using a decline on the treadmill, the most appropriate degree of decline is between three to seven degrees (Ebben, 2008; Ebben, Davies, & Clewien, 2008; Foran, 2001; Song-hua & Lei, 2005) with greater declines negatively impacting technical performance.

Rehabilitation

Tight ankle planterflexors require dorsiflexor muscles to work harder to dorsiflex the ankle which can lead to overuse injuries (Brukner & Khan, 2001). With Nigg et al’s., (1995) finding that treadmill runners plant their feet in a flatter position when TMR, the range of ankle dorsiflexion needed in preparation for heel strike is reduced. Thus, dorsiflexor muscle loading, especially against tight planterflexors, is reduced making the treadmill useful in reducing shin pathologies and as part of a rehabilitation program.

In addition, skeletal impact may be reduced through a reduction in GRF and, as such, may be beneficial for clients suffering from impact related pathologies. Support for this argument comes not only from the study by Riley et al., (2008) who found a decrease in GRF during TMR but also from research by Milgrom et al., (2003) that suggested a reduced chance of tibial stress fractures in TMR compared to HSR.

Furthermore, with a reduced need for stabilization when TMR compared to HSR (see above), TMR may be more suitable for clients undergoing ankle rehabilitation for conditions like recurrent ankle sprains. Conversely, the importance of returning clients to HSR training as part of later-stage rehabilitation, in order to ensure that they can meet the stabilization requirements of HSR, is also identified.

Treadmill Specific Injury Potential

Falling Off

Falling off a treadmill is a common injury and, while beginners are at a greater risk, experienced runners can suffer the same fate. Often the cause is an external influence like water bottles, towels or phones falling onto a treadmill while it is in operation. Likewise, turning the head to look in a mirror, watch television or talk to the trainer standing at the side of a treadmill may alter foot strike position and cause a client to misstep and fall.

Slipping

Cleaning agents, applied either directly (eg. using furniture polish to clean the belt) or indirectly (antiseptic spray when cleaning sweat off a machine) may cause the belts to become slippery. Slippery belts can not only cause the client to slip when running (usually on foot strike) but may also cause your client to alter their running technique to improve stability (wider stride width, shorter stride lengths and abducted shoulders with flaring elbows).

Children and Burns

For those using home studios extreme care must be taken with children (either your own or your clients) as children are vulnerable to friction burns when falling on or onto moving treadmill belts (Abbas, Bamberger, & Gebhart, 2004; Carman & Chang, 2001; Maguina, Palmieri, & Greenhalgh, 2004; Wong et al., 2007).

Practical Implementation

Safety and Education

• Ensure you, as a trainer/coach/user, are aware of the technical specifications of the treadmill you use (weight restrictions, spacing and power requirements, etc.). These can often be found on the manufacturer's web site if user manuals from purchase cannot be found.
• Teach your clients how to mount and dismount a moving belt (e.g. the ‘scooter’ method) and demonstrate the machine’s safety features (i.e. the ‘emergency stop’). Also, ensure clients are comfortable with changing speeds and the machine’s incline/decline grade while running.
• When providing instruction to a client on a treadmill, be aware of the impact your position may have on their technique and opt for a position that will allow the client to maintain as natural a running style as possible.
• Educate your clients on potential hazards caused by personal objects falling onto a moving belt and, if required, about the dangers to children.

Training Programs

• To reduce the potential for back and pelvis injuries ensure that core training for the trunk and pelvis (including the transverse abdominals and gluteus medius muscles) are part of your client's program. It's also important to include effective hip and adductor stretches (as required following an assessment). Also, ensure that you reduce your client’s running speed when you notice an increase in their stride support phase (foot stays on the treadmill longer and is dragged backward further).
• Even though metabolic parameters may be similar, there are still biomechanical and other physiological differences between HSR and TMR. With this in mind, a training program should—where possible—alternate between HSR and TMR rather than choosing one as the sole source of training, especially when training a client for a HSR event.
• TMR can be used as part of a considered speed assisted/overspeed training program as well as training method to increase stride frequency (neural patterning).
• While TMR may be useful in early stage rehabilitation, later stage rehabilitation should—where possible—include HSR to ensure the client is capable of meeting the proprioceptive and impact requirements of HSR.

Care and Maintenance

• Check the belt regularly. If you feel the belt slipping or a pause in the belt every time your foot strikes look at belt tightness and belt wear (loose and fraying). Use an alternate treadmill if excessive wear is noted. Be aware that some machines with weaker motors may always produce a notable belt lag at higher speeds.
• Educate clients on the best way to clean equipment.

Conclusion

TMR has many similarites to HSR and can be a useful addition to a general or rehabilitative program. With this in mind, there are some differences between the two modes of running and, whenever possible, TMR should be integrated with HSR to provide optimal training effect. Finally, trainers and coaches should ensure they are familiar with any treadmills they use with clients and educate their clients on treadmill use and safety.

References

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