DOUBLE POLE SCIENCE, WHAT WE KNOW SO FAR - Part 1

There are of course pitfalls with the “apples to apples” transfer of laboratory data to snow, but the greater internal validity, accuracy and reliability likely outweigh this issue. There are ongoing efforts by investigators to standardize data collection, thus maximizing control over such variables as; skiing sub-technique, participant skill set, track preparation and/or terrain (artificial or natural), intra-test nutrition/hydration and neuromuscular measurement tools, just to mention a few (Pellegrini B, Sandbakk O, et al. Methodological Guidelines Designed to Improve the Quality of Research on Cross Country Skiing. J Sci Sport Exer. 3:207-223,2021).  

Let’s look at what science has revealed about DP skiing thus far.

THE INFLUENCE OF FITNESS AND FATIGUE RESISTENCE ON POLING EFFIENCY AND PERFORMANCE: 

Lactate response to incline treadmill skiing using DP or DIA technique was investigated by Mittelstadt et al in 1995 (Lactate Response to Uphill Roller Skiing: Diagonal Stride verses Double Pole Techniques, Med Sci Sports Exer). They found that DP skiing at a 7.1% grade actually produced higher blood lactate levels than DIA but, when the gradient was lowered no significant differences were noted that would drive one technique over another in terms of a physiologic advantage. It should be noted that the requested effort of the DIA component in this study was 70% of technique specific VO2 max. 

This study exemplifies how metabolically demanding the act of DP skiing can be, given the terrain. The potential anaerobic nature of high intensity DP mandates terrain specific conditioning during intensity blocks. 

A study by Stoggl, Bjorklund and Holmberg (Scand J Med Sci Sports.2012), measured the relationship between muscle activation (upper and lower extremity groups), force production and O2 extraction in 9 “well-trained” male skiers while treadmill double poling at 70% and 90% of VO2 peak. They found that upper extremity O2 extraction (regional arterial vs. venous O2 concentrations) was lower than that of the legs at both intensities. Concurrently, they recorded higher lactate levels in the subject`s arms. 

These findings correlated to earlier studies by Van Hall et al (2003) and Calbet et al (2005) which noted significantly higher lactate levels in the arms and lower O2 extraction with DP verses DIA techniques. 

Mygind et al and Remme et al in separate studies demonstrated that peak arm VO2 (VO2 peak) is as much as 29% higher during DP verses DIA skiing (J Sports Sci.1991, Med Sci Sports Exer 1991), exemplifying the upper extremity intensity demands of “fast” DP. 

These studies exemplify the compromised abilities of smaller upper extremity muscles to utilize oxygen and clear lactate successfully due to lessor blood perfusion, smaller diffusion area and a greater diffusing distance, verses lower extremity groups (Calbet JA, Holmberg HC, et al. Why do arms extract less oxygen than legs during exercise. Am J Physiol Regul Integr Comp Physiol. 289:2005). 

These concepts should direct coaches and athletes to devise training modes which enhance DP “durability”, namely specific lactate threshold workouts for the upper extremities (single sticking uphill!). 

In order for the athlete to produce high DP force output for sustained periods of time, the energy system utilized must include oxygen uptake (VO2 max). Numerous studies have correlated elite level FIS points to an athlete`s VO2max and VO2 peak abilities (Rundell, Bilodeau, Gaskill, etc). Not surprisingly, studies reveal pronounced correlations between “max” (sustained) and “peak” (instantaneous) fitness and skier success, whether it be in sprinting or distance events (Staib J, Joohee IM, Caldwell Z (thanks Zach!), et al. J Strength Conditioning Research. 14(3), 2000). 

Studies point to a number of DP parameters altered by either repeated sprint bouts, or longer duration fatigue inducing skiing. Poling cycle length, rate, force and pole velocity all seem to be affected by fatigue, but in varying amounts (Stoggl et al 2006, Mognoni et al 2001, Van Hall et al 2003, Zory et al 2008).  What seems to be more universally affected by fatigue is body position and kinematics during a DP “cycle” (tip impact to tip impact).

