The Mechanical Loading Effect of Squatting With Long and Short Femurs

An interesting side-effect of the tremendous increase in the popularity of Olympic lifting in recent years is that more athletes with non-optimal body types are entering the sport and even competing at a high level. Although many principles of squatting do not change regardless of body type, the individual parameters of athlete warrant consideration. This is especially true as athlete training and program design become more specific and refined. Simply put, physics matter and a certain “optimal” styles of lifting that were developed and applied to a population of elite athletes selected based on specific traits may not be the best approach for every lifter in the gym.

It’s generally accepted that short femurs are a beneficial trait for Olympic weightlifting and has been a strong consideration when approaching athlete selection [2]. Femur length has a large impact on squatting because it determines the length of the lever between the hip and the knee, the primary joints in the squat. The impact of femur length is especially pronounced when the athlete is working to maintain an upright torso as it places a large mechanical demand on the knee joint and thus the quadriceps, the primary muscle group responsible for knee extension.

This article isn’t intended to get into technique or training methods, but instead investigates the impact of different femur lengths on the joint loads of the hip and knee during a high-bar back squat.

Methods

To directly compare the effect of segment length, I made a hypothetical lifter who is 100 kg (220 lbs) and 1.8 m tall (70.86 in). I ran three scenarios for the lifter: One where they have “average” segment lengths based on published textbook values, one where their femur length is 5% shorter, and one where their femur length is 5% longer. When I adjusted the femur length of the athlete, I adjusted the length of the trunk as well so the total height of the athlete is the same. Therefore the athlete with the short femurs also had a longer torso and vice versa.

The femur and trunk length of each instance is shown below. For the average case, the femur length was calculated as 23.2% of the athlete’s height and the trunk length was 30%. The way the calculation turned out, the short femur athlete had a thigh segment length 2 cm shorter than average and the long femur athlete has a thigh segment 2 cm longer than average.

Subject Femur Length Trunk Length
Short Femur .398 m .560 m
Average Femur .418 m .540 m
Long Femur .438 m .520 m

To compare the effect of the segment length changes, we’ll assume the athlete is holding the position with their thigh right at parallel (Figure 1). The trunk angle and shin angle will both be set to 60 degrees.

To calculate the moments at the hip and knee, I used inverse dynamics equations utilizing the known downward force of the barbell along with the lengths and orientations of the segments. For the purpose of this blog, I’m won’t go through the derivation however the methods were the same as the calculations done in published research by Lander (Figure 2) [1]. The calculated hip and knee moments for each instance are shown below.

Back Squat Image
Figure 1. Athlete at parallel position in back squat. Orange arrows denote primary force and moment vectors.
Screen Shot 2017-07-12 at 8.53.20 PM
Figure 2. Inverse dynamics calculations referenced from literature.

Results

Subject Hip Moment (N*m) Knee Moment (N*M) Knee Moment Change from Avg
Short Femur 635 384 -16%
Average Femur 613 458
Long Femur 590 532 +16%

To give context to these numbers, this is the amount of torque the muscles need to produce to support the athlete/barbell system in this position. Therefore the higher the moment at the knee, the more work the quadriceps must do. To produce upward motion, the athlete’s muscles must produce a torque greater than the calculated moment below.

Mechanical work at the knee is calculated as the knee moment multiplied by the angular velocity of the knee. This means that if all the athletes extend their knee at the same rate, the long femur does 16% more work than the average athlete, and the short femur athlete does 16% less work. There is no surprise then why athletes with short femurs are built better for squatting.

Discussion

So is it really that bad for athletes with long femurs?

No. A short femur athlete will not just naturally be able to squat 32% more weight than the athlete with long femurs. There are many factors that play into the difficulty of a squat that are not easy to quantify beyond muscle-moments such a where the muscles insert, muscle cross-sectional area, neuromuscular considerations, and motor unit recruitment.

Additionally athletes with longer femurs typically compensate by having a greater trunk tilt while squatting. This serves two purposes: A) Positions the bar more directly over the mid-foot, and B) Increases the moment at the hip, which decreases the moment at the knee. For example, if the athlete with long femurs in this example increases their trunk tilt by 5 degrees to an angle of 55 degrees (relative to horizontal), then the moment at the knee decreases to 445 Nm which is slightly less than the knee moment for the case of the average athlete. However, if the athlete with long femurs is trying to maintain the greatest upright trunk angle as possible as is generally desirable when catching a clean or snatch, then they must compensate with additional ankle flexion (range of motion that many athletes lack). This allows them to maintain the same trunk orientations as other athletes but they will pay the price with a greater joint moment at the knee, making the squat feel more difficult.

Conclusions

Individual athlete characteristics are important and will continue to become more important across all areas of athletics as training programs become more specialized and refined. Although parameters such as segment length are not something we can change, anatomical parameters are something that needs to be considered in the training approach for an athlete.

Key Take-aways
  • Long femurs increase the moment at the knee during the squat, and thus, it requires a greater muscular contribution from the quadriceps to stand up out of the bottom.
  • This is not an excuse not to squat or why your squat is low. If you have long femurs and want to be successful in weightlifting, you will likely need to increase your emphasis on training the squat to overcome this disposition. On the flip side, long femurs and a short torso are ideal for the pull and therefore the athlete would need to focus less on that aspect of the lift.
  • An athlete with long femurs and a short torso may be unable to maintain as upright a torso as other athletes. Although a more upright torso is ideal, and is something that should be pursued, it simply may not be possible for this type of athlete without a tremendous degree of ankle mobility. This is especially important to consider for new to mid-level athletes that want to be good at the lifts but have no aspirations or expectation to be elite.

 

A couple other great, related articles

References:

  1. Lander, Jeffrey E., R. Leslie Simonton, and J. K. Giacobbe. “The effectiveness of weight-belts during the squat exercise.” Medicine and science in sports and exercise 22.1 (1990): 117-126.
  2. Takano, Bob. “Athlete Selection for Weightlifting.” Olympic Weightlifting: Catalyst Athletics. N.p., 29 Aug. 2012. Web.

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