BLOGS

This article breaks down what changed, why a cycloidal drive makes sense for high-load leg joints, and what the redesign teaches integrators who are choosing between harmonic, planetary and cycloidal options for their own machines.

From bird knees to factory knees: form follows duty cycle

Earlier Digit generations were optimized for dynamic walking. The ostrich-style, backward-bending knee is excellent for that: it keeps the effective leg length long, stores and releases energy efficiently at pace, and suits a robot whose main job was moving totes along predictable routes.

Digit 5's target duty cycle is different. Agility built the new robot around what customers asked for after roughly three years of Digit 4 running on real production floors: heavy, repetitive lifting, deep squats to work near floor level, kneeling, and full coverage of single-person lift tasks under OSHA rules. That is a strength and statics problem as much as a locomotion problem — and it demands a leg that can hold large torques at low speed without overheating or failing under shock.

The result: Digit 5 stands 1.81 m tall, weighs 129 kg, reaches up to 2.2 m, and can repeatedly lift loads up to 22.7 kg (50 lb) — 40% more payload than the previous generation, according to Agility.

Why a cycloidal actuator for the legs

Agility's own materials describe the new legs as "powered by proprietary cycloidal actuator technology." The engineering logic is straightforward:

• Shock tolerance. Cycloidal reducers distribute load across many contact points and tolerate impact loads far better than strain-wave gears. A warehouse robot that drops a bin, misjudges a step, or catches a toe needs joints that survive events, not just cruise.
• Torque density in a compact housing. High reduction ratios in a short, flat package fit the hip and knee envelope without the bulk of a multi-stage planetary stack.
• Backdrive and stiffness balance. Leg joints need enough compliance for terrain contact yet enough stiffness for controlled squats and holds — a balance cycloidal drives hit well in the mid-to-high torque range.

The trade-off is precision at the output. Harmonic drives still win on backlash and compactness for the joints that need sub-arc-minute finesse — which is exactly why most serious humanoid programs, including reports around Agility's own platform, end up with hybrid architectures: cycloidal or planetary for high-torque hip and knee duty, harmonic for shoulders, wrists and other precision joints. We covered the selection logic in detail in Harmonic Drive vs Planetary Reducer: Which Robot Joint Module Fits Your Robot?

The industry is clearly moving this way. Chinese humanoid makers are shipping integrated cycloidal joint modules for legged locomotion, and 2026 market analyses consistently project cycloidal and quasi-direct-drive architectures taking share in lower-body duty while harmonic retains the precision-joint niche.

The numbers behind the redesign

Two of these deserve an actuator engineer's attention. First, the payload jump: 22.7 kg is roughly the ceiling of single-person manual lifts in OSHA-regulated facilities — Agility is targeting exactly the task envelope where a humanoid replaces, rather than assists, human labor. Second, the 10:1 run-to-charge ratio: batteries get the headlines, but sustained 20-hour duty is only useful if the joint modules can dissipate heat and hold efficiency across that cycle. Reducer efficiency and thermal design, not peak torque posters, decide whether a humanoid survives real shifts.

Safety: actuators are only half the story

Digit 5's "cooperatively safe" pitch rests on a 360° people-detection system, visual and audible intent signals, and — critically — an independent safety controller that supervises the main compute and can trigger avoidance, a stop, or a seated shutdown the moment a person gets too close. Agility is the first launch partner for NVIDIA's Halos for Robotics safety platform (pairing Digit 5 with IGX Thor), and contributes to the ANSI/A3 R15.108 technical report and ISO 25785-1, described as the first international safety standard written for humanoids.

The baseline Digit 4 already set

Context matters when judging the redesign: Digit 4 has logged more than 65,000 hours of paid work at customer sites including GXO, Schaeffler, Amazon and Toyota Motor Manufacturing Canada, and passed 100,000 cumulative totes at a GXO facility while holding roughly 98% task accuracy. Agility reports more than $300 million in multi-year orders for Digit 5 as of May 2026, with early access in the first half of 2027 and general availability by the end of 2027.

What joint module buyers should take away

At Ti5ROBOT, we build this selection logic into our product line: our Humanoid Robot Actuator family and lightweight joint modules — such as the 30-40 class joint module — pair high torque density with integrated encoders and drivers, so integrators can match joint topology to task without designing a reducer stack from scratch.

Digit 5 will not ship at scale until 2027, but its actuator choices are visible now — and they confirm where the industry has been heading: hybrid joint architectures, duty-cycle-driven reducer selection, and safety engineered at every level, from the servo loop to the system controller.