 Zoppirolli et al noted a reduction in the center of mass (COM) displacement, via 2D filming during the 2017 Marcialonga 58 km marathon, taking video samples at 7 km and 55 km marks. Analysis of the 15 fastest male participants formed the data pool. 

Late in the race, ankle angles increased as COM forward displacement decreased. Secondary to the COM deviation, the investigators noted altered arm angles at the 55 km mark. Specifically, there was a trend toward reduced elbow angulation (more extended) and elbow joint velocity in the early phase of the DP cycle. Correspondingly, greater shoulder angles were noted. 

Zory et al found similar alterations in hip and trunk positioning (reduced COM movement forward) following sprint bout induced fatigue, but without measurable change in upper extremity characteristics (Effect of fatigue on double pole kinematics in sprint cross country skiing. Human Movement Science 28.2009). Smith et al 1996, Holmberg et al 2005 and others have highlighted the importance of trunk and hip flexion contribution to DP propulsion starting from an extended, upright preparatory phase (hands up, hips up).

High hip and trunk flexion angular velocities seem to correlate with faster double poling. With the onset of muscular fatigue, specifically the gluteal and back extensor groups, the high/extended pre-poling position is more difficult to attain, thus detracting from hip/trunk flexion angular speeds and the subsequent loading of planted poles. While fatigued, skiers operate from a more “collapsed” hip and trunk posture. Knees and ankles remained flexed to counteract the balance challenge of a COM “in the back seat”. This limits the ability to place the poles vertically prior to the onset of propulsion and abbreviates the net horizontal phase of pole loading which provides forward thrust.  

It appears that whether fatigue is sprint or distance induced, trunk and hip activity is most consistently compromised. Training programs should contain adequate posterior muscle conditioning, both strength and endurance based, and rapid firing drills for the anterior musculature to help minimize this phenomenon. More on that later.

Stoggl TL and Muller E collected data on 24 elite skiers while performing a maximal anaerobic skiing treadmill test using skating and classic techniques, including DP (Med Sci Sports Exer Jul 41 (7), 2009). Skiers improved speed across all techniques via an increase in cycle rate while trying to “maintain” their cycle length. This was easier for them to do in DP compared to DIA, or SK skiing. The investigators also noted a strong correlation between an extended swing phase duration (loading, preparatory phase) and one`s ability to maintain speed via effective thrust despite abbreviated loading phases.

In a DP specific study in 2009, Lindinger SJ, et al collected force production, kinematic and velocity data on 12 male elite subjects, roller skiing at 9,15,21, and 27 km/hr (-1) as well as at maximum velocity (Vmax). 

They noted that both DP frequency and cycle length increased up to the 27km/hr mark, but thereafter poling frequency dictated velocity gains. Joint motion and angular velocities correspondingly increased with speed in an attempt to maximize pole force production in an abbreviated time frame. The investigators concluded that elite skiers employ a mix of cycle rate and length alteration to control sub-maximal speeds, with frequency playing a greater role at Vmax. Maximum speed generation was further assisted by rapid force development. (Control of speed during the double-poling technique performed by elite cross-country skiers. Med Sci Sports Exer. 41(1) 2009).  

Terrain also plays an obvious role in DP technique, rate and cycle length. Stoggl and Holmberg compared the biomechanics of 13 male elite skiers as they double-poled at “moderate” and “high” speeds on a treadmill which was either deemed flat (1 deg) or inclined (7 deg). 

They found skier “swing” times were much shorter (48%) when skiing “uphill” indicating the utilization of higher frequencies. Peak pole forces were, not surprisingly, 13% higher and occurred 68% later in the poling cycle. Force “impulse” throughout the phase was substantially higher at +87-123% max and directionally oriented 18% more efficiently for propulsion (vertical to horizontal). In general knee, ankle, elbow joint positions were more flexed, shoulders less flexed and less abducted, and torso positions more upright.

Center of mass (COM) raised 25% higher with DP up the incline. Pole plant timing was synchronized to the maximal COM apex, which was not noted while skiing on “flat terrain” (Double-Poling Biomechanics of Elite Cross-Country Skiers: Flat verses Uphill Terrain. Med Sci Sports Exer. 2016). Skiers in this study utilized a higher frequency and more powerful postures and movements to “climb”, again highlighting the need for extreme fitness. 

Biomechanical analysis of the worlds` best skiers has dominated the research for years. Everyone wants to know “what makes them so fast?” For the purposes of this article let`s touch on the points most relevant to coaches and athletes.  

 Double-pole skiing must include the whole body.

 Holmberg HC et al noted a 7.7% reduction in VO2 peak, a 9.4% reduction in Velocity max and a 11.7 % quicker time to exhaustion while DP treadmill skiing when they “braced” participant`s (n=11) ankles and knees, disallowing movement at those joints. Heart rate (HR) and lactate concentrations where also recorded as higher without a change in O2 consumption at sub-maximal efforts. Kinematically, they noted a 13.6% higher poling frequency, a 4.9% shorter poling phase, a 13.3% shorter recovery (swing + prep) phase and 10.9% lower pole force when attempting to ski at 85% Vmax from a locked position (Contribution of the legs to double-pole performance in elite cross-country skiers. Med Sci Sports Exer.2006).  

The dynamic contribution of ankles, knees and hips are inherently linked to fast and efficient DP skiing. This is of course dependent on having a stable foundation from which to generate force. The link between core, lower extremity and upper extremity muscle activity appears to be sequential in nature; slow to fast, central to peripheral (Henneman et al 1965). 

In a comprehensive study published in the journal of the American College of Sports Medicine in 2005, Holmberg HC, Lindinger S, et al gathered data on; ground reaction forces, joint angles, cycle characteristics, muscle electromyography (EMG) and velocity on 11 elite skiers while double poling on a treadmill at 85% of their max DP abilities. I believe it to be a must read for coaches and athletes who want to gain an understanding of DP mechanics and make relevant changes in their conditioning routines and technique suggestions (Biomechanical Analysis of Double Poling in Elite Cross-Country Skiers. Med Sci Sports Exer.37 (5) 2005).  

The information provided by this study is extensive. EMG data obtained from numerous core, upper and lower extremity muscle groups indicated levels of activation as well as specific patterns of utilization based on the phase of the poling cycle. Muscles with the highest activity as a percentage of maximum volitional contraction (MVC) included; abdominal groups of Rectus Abdominus and the Obliques, Teres Major (posterior shoulder), Lat Dorsi, Pec Major and Glut Maximus.

Regarding the sequence of activation, muscle contractions occurring as the skier was in the second half of their Recovery Time (RT), or the preparatory/ pre- loading phase, were closely analyzed. The total RT was noted to be 2.8 times longer than the actual poling time. All of the following muscles were active during the preparatory phase but heightened their EMG output at various points in the overall poling cycle. The first muscles to “turn on” were the abdominals, the Rectus Femoris (long quad muscle and hip flexor) and the posterior shoulder groups. They remained active until the midpoint of poling.  Hip flexors became very active immediately prior to pole strike with simultaneous, but lesser, activity from the pec, latissimus dorsi and gluteus max muscles. The Triceps Surae became very active at pole plant and was the last muscle to “turn off”, just prior to the conclusion of propulsion.  

This is an oversimplification of the firing patterns of major muscle contributors to propulsion, but it does exemplify the central to peripheral, slow to fast twitch trend exhibited by elite DP athletes, specifically, core positioning and stabilization, followed by powerful movements off that platform. 

Joint angular speeds (how quickly a joint completes its range of motion) of the trunk, hips and elbows corresponded to either force application or re-posturing phases (i.e., “crash down”, “hips up “, “quick hands”, “get full extension”).

Regarding joint positions, most notable was the timing of maximal elbow flexion (bend) at the end of the RT/ prep phase The skiers (sub-consciously) placed a quick stretch on the triceps to stimulate the Stretch Shortening Cycle (SSC) mechanism of the muscle. This neuromotor reflex adds to fiber recruitment and speed of contraction via bypassing the brain`s motor cortex.